Papers for Friday, Feb 20 2026

Cosmological analyses are moving past the well understood 2-point statistics to extract more information from cosmological fields. A natural step in extending inference pipelines to other summary statistics is to include higher order N-point correlation functions (NPCFs), which are computationally expensive and difficult to model. At the same time it is unclear how many NPCFs one would have to include to reasonably exhaust the cosmological information in the observable fields. An efficient alternative is given by learned and optimized summary statistics, largely driven by overparametrization through neural networks. This, however, largely abandons our physical intuition on the NPCF formalism and information extraction becomes opaque to the practitioner. We design a simulation-based inference pipeline, that not only benefits from the efficiency of machine learned summaries through optimization, but also holds on to the NPCF program. We employ the heavily constrained Cosmological Correlator Convolutional Neural Network (C3NN) which extracts summary statistics that can be directly linked to a given order NPCF. We present an application of our framework to simulated lensing convergence maps and study the information content of our learned summary at various orders in NPCFs for this idealized example. We view our approach as an exciting new avenue for physics-informed simulation-based inference.

Protoplanetary disks can become eccentric when planets open deep gaps within, but how eccentric are they? We answer this question by analyzing two-dimensional hydrodynamical simulations of planet-disk interaction. The steady state eccentricity of the outer disk (outside of the planet's orbit) is described as a balance between eccentricity excitation by the 1:3 eccentric Lindblad resonance and eccentricity damping by gas pressure. This eccentricity scales with $q(\frac{h_p}{r_p})^{(-1)}(\frac{r_{gap}}{r_p})^{(a-\frac{b}{2}-2)}$, where $q$ is the planet-to-star mass ratio, $\frac{h_p}{r_p}$ is the disk aspect ratio, $\frac{r_{gap}}{r_p}$ is the radial position of the outer gap edge divided by the planet's position, and $a$ and $b$ are the negative exponents in the disk's surface density and temperature power law profiles, respectively. We derive a semi-analytic eccentricity profile that agrees with numerical simulations to within 30%. Our result is a first step to quantitatively interpret observations of eccentric protoplanetary disks, such as MWC 758, HD 142527, IRS 48, and CI Tau.

The nascent methodology of applying the principles of causal discovery to astrophysical data has produced affirming results about deeply held theories concerning the causal nature behind the observed coevolution of supermassive black holes (SMBHs) with their host galaxies. The key results from observations have demonstrated an apparent causal reversal across different galaxy morphologies$\unicode{x2014}$SMBHs causally influence the evolution of the physical parameters of their spiral galaxy hosts, whereas SMBHs in elliptical galaxies are passive companions that grow in near lockstep with their hosts. To further explore and ascertain insights, it is necessary to utilize galaxy simulations to track the time evolution of the observed causal relations to learn more about the temporal nature of the changing SMBH/galaxy evolutionary directions. We conducted experiments with the NIHAO suite of cosmological zoom-in hydrodynamical simulations to follow the evolution of individual galaxies along with their central SMBH masses ($M_\unicode{x25CF}$) and properties, including central stellar velocity dispersion ($\sigma_0$). We reproduce the causal results from real galaxies, but add clarity by observing that the SMBH/galaxy causal directions are noticeably inverted between the epochs before and after the peak of star formation. The implications for causal reversal of the $M_\unicode{x25CF}\unicode{x2013}\sigma_0$ relation portend larger concerns about the reliability of SMBH masses estimated at high redshifts and presumptions of overmassive black holes at early epochs. Toward this problem, we apply updated causally-informed scaling relations that predict high-$z$ black hole masses that are approximately two orders of magnitude less massive, and thus not overmassive with respect to local $z=0$ SMBH$\unicode{x2013}$galaxy mass ratios.

The solar-stellar connection provides a unique framework for understanding magnetic activity and atmospheric heating across a broad spectrum of stars. Solar Dynamics Observatory (SDO) of NASA, equipped with the Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly, and Extreme ultraviolet Variability Experiment, has enabled detailed Sun-as-a-star studies that bridge solar and stellar physics. Integrating spatially resolved solar observations into disk-integrated datasets, these studies provide insights into magnetic activity occurring in distant stars. This review highlights key results from recent analyses that employed all three SDO instruments to characterize active regions, quantify universal heating relationships, and reconstruct stellar X-ray and ultraviolet spectra. We discuss how these findings advance our understanding of stellar magnetic activity, provide predictive tools for exoplanetary environments, and outline future directions for applying solar-based frameworks to diverse stellar populations.

Modern cosmology and galaxy formation rely on an understanding of how cosmic baryons are distributed, a significant portion of which exist in the diffuse gas confined to halos. Fast Radio Bursts (FRBs) are a promising probe of the Universe's ionized gas. At low redshift, the contribution to the dispersion measure (DM) from the intergalactic medium (IGM) and intervening halos is subdominant, allowing us to study the circumgalactic media (CGM) of the host galaxies. We select a sample of five local universe FRBs whose host interstellar medium (ISM) DM is negligible and use these to constrain the mass of the CGM in each halo. We find that one of our sources, the only massive elliptical host galaxy, has been evacuated of its baryons ($M_\mathrm{gas}=0.02^{+0.02}_{-0.02}M_\mathrm{h}$, corresponding to $\sim$10$\%$ of the cosmological average $\frac{\Omega_b}{\Omega_m}$). This galaxy shows evidence of a past episode of AGN activity, consistent with the picture of strong AGN feedback in galaxy group-scale halos. The other sources are consistent with existing multiwavelength data and tentatively support more baryon retention in $L_*$ galaxies compared to group-scale halos. We show that FRBs can measure the halo gas fraction $f_\mathrm{gas}$ in halos of mass $M_\mathrm{h}\sim10^{11-13}M_\odot$, and up to $\sim10^{14}M_\odot$ if galaxy cluster hosts are included, which is a larger range than other gas probes can access. Finally, we demonstrate that a large sample of local universe FRBs, such as those expected from upcoming all-sky radio telescopes, will enable precision measurements of halo gas, which is crucial for understanding the effects of feedback.

Accretion-driven outbursts in young stellar objects remain poorly understood, largely limited by a statistically small sample of closely followed-up events. This underscores the importance of a thorough exploration of each outbursting object. We studied a peculiar outbursting system, Gaia24ccy, which exhibited two $\Delta g \sim$ 3.8 mag outbursts in 2019 and 2024. The system consists of two unresolved, nearly identical, and rapidly rotating young stars: Gaia24ccy A (1.1419 days) and Gaia24ccy B (1.7898 days). Periodogram analyses just before the onset of the outbursts suggest Gaia24ccy B to be the outbursting component. Unlike any previously known EXor sources, the two outburst profiles show a very similar evolution: both rose at the same rate for the first 15 days, followed by many 'sub-bursts' on the timescale of 10-20 days. The 2019 outburst lasted 145-255 days, while the 2024 outburst persisted for 367 days. We infer the unstable region to lie at $r_{\rm trigger} \simeq 0.019-0.047$ au ($\sim5-12.3 R_\star$). The accreted mass per event $M_{\rm acc}\sim10^{-5} M_\odot$ can be provided by a compact inner-disk reservoir. The photometric rise/decay timescales and the mid-infrared color evolution favor a thermal-viscous trigger in a hot inner disk, while the appearance of rich emission-line spectra indicates concurrent magnetospheric compression - together best described by a hybrid picture. Finally, we explain the reddening of the mid-infrared color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.

The Laser Interferometer Space Antenna is expected to observe gravitational waves from massive black hole binaries across cosmic time. Many are anticipated to be detectable hours to weeks before coalescence. We present a fast algorithm for the pre-merger detection and preliminary characterization of such binaries. The method performs a search for excess power with a chirping time-frequency morphology in short-time Fourier transform spectrograms. By tiling the time-frequency plane with slices defined by the quadrupole frequency evolution, we define a signal significance relative to a fitted background distribution of instrumental noise and Galactic foreground. Individual search triggers are followed by a coherence tracker, which groups over time triggers consistent with the same physical signal . Doing so, our analysis provides progressively refined estimates of the chirp mass and coalescence time. We validate our algorithm on the Sangria LISA Data Challenge dataset, successfully detecting all 15 injected MBHBs: 14 of them hours-to-weeks before merger, while one is only detected after the binary coalescence. The algorithm yields chirp mass relative errors below $3\%$ for high-SNR sources and coalescence time uncertainties of up to a few hours. With a computational cost of less than a second to process a 10-day data segment on single core, our approach is suitable for generating real-time alerts, trigger protected observational periods, and provide informative priors for Bayesian parameter estimation.

