Submitted to Physical Review X; 24 pages, 5 figures
The relaxation of a weakly collisional plasma is described by the Boltzmann-Poisson equations with the Lenard-Bernstein collision operator. We perform a perturbative analysis of these equations, and obtain, for the first time, exact analytic solutions, enabling definitive resolutions to long-standing controversies regarding the impact of weak collisions on continuous spectra and Landau eigenmodes. Unlike some previous studies, we retain both damping and diffusion terms in the collision operator. We find that the linear response is a temporal convolution of a continuum that depends on the continuous velocities of particles, and discrete normal modes that encapsulate coherent oscillations. The normal modes are exponentially damped over time due to collective effects (Landau damping) as well as collisional dissipation. The continuum is also damped by collisions but somewhat differently. Up to a collision time, which is the inverse of the collision frequency νc, the continuum decay is driven by velocity diffusion and occurs super-exponentially over a timescale ∼ν−1/3c. After a collision time, however, the continuum decay is driven by the collisional damping of particle velocities and diffusion of their positions, and occurs exponentially over a timescale ∼νc. This hitherto unknown, slow exponential decay causes perturbations to damp the most on scales comparable to the mean free path, but very slowly on larger scales, which establishes the local thermal equilibrium, the essence of the fluid limit. The long-term decay of the response is driven by the normal modes on scales smaller than the mean free path, but, on larger scales, is governed by the slowly decaying continuum and the least damped normal mode. Our analysis firmly establishes a long-sought connection between the collisionless and fluid limits of weakly collisional plasmas.
A new type of self-similarity is found in the problem of a plane-parallel, ultra-relativistic blast wave, propagating in a powerlaw density profile of the form ρ∝z−k. Self-similar solutions of the first kind can be found for k<7/4 using dimensional considerations. For steeper density gradients with k>2, second type solutions are obtained by eliminating a singularity from the equations. However, for intermediate powerlaw indices 7/4<k<2 the flow does not obey any of the known types of self-similarity. Instead, the solutions belong to a new class in which the self-similar dynamics are dictated by the non self-similar part of the flow. We obtain an exact solution to the ultra-relativistic fluid equations and find that the non self-similar flow is described by relativistic expansion into vacuum, composed of (1) an accelerating piston that contains most of the energy and (2) a leading edge of fast material that coincides with the interiors of the blastwave and terminates at the shock. The dynamics of the piston itself are self-similar and universal, and do not depend on the external medium. The exact solution of the non self-similar flow is used to solve for the shock in the new class of solutions.
13 pages, 9 figures. Accepted for publication in A&A
Magnetic neutron stars (NSs) often exhibit a cyclotron resonant scattering feature (CRSF) in their X-ray spectra, but the site of the CRSF formation is still an open puzzle. A promising candidate for high-luminosity sources has always been the radiative shock (RS) in the accretion column. Yet, no quantitative calculations of spectral formation at the RS have been performed so far. Here we explore the scenario where the shock is the site of the CRSF formation. We study spectral formation at the RS and the emergent spectral shape across a wide range of the parameter space. We developed a Monte Carlo code to conduct radiation transfer simulations at the RS, adopting a fully relativistic scheme for the interaction between radiation and electrons. We properly treated bulk-motion Comptonization in the pre-shock region, thermal Comptonization in the post-shock region, and resonant Compton scattering in both regions. We calculated the angle- and energy-dependent emergent X-ray spectrum from the RS, focusing on both the CRSF and the X-ray continuum, under diverse conditions. We find that a power law, hard X-ray continuum and a CRSF are naturally produced by the first-order Fermi energization as the photons criss-cross the shock. The CRSF shape depends mainly on the transverse optical depth and the post-shock temperature. We show that the CRSF energy centroid is shifted by ~(20-30)% to lower energies compared to the classical cyclotron energy, due to the Doppler boosting between the shock frame and the bulk-motion frame. We demonstrate that a "bump" feature arises in the right wing of the CRSF due to upscattering of photons by the accreting plasma and extends to higher energies for larger optical depths and post-shock temperatures. The implications of the Doppler effect on the centroid of the emergent CRSF must be considered if an accurate determination of the magnetic field strength is desired.
