Papers for Tuesday, Feb 16 2021

Velocity field provides a complementary avenue to constrain cosmological information, either through the peculiar velocity surveys or the kinetic Sunyaev Zel'dovich effect. One of the commonly used statistics is the mean radial pairwise velocity. Here, we consider the three-point mean relative velocity, i.e. the mean relative velocities between pairs in a triplet. Using halo catalogs from the Quijote suite of N-body simulations, we first showcase how the analytical prediction for the mean relative velocities between pairs in a triplet achieve better than 4-5% accuracy using standard perturbation theory at leading order for triangular configurations with a minimum separation of $r \geq 50\ h^{-1}$Mpc. Furthermore, we present the three-point relative velocity as a novel probe of neutrino mass estimation. We explore the full cosmological information content of the halo mean pairwise velocities, and the mean relative velocities between halo pairs in a triplet. We undertake this through the Fisher-matrix formalism using 22,000 simulations from the Quijote suite, and considering all triangular configurations with a minimum and a maximum separation of $20\ h^{-1}$Mpc and $120\ h^{-1}$Mpc, respectively. We find that the mean relative velocities in a triplet allows a 1$\sigma$ neutrino mass ($M_\nu$) constraint of 0.065 eV, that is roughly 13 times better than the mean pairwise velocity constraint (0.877 eV). This information gain is not limited only to neutrino mass, but extends to other cosmological parameters: $\Omega_{\mathrm{m}}$, $\Omega_{\mathrm{b}}$, $h$, $n_{\mathrm{s}}$ and $\sigma_{8}$ achieving a gain of 8.9, 11.8, 15.5, 20.9 and 10.9 times respectively. These results illustrate the possibility of exploiting the mean three-point relative velocities for constraining the cosmological parameters accurately from future cosmic microwave background experiments and peculiar velocity surveys.

We present the results regarding the analysis of the fast X-ray/infrared (IR) variability of the black-hole transient MAXI J1535$-$571. The data studied in this work consist of two strictly simultaneous observations performed with XMM-Newton (X-rays: 0.7$-$10 keV), VLT/HAWK-I ($K_{\rm s}$ band, 2.2 $\mu$m) and VLT/VISIR ($M$ and $PAH2$_$2$ bands, 4.85 and 11.88 $\mu$m respectively). The cross-correlation function between the X-ray and near-IR light curves shows a strong asymmetric anti-correlation dip at positive lags. We detect a near-IR QPO (2.5 $\sigma$) at $2.07\pm0.09$ Hz simultaneously with an X-ray QPO at approximately the same frequency ($f_0=2.25\pm0.05$). From the cross-spectral analysis a lag consistent with zero was measured between the two oscillations. We also measure a significant correlation between the average near-IR and mid-IR fluxes during the second night, but find no correlation on short timescales. We discuss these results in terms of the two main scenarios for fast IR variability (hot inflow and jet powered by internal shocks). In both cases, our preliminary modelling suggests the presence of a misalignment between disk and jet.

We present a comprehensive study on how perturbations due to a distribution of $\Lambda$CDM dark matter subhalos can lead to star clusters deviating from their orbits. Through a large suite of massless test particle simulations, we find that (1) subhalos with masses less than $10^8 M_{\odot}$ negligibly affect test particle orbits, (2) perturbations lead to orbital deviations only in environments with substructure fractions $f_{sub} \geq 1\%$, (3) perturbations from denser subhalos produce larger orbital deviations, and (4) subhalo perturbations that are strong relative to the background tidal field lead to larger orbital deviations. To predict how the variation in test particle orbital energy $\sigma_e(t)$ increases with time, we test the applicability of theory derived from single-mass subhalo populations to populations where subhalos have a mass spectrum. We find $\sigma_e(t)$ can be predicted for test particle evolution within a mass spectrum of subhalos by assuming subhalos all have masses equal to the mean subhalo mass and by using the local mean subhalo separation to estimate the change in test particle velocities due to subhalo interactions. Furthermore, the orbital distance variation at an orbital distance $r$ can be calculated via $\sigma_r=2.98 \times 10^{-5} \pm 8 \times 10^{-8} (\rm kpc^{-1} km^{-2} s^{2}) \times r \times \sigma_e$ with a dispersion about the line of best fit equalling 0.08 kpc. Finally, we conclude that clusters that orbit within 100 kpc of Milky Way-like galaxies experience a change no greater than $2\%$ in their dissolution times.

We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the impact of black hole spin on the dynamical formation and evolution of minidisks. We find that during the late inspiral the sizes of minidisks are primarily determined by the interplay between the tidal field and the effective innermost stable orbit around each black hole. Our calculations support that a minidisk forms when the Hill sphere around each black hole is significantly larger than the black hole's effective innermost stable orbit. As the binary inspirals, the radius of the Hill sphere decreases, and minidisk sconsequently shrink in size. As a result, electromagnetic signatures associated with minidisks may be expected to gradually disappear prior to merger when there are no more stable orbits within the Hill sphere. In particular, a gradual disappearance of a hard electromagnetic component in the spectrum of such systems could provide a characteristic signature of merging black hole binaries. For a binary of given total mass, the timescale to minidisk "evaporation" should therefore depend on the black hole spins and the mass ratio. We also demonstrate that accreting binary black holes with spin have a higher efficiency for converting accretion power to jet luminosity. These results could provide new ways to estimate black hole spins in the future.

The upcoming Square Kilometre Array (SKA-Low) will map the distribution of neutral hydrogen during reionization, and produce a tremendous amount of 3D tomographic data. These images cubes will be subject to instrumental limitations, such as noise and limited resolution. Here we present SegU-Net, a stable and reliable method for identification of neutral and ionized regions in these images. SegU-Net is a U-Net architecture based convolutional neural network (CNN) for image segmentation. It is capable of segmenting our image data into meaningful features (ionized and neutral regions) with greater accuracy compared to previous methods. We can estimate the true ionization history from our mock observation of SKA with an observation time of 1000 h with more than 87 per cent accuracy. We also show that SegU-Net can be used to recover various topological summary statistics, such as size distributions and Betti numbers, with a relative difference of only a few per cent. These summary statistics characterise the non-Gaussian nature of the reionization process.

Primordial black holes (PBHs), formed out of large overdensities in the early Universe, are a viable dark matter (DM) candidate over a broad range of masses. Ultra-light, asteroid-mass PBHs with masses around $10^{17}$ g are particularly interesting as current observations allow them to constitute the entire DM density. PBHs in this mass range emit $\sim$ MeV photons via Hawking radiation which can directly be detected by the gamma ray telescopes, such as the upcoming AMEGO. In this work we forecast how well an instrument with the sensitivity of AMEGO will be able to detect, or rule out, PBHs as a DM candidate, by searching for their evaporating signature when marginalizing over the Galactic and extra-Galactic gamma-ray backgrounds. We find that an instrument with the sensitivity of AMEGO could exclude non-rotating PBHs as the only DM component for masses up to $7 \times 10^{17}$ g at 95% confidence level (C.L.) for a monochromatic mass distribution, improving upon current bounds by nearly an order of magnitude. The forecasted constraints are more stringent for PBHs that have rotation, or which follow extended mass distributions.

We study protostellar envelope and outflow evolution using Hubble Space Telescope NICMOS or WFC3 images of 304 protostars in the Orion Molecular clouds. These near-IR images resolve structures in the envelopes delineated by the scattered light of the central protostars with 80 AU resolution and they complement the 1.2-870 micron spectral energy distributions obtained with the Herschel Orion Protostar Survey program (HOPS). Based on their 1.60 micron morphologies, we classify the protostars into five categories: non-detections, point sources without nebulosity, bipolar cavity sources, unipolar cavity sources, and irregulars. We find point sources without associated nebulosity are the most numerous, and show through monochromatic Monte Carlo radiative transfer modeling that this morphology occurs when protostars are observed at low inclinations or have low envelope densities. We also find that the morphology is correlated with the SED-determined evolutionary class with Class 0 protostars more likely to be non-detections, Class I protostars to show cavities and flat-spectrum protostars to be point sources. Using an edge detection algorithm to trace the projected edges of the cavities, we fit power-laws to the resulting cavity shapes, thereby measuring the cavity half-opening angles and power-law exponents. We find no evidence for the growth of outflow cavities as protostars evolve through the Class I protostar phase, in contradiction with previous studies of smaller samples. We conclude that the decline of mass infall with time cannot be explained by the progressive clearing of envelopes by growing outflow cavities. Furthermore, the low star formation efficiency inferred for molecular cores cannot be explained by envelope clearing alone.