We propose and investigate a class of dynamical dark energy models in which the cosmological constant evolves from negative values in the early Universe to a positive value at low redshifts. This framework includes a generalised ladder-step evolution, as well as smooth-transition scenarios, providing a unified description of sign-changing cosmological constants. We analyse the theoretical construction and background dynamics of these models using cosmographic diagnostics. Extending this study to the linear perturbation regime, we solve the perturbation equations from the radiation-dominated era with adiabatic initial conditions. We examine the evolution of the matter density contrast, gravitational potential, and the $f\sigma_8$ observable. Our results are compared against the standard $\Lambda$CDM model and confronted with current observational data, illustrating the phenomenological viability of sign-changing dark energy models and revealing distinctive imprints on cosmic structure formation arising from the transition of the cosmological constant.

Protoplanetary discs are thought to evolve either through angular momentum transport driven by viscous processes or through angular momentum removal induced by magnetohydrodynamic (MHD) winds. One proposed method to distinguish between these two evolutionary pathways is by comparing mass accretion rates and disc sizes, but observational constraints complicate this distinction. In this study, we investigate how extreme ultraviolet (EUV) external photoevaporation affects the evolution of protoplanetary discs, particularly in environments such as the Orion Nebula Cluster. Using a combination of analytical derivations and 1D numerical simulations, we explore the impact of externally induced mass-loss on disc structure and accretion dynamics. We demonstrate that, in the viscous scenario, there exists a clear, near one-to-one correlation between the mass-loss rate due to external photoevaporative outflows and the mass accretion rate onto the central star. In contrast, MHD wind-driven discs do not exhibit such trend, leading to a distinct evolutionary path. External photoevaporative mass-loss rates and mass accretion rates can both be accurately measured for a population of discs, without a strong model dependence. Thus, our findings provide a robust observational test to distinguish between viscous and MHD wind-driven disc evolution, offering a new approach to constraining angular momentum transport mechanisms in protoplanetary discs. Applying this diagnostic observationally requires joint measurements of $\dot{M}_{\rm acc}$ and $\dot{M}_{\rm pe}$ for the same objects, which are currently scarce in bright HII regions due to contamination and sensitivity limitations.

An important next step for pulsar timing arrays (PTAs) is to measure anisotropies in the gravitational wave background (GWB) at $\sim$ nano-Hz frequencies. We calculate the expected GWB anisotropies using empirically calibrated models for the merger rates of supermassive black hole binaries (SMBHBs). The anisotropies reflect both shot-noise in the discrete SMBHB populations while also tracing, in part, the large-scale structure (LSS) of the universe. The shot-noise term is sensitive to the high-mass end of the merging SMBH mass function, depends somewhat on the low-redshift tail of the merger distribution, and is a strong function of observing frequency. The precise frequency dependence provides a test of SMBHB residence times. In our models, the mean shot-noise anisotropy typically lies close to or above the broad frequency-band NANOGrav upper limits. Consequently, near-future PTA data, and potentially re-analyses of existing measurements using frequency-dependent shot-noise anisotropy templates, should be capable of detecting this signal or placing meaningful constraints on SMBHB merger models. A full interpretation, however, will require modeling the probability distribution of shot-noise amplitudes rather than relying solely on ensemble-averaged predictions. The LSS-induced anisotropies are at least two to three orders of magnitude smaller. Although the LSS contribution contains valuable information regarding the redshift distribution and clustering bias of the merging SMBHBs, detecting this component will be challenging.

Close-in exoplanets with H/He atmospheres often undergo hydrodynamic escape. In extreme cases, it is hypothesized that the mass loss can be high enough for the escaping planetary material to wrap around the star, forming a long-lasting circumstellar torus. In this work, we develop a physical model of such circumstellar tori and use a ray tracing scheme to calculate the attenuation of stellar light passing through them. We show that the presence of a circumstellar torus significantly increases the equivalent width of the observed stellar He I 10830~Å~line. When combined with observations of the star's Ca II H & K lines, these systems can typically be distinguished from field stars. Based on these results, we propose a survey of stars hosting close-in planets, combining observations of the He I 10830~Å~and Ca II H & K lines to search for circumstellar tori generated from planetary mass-loss in these systems.

Asteroid activity (e.g., thermo-mechanical breakdown, impacts, rotational shedding, tidal disruption, etc.) can inject meteoroids into near-Earth space and leave detectable signatures in orbit catalogs. We searched for such recent signatures using orbit-similarity statistics and explicit null-hypothesis testing applied to shower-removed, asteroidal video-meteor datasets. Our sample comprises 235{,}271 meteors and fireballs from four all-sky video networks (GMN, CAMS, EDMOND, and SonotaCo). For meteors we use the geocentric dissimilarity criterion $D_N$ and construct KDE-based sporadic null realizations to evaluate (i) global cumulative similarity distributions and (ii) localized $D_N$-conditioned ($D_N<0.015$) pair-excess maps in the $(U,\lambda_\odot)$ plane; we additionally apply DBSCAN ($\epsilon=0.03$, $\mathrm{min\_samples}=2$) to isolate the coherent, statistically significant structures. We find no survey-consistent, stream-like signature in the Earth-like, low-inclination region expected for a distinct \emph{recent} tidal-disruption family; instead, significant-bin membership implies, under our adopted detection thresholds and binning, a conservative combined upper limit of $\leq 53/235{,}271$ ($\leq 2.3\times10^{-4}$) for sporadic asteroidal meteors plausibly attributable to a detectable recent tidal-disruption-like contribution. In contrast, we confirm the detection of a new diffuse southern Virginid-region stream: GMN exhibits a local z-score of 6.32 relative to the KDE-null mean in the $U-\lambda_\odot$ phase space (global significance of 5.3~$\sigma$), with weaker supporting excess in SonotaCo and EDMOND. DBSCAN isolates $N=282$ members (243 GMN plus additional SonotaCo, CAMS, and EDMOND) on a low-perihelion, asteroidal orbit ($q=0.22\pm0.01$ au, $i=12.3^{\circ}\pm1.8^{\circ}$, $T_J=4.6\pm0.3$) consistent with near-Sun thermo-mechanical ``rock-comet'' activity.

We perform the first systematic study of the minute-to-hours-timescale stochastic variability observed in the X-ray luminosity of tidal disruption events (TDEs) using XMM-Newton data and Fourier analysis methods. We measure the spectral properties, power spectral densities (PSDs), fractional variability amplitudes, and energy dependence of the variability for 18 TDEs spanning 54 observations, of which 27 occur in thermal disk-dominated states and 27 show a nonthermal hard X-ray corona. Compared to pure thermal sources, we find TDEs with coronae are more X-ray variable and show steeper PSDs indicating longer correlation timescales. This state-transition behavior is qualitatively similar to X-ray binaries, which show higher fractional variability in the hard state than in the soft state. However, newborn TDE coronae show systematically flatter PSDs and softer energy spectra than their long-lived AGN counterparts. We also show that the variability amplitude of thermal TDEs increases with photon energy, consistent with variations sourced by local temperature fluctuations and exponentially enhanced in the Wien tail. Our work demonstrates that combining spectral and timing properties of X-ray TDEs can probe the microphysics of newly formed accretion flows around supermassive black holes, and that the coronae formed in TDEs fundamentally differ from those in AGN.

We test the Etherington cosmic distance-duality relation (CDDR), by comparing Type Ia supernova (SNIa) luminosity-distance information from the Pantheon+ compilation with an angular-diameter-distance reconstructed from localized Fast Radio Bursts (FRBs). The core of our methodology is a data-driven reconstruction from FRBs using artificial neural networks (ANNs): we infer a smooth mean extragalactic dispersion-measure relation and use its redshift derivative to recover $H(z)$ and hence $D_\mathrm{A}^{\rm FRB}(z)$ without assuming a parametric form for the expansion history. Possible deviations from CDDR are parameterized through three one-parameter models of $\eta(z)\equiv D_\mathrm{L}/[(1+z)^2D_\mathrm{A}]$. We implement two complementary likelihoods: (i) a direct approach using individual SNIa with the full Pantheon+ covariance, and (ii) a machine-learning approach in which we reconstruct the SN Hubble diagram on the FRB redshift grid, propagating SN and FRB uncertainties into non-diagonal covariance matrices via Monte Carlo and bootstrap realizations. Within the FRB reconstruction, we anchor the mean extragalactic dispersion measure at $z=0$, which yields a data-driven constraint on the average host/near-source contribution $\mathrm{DM}_{\rm host}=128.8\pm 34.1\,\mathrm{pc\,cm^{-3}}$ ($3\sigma$ of statistical confidence). We find that both likelihood implementations give consistent posteriors and no statistically significant evidence for departures from CDDR at the current precision.