15 pages, including 5 figures
In this paper, we report Magnetospheric Multiscale (MMS) observations of the dayside magnetosphere when the upstream Alfv\'en speed rises above the solar wind speed (sub-Alfv\'enic), causing the windsock-like magnetosphere to transform into Alfv\'en wings. The event occurred in the magnetic cloud of a Coronal Mass Ejection (CME) on April 24, 2023. We highlight the following outstanding features: (1) a layer of accelerated cold CME flow directly adjacent to the wing and to the magnetopause, which represents a rare regime of the terrestrial magnetosphere interaction with unshocked CME plasma; (2) moving filaments of the Alfv\'en wing created by magnetic reconnection, which represent new channels of connection between Earth's magnetosphere and the foot points of the Sun's erupted flux rope; (3) cold CME ion deceleration with little heating across the magnetopause. The reported MMS measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to further understand how sub-Alfv\'enic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets that are close to their host stars.
20 pages, 9 Figures, submitted to ApJ. Comments welcome!
We investigate the connection between galaxies, dark matter halos, and their large-scale environments with Illustris TNG300 hydrodynamic simulation data. We predict stellar masses from subhalo properties to test two types of machine learning (ML) models: Explainable Boosting Machines (EBMs) with simple galaxy environment features and E(3)-invariant graph neural networks (GNNs). The best-performing EBM models leverage spherically averaged overdensity features on 3 Mpc scales. Interpretations via SHapley Additive exPlanations (SHAP) also suggest that, in the context of the TNG300 galaxy--halo connection, simple spherical overdensity on ∼3 Mpc scales is more important than cosmic web distance features measured using the DisPerSE algorithm. Meanwhile, a GNN with connectivity defined by a fixed linking length, L, outperforms the EBM models by a significant margin. As we increase the linking length scale, GNNs learn important environmental contributions up to the largest scales we probe (L=10 Mpc). We conclude that 3 Mpc distance scales are most critical for describing the TNG galaxy--halo connection using the spherical overdensity parameterization but that information on larger scales, which is not captured by simple environmental parameters or cosmic web features, can further augment these models. Our study highlights the benefits of using interpretable ML algorithms to explain models of astrophysical phenomena, and the power of using GNNs to flexibly learn complex relationships directly from data while imposing constraints from physical symmetries.
19 pages, 13 figures, accepted by The Astrophysical Journal
We present a focused study of radially-resolved varying PAH emission in the low-luminosity AGN-host NGC 4138 using deep Spitzer/IRS spectral maps. Using new model PAH spectra, we investigate whether these variations could be associated with changes to the PAH grain size distribution due to photodestruction by the AGN. Separately, we model the effects of the varying radiation field within NGC 4138, and we use this model to predict the corresponding changes in the PAH emission spectrum. We find that PAH band ratios are strongly variable with radius in this galaxy with short-to-long wavelength band ratios peaking in the starburst ring. The changing mix of starlight appears to have a considerable effect on the trends in these band ratios, and our radiation model predicts the shapes of these trends. However, the amplitude of observed variation is ~2.5 times larger than predicted for some ratios. A cutoff of small grains in the PAH size distribution, as has been suggested for AGN, together with changes in PAH ionization fraction could explain the behavior of the shorter bands, but this model fails to reproduce longer band behaviors. Additionally, we find that short-to-long wavelength PAH band ratios increase slightly within ~270pc of the center, suggesting that the AGN may directly influence PAH emission there.
27 pages, 18 figures, 2 appendices, submitted to A\&A
Number counts of galaxy clusters across redshift are a powerful cosmological probe, if a precise and accurate reconstruction of the underlying mass distribution is performed -- a challenge called mass calibration. With the advent of wide and deep photometric surveys, weak gravitational lensing by clusters has become the method of choice to perform this measurement. We measure and validate the weak gravitational lensing (WL) signature in the shape of galaxies observed in the first 3 years of the DES Y3 caused by galaxy clusters selected in the first all-sky survey performed by SRG/eROSITA. These data are then used to determine the scaling between X-ray photon count rate of the clusters and their halo mass and redshift. We empirically determine the degree of cluster member contamination in our background source sample. The individual cluster shear profiles are then analysed with a Bayesian population model that self-consistently accounts for the lens sample selection and contamination, and includes marginalization over a host of instrumental and astrophysical systematics. To quantify the accuracy of the mass extraction of that model, we perform mass measurements on mock cluster catalogs with realistic synthetic shear profiles. This allows us to establish that hydro-dynamical modelling uncertainties at low lens redshifts (z<0.6) are the dominant systematic limitation. At high lens redshift the uncertainties of the sources' photometric redshift calibration dominate. With regard to the X-ray count rate to halo mass relation, we constrain all its parameters. This work sets the stage for a joint analysis with the number counts of eRASS1 clusters to constrain a host of cosmological parameters. We demonstrate that WL mass calibration of galaxy clusters can be performed successfully with source galaxies whose calibration was performed primarily for cosmic shear experiments.