Sub-Chandrasekhar mass carbon-oxygen white dwarfs (CO WDs) with a surface helium (He) shell have been proposed as progenitors of Type Ia supernovae (SNe Ia). If true, the resulting thermonuclear explosions should be able to account for at least some of the range of SNe Ia observables. To study this, we conduct a parameter study based on 3D simulations of double detonations in CO WDs with a He shell, assuming different core and shell masses. An admixture of C to the shell and solar metallicity are included in the models. The hydrodynamic simulations are carried out using the AREPO code. This allows us to follow the He shell detonation with high numerical resolution, and improves the reliability of predicted nucleosynthetic shell detonation yields. The addition of C to the shell leads to a lower production of 56Ni while including solar metallicity increases the production of IMEs. The production of higher mass elements is further shifted to stable isotopes at solar metallicity. Moreover, we find different core detonation ignition mechanisms depending on the core and shell mass configuration. This has an influence on the ejecta structure. We present the bolometric light curves predicted from our explosion simulations using the radiative transfer code ARTIS, and make comparisons with SNe Ia data. The bolometric light curves of our models show a range of brightnesses, able to account for sub-luminous to normal SNe Ia. We show the model bolometric width-luminosity relation compared to data for a range of viewing angles. We find that, on average, our brighter models lie within the observed data. The ejecta asymmetries produce a wide distribution of observables, which might account for outliers in the data. However, the models overestimate the extent of this compared to data. We also find the bolometric decline rate over 40 days appears systematically faster than data. (abridged)

Recent evidence based on APOGEE data for stars within a few kpc of the Galactic centre suggests that dissolved globular clusters (GCs) contribute significantly to the stellar mass budget of the inner halo. In this paper we enquire into the origins of tracers of GC dissolution, N-rich stars, that are located in the inner 4 kpc of the Milky Way. From an analysis of the chemical compositions of these stars we establish that about 30% of the N-rich stars previously identified in the inner Galaxy may have an accreted origin. This result is confirmed by an analysis of the kinematic properties of our sample. The specific frequency of N-rich stars is quite large in the accreted population, exceeding that of its in situ counterparts by near an order of magnitude, in disagreement with predictions from numerical simulations. We hope that our numbers provide a useful test to models of GC formation and destruction.

The formation mechanism of the W50/SS433 complex has long been a mystery. We propose a new scenario in which the SS433 jets themselves form the W50/SS433 system. We carry out magnetohydrodynamics simulations of two-side jet propagation using the public code CANS+. As found in previous jet studies, when the propagating jet is lighter than the surrounding medium, the shocked plasma flows back from the jet tip to the core. We find that the morphology of light jets is spheroidal at early times, and afterward, the shell and wings are developed by the broadening spherical cocoon. The morphology strongly depends on the density ratio of the injected jet to the surrounding medium. Meanwhile, the ratio of the lengths of the two-side jets depends only on the density profile of the surrounding medium. We also find that most of the jet kinetic energy is dissipated at the oblique shock formed by the interaction between the backflow and beam flow, rather than at the jet terminal shock. The position of the oblique shock is spatially consistent with the X-ray and TeV gamma-ray hotspots of W50.

Convection and turbulence in core-collapse supernovae (CCSNe) are inherently three-dimensional in nature. However, 3D simulations of CCSNe are computationally demanding. Thus, it is valuable to modify simulations in spherical symmetry to incorporate 3D effects using some parametric model. In this paper, we report on the formulation and implementation of general relativistic (GR) neutrino-driven turbulent convection in the spherically symmetric core-collapse supernova code \texttt{GR1D}. This is based upon the recently proposed method of Supernova Turbulence in Reduced-dimensionality (\textit{STIR}) in Newtonian simulations from \cite{Couch2020_STIR}. When the parameters of this model are calibrated to 3D simulations, we find that our GR formulation of \textit{STIR} requires larger turbulent eddies to achieve a shock evolution similar to the original \textit{STIR} model. We also find that general relativity may alter the correspondence between progenitor mass and successful vs.~failed explosions.

The origin of the Chicxulub impactor, which is attributed as the cause of the K/T mass extinction event, is an unsolved puzzle. The background impact rates of main-belt asteroids and long-period comets have been previously dismissed as being too low to explain the Chicxulub impact event. Here, we show that a fraction of long-period comets are tidally disrupted after passing close to the Sun, each producing a collection of smaller fragments that cross the orbit of Earth. This population could increase the impact rate of long-period comets capable of producing Chicxulub impact events by an order of magnitude. This new rate would be consistent with the age of the Chicxulub impact crater, thereby providing a satisfactory explanation for the origin of the impactor. Our hypothesis explains the composition of the largest confirmed impact crater in Earth's history as well as the largest one within the last million years. It predicts a larger proportion of impactors with carbonaceous chondritic compositions than would be expected from meteorite falls of main-belt asteroids.

In this letter, we propose an accreting stellar binary model for understanding the active periodic Fast radio bursts (FRBs). The system consists of a stellar compact object (CO) and a donor star (DS) companion in an eccentric orbit, where the DS fills its own Roche lobe near the periastron. The CO will accrete the material of the DS and then drive magnetic blobs. FRBs would be produced by the shock process between the magnetic blobs and the stellar wind of the DS through the synchrotron maser mechanism. We show that this model can in principle sufficiently produce highly active FRBs with a long lifetime, and also can naturally explain the periodicity and the duty cycle of the activity as appeared in FRBs 180916.J0158+65 and 121102. The radio nebula excited by the long-term injection of magnetic blobs into the surrounding environment also can account for the persistent radio source associated with FRB 121102. In addiction, we discuss in detail the possible multi-wavelength counterparts of FRB 180916.J0158+65 in the context of this model. Multi-wavelength observations in the future will verify or falsify this model.

We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit observations of the intergalactic medium (IGM) in the Stephan's Quintet (SQ) galaxy group. SQ hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km/s, where large amounts of cold (10-100 K) and warm (100-5000 K) molecular gas coexist with a hot plasma. COS spectroscopy along five lines-of-sight, probing 1 kpc-diameter regions in the IGM, reveals very broad (~2000 km/s) and powerful Ly$\alpha$ line emission with complex line shapes. These Lyman-alpha line profiles are often similar to, or sometimes much broader than line profiles obtained in H$\beta$, [CII], and CO (1-0) emission along the same lines-of-sight. In these cases, the breadth of the Ly$\alpha$ emission, compared with H$\beta$, implies resonance scattering. Line ratios of Ly$\alpha$/H$\beta$ for the two COS pointings closest to the center of the shocked ridge are close to the Case B recombination value, suggesting that at these positions Ly$\alpha$ photons escape through scattering in a low density medium free of dust. Some Ly$\alpha$ spectra show suppressed velocity components compared with [CII] and H$\beta$, implying that some of the Ly$\alpha$ photons are absorbed. Scattering indicates that the neutral gas of the IGM is clumpy, with multiple clumps along a given line of sight. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by the multi-phase IGM in X-rays, Ly$\alpha$, H$_2$, [CII] are comparable within a factor of a few. We suggest that both shocks and mixing layers co-exist and contribute to the energy dissipation associated with a turbulent energy cascade. This may be important for the cooling of gas at higher redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence more common.

The NASA/IPAC Extragalactic Database (NED) is an impressive tool for finding near-exhaustive information on millions of astrophysical objects. Here we outline a small systematic error that occurs in NED because a low-redshift approximation is used when making the correction from redshifts in the heliocentric frame to the cosmic microwave background (CMB) rest frame. It means that historically NED systematically misreported the values of CMB-frame redshifts by up to $\sim10^{-3}z$ (about 0.001 at redshift of 1). While the error is generally negligible for most applications, it is still important to remove systematic redshift biases, as they propagate to cosmological parameters. We have consulted with the NED team and they are updating the software to remove this systematic error so these corrections are accurate at all redshifts. Here we explain the changes and how they impact the redshift values NED currently reports.