OB stars generally form in open clusters within the Milky Way's thin disk, so when they are found at high Galactic latitudes, it is thought that they were ejected from their birth clusters during the past few tens of millions of years. Using Gaia Data Release 3 (hereafter DR3) data, we traced the kinematic trajectories of 39 high-latitude B-type stars and 447 Galactic open clusters with high-quality astrometry to search for moments of past intersection. In cases where we found matching trajectories, we also considered the clusters' HR diagrams to confirm parent-orphan pairs have matching ages. Further analysis of the clusters' core environments allowed us to determine a probable ejection mechanism. Through these paternity tests, we have identified possible origins for five of these orphaned B-type stars. Here we present the likely travel times, ejection velocities, and a discussion of the runaway mechanism for each case. We also identify one star whose trajectory did not bring it near the disk during the time period of our analysis, and we discuss its possible origins as well.

We discuss the behavior of the default ``wavecal'' spectra obtained together with most STIS spectroscopic exposures, which are needed for proper wavelength calibration of the science data. Because the Pt/Cr-Ne lamps used for the wavecals have been fading (especially at the shortest wavelengths), some changes in the default lamp and/or exposure time have been implemented in recent years to maintain accurate calibrations. To assess whether additional changes might be appropriate, we examine the distribution of the SHIFTA1, SHIFTA2 values derived from the wavecals (the x and y offsets of the spectral image on the detector), we re-visit the wavelength-dependent fading of the lamps, and we perform simulations to estimate the exposure times that would be needed to obtain accurate SHIFTA values. While the current wavecals do appear to yield reasonable SHIFTA, increases in the default exposure times for some of the shortest-wavelength settings would help to ensure reliable wavelength zero points as the lamps continue to fade.

A theory of the spectral "zebra" pattern of the Crab pulsar's high-frequency interpulse (HFIP) radio emission is developed. The observed emission bands are interference maxima caused by multiple ray propagation through the pulsar magnetosphere. The high-contrast interference pattern is the combined effect of gravitational lensing and plasma de-lensing of light rays. The model enables space-resolved tomography of the pulsar magnetosphere, yielding a radial plasma density profile of $n_{e}\propto r^{-3}$, which agrees with theoretical insights. We predict the zebra pattern trend to change at a higher frequency when the ray separation becomes smaller than the pulsar size. This frequency is predicted to be in the range between 42 GHz and 650 GHz, which is within the reach of existing facilities like ALMA and SMA. These observations hold significant importance and would contribute to our understanding of the magnetosphere. Furthermore, they offer the potential to investigate gravity in the strong field regime near the star's surface.

This is a personal account of the early work that led to what is now known as the ``preheating stage" of inflationary cosmology. The broader applicability of the underlying instability mechanisms in cosmology are indicated.

This study characterises a series of type~II radio bursts associated with a CME that occurred on 14 May, focusing on the coronal conditions during the event and identifying the likely location of the shocks where the radio bursts are generated. The CME was tracked using a combination of white light and extreme ultraviolet observations of the solar corona taken by three instruments: GOES-SUVI, two coronagraphs of the SOHO-LASCO, together with ground-based radio observations between 10-240~MHz from I-LOFAR. The radial distances of the radio sources were examined using a series of density models, with both PFSS and MHD models used to examine the coronal plasma conditions. Four type~II bursts were identified in the I$-$LOFAR radio dynamic spectrum over $\sim$15~minutes, exhibiting features such as band splitting, herringbones, and fragmentation. The shocks were found to have speeds ranging between $\sim$443$-$2075~km s$^{-1}$, with drift rates of $\sim-$361 to -78~kHz~s$^{-1}$. The shocks were found to have a $M_A \approx$ 3.21$-$3.57. indicating that they were super-Alfvénic. The first type~II burst was triggered $\sim$18~minutes after the CME launch, with each burst appearing to have been generated at a different height in the corona. Analysis of the derived kinematics and modelling results suggests that the type~II bursts were likely produced at the shoulders of the CME near the flanks, where open magnetic field lines and relatively low Alfvén speeds facilitated shock formation. This multi-instrument study shows that multiple type II bursts from a single CME originated at different coronal heights, with modelling indicating their generation near the CME flanks, where low Alfvén speeds and open magnetic field lines facilitated shock formation.

We present results from imaging observations of three quadruply lensed quasars by Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) at redshifts $z = 2.550$, 2.975, and 1.500. We model our targets assuming a singular isothermal ellipsoid mass profile and an elliptical SÃ\c{opyright}rsic profile for the lensing galaxies, and reconstruct the geometric configuration of each system with measured Einstein radii of 0.44$'$, 0.58$'$, and 0.49$'$. While no spectroscopic measurements are available for the lenses, we constrain the redshift of each lens to $0.5 < z < 1.2$, $1.0 < z < 1.5$, and $0.4 < z < 0.9$. For all three lenses, the best-fit light model yield a typical de Vaucouleurs $n_{\rm S\acute{e}rsic} \sim 4$ profile and an effective radius $R_e$ around $\sim 1.5 - 3.5$ kpc. We accordingly classify the three lenses as early-type galaxies at an intermediate to high redshift, a common type for strong lensing galaxies. Compared to other known quadruple lenses, the lensing galaxies in this work are at the lower end of the distribution of Einstein radii and upper end of the distribution of the lens redshifts. They represent an interesting quadrant of subarcsecond-separation lenses in the population of single-galaxy strong lensing which have been largely unexplored yet and will be great targets of interest in upcoming high-resolution lensing surveys.

We used N-body simulations to model the 4.5 Gyr orbital evolution of the early Kuiper Belt, incorporating a massive protoplanetary disk, the four giant planets, and 1500 primordial Pluto-class bodies ("Plutos") that drove Neptune's grainy migration. The analysis of 67 simulated systems revealed key insights: (1) All systems featured the primary trans-Neptunian object (TNO) populations: cold/hot classical, resonant, scattered, and detached; (2) Captures into stable resonant orbits favored close Neptunian mean motion resonances (MMRs; e.g., 3:2, 2:1), while distant ones beyond 50 au (e.g., 5:2 MMR) were underpopulated; (3) Optimal matches to observed resonant fractions and the classical region (including the kernel) arose from models considering a jumping Neptune, self-gravitating Plutos, and an initial disk edge at 45-47 au; (4) Models including primordial scattered disks boosted distant MMR captures but overproduced scattered objects; (5) All models were inefficient at producing the detached (q > 40 au) and high-i (i > 45 deg) populations and failed to populate observed niches, such as distant detached (a > 245 au), low-i detached (i < 20 deg), low-i scattered with q = 37-40 au (i < 20 deg), and extreme (q > 50 au or i > 50 deg) TNOs; (6) Grainy migration effects peaked early, fading as the Plutos were removed; (7) With a few primordial Plutos surviving inside 50 au, the initial population was estimated at ~150-500 to explain Pluto's solitary status. Although our four-giant-planet models reasonably replicate the trans-Neptunian structure within 50 au, they fail to account for detached, high-i, and extreme TNOs. Additional processes (e.g., a distant undiscovered planet) are required for a comprehensive outer solar system framework.

Shock chemistry is an excellent tool to shed light on the formation and destruction mechanisms of complex organic molecules (COMs). The L1157-mm outflow is the only low-mass protostellar outflow that has extensively been studied in this regard. Using the data taken as part of the PRODIGE (PROtostars & DIsks: Global Evolution) large program, we aim to map COM emission and derive the molecular composition of the protostellar outflow driven by the Class 0 protostar NGC1333 IRAS4B1 to introduce it as a new laboratory to study the impact of shocks on COM chemistry. In addition to typical outflow tracers such as SiO and CO, outflow emission is seen from H2CO, HNCO, and HC3N, as well as from the COMs CH3OH, CH3CN, and CH3CHO, and even from deuterated species such as DCN, D2CO, and CH2DOH. Maps of integrated intensity ratios between CH3OH and DCN, D2CO, and CH3CHO reveal gradients with distance from the protostar. Intensity ratio maps of HC3N and CH3CN with respect to CH3OH peak in the southern lobe where temperatures are highest. Rotational temperatures derived towards two positions, one in each lobe, are found in the range ~50-100 K. Abundances with respect to CH3OH are higher by factors of a few than for the L1157-B1. In conclusion, for the first time, we securely detected the COMs CH3CN, CH3CHO, and CH2DOH in the IRAS 4B1 outflow, serendipitously with limited sensitivity and bandwidth. Targeted observations will enable the discovery of new COMs and a more detailed analysis of their emission. Morphological differences between molecules in the IRAS 4B1 outflow lobes and their relative abundances provide first proof that this outflow is a promising new laboratory for shock chemistry, which will offer crucial information on COM formation and destruction as well as outflow structure and kinematics.