26 pages, 7 figures and 2 tables, accepted for publication in Physics and Astronomy Reports
Accepted for publication in Space Weather Journal
15 pages, 4 figures, 1 table, 6 data files attached
accepted for publication in the Astrophysical Journal, 29 pages, 12 figures
20 pages, 8 figures. Submitted to MNRAS; second revision
Under review in MNRAS. 15 pages, 10 figures, and 2 tables
11 pages, 4 figures
Submitted to A&A
To be submitted to ApJ in Feb. 2024. 26 pages, 6 figures, 5 Tables
15 pages, 15 figures
Resubmitted to ApJ; 28 pages, 16 figures
19 pages, 13 figures, submitted to A&A. Code available at this https URL
Accepted for publication in the Astronomical Journal. The HPIC is hosted on the NASA Exoplanet Archive at this https URL
Submitted to ApJ
Accepted for publication on MNRAS
accepted version February 09, 2024
24 pages, 21 figures
11 pages, 12 figures
24 pages, 16 figures, 3 tables, accepted for publication in AJ
15 pages, 10 figures, 2 tables, 5 appendices (4 additional pages, 6 additional figures). Accepted for publication in A&A
Comments welcome
Accepted for publication in ApJ
30 pages, 11 figures, 5 tables
14 pages, 4 figures, accepted for publication in ApJ
7 pages, 4 figures, Accepted for publication in MNRAS
29 pages, 19 figures, 8 Tables, Accepted for publication in The Astrophysical Journal
26 pages, 11 figures. accepted for publication in ApJ
4 figures, 5 tables, accepted for publication in AJ
20 pages, 25 figures, 5 tables, accepted for publication in A&A
21 pages. Submitted to MNRAS. Comments welcomed
20 pages, 30 figures
17 pages, 1 figure, 1 table. Submitted to ApJ. Scripts for calculation are available on Zenodo (see link in introduction)
12 pages, 7 figures, accepted for publication in Astronomy and Astrophysics
Accepted for publication in A&A. 14 pages, 11 figures, 8 tables. A minimum driver program is available at this https URL
14 pages, 5 figures. Submitted to JCAP. Welcome comments!
20 pages, 8 figures. Submitted to MNRAS 13/02/24
in press in A&A. 27 pages, 21 figures, 3 tables
36 pages, 23 figures, 6 tables. Submitted to A&A
A&A in press
22 pages, 23 figures
24 pages, 31 figures, 2 tables. Submitted to A&A
43 pages, 22 figures, submitted to A&A
13 pages, 6 figures, submitted to A&A
Submitted to A&A, 17 pages, 15 figures
15 pages, 8 figures; accepted for publication in A&A
Accepted for publication in MNRAS
Submitted to the Nordic Machine Intelligence journal. The code is available at this https URL Comments welcome
13 pages, 12 figures, in press, accepted by Astronomy and Astrophysics for publication
16 pages, 6 figures, accepted for publication on ApJ
21 pages, 14 figures, accepted to Astronomy Letters at 2023 November 21
12 pages, 10 sub figures and one animation across 5 figures. Animation may not work in all pdf-viewing softwares. Accepted in ApJ
12 pages, 10 figures, 1 table, accepted for publication in MNRAS
52 pages, 14 figures
Submitted to A&A
21 pages, 18 figures, submitted to Monthly Notices of the Royal Astronomical Society
10 pages, 6 figures
15 pages, 8 figures
15+1 pages and 10 figures
12 pages, 4 figures
15 pages