The phenomenon of peripheral coronal loop contraction during solar flares and eruptions, recently discovered in observations, gradually intrigues solar physicists. However, its underlying physical mechanism is still uncertain. One is Hudson (2000)'s implosion conjecture which attributes it to magnetic pressure reduction in the magnetic energy liberation core, while other researchers proposed alternative explanations. In previous observational studies we also note the disappearance of peripheral shrinking loops in the late phase, of which there is a lack of investigation and interpretation. In this paper, we exploit a full MHD simulation of solar eruption to study the causes of the two phenomena. It is found that the loop motion in the periphery is well correlated with magnetic energy accumulation and dissipation in the core, and the loop shrinkage is caused by a more significant reduction in magnetic pressure gradient force than in magnetic tension force, consistent with the implosion conjecture. The peripheral contracting loops in the late phase act as inflow to reconnect with central erupting structures, which destroys their identities and naturally explains their disappearance. We also propose a positive feedback between the peripheral magnetic reconnection and the central eruption.

We present a joint likelihood analysis of the real-space power spectrum and bispectrum measured from a variety of halo and galaxy mock catalogs. A novel aspect of this work is the inclusion of nonlinear triangle configurations for the bispectrum, made possible by a complete next-to-leading order ("one-loop") description of galaxy bias, as is already common practice for the power spectrum. Based on the goodness-of-fit and the unbiasedness of the parameter posteriors, we accomplish a stringent validation of this model compared to the leading order ("tree-level") bispectrum. Using measurement uncertainties that correspond to an effective survey volume of $6\,(\mathrm{Gpc}/h)^3$, we determine that the one-loop corrections roughly double the applicable range of scales, from $\sim 0.17\,h/\mathrm{Mpc}$ (tree-level) to $\sim 0.3\,h/\mathrm{Mpc}$. This converts into a $1.5 - 2$x improvement on constraints of the linear bias parameter at fixed cosmology, and a $1.5 - 2.4$x shrinkage of uncertainties on the amplitude of fluctuations $A_s$, which clearly demonstrates the benefit of extracting information from nonlinear scales despite having to marginalize over a larger number of bias parameters. Besides, our precise measurements of galaxy bias parameters up to fourth order allow for thorough comparisons to coevolution relations, showing excellent agreement for all contributions generated by the nonlocal action of gravity. Using these relations in the likelihood analysis does not compromise the model validity and is crucial for obtaining the quoted improvements on $A_s$. We also analyzed the impact of higher-derivative and scale-dependent stochastic terms, finding that for a subset of our tracers the former can boost the performance of the tree-level model with constraints on $A_s$ that are only slightly degraded compared to the one-loop model.

This topical collection summarizes recent advances in observing and modeling irradiance variations of the Sun and Sun-like stars, emphasizing the links between surface magnetic fields and the resulting solar and stellar variability. In particular, the articles composing this collection summarize recent progress in i) solar-irradiance measurements; ii) modeling of solar- and stellar-irradiance variability; and iii) understanding of the effects of such variability on Earth's climate and exoplanet environments. This topical-collection overview article gives background and more details on these aspects of variability.

The loss and gain of volatile elements during planet formation is key for setting their subsequent climate, geodynamics, and habitability. Two broad regimes of volatile element transport in and out of planetary building blocks have been identified: that occurring when the nebula is still present, and that occurring after it has dissipated. Evidence for volatile element loss in planetary bodies after the dissipation of the solar nebula is found in the high Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's minor bodies. This volatile loss is expected to occur when the bodies are heated by planetary collisions and short-lived radionuclides, and enter a global magma ocean stage early in their history. The bulk composition of exo-planetary bodies can be determined by observing white dwarfs which have accreted planetary material. The abundances of Na, Mn, and Mg have been measured for the accreting material in four polluted white dwarf systems. Whilst the Mn/Na abundances of three white dwarf systems are consistent with the fractionations expected during nebula condensation, the high Mn/Na abundance ratio of GD362 means that it is not (>3 sigma). We find that heating of the planetary system orbiting GD362 during the star's giant branch evolution is insufficient to produce such a high Mn/Na. We, therefore, propose that volatile loss occurred in a manner analogous to that of the solar system bodies, either due to impacts shortly after their formation or from heating by short-lived radionuclides. We present potential evidence for a magma ocean stage on the exo-planetary body which currently pollutes the atmosphere of GD362.

Imaging surveys of CO and other molecular transition lines are fundamental to measuring the large-scale distribution of molecular gas in the Milky Way. Due to finite angular resolution and sensitivity, however, observational effects are inevitable in the surveys, but few studies are available on the extent of uncertainties involved. The purpose of this work is to investigate the dependence of observations on angular resolution (beam sizes), sensitivity (noise levels), distances, and molecular tracers. To this end, we use high-quality CO images of a large-scale region (25.8 <l< 49.7 deg and |b|<5 deg) mapped by the Milky Way Imaging Scroll Painting (MWISP) survey as a benchmark to simulate observations with larger beam sizes and higher noise levels, deriving corresponding beam filling and sensitivity clip factors. The sensitivity clip factor is defined to be the completeness of observed flux. Taking the entire image as a whole object, we found that 12CO has the largest beam filling and sensitivity clip factors and C18O has the lowest. For molecular cloud samples extracted from images, the beam filling factor can be described by a characteristic size, $l_{1/4}$=0.762 (in beam size), at which the beam filling factor is approximately 1/4. The sensitivity clip factor shows a similar relationship but is more correlated with the mean voxel signal-to-noise ratio of molecular clouds. This result may serve as a practical reference on beam filling and sensitivity clip factors in further analyses of the MWISP data and other observations.

We report here the results of an Effective Field Theory (EFT) WIMP search analysis using LUX data. We build upon previous LUX analyses by extending the search window to include nuclear recoil energies up to $\sim$180 keV$_{nr}$, requiring a reassessment of data quality cuts and background models. In order to use a binned Profile Likelihood statistical framework, the development of new analysis techniques to account for higher-energy backgrounds was required. With a 3.14$\times10^4$ kg$\cdot$day exposure using data collected between 2014 and 2016, we set 90\% C.L. exclusion limits on non-relativistic EFT WIMP couplings to neutrons and protons, providing the most stringent constraints on a significant fraction of the possible EFT WIMP interactions. Additionally, we report world-leading exclusion limits on inelastic EFT WIMP-nucleon recoils.

We develop a model of the generation of coherent radio emission in the Crab pulsar, magnetars and Fast Radio Bursts (FRBs). Emission is produced by a reconnection-generated beam of particles via a Free Electron Laser (FEL) mechanism, operating in a weakly-turbulent, guide-field dominated plasma; the radio emission is not rotationally but reconnection powered. Alfvenic turbulence creates both the FEL wiggler and, via a ponderomotive force, charge bunches in the beam that Compton scatter the wiggler field coherently. The model is both robust to the underlying plasma parameters and succeeds in reproducing a number of subtle observed features: (i) emission frequencies depend mostly on the properties of turbulence and the Lorentz factor of the reconnection generated beam, $\omega \sim \gamma_0^2 ( c k_w) $ - the emission frequency is independent of the absolute value of the underlying magnetic field. (ii) The model explains both broadband emission and the presence of emission stripes, including multiple stripes observed in the High Frequency Interpulse of the Crab pulsar. (iii) The model reproduces correlated polarization properties: presence of narrow emission bands in the spectrum favors linear polarization, while broadband emission can have arbitrary polarization; this matches the FRB phenomenology. (iv) The mechanism is robust to the momentum spread of the particle beam. (v) The model even reproduces brightness temperatures observed in FRB and Crab GPs ($\sim 10^{40}$ K and $\sim 10^{33}$ K correspondingly). Presence of several distinct harmonic wigglers may further enhance the efficiency of the FEL due to large amplitude beat oscillations and the creation of longer living density patterns. We also discuss a model of wigglers as non-linear force-free Alfven solitons, limited both in the transverse and longitudinal directions.