The Kolmogorov stochasticity parameter (KSP) as a sensitive descriptor of degree of randomness of signals is used to analyze the properties of the NANOGrav pulsar timing data associated to a stochastic gravitational wave background. The time of arrival (TOA) data of white noise for 68 pulsars are analyzed regarding their KSP properties. The analysis enables to obtain the degree of randomness of the white noise for various pulsars and to reveal its inhomogeneity, i.e. pulsars with low and high randomness of the white noise. The time-dependence of the randomness in the white noise is also studied, indicating the existence of non-stationary physical processes influencing the pulsar timing. The KSP thus is acting as an indicator for the degree of the agreement between the observations and the timing models and as a test in revealing the contribution of various physical processes in the stochastic background signal.

The detection limit of astronomical imaging observations is limited by several noise sources. Some of that noise is correlated between neighbouring image pixels and exposures, so in principle could be learned and corrected. We present an astronomical self-supervised transformer-based denoising algorithm (ASTERIS), that integrates spatiotemporal information across multiple exposures. Benchmarking on mock data indicates that ASTERIS improves detection limits by 1.0 magnitude at 90% completeness and purity, while preserving the point spread function and photometric accuracy. Observational validation using data from the James Webb Space Telescope (JWST) and Subaru telescope identifies previously undetectable features, including low-surface-brightness galaxy structures and gravitationally-lensed arcs. Applied to deep JWST images, ASTERIS identifies three times more redshift > 9 galaxy candidates, with rest-frame ultraviolet luminosity 1.0 magnitude fainter, than previous methods.

The gravitational wave (GW) event S250206dm, as the first well-localized neutron star merger candidate potentially located in the mass gap, presented a unique opportunity to probe the electromagnetic signatures from such a system. Here we report a deep, multiband search with the new 2.5-meter Wide Field Survey Telescope (WFST), covering about 64% of the localization region up to a 5-sigma limiting magnitude of 23 mag. In total, 12 potential candidates have been identified while none of them are likely related to S250206dm. This non-detection provides the most stringent constraint to date on any associated kilonova. Crucially, an AT 2017gfo-like event at 269 Mpc can be excluded by WFST observations alone. Based on ejecta mass limits, a neutron star-black hole with a large mass ratio (Q >= 3.2) is disfavored. This optical-derived constraint on the mass ratio reaches, for the first time, a precision comparable to that inferred from the GW signal. This work presents the best observation of this type of events until now, and demonstrates the power of rapid, deep follow-up observations to constrain the properties of compact binary progenitors, offering key insights into the constituents of the mass gap.

In this paper we present 16 early-phase type Ia supernovae (SNe Ia) discovered during the pilot survey of the 2.5-meter Wide Field Survey Telescope (WFST-PS) from March 4 to July 10, 2024, including three SNe Ia with early-excess emission features (EExSNe Ia). The discovery magnitude of the 16 WFST-PS early-phase SNe is at least 3 mag fainter than their peak brightness. A large scatter of color indices is found in approximately the first 10 days of supernova explosions, indicating diverse photometric behaviors in the early phase. Three EExSNe Ia show relatively brighter peak luminosities and longer rise time compared to those of non-EExSNe Ia. The results indicate that current theoretical models require further refinement to fully capture the early photometric evolution of SNe Ia. Based on the initial high-cadence ugr-band data from the WFST-PS survey, we emphasize that early near-ultraviolet (NUV) observations are indispensable for placing tight constraints on the explosion mechanisms and progenitor systems of SNe Ia.

We present comprehensive photometric and spectroscopic observations of a transitional type Ia SN 2024aedt, discovered by the 2.5-meter Wide Field Survey Telescope (WFST) within one day of the explosion. Its light curve is characterized by a peak absolute magnitude of $M_B = -18.49 \pm 0.03$ mag and a decline rate of $\Delta m_{15}(B) = 1.53 \pm 0.36$ mag, placing the object on the $\Delta m_{15}(B)$--$M_B$ diagram in the transition region between normal and subluminous SNe Ia. Furthermore, the early-color evolution and host galaxy environment of SN 2024aedt underscore its transitional nature, sharing properties with both normal and 91bg-like SNe Ia. Light-curve modeling with MOSFiT yields a synthesized $^{56}\mathrm{Ni}$ mass of $0.414 \pm 0.042\,M_{\odot}$ and a total ejecta mass of $0.548 \pm 0.108\,M_{\odot}$. A comparison with theoretical models suggests that the evolutionary trend can be broadly explained by both delayed-detonation (DDT) and double-detonation (DDet) scenarios while possible early-excess emissions predicted by DDet cannot be identified given the limited detections soon after the SN explosion. Although the overall spectral evolution of SN 2024aedt is similar to that of other transitional SNe Ia, the spectroscopic comparison reveals diversity in the early-phase blue-end features, which becomes more homogeneous at later phases. The result indicates the importance of early-time observations in understanding the origin of SN Ia diversity.

We investigate the multiband photometric properties of seven supernovae (SNe) showing double-peaked light-curve evolution and prominent shock-cooling emission, observed by the Wide Field Survey Telescope (WFST) during its first year of operation. By jointly employing an analytic early shock-cooling model and the Arnett radioactive-diffusion model, we fit the bolometric light curves and infer ejecta masses in the range $1.1$-$2.6 M_\odot$, consistent with a transitional population between ultra-stripped supernovae (USSNe) and normal stripped-envelope supernovae (SESNe). The envelope masses are estimated to be $M_{\rm env}=0.1$-$0.4 M_\odot$, while the progenitors are constrained to be yellow or blue supergiants (YSGs/BSGs) with radii of $R=120$-$300 R_\odot$. Using empirical relations, we estimate progenitor luminosities of $L=10^{4.6}$-$10^{4.9} L_\odot$, corresponding to zero-age main-sequence (ZAMS) masses of $8$-$20 M_\odot$. Theoretical models suggest that such progenitors are more naturally produced through binary evolution channels, as single-star evolutionary pathways are unable to yield ejecta masses this low.

The 21 cm bispectrum (BS) offers a powerful probe of the Epoch of Reionization (EoR), but its observational access is severely hindered by dominant astrophysical foregrounds. Considering Murchison Widefield Array (MWA) observations at $154.2~\mathrm{MHz}$ ($z=8.2$), we mitigate the foregrounds with Smooth Component Filtering (SCF) and estimate the 21 cm BS. We validate the pipeline using a simulated 21 cm signal and show that the input BS is recovered for modes $k_{\parallel} \ge [k_\parallel]_f=0.135~{\rm Mpc}^{-1}$. Applied to actual data, the SCF produces substantial foreground suppression, reducing the amplitude of the cylindrical BS $B(k_{1\perp},k_{2\perp},k_{3\perp},k_{1\parallel},k_{2\parallel})$ by $3-4$ orders of magnitude. The artifacts due to the missing frequency channels in the data are also suppressed. The resulting EoR window is significantly cleaner at small $k_{\perp}$. We adopt the region $(k_{1 \perp},k_{2 \perp},k_{3 \perp})\leq 0.026~{\rm Mpc}^{-1}$ and $(k_{1\parallel},k_{2\parallel},k_{3\parallel})>0.135~{\rm Mpc}^{-1}$ to evaluate the 3D spherical BS and constrain the EoR signal. By combining estimates over all triangle shapes, we place the lower and upper limits on the mean cube brightness temperature fluctuations $\Delta^3$. The estimates are consistent with statistical fluctuations from system noise. The most stringent lower limit $\Delta^3_{\rm LL}=-(1.25\times 10^4)^3~{\rm mK}^3$ and upper limit $\Delta^3_{\rm UL}=(1.22\times 10^4)^3~{\rm mK}^3$ are obtained at $k_1=0.281~{\rm Mpc}^{-1}$. Additional observing time will reduce the noise level and enable substantially tighter constraints on the EoR signal.

