adsabs/SciX_UAT_keywords huggingface dataset:
Can We Trust MHD Jump Conditions for Collisionless Shocks?

When applied to compute the density jump of a shock, the standard magnetohydrodynamic (MHD) formalism assumes (1) that all the upstream material passes downstream, together with the momentum and energy it carries, and (2) that pressures are isotropic. In a collisionless shock, shock-accelerated particles going back and forth around the front can invalidate the first assumption. In addition, an external magnetic field can sustain stable pressure anisotropies, invalidating the second assumption. It is therefore unclear whether or not the density jump of a collisionless shock fulfills the MHD jump. Here we try to clarify this issue. A literature review is conducted on 68 articles dealing with Particle-In-Cell simulations of collisionless shocks. We analyze the factors triggering departure from the MHD density jump and quantify their influence on ΔRH, the relative departure from the Rankine-Hugoniot (RH) jump. For small departures we propose ${{\rm{\Delta }}}_{\mathrm{RH}}=+{ \mathcal O }({10}^{-1-3.7\kappa }){t}^{\kappa }-\sigma { \mathcal O }(1)$ , where t is the timescale of the simulation, σ is the magnetization parameter and κ is a constant of order unity. The first term stems from the energy leakage into the accelerated particle. The second term stems from the downstream anisotropy triggered by the field (assuming an isotropic upstream). This relation allows us to assess to what extent a collisionless shock fulfills the RH density jump. In the strong field limit and for parallel shocks, the departure caused by the field saturates at a finite, negative value. For perpendicular shocks, the departure goes to zero at small and high σ's so that we find here a departure window. The results obtained have to be checked against full 3D simulations.

Keywords: shocks; magnetohydrodynamics; plasma astrophysics; plasma physics; high energy astrophysics
A Pluto-Charon Sonata IV. Improved Constraints on the Dynamical Behavior and Masses of the Small Satellites

We discuss a new set of ~500 numerical n-body calculations designed to constrain the masses and bulk densities of Styx, Nix, Kerberos, and Hydra. Comparisons of different techniques for deriving the semimajor axis and eccentricity of the four satellites favor methods relying on the theory of Lee & Peale, where satellite orbits are derived in the context of the restricted three-body problem (Pluto, Charon, and one massless satellite). In each simulation, we adopt the nominal satellite masses derived in Kenyon & Bromley, multiply the mass of at least one satellite by a numerical factor f ≥ 1, and establish whether the system ejects at least one satellite on a timescale ≤4.5 Gyr. When the total system mass is large (f ≫ 1), ejections of Kerberos are more common. Systems with lower satellite masses (f ≈ 1) usually eject Styx. In these calculations, Styx often "signals" an ejection by moving to higher orbital inclination long before ejection; Kerberos rarely signals in a useful way. The n-body results suggest that Styx and Kerberos are more likely to have bulk densities comparable with water ice, ρ SK ≲ 2 g cm-3, than with rock. A strong upper limit on the total system mass, M SNKH ≲ 9.5 × 1019 g, also places robust constraints on the average bulk density of the four satellites, ρ SNKH ≲ 1.4 g cm-3. These limits support models where the satellites grow out of icy material ejected during a major impact on Pluto or Charon.

Keywords: pluto; plutonian satellites; dynamical evolution; natural satellite formation
The Organization of Cloud-scale Gas Density Structure: High-resolution CO versus 3.6 μm Brightness Contrasts in Nearby Galaxies