Sgr B1 is a luminous H II region in the Galactic Center immediately next to the massive star-forming giant molecular cloud Sgr B2 and apparently connected to it from their similar radial velocities. In 2018 we showed from SOFIA FIFI-LS observations of the [O III] 52 and 88 micron lines that there is no central exciting star cluster and that the ionizing stars must be widely spread throughout the region. Here we present SOFIA FIFI-LS observations of the [O I] 146 and [C II] 158 micron lines formed in the surrounding photodissociation regions (PDRs). We find that these lines correlate neither with each other nor with the [O III] lines although together they correlate better with the 70 micron Herschel PACS images from Hi-GAL. We infer from this that Sgr B1 consists of a number of smaller H II regions plus their associated PDRs, some seen face-on and the others seen more or less edge-on. We used the PDR Toolbox to estimate densities and the far-ultraviolet intensities exciting the PDRs. Using models computed with Cloudy, we demonstrate possible appearances of edge-on PDRs and show that the density difference between the PDR densities and the electron densities estimated from the [O III] line ratios is incompatible with pressure equilibrium unless there is a substantial pressure contribution from either turbulence or magnetic field or both. We likewise conclude that the hot stars exciting Sgr B1 are widely spaced throughout the region at substantial distances from the gas with no evidence of current massive star formation.

Strong line metallicity calibrations are widely used to determine the gas phase metallicities of individual HII regions and entire galaxies. Over a decade ago, based on the Sloan Digital Sky Survey Data Release 4 (SDSS DR4), Kewley \& Ellison published the coefficients of third-order polynomials that can be used to convert between different strong line metallicity calibrations for global galaxy spectra. Here, we update the work of Kewley \& Ellison in three ways. First, by using a newer data release (DR7), we approximately double the number of galaxies used in polynomial fits, providing statistically improved polynomial coefficients. Second, we include in the calibration suite five additional metallicity diagnostics that have been proposed in the last decade and were not included by Kewley \& Ellison. Finally, we develop a new machine learning approach for converting between metallicity calibrations. The random forest algorithm is non-parametric and therefore more flexible than polynomial conversions, due to its ability to capture non-linear behaviour in the data. The random forest method yields the same accuracy as the (updated) polynomial conversions, but has the significant advantage that a single model can be applied over a wide range of metallicities, without the need to distinguish upper and lower branches in $R_{23}$ calibrations. The trained random forest is made publicly available for use in the community.

We present new X-ray and UV observations of the Wolf-Rayet + black hole binary system NGC 300 X-1 with the Chandra X-ray Observatory and the Hubble Space Telescope Cosmic Origins Spectrograph. When combined with archival X-ray observations, our X-ray and UV observations sample the entire binary orbit, providing clues to the system geometry and interaction between the black hole accretion disk and the donor star wind. We measure a binary orbital period of 32.7921$\pm$0.0003 hr, in agreement with previous studies, and perform phase-resolved spectroscopy using the X-ray data. The X-ray light curve reveals a deep eclipse, consistent with inclination angles of $i=60-75^{\circ}$, and a pre-eclipse excess consistent with an accretion stream impacting the disk edge. We further measure radial velocity variations for several prominent FUV spectral lines, most notably He II $\lambda$1640 and C IV $\lambda$1550. We find that the He II emission lines systematically lag the expected Wolf-Rayet star orbital motion by a phase difference $\Delta \phi\sim0.3$, while C IV $\lambda$1550 matches the phase of the anticipated radial velocity curve of the Wolf-Rayet donor. We assume the C IV $\lambda$1550 emission line follows a sinusoidal radial velocity curve (semi-amplitude = 250 km s$^{-1}$) and infer a BH mass of 17$\pm$4 M$_{\odot}$. Our observations are consistent with the presence of a wind-Roche lobe overflow accretion disk, where an accretion stream forms from gravitationally focused wind material and impacts the edge of the black hole accretion disk.

Namibia is world-renowned for its incredibly dark skies by the astronomy community, and yet, the country is not well recognised as a dark sky destination by tourists and travellers. Forged by a collaboration between the Universities of Oxford and Namibia, together we are using astronomy as a means for capacity-building and sustainable socio-economic growth via educating tour guides and promoting dark sky tourism to relevant stakeholders.

We present simultaneous measurements of emission from dust continuum at 230 GHz and the J=2-1 $^{12}$CO, $^{13}$CO and C$^{18}$O isotopologues at $\sim$ 15 pc resolution from individual Giant Molecular Clouds (GMCs) in the Andromeda galaxy (M31). These observations were obtained in an ongoing survey of this galaxy being conducted with the Submillimeter Array (SMA). Initial results describing the continuum and $^{12}$CO emission were published earlier. Here we primarily analyze the observations of $^{13}$CO and C$^{18}$O emission and compare them to the measurements of dust continuum and $^{12}$CO emission. We also report additional dust continuum and CO measurements from newly added GMCs to the M31 sample. We detect spatially resolved $^{13}$CO emission with high signal-to-noise in 31 objects. We find the extent of the $^{13}$CO emission to be nearly comparable to that of $^{12}$CO, typically covering 75\% of the area of the $^{12}$CO emission. We derive $^{13}$CO and C$^{18}$O abundances of 2.9 $\times 10^{-6}$ and 4.4 $\times 10^{-7}$ relative to H$_2$, respectively, by comparison with hydrogen column densities of the same regions derived from the dust continuum observations assuming a Milky Way gas-to-dust ratio. We find the isotopic abundance ratio [$^{13}$CO]/[C$^{18}$O] = 6.7$\pm$2.9 to be consistent with the Milky Way value (8.1). Finally, we derive the mass-to-light conversion factors for all three CO species to be $\alpha_{12} = 8.7 \pm 3.9$, $\alpha_{13} = 48.9 \pm 20.4$ and $\alpha_{18} = 345^{+25}_{-31}$ M$_\odot$ (K km s$^{-1}$pc$^2$)$^{-1}$ for the J=2-1 transitions of $^{12}$CO, $^{13}$CO and C$^{18}$O, respectively.

We introduce statistical techniques required to handle complex computer models with potential applications to astronomy. Computer experiments play a critical role in almost all fields of scientific research and engineering. These computer experiments, or simulators, are often computationally expensive, leading to the use of emulators for rapidly approximating the outcome of the experiment. Gaussian process models, also known as Kriging, are the most common choice of emulator. While emulators offer significant improvements in computation over computer simulators, they require a selection of inputs along with the corresponding outputs of the computer experiment to function well. Thus, it is important to select inputs judiciously for the full computer simulation to construct an accurate emulator. Space-filling designs are efficient when the general response surface of the outcome is unknown, and thus they are a popular choice when selecting simulator inputs for building an emulator. In this tutorial we discuss how to construct these space filling designs, perform the subsequent fitting of the Gaussian process surrogates, and briefly indicate their potential applications to astronomy research.

Self-similar solutions to converging (implosions) and diverging (explosions) shocks have been studied before, in planar, cylindrical or spherical symmetry. Here we offer a unified treatment of these apparently disconnected problems . We study the flow of an ideal gas with adiabatic index $\gamma$ with initial density $\rho\sim r^{-\omega}$, containing a strong shock wave. We characterize the self-similar solutions in the entirety of the parameter space $\gamma,\omega$, and draw the connections between the different geometries. We find that the so-called "gap" in diverging shocks does not exist in planar geometry, and that only type II self-similar solutions are valid in converging shocks. We also find that in some cases, a converging shock might not create a reflected shock after its convergence. Finally, we derive analytical approximations for the similarity exponent in the entirety of parameter space.

We have developed a method for estimating the properties of the progenitor dwarf galaxy from the tidal stream of stars that were ripped from it as it fell into the Milky Way. In particular, we show that the mass and radial profile of a progenitor dwarf galaxy evolved along the orbit of the Orphan Stream, including the stellar and dark matter components, can be reconstructed from the distribution of stars in the tidal stream it produced. We use MilkyWay@home, a PetaFLOPS-scale distributed supercomputer, to optimize our dwarf galaxy parameters until we arrive at best-fit parameters. The algorithm fits the dark matter mass, dark matter radius, stellar mass, radial profile of stars, and orbital time. The parameters are recovered even though the dark matter component extends well past the half light radius of the dwarf galaxy progenitor, proving that we are able to extract information about the dark matter halos of dwarf galaxies from the tidal debris. Our simulations assumed that the Milky Way potential, dwarf galaxy orbit, and the form of the density model for the dwarf galaxy were known exactly; more work is required to evaluate the sources of systematic error in fitting real data. This method can be used to estimate the dark matter content in dwarf galaxies without the assumption of virial equilibrium that is required to estimate the mass using line-of-sight velocities. This demonstration is a first step towards building an infrastructure that will fit the Milky Way potential using multiple tidal streams.