We investigate the detection and nature of 5.6~\mu m MIRI-red sources in the MIRI Deep Imaging Survey (MIDIS), covering 2.4~arcmin$^2$ in the Hubble Ultra Deep Field. MIDIS is the deepest JWST/MIRI survey to date, probing faint limits and enabling studies of rare high-redshift galaxy populations. We define MIRI-red sources as those detected at 5$\sigma$ significance in MIRI/F560W with red colors: $m_{\rm F444W} - m_{\rm F560W} \ge 0.5$. Using an empirical methodology, we estimate the purity and completeness of MIRI detections and find that a 5-sigma detection at 28.75 mag has a purity of 92\% and completeness of 54\%. We identify seven MIRI-red galaxy candidates, including an F115W dropout consistent with a high-redshift galaxy candidate. We explore possible physical origins for the MIRI-red population, including active galactic nuclei, dust-obscured galaxies, extreme emission-line galaxies, evolved stellar populations, and Little Red Dots (LRDs). Given the proximity of the F444W and F560W filters and the depth of MIDIS, MIRI-red galaxies are consistent with emission-line galaxies with $EW_0(H\alpha) \ge 750$ Å or $EW_0(H\beta + [OIII]) \ge 600$ Å, or high-redshift Balmer breaks of at least 1.6. We also discuss an extreme MIRI-red galaxy undetected in F444W, a potential MIRI-only source, for which we derive $EW_0(H\alpha) \sim 6000$ Å and $EW_0(H\beta + [OIII]) \sim 4000$ Å, or high-$z$ LRD analogs with Balmer breaks of 6.3. Finally, we find fewer MIRI-red detections than expected from extrapolations of the H$\alpha$ or H$\beta$+[OIII] line luminosity functions, consistent with previous deep searches, while the absence of $z>10$ LRD candidates agrees with theoretical expectations for the MIDIS volume.

The formation of planetesimals was an integral part of the cascading series of processes that built the terrestrial planets. To illuminate planetesimal formation, here we develop a refined thermal evolution model to calculate the formation ages of meteorite parent planetesimals. This model includes chemical reactions and phase changes during heating, as well as natural variations in the proportions of the constituent phases of these planetesimals. We find that the parent bodies of non-carbonaceous (NC) and carbonaceous (CC) iron meteorites start forming at very similar times (~0.95 Myr after calcium-aluminium-rich inclusion [CAI] formation) and occupy overlapping time windows. NC and CC chondrite parent bodies formed later during non-overlapping periods. We combine these ages with proportions of isotopic end-members we recover from mixing models to construct records of motion throughout the protoplanetary disk. These records argue that NC and CC material traversed the barrier in the disk after ~0.95 Myr after CAI formation. The onset of this motion coincided with planetesimal formation, indicating that the phenomenon that drove motion also triggered planetesimal formation. We argue that this feature also served as the semi-permeable barrier in the disk. Although its identity is uncertain, the effects this phenomenon had on the timing of planetesimal formation and motion through the disk can now serve as constraints on models of disk evolution. Models that reproduce these effects would elucidate the nature and implications of this phenomenon, which is key to unlocking a holistic model of terrestrial planet building.

The advent of integral field data has revolutionised the study of galaxy evolution. A key component of this is dynamical modelling methods which have allowed for crucial insights to be made from kinematic data. Despite this importance, most dynamical models make a number of key assumptions which do not hold for real galaxies. These include assumptions about the geometry (axisymmetry or triaxiality), the shape of the velocity ellipsoid, and the shape of the underlying stellar distribution. At the same time, machine learning methods are becoming increasingly powerful, with many applications appearing in astronomy. As a first step towards building new dynamical modelling methods with machine learning, it is important to understand the types of machine learning architectures that are best fit for dynamical modelling. To investigate this, we construct a training set of dynamical models of early-type galaxies using Jeans Anisotropic Modelling (JAM). We then train a neural network on this data using the parameters of JAM and mock photometry as the input. We are able to accurately model JAM galaxies with relatively simple machine learning architectures, leading to a significant speed increase over traditional JAM modelling.

Atmospheric escape driven by extreme ultraviolet (EUV) radiation is a critical process shaping the evolution of close-in exoplanets. Recent observations have detected helium triplet absorption in numerous (>20) close-in exoplanets, highlighting the importance of understanding upper atmospheric thermo-chemical structure. While super-solar metallicity has been observed in the atmospheres of some close-in exoplanets, the impact of metal species on both atmospheric escape dynamics and observed absorption features remains poorly understood. In this study, we derive a simplified yet accurate formula for the equivalent width of helium absorption in the limit of an isothermal temperature for the upper atmosphere. Our results demonstrate that planets with lower temperature (metal-rich atmosphere) exhibit lower mass-loss rate although the equivalent width of helium triplet absorption remains largely independent of atmospheric temperature (metallicity) because the low temperatures in these atmospheres enhance the fraction of helium in its triplet state. Additionally, we present a hydrodynamic model based on radiation-hydrodynamics simulations that incorporates the effects of metal cooling. Our analytical model can predict the helium triplet equivalent width of the atmosphere of simulations. The analytical model provides a comprehensive framework for understanding how metal cooling in the upper atmosphere influences the thermo-chemical structure and observable helium features of close-in exoplanetary atmospheres, offering valuable insights for interpreting current and future observational data.

Complex organic molecules are key markers of molecular diversity, and their formation conditions in protoplanetary disks remain an active area of research. These molecules have been detected on a variety of celestial bodies, including icy moons, and may play a crucial role in shaping the current composition of the Galilean moons. Experimental studies suggest that their formation could result from UV irradiation or thermal processing of NH3:CO2 ices. In this context, we investigate the formation of complex organic molecules in the protosolar nebula and their subsequent transport to the Jupiter system region. Lagrangian transport and irradiation simulations of 500 individual particles are performed using a two-dimensional disk evolution model. Based on experiments with UV irradiation and thermal processing of CO2:NH3 ice, this model allows us to estimate the estimate the potential for the formation of complex organic molecules through these processes. Almost none of the particles released at a local temperature of 20 K (corresponding to ~12 AU from the Sun) reach the location of the system of Jupiter. However, when released at a local temperature of 80 K (~7 AU), approximately 45% of the centimetric particles and 30% of the micrometric particles can form complex organic molecules via thermal processing, subsequently reaching the location of the system of Jupiter within 300 kyr. Assuming that the Galilean moons formed in a cold circumplanetary disk around Jupiter, the nitrogen-bearing species potentially present in their interiors could have originated from the formation of complex organic molecules in the protosolar nebula.

Saturn's moon Titan exhibits remarkable parallels to the Earth in many geophysical and geological processes not found elsewhere in the solar system at the present day. These include a nitrogen atmosphere with a condensible gas - methane - replacing the Earth's water, leading to an active meteorology with rainfall and surface manifestations including rivers, lakes and seas, and the dissolution of karstic terrain. Other phenomena such as craters, dunes, and tectonic features are found elsewhere - e.g. on Mars and Venus - but their continuing alteration by pluvial, fluvial and lacustrine processes can be studied only on Earth and Titan. Meanwhile Titan also hosts an interior liquid water ocean with similarities to the Earth as well as to ocean worlds such as Europa and Enceladus. Our focus in this review paper is twofold: to describe the geophysical and geological parallels between Earth and Titan, and to evaluate the yet-underexploited possibilities for field analog research to gain new knowledge about these processes. To date, Titan's much colder temperature and different atmospheric and crustal materials have led to a skepticism that useful analogs can be found on Earth. Our conclusion, however, is that a much larger range of useful analog field work is possible and this work will substantially enhance our knowledge of both worlds. Such investigation will supplement the existing sparse data for Titan returned by space missions, will greatly enhance our understanding of such datasets, and will help to provide science impetus and goals for future missions.

Lithium plays a unique role in astrophysics, as it is a powerful diagnostic for the physics and evolution of low-mass stars, Galactic archaeology, and cosmology. We review the Li observations in stars at different phases of their evolution, the strengths and the limitations of the current theoretical stellar models to explain the Li abundance data, our understanding of the Li sources and of the evolution of Li through- out the Galactic history. Key takeaways from the current state of the research in the field are: 1) Stellar evolution models accounting for fundamental transport processes of chemical species and angular momentum hold the promise of providing a common stellar Li depletion explanation to the Li abundance patterns observed in all Galactic stellar populations, including the dip and the plateau(s). 2) Novae are most probably the main source of Li in the Galaxy, on observational (but not yet theoretically established) grounds. 3) Radial migration of stars in the Galactic disk holds the key to understand many aspects of the Li evolution in the Milky Way.