In this paper we examine the factors that shape the distribution of molecular gas surface densities on the 150 pc scale across 67 morphologically diverse star-forming galaxies in the PHANGS-ALMA CO (2-1) survey. Dividing each galaxy into radial bins, we measure molecular gas surface density contrasts, defined here as the ratio between a fixed high percentile of the CO distribution and a fixed reference level in each bin. This reference level captures the level of the faint CO floor that extends between bright filamentary features, while the intensity level of the higher percentile probes the structures visually associated with bright, dense interstellar medium features like spiral arms, bars, and filaments. We compare these contrasts to matched percentile-based measurements of the 3.6 μm emission measured using Spitzer/IRAC imaging, which trace the underlying stellar mass density. We find that the logarithms of CO contrasts on 150 pc scales are 3-4 times larger than, and positively correlated with, the logarithms of 3.6 μm contrasts probing smooth nonaxisymmetric stellar bar and spiral structures. The correlation appears steeper than linear, consistent with the compression of gas as it flows supersonically in response to large-scale stellar structures, even in the presence of weak or flocculent spiral arms. Stellar dynamical features appear to play an important role in setting the cloud-scale gas density in our galaxies, with gas self-gravity perhaps playing a weaker role in setting the 150 pc scale distribution of gas densities.

Keywords: disk galaxies; spiral galaxies; interstellar medium; galaxy structure; molecular gas
A Misfired Outburst in the Neutron Star X-Ray Binary Centaurus X-4

We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 yr. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disk; we detect short-duration flares, likely originating also in the disk, superimposed with a small-amplitude (<0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (~2300 days) downward trend, followed by a shorter (~1000 days) upward one, is observed in the disk light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disk instability model. For Cen X-4, the rise of the optical flux proceeded for ~3 yr, and culminated in a flux increase at all wavelengths (optical-UV-X-rays) at the end of 2020. This increase faded after ~2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disk. The propagation might have stalled soon after the ignition due to the increasing surface density in the disk that the front encountered while propagating outward. The stall was likely eased by the low-level irradiation of the outer regions of the large accretion disk, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.

Keywords: neutron stars; low-mass x-ray binary stars; stellar accretion disks
Quiescent Galaxy Size, Velocity Dispersion, and Dynamical Mass Evolution

We use surveys covering the redshift range 0.05 < z < 3.8 to explore quiescent galaxy scaling relations and the redshift evolution of the velocity dispersion, size, and dynamical mass at fixed stellar mass. For redshift z < 0.6, we derive mass-limited samples and demonstrate that these large samples enhance constraints on the evolution of the quiescent population. The constraints include 2985 new velocity dispersions from the SHELS F2 survey. In contrast with the known substantial evolution of size with redshift, evolution in the velocity dispersion is negligible. The dynamical-to-stellar-mass ratio increases significantly as the universe ages, in agreement with recent results that combine high-redshift data with the Sloan Digital Sky Survey. Like other investigators, we interpret this result as an indication that the dark matter fraction within the effective radius increases as a result of the impact of the minor mergers that are responsible for size growth. We emphasize that dense redshift surveys covering the range 0.07 < z < 1 along with strong and weak lensing measurements could remove many ambiguities in evolutionary studies of the quiescent population.

Keywords: galaxy evolution; galaxy dynamics; galaxy structure; galaxy spectroscopy; galaxy photometry; dark matter; galaxy dark matter halos; early-type galaxies
Chromatic Microlensing Time Delays

Due to the finite size of the disk and the temperature fluctuations producing the variability, microlensing changes the actual time delays between images of strongly lensed active galactic nuclei on the ∼day(s) light-crossing timescale of the emission region. This microlensing-induced time delay depends on the disk model, primarily the disk size Rdisk, which has been found to be larger than predicted by the thin-disk model. In this work, we propose that light curves measured in different bands will give different time delays because Rdisk is a function of wavelength; by measuring the time-delay differences between bands, one can (1) directly verify such an new effect, and (2) test the thin-disk model of quasars. For the second goal, our method can avoid the potential inconsistency between multi-band light curves that may bias the results by continuum reverberation mapping. We conduct a simulation based on a PG 1115+080-like lensed quasar, calculating the theoretical distributions of time-delay differences between two bands: u and i centered around 354 nm and 780 nm, under and beyond the thin-disk model, respectively. Assuming that the disk size is twice the size of the standard one, we find that with a precision of 2 days in the time-delay difference measurements, the microlensing time delay effect can be verified with ∼4 measurements, while with ∼35 measurements the standard model can be excluded. This approach could be realized in the ongoing and upcoming multi-band wide-field surveys with follow-up observations.