The advent of global mm-band Very Long Baseline Interferometry (VLBI) in recent years has finally revealed the morphology of the base of the two most prominent nearby, bright, extragalactic radio jets in M\,87 and 3C\,84. The images are quite surprising considering the predictions of jet theory and current numerical modeling. The jet bases are extremely wide compared to expectations and the nucleus of 3C\,84 is very complicated. It appears as a double in 86\,GHz observations with 50\,$\mu$as resolution and a triple nucleus with 30\,$\mu$as resolution with space-based VLBI by RadioAstron at 22\,GHz. What is even odder is that the double and triple are arranged along an east-west line that is approximately orthogonal to the north-south large scale jet on 150\,$\mu$as $-$ 4\,mas scales. We explore the emergence of an (east-west) double nucleus in the lower resolution 43\,GHz Very Long Baseline Array (VLBA) imaging from August 2018 to April 2020. The double is marginally resolved. We exploit the east-west resolution associated with the longest baselines, $\sim 0.08$\,mas, to track a predominantly east-west separation speed of $\approx 0.086\pm 0.008$\,c. We estimate that the observed mildly relativistic speed persists over a de-projected distance of $\sim 1900-9800$ times the central, supermassive black hole, gravitational radius ($\sim 0.3-1.5$\,lt-yrs) from the point of origin.

We generate constrained realizations (CRs) of the density and peculiar velocity fields within $200 \; h^{-1} \, \mathrm{Mpc}$ from the final release of the Two-Micron All-Sky Redshift Survey (2MRS) $-$ the densest all-sky redshift survey to date. The CRs are generated by combining a Wiener filter estimator in spherical Fourier-Bessel space with random realizations of log-normally distributed density fields and Poisson-sampled galaxy positions. The algorithm is tested and calibrated on a set of semi-analytic mock catalogs mimicking the environment of the Local Group (LG), to rigorously account for the statistical and systematic errors of the reconstruction method. By comparing our peculiar velocity CRs with the observed velocities from the Cosmicflows-3 catalog, we constrain the normalized linear growth rate to $f \sigma_8^\mathrm{lin} = 0.363 \pm 0.070$, which is consistent with the latest Planck results as well as other direct probes. Simultaneously, we estimate a bulk flow contribution from sources beyond the 2MRS reconstruction volume of $B^\mathrm{ext} = 164 \pm 68 \; \mathrm{km} \, \mathrm{s}^{-1}$ towards $l = 311 \pm 24^\circ$, $b = 0 \pm 23^\circ$. The total reconstructed velocity field at the position of the LG, smoothed with a $1 \; h^{-1} \, \mathrm{Mpc}$ Gaussian, is $552 \pm 71 \; \mathrm{km} \, \mathrm{s}^{-1}$ towards $l = 274.3 \pm 7.9^\circ$, $b = 33.9 \pm 8.1^\circ$, in good agreement with the observed CMB dipole. The total reconstructed bulk flow within different radii is compatible with other measurements. Within a $50 \; h^{-1} \, \mathrm{Mpc}$ Gaussian window we find a bulk flow of $229 \pm 45 \; \mathrm{km} \, \mathrm{s}^{-1}$ towards $l = 295 \pm 11^\circ$, $b = 5 \pm 11^\circ$. The code used to generate the CRs and obtain these results, dubbed CORAS, is made publicly available.

Building on recent work by Chandar et al. (2020), we construct X-ray luminosity functions (XLFs) for different classes of X-ray binary (XRB) donors in the nearby star-forming galaxy M83 through a novel methodology: rather than classifying low- vs. high-mass XRBs based on the scaling of the number of X-ray sources with stellar mass and star formation rate, respectively, we utilize multi-band Hubble Space Telescope imaging data to classify each Chandra-detected compact X-ray source as a low-mass (i.e. donor mass <~ 3 solar masses), high-mass (donor mass >~ 8 solar masses) or intermediate-mass XRB based on either the location of its candidate counterpart on optical color-magnitude diagrams or the age of its host star cluster. In addition to the the standard (single and/or truncated) power-law functional shape, we approximate the resulting XLFs with a Schechter function. We identify a marginally significant (at the 1-to-2 sigma level) exponential downturn for the high-mass XRB XLF, at logLx ~ 38.48^{+0.52}_{-0.33} (in log CGS units). In contrast, the low- and intermediate-mass XRB XLFs, as well as the total XLF of M83, are formally consistent with sampling statistics from a single power-law. Our method suggests a non-negligible contribution from low- and possibly intermediate-mass XRBs to the total XRB XLF of M83, i.e. between 20 and 50%, in broad agreement with X-ray based XLFs. More generally, we caution against considerable contamination from X-ray emitting supernova remnants to the published, X-ray based XLFs of M83, and possibly all actively star-forming galaxies.

We report on the X-ray properties of the new transient Swift J0840.7$-$3516, discovered with Swift/BAT in 2020 February, using extensive data of Swift, MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after the discovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, and then remained almost constant over 9 months. Large-amplitude short-term variations on time scales of 1--$10^4$ s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law shaped spectrum with a photon index of $\sim 1.0$ extending up to $\sim 30$ keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became $\sim 2$ at the end of the decay. A spectral absorption feature at 3--4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.

By fitting high-quality and simultaneous multi-wavelength (MWL) spectral energy distributions (SEDs) at multiple epochs with a one-zone leptonic jet model, we study jet properties of the three famous blazars Mrk 421, 3C 454.3 and 3C 279. In the jet model, the emitting electron energy distributions (EEDs) are calculated by solving the kinetic equation of electron injection, escape, adiabatic and radiative energy losses. To explore multi-dimensional parameter space systematically, we employ a Markov chain Monte Carlo (MCMC) fitting technique. The properties of emission regions we derived here are consistent with those in previous studies, e.g., the particle-dominated and low-magnetization jet. The new finding is that there is a tight correlation between $\gamma$-ray luminosity and electron injection power and an anti-correlation between $\gamma$-ray luminosity and jet magnetization parameter. The results suggest that same energy-dissipative mechanism (like a shock) could be operating in the jets of different types of blazars, and the origin of $\gamma$-ray flares is associated with the particle acceleration process.

We present the analyses of two microlensing events, OGLE-2018-BLG-0567 and OGLE-2018-BLG-0962. In both events, the short-lasting anomalies were densely and continuously covered by two high-cadence surveys. The light-curve modeling indicates that the anomalies are generated by source crossings over the planetary caustics induced by planetary companions to the hosts. The estimated planet/host separation (scaled to the angular Einstein radius $\theta_{\rm E}$) and mass ratio are $(s, q) = (1.81, 1.24\times10^{-3})$ and $(s, q) = (1.25, 2.38\times10^{-3})$, respectively. From Bayesian analyses, we estimate the host and planet masses as $(M_{\rm h}, M_{\rm p}) = (0.24_{-0.13}^{+0.16}\,M_{\odot}, 0.32_{-0.16}^{+0.34}\,M_{\rm J})$ and $(M_{\rm h}, M_{\rm p}) = (0.55_{-0.29}^{+0.32}\,M_{\odot}, 1.37_{-0.72}^{+0.80}\,M_{\rm J})$, respectively. These planetary systems are located at a distance of $7.07_{-1.15}^{+0.93}\,{\rm kpc}$ for OGLE-2018-BLG-0567 and $6.47_{-1.73}^{+1.04}\,{\rm kpc}$ for OGLE-2018-BLG-0962, suggesting that they are likely to be near the Galactic bulge. The two events prove the capability of current high-cadence surveys for finding planets through the planetary-caustic channel. We find that most published planetary-caustic planets are found in Hollywood events in which the source size strongly contributes to the anomaly cross section relative to the size of the caustic.