We present a new method for solving the relativistic Vlasov--Maxwell system of equations, applicable to a wide range of extreme high-energy-density astrophysical and laboratory environments. The method directly discretizes the kinetic equation on a high-dimensional phase-space grid using a discontinuous Galerkin finite element approach, yielding a high-order, conservative numerical scheme that is free from the Poisson noise inherent to traditional Monte-Carlo methods. A novel and flexible velocity-space mapping technique enables the efficient treatment of the wide range of energy scales characteristic of relativistic plasmas, including QED pair-production discharges, instabilities in strongly magnetized plasmas surrounding neutron stars, and relativistic magnetic reconnection. Our noise-free approach is capable of providing unique insight into plasma dynamics, enabling detailed analysis of electromagnetic emission and fine-scale phase-space structure.

UV slopes ($\beta$) are a powerful diagnostics for galaxies at the Epoch of Reionization, tracing star formation, ISM ionization, and the escape fraction $f_{esc}$ of ionizing photons. Studies at low and intermediate z find a gradual $\beta$ reddening with time and steeper slopes for fainter galaxies, however recent JWST studies reveal a flattening of this trend at $z>7$. We want measure $\beta$ for galaxies at $z>7.5$ using the strong lensing around massive galaxy clusters to observe high-redshift and faint galaxies. The low-brightness regime is of particular interest for reionization, as most of the recent models of this process posit that numerous faint galaxies are the prime drivers of reionization. We use NIRCam and NIRSpec data from CANUCS, Technicolor, JUMPS, Silver Bullet, UNCOVER and MEGASCIENCE across 7 strong lensing fields. We find galaxies down to $M_{UV}\sim-16$ and 7.5<z<12.5. We measure \b{eta} with a forward-modelling procedure and estimate $f_{esc}$ for a subsample with emission line data using a relation, calibrated from a low-z sample, with UV slope, galaxy size and H$\beta$ equivalent width. We find 378 galaxies (45 with spectrum), yielding average values $\beta=-2.3\pm0.4$, $z=8.5\pm1.0$, and $M_{UV}=-18\pm1$. We find no significant $\beta$ evolution across our redshift range, suggesting a flattening of the $\beta-z$ trend above $z\sim7.5$. We find a weak negative trend between $\beta$ and $M_{UV}$. For 14 galaxies we estimate an average $f_{esc}=0.26\pm0.22$. The flat trend of $\beta$ at $z>7.5$ suggests similar properties between $300$ and $600 Myr$ after the Big Bang. The weak trend between $\beta$ and $M_{UV}$ suggests an analogous composition for low- and high-mass galaxies' ISM, likely due to a lack of time for dust buildup. While average $f_{esc}$ is higher than necessary to ionize the IGM by z~6, the model extrapolated at low-z may overestimate its value.

The formation pathways of sulfur-bearing species in the interstellar medium are crucial to understand astrochemical processes in cold molecular clouds and to gain new insights about the sulfur budget in these regions. We aim to explore the recently detected, thioethanal (CH$_{3}$CHS) formation mechanisms from thioethanol (CH$_{3}$CH$_{2}$SH) as a precursor in addition to secondary sulfur products. The electronic structure methods and density functional theory for both gas-phase and ice-grain surface environments is employed. To mimic interstellar ice-mantles, we use medium (W6) and large amorphized (W22) water clusters as implemented in Binding Energy Evaluation protocol. A barrierless formation mechanism for CH$_{3}$CHS under low-temperature interstellar conditions is identified, in the gas phase. Surface environments modulate activation barriers in a site-specific manner, elucidated through both Langmuir-Hinshelwood and Eley-Rideal initiated surface reaction pathways. Compared to oxygen analogs, sulfur chemistry enables alternate pathways due to weaker S-H bonding, with a competing route forming ethane-1,1-di-thiol (CH$_{3}$CH(SH)SH), on the ice-grain surface, potentially reducing CH$_{3}$CHS yields. The first accurate binding energy for thioethanol on water ice is also reported, confirming its greater volatility than ethanol. The proposed mechanism offers a tentative hypothesis for the apparent mutual exclusive detections of the CH$_{3}$CH$_{2}$SH and CH$_{3}$CHS in TMC-1, Orion, and Sgr B2(N), that further requires validation through quantitative astrochemical modeling and also to distinguish this chemical differentiation from observational sensitivity limitations. These qualitative findings highlight the multifaceted chemical behavior of sulfur-bearing organics in the interstellar medium and support CH$_{3}$CH(SH)SH as promising astro-chemical targets.

The Seyfert galaxy Markarian 590 offers a rare glimpse into the dynamic life cycle of black hole accretion, captured across multiple wavelengths from the years 1975 to 2025. Using the decade-long multi-band observations from the Swift observatory, we capture a clear spectral state transition analogous to those seen in X-ray binaries but seldom observed in a single AGN. We track a complete AGN state transition in real time, as the source evolves from a faint, hard X-ray state to a bright, UV- and soft X-ray dominated phase. The X-ray loudness parameter $\alpha_{ox}$ follows a pronounced 'V'-shaped dependence on Eddington ratio $\lambda_{Edd}$, with a break at $\lambda_{Edd}$ = 0.021 +/- 0.008, coinciding with thresholds identified in population studies of changing-look quasars and X-ray binaries. Across this transition, Mkn 590 evolves through distinct accretion regimes in the Hardness Intensity Diagrams (HID): faint, flaring, transitional, and bright, on a timescale of $\sim$10 years, which is well below classical viscous timescales for a geometrically thin disk but in agreement with the propagation of thermal wavefronts in the inner disk. When placed on the Fundamental Plane of black hole activity, the source broadly follows the expected radio/X-ray mass scaling, though with a 70% flatter slope, pointing towards a persistent coronal-jet coupling even in radiatively efficient states. Together, our results establish Mkn 590 as a rare, time-resolved case of AGN state transitions and offer compelling evidence for scale-invariant accretion physics across the black hole mass spectrum.

We investigate the azimuthal dependence of H I and O VI-bearing gas in the circumgalactic medium (CGM) of 113 isolated galaxies in the redshift range 0.12 < z < 0.75, including 91 new measurements from the MUSE Quasar-fields Blind Emitters Survey (MUSEQuBES). The H I covering fraction (k_HI) within the virial radius (Rvir) of low-mass (7 < log10(M*/Msun)< 9) galaxies, for a threshold column density of log10(N(HI)/cm^-2) = 14.5, exhibits an enhancement along both the disk plane (azimuthal angle phi < 20 degree) and in the polar direction (phi > 70 degree). In contrast, such a bimodal distribution is not observed for higher mass galaxies (9 < log10(M*/Msun) < 11.3). Similarly, the O VI covering fraction (k_OVI), for a threshold of log10(N(OVI)/cm^-2) = 14.0, shows a tentative enhancement along both the projected major and minor axes for low-mass galaxies. In contrast, O VI-bearing gas around higher- mass galaxies appears more uniformly distributed, with no significant azimuthal dependence. Finally, using the halo circular-velocity-normalized pixel-velocity two-point correlation function (TPCF), we find that O VI absorbers are kinematically narrower along the disk plane compared to the polar directions of the host galaxies with similar stellar mass distributions. The observed isotropic distribution of O VI in high-mass halos suggests that its spatial distribution is governed by global halo properties; however, the O VI kinematics retain memory of the site of origin.

This paper describes progress achieved in early investigations of the orbital motion and light curve of comet C/2026 A1 (MAPS), the third ground-based discovery of a Kreutz sungrazer in the 21st century. The highly unusual trait of the comet that has so far been ascertained is its extraordinarily long orbital period. The most recent orbital computations make it increasingly likely that the object is a fragment of one of the comets observed by Ammianus Marcellinus in AD 363, thereby strengthening evidence in support of the contact-binary hypothesis of the Kreutz system. In this context, the comet is the only second-generation fragment of Aristotle's comet that we are aware of to appear after the 12th century. It does not look like a major fragment, but rather like an outlying fragment of a much larger sungrazer. In 363 it apparently separated from a parent different from the lineage of comet Pereyra. The light curve of comet MAPS has so far been fairly smooth, without outbursts. To reach the brightness of comet Ikeya-Seki, the comet would have to follow an r^(-17) law in the coming weeks, which is unlikely.