Symbiotic stars show emission across the electromagnetic spectrum from a wide array of physical processes. At cm-waves both synchrotron and thermal emission is seen, often highly variable and associated with outbursts in the optical and X-rays. Most models of the radio emission include an ionized region within the dense wind of the red giant star, that is kept ionized by activity on the white dwarf companion or its accretion disk. In some cases there is on-going shell burning on the white dwarf due to its high mass accretion rate or a prior nova eruption, in other cases nuclear fusion occurs only occasionally as recurrent nova events. In this study we measure the spectral indices of a sample of symbiotic systems in the Southern Hemisphere using the Australia Telescope Compact Array. Putting our data together with results from other surveys, we derive the optical depths and brightness temperatures of some well-known symbiotic stars. Using parallax distances from Gaia Data Release 3, we determine the sizes and characteristic electron densities in the radio emission regions. The results show a range of a factor of 10^4 in radio luminosity, and a factor of 100 in linear size. These numbers are consistent with a picture where the rate of shell burning on the white dwarf determines the radio luminosity. Therefore, our findings also suggest that radio luminosity can be used to determine whether a symbiotic star is powered by accretion alone or also by shell burning.

We have conducted a systematic survey for z $<$ 0.04 active Galactic nuclei (AGNs) that may have changed spectral class over the past decade. We use SkyMapper, Pan-STARRS and the V\'eron-Cetty & V\'eron (2010) catalogue to search the entire sky for these ``changing-look'' AGNs using a variety of selection methods, where Pan-STARRS has a coverage of 3$\pi$ steradians (sky north of Declination $-30^\circ$) and SkyMapper has coverage of $\sim$ 21,000$~\rm{deg^2}$ (sky south of Declination $0^\circ$). We use small aperture photometry to measure how colour and flux have changed over time, where a change may indicate a change in spectral type. Optical colour and flux are used as a proxy for changing H$\alpha$ equivalent width, while WISE 3.4 $\mu$m flux is used to look for changes in the hot dust component. We have identified four AGNs with varying spectra selected using our optical colour selection method. Three AGNs were confirmed from recent observations with WiFeS on the 2.3 m telescope at Siding Spring and the other was identified from archival spectra alone. From this, we identify two new changing look AGNs; NGC 1346 and 2MASX J20075129-1108346. We also recover Mrk 915 and Mrk 609, which are known to have varying spectra in the literature, but they do not meet our specific criteria for changing look AGNs.

We present an overview of the SkyMapper optical follow-up program for gravitational-wave event triggers from the LIGO/Virgo observatories, which aims at identifying early GW170817-like kilonovae out to $\sim 200$ Mpc distance. We describe our robotic facility for rapid transient follow-up, which can target most of the sky at $\delta<+10\deg $ to a depth of $i_\mathrm{AB}\approx 20$ mag. We have implemented a new software pipeline to receive LIGO/Virgo alerts, schedule observations and examine the incoming real-time data stream for transient candidates. We adopt a real-bogus classifier using ensemble-based machine learning techniques, attaining high completeness ($\sim$98%) and purity ($\sim$91%) over our whole magnitude range. Applying further filtering to remove common image artefacts and known sources of transients, such as asteroids and variable stars, reduces the number of candidates by a factor of more than 10. We demonstrate the system performance with blind-search data obtained for GW190425, a binary neutron star merger detected during the LIGO/Virgo O3 observing campaign. In time for the LIGO/Virgo O4 run, we will have deeper reference images allowing transient detection to $i_\mathrm{AB}\approx $21 mag.

The light curves (LC) for Supernova (SN) can be modeled adopting the conversion of the flux of kinetic energy into radiation. This conversion requires an analytical or a numerical law of motion for the expanding radius of the SN. In the framework of conservation of energy for the thin layer approximation we present a classical trajectory based on a power law profile for the density, a relativistic trajectory based on the Navarro--Frenk--White profile for the density, and a relativistic trajectory based on a power law behaviour for the swept mass. A detailed simulation of the LC requires the evaluation of the optical depth as a function of time. We modeled the LC of SN~1993J in different astronomical bands, the LC of GRB 050814 and the LC GRB 060729 in the keV region. The time dependence of the magnetic field of equipartition is derived from the theoretical formula for the luminosity.

For the duration of the Fermi Gamma-Ray Space Telescope's mission, approximately one-third of the point sources detected have been noted as "unassociated," meaning that they seem to have no known counterpart at any other wavelength/frequency. This mysterious part of the gamma-ray sky is perhaps one of the largest unknowns in current astronomical pursuits, and as such has been probed extensively by various techniques at various frequencies. Radio frequencies have been perhaps one of the most fruitful, producing a large fraction of the identified and associated Active Galactic Nuclei (AGN) and pulsars noted in each update of the point source catalogs. Here we present a catalog of 7432 radio counterpart candidates for unassociated gamma-ray fields in the 2nd Data Release of the 4th Fermi Point Source Catalog (4FGL-DR2). A description of the catalog and source types is provided followed by a discussion that demonstrates how the results of this work will aid new associations and identifications. As part of this work, we also present a first catalog derived from "quicklook" images of the Very Large Array Sky Survey (VLASS).

We show that black holes with a Schwarzschild radius of the order of the electroweak scale may act as seeds for the baryon number violation within the Standard model via sphaleron transitions. The corresponding rate is faster than the one in the pure vacuum and baryon number violation around black holes can take place during the evolution of the universe after the electroweak phase transition. We show however that this does not pose any threat for a pre-existing baryon asymmetry in the universe.

The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, $A^+_{\mathrm{rh}}$, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate $A^+_{\mathrm{rh}}$ for 12 open clusters (OCs) using main-sequence turn-off stars as a reference population. The BSS as well as the main-sequence turn-off stars for these clusters are identified using the proper motions and parallaxes from the Gaia DR2 data, with spectroscopically confirmed BSS populations for some of these clusters available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of $A^+_{\mathrm{rh}}$ and other markers of dynamical ages of the clusters, i.e., the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors.

Published high-resolution rotation-vibration transitions of \spec{h212c16o}, the principal isotopologue of methanal, are analyzed using the MARVEL (Measured Active Rotation-Vibration Energy Levels) procedure. The literature results are augmented by new, high-accuracy measurements of pure rotational transitions within the ground, $\nu_3$, $\nu_4$, and $\nu_6$ vibrational states. Of the \nbNonRedTr\ non-redundant transitions processed, which come from \nbSr\ sources including the present work, \nbValTr\ could be validated, providing \nbEl\ empirical energy levels of \spec{h212c16o}\ with statistically well-defined uncertainties. All the empirical rotational-vibrational energy levels determined are used to improve the accuracy of ExoMol's AYTY line list for hot formaldehyde. The complete list of collated experimental transitions, the empirical energy levels determined, as well as the extended and improved line list are provided as Supplementary Material.

We report the discovery of possible periodic X-ray dips in a pulsating ultraluminous X-ray source, M51 ULX-7, with the archival Changra observations. With ~20 days of monitoring in the superorbital descending state, we discovered three dips with separations of ~2 and ~8 days via the Bayesian block technique. A phase-dispersion minimization and a $\chi^2$ test suggest that the dip is likely recurrent with a period of ~2 days, consistent with the orbital period of M51 ULX-7. We interpret the dip as an obscuring of the emission from the pulsar by the vertical structure on the stream-disk interaction region or the atmosphere of the companion star. Both interpretations suggest the viewing angle to be ~60 degrees. Given that the magnetic field of M51 ULX-7 is moderately high, $B\sim10^{13}$ G, low geometric beaming with $b\lesssim1/2$ is sufficient to explain the observed flux and the presence of dips. Obscuration of the stellar wind remains an alternative possible origin and further monitoring of the dips will be required.

Accurate forecasting of the arrival time and arrival speed of coronal mass ejections (CMEs) is a unsolved problem in space weather research. In this study, a comparison of the predicted arrival times and speeds for each CME based, independently, on the inputs from the two STEREO vantage points is carried out. We perform hindcasts using ELlipse Evolution model based on Heliospheric Imager observations (ELEvoHI) ensemble modelling. An estimate of the ambient solar wind conditions is obtained by the Wang-Sheeley-Arge/Heliospheric Upwind eXtrapolation (WSA/HUX) model combination that serves as input to ELEvoHI. We carefully select 12 CMEs between February 2010 and July 2012 that show clear signatures in both STEREO-A and STEREO-B HI time-elongation maps, that propagate close to the ecliptic plane, and that have corresponding in situ signatures at Earth. We find a mean arrival time difference of 6.5 hrs between predictions from the two different viewpoints, which can reach up to 9.5 hrs for individual CMEs, while the mean arrival speed difference is 63 km s$^{-1}$. An ambient solar wind with a large speed variance leads to larger differences in the STEREO-A and STEREO-B CME arrival time predictions ($cc~=~0.92$). Additionally, we compare the predicted arrivals, from both spacecraft, to the actual in situ arrivals at Earth and find a mean absolute error of 7.5 $\pm$ 9.5 hrs for the arrival time and 87 $\pm$ 111 km s$^{-1}$ for the arrival speed. There is no tendency for one spacecraft to provide more accurate arrival predictions than the other.