We present a hydrodynamical description of spherical void evolution in modified gravity (MG), extending the standard General Relativity (GR) and dynamical dark energy treatment by encoding gravity modifications into effective couplings that enter the Euler and Poisson equations. This yields a compact non-linear evolution equation for the Eulerian density contrast, controlled by a time- and density-dependent effective gravitational strength, and provides a direct map between model functions and void observables. We apply the framework to the luminal Galileon class of models, where derivative self-interactions generate Vainshtein screening and might lead to a breakdown of the physical branch in sufficiently underdense regions. Exploiting this feature, we apply the void-informed viability requirement that translates into bounds on the theory parameter space and, equivalently, on the minimum attainable void depth as a function of redshift. For viable parameters of a concrete model, we quantify the impact of MG on isolated void evolution, the Lagrangian to Eulerian mapping, and the shell-crossing threshold. Relative to GR, we find a clear hierarchy of MG effects, with ${\cal O}(10\%)$ modifications in the gravitational couplings, percent-level shifts in the void density evolution, and sub-percent deviations in both the mapping and the shell-crossing thresholds. Moreover, within the adopted parametrization, we show analytically that voids always lie in an unscreened regime on the physical branch. Overall, the formalism provides a self-consistent route to predict void dynamics and consistency constraints in a broad class of MG models.

Deep, wide-area photometric surveys have uncovered a population of compact ($r_{1/2} \approx$ 1-15 pc), extremely-low-mass ($M_* \approx$ 20-4000 $M_{\odot}$) stellar systems in the Milky Way halo that are smaller in size than known ultra-faint dwarf galaxies (UFDs) and substantially fainter than most classical globular clusters (GCs). Very little is known about the nature and origins of this population of "Ultra-Faint Compact Satellites" (UFCSs) owing to a dearth of spectroscopic measurements. Here, we present the first spectroscopic census of these compact systems based on Magellan/IMACS and Keck/DEIMOS observations of 19 individual UFCSs, representing $\sim$2/3 of the known population. We securely measure mean radial velocities for all 19 systems, velocity dispersions for 15 (predominantly upper limits), metallicities for 17, metallicity dispersions for 8, and $\textit{Gaia}$-based mean proper motions for 18. This large new spectroscopic sample provides the first insights into population-level trends for these extreme satellites. We demonstrate that: (1) the UFCSs are kinematically colder, on average, than the UFDs, disfavoring very dense dark matter halos in most cases, (2) the UFCS population is chemically diverse, spanning a factor of $\sim$300 in mean iron abundance ($\rm -3.3 \lesssim [Fe/H] \lesssim -0.8$), with multiple systems falling beneath the "metallicity floor" proposed for GCs, and (3) while some higher-metallicity and/or younger UFCSs are clearly star clusters, the dynamical and/or chemical evidence allows the possibility that up to $\sim$50% of the UFCSs in our sample (9 of 19) may represent the smallest and least-massive galaxies yet discovered.

Neutron stars provide a unique environment to probe the properties of dense nuclear matter. In this work, we present a comparative study between two approaches to modeling the neutron star structure: a Chiral Effective Field Theory based approach and the MUSES Calculation Engine framework, which uses three different approaches for the three density regions. We analyze the resulting mass-radius relations, discussing the respective advantages and limitations of the two methods.

We consider a dark sector consisting of fermionic dark matter (DM) charged under a broken dark $U(1)_D$ gauge symmetry, interacting with the Standard Model through kinetic mixing. In such models, the DM annihilation cross section is typically suppressed by the small kinetic mixing and or a heavy mediator, often leading to an overabundant relic density. We show that the observed DM abundance can be achieved if the dark Higgs undergoes a strong first order phase transition after DM freeze-out. In this scenario, the relic abundance is set by thermal freeze-out in the symmetric phase and subsequently reduced by entropy injection from the phase transition, rather than by annihilation in the broken phase. We find that to reproduce the observed relic abundance, the required phase transition is generically supercooled. The resulting stochastic gravitational wave signal lies within the sensitivity of future experiments, providing a complementary probe of this framework. Moreover, a strongly supercooled phase transition can potentially account for the NANOGrav signal for DM masses below $O(10)$ GeV.

This paper presents a low-thrust trajectory optimization strategy to achieve a near-circular lunar orbit for a CubeSat injected into a lunar flyby trajectory. The 12U CubeSat HORYU-VI is equipped with four Hall-effect thrusters and designed as a secondary payload on NASA's Space Launch System under the Artemis program. Upon release, the spacecraft gains sufficient energy to escape the Earth-Moon system after a lunar flyby. The proposed trajectory is decomposed into three phases: (1) pre-flyby deceleration to avoid heliocentric escape, (2) lunar gravitational capture, and (3) orbit circularization to the science orbit. For each phase, an impulsive-burn solution is first computed as an initial guess, which is then refined through finite-burn optimization using Sequential Quadratic Programming (SQP). The dynamical model incorporates Earth-Moon-Sun-Jupiter gravitational interactions and a high-fidelity lunar gravity field. All trajectories are independently verified with NASA's General Mission Analysis Tool (GMAT). Results demonstrate that HORYU-VI achieves lunar capture within 200 days, establishes a stable science orbit at 280 days, and can spiral down to a near-circular 100 km orbit by 450 days, using a total Delta-V of 710 m/s, well within the capability of the electric propulsion system.

Gamma-ray bursts (GRBs) are among the most energetic transient phenomena in the universe and serve as powerful probes for high-energy astrophysical processes. In particular, faint GRBs originating from a distant universe may provide unique insights into the early stages of star formation. However, detecting and localizing such weak sources remains challenging owing to low photon statistics and substantial background noise. Although recent machine learning models address individual aspects of these challenges, they often struggle to balance the trade-off between statistical robustness and noise suppression. Consequently, we propose ComptonUNet, a hybrid deep learning framework that jointly processes raw data and reconstructs images for robust GRB localization. ComptonUNet was designed to operate effectively under conditions of limited photon statistics and strong background contamination by combining the statistical efficiency of direct reconstruction models with the denoising capabilities of image-based architectures. We perform realistic simulations of GRB-like events embedded in background environments representative of low-Earth orbit missions to evaluate the performance of ComptonUNet. Our results demonstrate that ComptonUNet significantly outperforms existing approaches, achieving improved localization accuracy across a wide range of low-statistic and high-background scenarios.

The inspiral of a compact object into a black hole is a key source of low-frequency gravitational waves for future space-based detectors like LISA. While models of this process have advanced, they typically focus on asymptotically flat spacetimes. In this paper, we explore how the absence of asymptotic flatness affects the slow, adiabatic orbital evolution due to radiation reaction. This lack of asymptotic flatness can arise from external environments or an expanding universe. Using the Schwarzschild-de Sitter (SdS) spacetime, where the deviation from flatness is governed by the cosmological constant, we study bound orbits characterized by their semi-latus rectum $p$ and eccentricity $e$. We calculate how the cosmological constant shifts the separatrix between bound and plunging orbits and alters the relationship between the binary's binding energy, angular momentum, and orbital parameters. Assuming the orbital timescale is much shorter than the inspiral timescale, we apply a modified quadrupole formula to examine the impact of a small positive cosmological constant on the orbital evolution in the weak-field limit. We find that the cosmological constant accelerates the decrease in eccentricity, reducing inspiral plunge times, which could influence event rate estimates for space-based detectors.

We present g4chargeit, a kinetic Monte Carlo framework built on Geant4 for self-consistent simulation of time-dependent electrostatic charging in dielectric materials. The model explicitly incorporates stochastic particle transport and scattering processes using validated Geant4 cross-sections, while self-consistently evolving the electric potential and field. As a representative application, we simulate the charging of regolith grains under average dayside conditions on the Moon. The surface of the Moon, in addition to other airless planetary bodies, are regularly exposed to solar ultraviolet photons and solar-wind plasma, creating a radiation environment in which electrostatic interactions among regolith grains become significant. Until now, simulations of regolith charging have often relied on analytical approximations that oversimplify grain geometry and interaction mechanisms. Our Geant4-based simulations reveal charge accumulation within intergrain micro-cavities, leading to repulsive electrostatic forces consistent with experimental observations. The framework establishes a multiscale approach that links microscopic scattering events to the continuity equation of surface charge density and to the formation of macroscopic surface charge patches in complex grain geometries. Although demonstrated here for planetary regolith, the method is general and applicable to a broad range of dielectric charging problems. The code is openly available at this https URL.

We present Nested Sampling with Slice-within-Gibbs (NS-SwiG), an algorithm for Bayesian inference and evidence estimation in high-dimensional models whose likelihood admits a factorization, such as hierarchical Bayesian models. We construct a procedure to sample from the likelihood-constrained prior using a Slice-within-Gibbs kernel: an outer update of hyperparameters followed by inner block updates over local parameters. A likelihood-budget decomposition caches per-block contributions so that each local update checks feasibility in constant time rather than recomputing the global constraint at linearly growing cost. This reduces the per-replacement cost from quadratic to linear in the number of groups, and the overall algorithmic complexity from cubic to quadratic under standard assumptions. The decomposition extends naturally beyond independent observations, and we demonstrate this on Markov-structured latent variables. We evaluate NS-SwiG on challenging benchmarks, demonstrating scalability to thousands of dimensions and accurate evidence estimates even on posterior geometries where state-of-the-art gradient-based samplers can struggle.

Dynamical structures in the circular restricted three-body problem (CR3BP) are fundamental for designing low-energy transfers, as they aid in analyzing phase space transport and designing desirable trajectories. This study focuses on lobe dynamics to exploit local chaotic transport around celestial bodies, and proposes a new method for systematically designing low-energy transfers by combining multiple lobe dynamics. A graph-based framework is constructed to explore possible transfer paths between departure and arrival orbits, reducing the complexity of the combinatorial optimization problem for designing fuel-efficient transfers. Based on this graph, low-energy transfer trajectories are constructed by connecting chaotic orbits within lobes. The resulting optimal trajectory in the Earth--Moon CR3BP is then converted into an optimal transfer in the bicircular restricted four-body problem using multiple shooting. The obtained transfer is compared with existing optimal solutions to demonstrate the effectiveness of the proposed method.

Observational data in cosmology indicate a small, positive, and nonvanishing cosmological constant that dominates the energy budget of the present universe. The origin of the cosmological constant from a quantum perspective remains unresolved, with a discrepancy of approximately 120 orders of magnitude between its observed value and theoretical estimates. Motivated by earlier work of Gibbons, we analyze the cosmological constant problem within a quantum-gravitational framework based on Schwarzian theory and its ensemble averaging. We then derive the phenomenological value of the dark energy density and obtain the corresponding equation of state. In this model, the cosmological constant arises from the ensemble average of time-reparametrization modes.

Through 2D3V PIC simulations of freely decaying sub-ion turbulence, intermittent localized regions with $\mathbf{E} \cdot \mathbf{B} \neq 0$ are found to be statistically associated with reductions in the magnitude of magnetic helicity while evolving in the early electron-scale interaction phase. Motivated by this behavior, we propose a source-compensated, history-dependent helicity density that satisfies an exact local balance identity by construction, enabling Saffman-type two-point correlation integrals which, under standard flux-decorrelation assumptions, can exhibit intermediate-scale plateaus that are roughly time-independent. In our simulations we demonstrate such plateaus to remain approximately invariant even as the usual Saffman helicity integral plateau value $I_H$ evolves during the early kinetic stage. Under approximate single-scale self-similarity, the plateau behavior of the magnetic integral is consistent with the 2D decay constraint $BL \sim \text{const}$. For initially net-helical configurations, we observe rapid development of mixed-signed magnetic helicity patches and a decrease of the global fractional helicity, such that the decay over the kinetic interval is again most consistent with the cancellation-dominated scaling constraint.

New exotic particles with MeV masses, such as axion-like particles or light dark matter, can be emitted from core-collapse supernovae (SNe) with semi-relativistic velocities. Due to their speed dispersion, they would arrive at Earth as an extended packet with a time spread that can be as large as tens of millennia for typical detectors. It has been argued in the literature that the superposition of packets from all galactic SNe would give rise to a smooth and stationary diffuse flux that could be observable on terrestrial experiments. In this article, we critically examine this hypothesis by carrying out a numerical simulation of the galactic history of SN explosions. We show that, although the particle packets do overlap, due to the short observational time window, each of them only contributes with a very narrow range of energies and with an intensity that depends on the SN distance. As a consequence, the energy dependence of the resulting flux is extremely sensitive to the stochastic nature of the SN population and far from smooth. This has profound implications for the expected signature in terrestrial experiments, which displays a spectral shape that is not properly described by the smooth approximation. We develop a numerical tool to compute this stochastic galactic flux for generic semi-relativistic particles, which also allows us to explore sub-MeV particles, where the smooth diffuse flux approach does not hold. To test this framework, we revisit existing bounds on axion-like particles and fermionic dark matter, finding weaker constraints than previously reported.

Characterizing compressible fluctuations in the solar wind is essential for understanding their role in solar wind acceleration and heating, yet their origin and evolution across different turbulence regimes remain poorly understood. In this study, we carry out a statistical analysis of the properties of compressible fluctuations in solar wind dominated by balanced and imbalanced turbulence. Using in-situ measurements from Wind, Solar Orbiter, and Parker Solar Probe, we investigate the scale dependence of density and magnetic field fluctuations and their correlations with plasma beta and radial distance. Our results indicate that solar wind compressibility is likely affected by both expansion effects and compressible dynamics governed by local plasma conditions. The non-Alfvenic wind is dominated by anti-correlated fluctuations, whereas the Alfvenic wind contains a mixture of correlated and anti-correlated fluctuations, though the latter remain prevalent. While the anti-correlated component is consistent with MHD slow magnetosonic modes, the correlated (fast mode-like) component is not reproduced by predictions from either linear MHD theory or nonlinear models of forced compressible fluctuations. Nevertheless, the dominant slow mode component explains the observed dependence on beta and the enhanced density fluctuations measured by Parker Solar Probe. This further suggests that slow mode waves contribute significantly to the compressible energy budget near the Sun and may play an important role in solar wind heating and acceleration close to the Sun.

We present theoretical and experimental evidence for a new phase of matter in neutron-rich systems that we call multimodal superfluidity. Using ab initio lattice calculations, we show that the condensate consists of coexisting s-wave pairs, p-wave pairs in entangled double pair combinations, and quartets composed of bound states of two s-wave pairs. We identify multimodal superfluidity as a general feature of single-flavor spin-1/2 fermionic systems with attractive s-wave and p-wave interactions, provided the system is stable against collapse into a dense droplet. Beyond neutrons at sub-saturation densities, we demonstrate that this phase appears in generalized attractive extended Hubbard models in one, two, and three dimensions. We elucidate the mechanism for this coexistence using self-consistent few-body Cooper models and compare with Bardeen-Cooper-Schrieffer theory. We also derive the form of the effective action and show that spin, rotational, and parity symmetries remain unbroken. Finally, we analyze experimental data to show that p-wave pair gaps and quartet gaps are present in atomic nuclei, and we discuss the consequences of this new phase for the structure and dynamics of neutron star crusts.

Recent experimental observations, most notably those reported by the ATOMKI and Positron Annihilation into Dark Matter Experiment (PADME) collaborations, have hinted anomalies that may indicate the presence of a new resonance with a mass around $17\,\text{MeV}$, potentially interacting with both nucleons and electrons. Since 2020, ATOMKI has observed this resonance in nuclear transitions from excited to ground states in ${}^{8}\mathrm{Be}$, ${}^{4}\mathrm{He}$, and ${}^{12}\mathrm{C}$. More recently, in 2025, PADME, operating at the Laboratori Nazionali di Frascati, has also hinted a similar excess, in this case in the $e^{+}e^{-}$ final-state events originating from positron annihilation on fixed-target atomic electrons of Carbonium. This concordance strengthens the case for a common underlying origin, potentially involving a new boson, conventionally referred to as $X_{17}$. Despite these intriguing developments, the global experimental landscape remains highly dynamic, particularly in light of recent MEG~II constraints, and a definitive confirmation or exclusion of the $X_{17}$ hypothesis is still lacking. Within this evolving and exciting context, this thesis investigates whether a hypothetical $17\,\text{MeV}$ particle, coupled to electrons as suggested by the PADME observations, could function as a mediator between the Standard Model and previously unexplored hidden sectors. Such a mediator could, in principle, offer a novel pathway toward addressing one of the principal outstanding inconsistencies of the Standard Model: the nature and origin of dark matter.