In solar physics, a severe numerical challenge for modern simulations is properly representing a transition region between the million-degree hot corona and a much cooler plasma of about 10000 K (e.g., the upper chromosphere or a prominence). In previous 1D hydrodynamic simulations, the transition region adaptive conduction (TRAC) method has been proven to capture aspects better that are related to mass evaporation and energy exchange. We aim to extend this method to fully multidimensional magnetohydrodynamic (MHD) settings, as required for any realistic application in the solar atmosphere. Because modern MHD simulation tools efficiently exploit parallel supercomputers and can handle automated grid refinement, we design strategies for any-dimensional block grid-adaptive MHD simulations. We propose two different strategies and demonstrate their working with our open-source MPI-AMRVAC code. We benchmark both strategies on 2D prominence formation based on the evaporation--condensation scenario, where chromospheric plasma is evaporated through the transition region and then is collected and ultimately condenses in the corona. A field-line-based TRACL method and a block-based TRACB method are introduced and compared in block grid-adaptive 2D MHD simulations. Both methods yield similar results and are shown to satisfactorily correct the underestimated chromospheric evaporation, which comes from a poor spatial resolution in the transition region. Because fully resolving the transition region in multidimensional MHD settings is virtually impossible, TRACB or TRACL methods will be needed in any 2D or 3D simulations involving transition region physics.

Coronal Mass Ejections (CMEs) are large-scale eruptions from the Sun into interplanetary space. Despite being major space weather drivers, our knowledge of the CME properties in the inner heliosphere remains constrained by the scarcity of observations at distances other than 1 au. Furthermore, most CMEs are observed in situ by single spacecraft, requiring numerical models to complement the sparse observations available. We aim to assess the ability of the linear force-free spheromak CME model in EUHFORIA to describe the radial evolution of interplanetary CMEs, yielding new context for observational studies. We model one well-studied CME, and investigate its radial evolution by placing virtual spacecraft along the Sun-Earth line in the simulation domain. To directly compare observational and modelling results, we characterise the interplanetary CME signatures between 0.2 and 1.9 au from modelled time series, exploiting techniques traditionally employed to analyse real in situ data. Results show that the modelled radial evolution of the mean solar wind and CME values is consistent with observational and theoretical expectations. The CME expands as a consequence of the decaying pressure in the surrounding wind: the expansion is rapid within 0.4 au, and moderate at larger distances. The early rapid expansion could not explain the overestimated CME radial size in our simulation, suggesting this is an intrinsic limitation of the spheromak geometry used. The magnetic field profile indicates a relaxation of the CME during propagation, while ageing is most probably not a substantial source of magnetic asymmetry beyond 0.4 au. We also report a CME wake that is significantly shorter than suggested by observations. Overall, EUHFORIA provides a consistent description of the radial evolution of solar wind and CMEs; nevertheless, improvements are required to better reproduce the CME radial extension.

Aims. Validate the use of ExPRES as an observation planning tool for the JUICE mission. Methods. We simulate the occultations of the Jovian auroral radio emissions during the Galileo flybys of the Galilean moons of Jupiter. The ExPRES simulations runs are configured using fixed typical parameters for the main aurora radio emissions. We compare the simulation run results with the actual Galileo PWS observations. Results. The ExPRES code accurately models the temporal occurrence of the occultations in the whole spectral range observed by Galileo PWS. Conclusions. The method can be applied for preparing the JUICE moon flyby science operation planning.

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster.

The CM carbonaceous chondrite meteorites provide a record of low temperature aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of 200C to over 750C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300 to 500C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (over 500C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from 13 wt percent to 3 wt percent and a subsequent reduction in the intensity of the 3 micron feature in IR spectra. We estimate that the heated CM chondrites have lost 15 to 65 percent of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of 20 to 50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu.

Classification of sources is one of the most important tasks in astronomy. Sources detected in one wavelength band, for example using gamma rays, may have several possible associations in other wavebands or there may be no plausible association candidates. In this work, we aim to determine probabilistic classification of unassociated sources in the third and the fourth Fermi Large Area Telescope (LAT) point source catalogs (3FGL and 4FGL-DR2) into two classes (pulsars and active galactic nuclei (AGNs)) or three classes (pulsars, AGNs, and other sources). We use several machine learning (ML) methods to determine probabilistic classification of Fermi-LAT sources. We evaluate the dependence of results on meta-parameters of the ML methods, such as the maximal depth of the trees in tree-based classification methods and the number of neurons in neural networks. We determine probabilistic classification of both associated and unassociated sources in 3FGL and 4FGL-DR2 catalogs. We cross-check the accuracy by comparing the predicted classes of unassociated sources in 3FGL that have associations in 4FGL-DR2. We find that in the 2-class case it is important to correct for the presence of other sources among the unassociated ones in order to realistically estimate the number of pulsars and AGNs. In particular, the estimated number of pulsars in the 3FGL (4FGL-DR2) catalog is 270 (481) in the 2-class case without corrections for the other sources and 158 (214) in the 3-class case. Provided that the number of associated pulsars is 167 (271) in the 3FGL (4FGL-DR2) catalog, the number of pulsars among the unassociated sources is expected to be similar or larger than the number of associated ones.

We describe a comprehensive model for systems locked in the Laplace resonance. The framework is based on the simplest possible dynamical structure provided by the Keplerian problem perturbed by the resonant coupling truncated at second order in the eccentricities. The reduced Hamiltonian, constructed by a transformation to resonant coordinates, is then submitted to a suitable ordering of the terms and to the study of its equilibria. Henceforth, resonant normal forms are computed. The main result is the identification of two different classes of equilibria. In the first class, only one kind of stable equilibrium is present: the paradigmatic case is that of the Galilean system. In the second class, three kinds of stable equilibria are possible and at least one of them is characterised by a high value of the forced eccentricity for the `first planet': here the paradigmatic case is the exo-planetary system GJ-876, in which the combination of libration centers admits triple conjunctions otherwise not possible in the Galilean case. The normal form obtained by averaging with respect to the free eccentricity oscillations, describes the libration of the Laplace argument for arbitrary amplitudes and allows us to determine the libration width of the resonance. The agreement of the analytic predictions with the numerical integration of the toy models is very good.

We constrain the redshift dependence of gas pressure bias $\left\langle b_{y} P_{\mathrm{e}}\right\rangle$ (bias-weighted average electron pressure), which characterises the thermodynamics of intergalactic gas, through a combination of cross-correlations between galaxy positions and the thermal Sunyaev-Zeldovich (tSZ) effect, as well as galaxy positions and the gravitational lensing of the cosmic microwave background (CMB). The galaxy sample is from the 4th data release of the Kilo-Degree Survey (KiDS). The tSZ $y$ map and the CMB lensing map are from the Planck 2015 and 2018 data releases, respectively. The measurements are performed in five redshift bins with $z\lesssim1$. With these measurements, combining galaxy-tSZ and galaxy-CMB lensing cross-correlations allows us to break the degeneracy between galaxy bias and gas pressure bias, and hence constrain them simultaneously. In all redshift bins, the best-fit values of $\left\langle b_{y} P_{\mathrm{e}}\right\rangle$ are at a level of $\sim 0.3\, \mathrm{meV/cm^3}$ and increase slightly with redshift. The galaxy bias is consistent with unity in all the redshift bins. Our results are not sensitive to the non-linear details of the cross-correlation, which are smoothed out by the Planck beam. Our measurements are in agreement with previous measurements as well as with theoretical predictions. We also show that our conclusions are not changed when CMB lensing is replaced by galaxy lensing, which shows consistency of the two lensing signals despite their radically different redshift range. This study demonstrates the feasibility of using CMB lensing to calibrate the galaxy distribution such that the galaxy distribution can be used as a mass proxy without relying on the precise knowledge of the matter distribution.

The hypothesis that strange quark matter is the true ground state of matter has been investigated for almost four decades, but only a few works have explored the dynamics of binary systems of quark stars. This is partly due to the numerical challenges that need to be faced when modelling the large discontinuities at the surface of these stars. We here present a novel technique in which the EOS of a quark star is suitably rescaled to produce a smooth change of the specific enthalpy across a very thin crust. The introduction of the crust has been carefully tested by considering the oscillation properties of isolated quark stars, showing that the response of the simulated quark stars matches accurately the perturbative predictions. Using this technique, we have carried out the first fully general-relativistic simulations of the merger of quark-star binaries finding several important differences between quark-star binaries and hadronic-star binaries with the same mass and comparable tidal deformability. In particular, we find that dynamical mass loss is significantly suppressed in quark-star binaries. In addition, quark-star binaries have merger and post-merger frequencies that obey the same quasi-universal relations derived from hadron stars if expressed in terms of the tidal deformability, but not when expressed in terms of the average stellar compactness. Hence, it may be difficult to distinguish the two classes of stars if no information on the stellar radius is available. Finally, differences are found in the distributions in velocity and entropy of the ejected matter, for which quark-stars have much smaller tails. Whether these differences in the ejected matter will leave an imprint in the electromagnetic counterpart and nucleosynthetic yields remains unclear, calling for the construction of an accurate model for the evaporation of the ejected quarks into nucleons.

We study the approach to scaling in axion string networks in the radiation era, through measuring the root-mean-square velocity $v$ as well as the scaled mean string separation $x$. We find good evidence for a fixed point in the phase-space analysis in the variables $(x,v)$, providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. The string length has also been commonly expressed in terms of a dimensionless string length density $\zeta$, proportional to the number of Hubble lengths of string per Hubble volume. In simulations with initial conditions far from the fixed point $\zeta$ is still evolving after half a light-crossing time, which has been interpreted in the literature as a long-term logarithmic growth. We show that all our simulations, even those starting far from the fixed point, are accounted for by a VOS model with an asymptote of $\zeta_*=1.20\pm0.09$ (calculated from the string length in the cosmic rest frame) and $v_* = 0.609\pm 0.014$.

We consider the following question: may two different black holes (BHs) cast exactly the same shadow? In spherical symmetry, we show the necessary and sufficient condition for a static BH to be shadow-degenerate with Schwarzschild is that the dominant photonsphere of both has the same impact parameter, when corrected for the (potentially) different redshift of comparable observers in the different spacetimes. Such shadow-degenerate geometries are classified into two classes. The first shadow-equivalent class contains metrics whose constant (areal) radius hypersurfaces are isometric to those of the Schwarzschild geometry, which is illustrated by the Simpson and Visser (SV) metric. The second shadow-degenerate class contains spacetimes with different redshift profiles and an explicit family of metrics within this class is presented. In the stationary, axi-symmetric case, we determine a sufficient condition for the metric to be shadow degenerate with Kerr for far-away observers. Again we provide two classes of examples. The first class contains metrics whose constant (Boyer-Lindquist-like) radius hypersurfaces are isometric to those of the Kerr geometry, which is illustrated by a rotating generalization of the SV metric, obtained by a modified Newman-Janis algorithm. The second class of examples pertains BHs that fail to have the standard north-south $\mathbb{Z}_2$ symmetry, but nonetheless remain shadow degenerate with Kerr. The latter provides a sharp illustration that the shadow is not a probe of the horizon geometry. These examples illustrate that non-isometric BH spacetimes can cast the same shadow, albeit the lensing is generically different.

We study Landau gauge gluon propagators in two-color QCD at finite quark chemical potential ($\mu_q$) and temperature ($T$). We include medium polarization effects at one-loop by quarks into massive gluon propagators, and compared the analytic results with the available lattice data. We particularly focus on the high density phase of color-singlet diquark condensates whose critical temperature is $\sim 100$ MeV with weak dependence on $\mu_q$. At zero temperature the color singlet condensates protect the IR limit of electric and magnetic gluon propagators from the medium screening effects. At finite temperature, this behavior remains true for the magnetic sector, but the electric screening mass should be generated by thermal, and hence gapless, particles which are unbound from the diquark condensates. Treating thermal excitations as quasi-quarks, we found that the electric screening develops too fast compared to the lattice results. Beyond the critical temperature for diquark condensates the analytic results are consistent with the lattice results.

In the past few years, the detection of gravitational waves from compact binary coalescences with the Advanced LIGO and Advanced Virgo detectors has become routine. Future observatories will detect even larger numbers of gravitational-wave signals, which will also spend a longer time in the detectors' sensitive band. This will eventually lead to overlapping signals, especially in the case of Einstein Telescope (ET) and Cosmic Explorer (CE). Using realistic distributions for the merger rate as a function of redshift as well as for component masses in binary neutron star and binary black hole coalescences, we map out how often signal overlaps of various types will occur in an ET-CE network over the course of a year. We find that a binary neutron star signal will typically have tens of overlapping binary black hole and binary neutron star signals. Moreover, it will happen up to tens of thousands of times per year that two signals will have their end times within seconds of each other. In order to understand to what extent this would lead to measurement biases with current parameter estimation methodology, we perform injection studies with overlapping signals from binary black hole and/or binary neutron star coalescences. Varying the signal-to-noise ratios, the durations of overlap, and the kinds of overlapping signals, we find that in most scenarios the intrinsic parameters can be recovered with negligible bias. However, biases do occur for a short binary black hole or a quieter binary neutron star signal overlapping with a long and louder binary neutron star event when the merger times are sufficiently close. Hence our studies show where improvements are required to ensure reliable estimation of source parameters for all detected compact binary signals as we go from second-generation to third-generation detectors.

A set of unified relativistic mean-field equations of state for hyperonic compact stars recently built in [M. Fortin, Ad. R. Raduta, S. Avancini, and C. Providencia, Phys. Rev. D {\bf 101}, 034017 (2020)] is used to study the thermal evolution of non-magnetized and non-rotating spherically-symmetric isolated and accreting neutron stars under different hypothesis concerning proton $S$-wave superfluidity. These equations of state have been obtained in the following way: the slope of the symmetry energy is in agreement with experimental data; the coupling constants of $\Lambda$ and $\Xi$-hyperons are determined from experimental hypernuclear data; uncertainties in the nucleon-$\Sigma$ interaction potential are accounted for; current constraints on the lower bound of the maximum neutron star mass are satisfied. Within the considered set of equations of state, the presence of hyperons is essential for the description of the cooling/heating curves. One of the conclusions we reach is that the criterion of best agreement with observational data leads to different equations of states and proton $S$-wave superfluidity gaps when applied separately for isolated neutron stars and accreting neutron stars in quiescence. This means that at least in one situation the traditional simulation framework that we employ is not complete and/or the equations of state are inappropriate. Another result is that, considering equations of state which do not allow for nucleonic dUrca or allow for it only in very massive NS, the low luminosity of SAX J1808 requires a repulsive $\Sigma$-hyperon potential in symmetric nuclear matter in the range $U_\Sigma^{(N)}\approx 10-30$ MeV. This range of values for $U_\Sigma^{(N)} $ is also supported by the criterion of best agreement with all available data from INS and XRT.

The observation of gravitational waves from LIGO and Virgo detectors inferred the mergers rates to be $23.9^{+14.9}_{-8.6}$ Gpc$^{-3}$ yr$^{-1}$ for binary black holes and $320^{+490}_{-240}$ Gpc$^{-3}$ yr$^{-1}$ for binary neutron stars. These rates suggest that there is a significant chance that two or more of these signals will overlap with each other during their lifetime in the sensitivity-band of future gravitational-wave detectors such as the Cosmic Explorer and Einstein Telescope. The detection pipelines provide the coalescence time of each signal with an accuracy $\mathcal{O}(10\,\rm ms)$. We show that using the information of the coalescence time, it is possible to correctly infer the properties of these "overlapping signals" with the current data-analysis infrastructure. Studying different configurations of the signals, we conclude that the inference is robust provided that the two signals are not coalescing within less than $\sim 1-2\,\mathrm{s}$. Signals whose coalescence epochs lie within $\sim 0.5\,\rm s$ of each other suffer from significant biases in parameter inference, and new strategies and algorithms are required to overcome such biases.