Keywords: gravitational lensing; quasar microlensing
Minidisk Accretion onto Spinning Black Hole Binaries: Quasi-periodicities and Outflows

We perform a full 3D general relativistic magnetohydrodynamical (GRMHD) simulation of an equal-mass, spinning, binary black hole approaching merger, surrounded by a circumbinary disk and with a minidisk around each black hole. For this purpose, we evolve the ideal GRMHD equations on top of an approximated spacetime for the binary that is valid in every position of space, including the black hole horizons, during the inspiral regime. We use relaxed initial data for the circumbinary disk from a previous long-term simulation, where the accretion is dominated by a m = 1 overdensity called the lump. We compare our new spinning simulation with a previous non-spinning run, studying how spin influences the minidisk properties. We analyze the accretion from the inner edge of the lump to the black hole, focusing on the angular momentum budget of the fluid around the minidisks. We find that minidisks in the spinning case have more mass over a cycle than the non-spinning case. However, in both cases we find that most of the mass received by the black holes is delivered by the direct plunging of material from the lump. We also analyze the morphology and variability of the electromagnetic fluxes, and we find they share the same periodicities of the accretion rate. In the spinning case, we find that the outflows are stronger than the non-spinning case. Our results will be useful to understand and produce realistic synthetic light curves and spectra, which can be used in future observations.

Keywords: supermassive black holes; jets; compact objects; magnetohydrodynamical simulations
Mass Flows in Expanding Coronal Loops

An expansion of the cross-sectional area directly impacts the mass flow along a coronal loop and significantly alters the radiative and hydrodynamic evolution of that loop as a result. Previous studies have found that an area expansion from the chromosphere to the corona significantly lengthens the cooling time of the corona and appears to suppress draining from the corona. In this work, we examine the fluid dynamics to understand how the mass flow rate, the energy balance, and the cooling and draining timescales are affected by a nonuniform area. We find that in loops with moderate or large expansion (cross-sectional area expansion factors of 2, 3, 10, 30, 100 from the photosphere to the apex), impulsive heating, for either direct thermal heating or electron beam heating, induces a steady flow into the corona, so that the coronal density continues to rise during the cooling phase, whereas a uniform loop drains during the cooling phase. The induced upflow carries energy into the corona, balancing the losses from thermal conduction, and continues until thermal conduction weakens enough so that it can no longer support the radiative losses of the transition region. As a result, the plasma cools primarily radiatively until the onset of catastrophic collapse. The speed and duration of the induced upflow both increase in proportion to the rate of area expansion. We argue that observations of blueshifted spectral lines, therefore, could place a constraint on a loop's area expansion.

Keywords: astrophysical fluid dynamics; solar corona; solar transition region
Fundamentalization of Periods for First- and Second-overtone Classical Cepheids

Almost half of all classical Cepheids do not pulsate in fundamental mode, and nowadays, the fundamentalization of their higher-mode periods is frequently applied to increase the sample size in astrophysical investigations and allow for comparison with fundamental-mode Cepheids. On the other hand, the relations used to obtain fundamentalized periods are either old or based on small samples that cover narrow period ranges. We used available data of 989 Cepheids pulsating in at least two modes to obtain modern, high-quality empirical fundamentalization relations applicable in a wide range of periods of first- and second-overtone Cepheids for metallicities typical for the Milky Way and Magellanic Clouds. A clear correlation between the features of these relations and metallicity is seen, and periods with lower sensitivity to metallicity are identified. We also compare our results with double-mode Cepheids from the M31 and M33 galaxies. For the first galaxy, this indicates Cepheids have metallicities from supersolar to typical for the LMC, while for the latter, from solar to typical for the SMC. A general discussion of the usage of different types of fundamentalization relations, depending on the scientific problem, is included.

Keywords: cepheid variable stars; pulsating variable stars; double-mode cepheid variable stars; large magellanic cloud; small magellanic cloud; milky way galaxy; andromeda galaxy; triangulum galaxy
High-energy Neutrino Emission from Espresso-reaccelerated Ions in Jets of Active Galactic Nuclei

We present a bottom-up calculation of the flux of ultrahigh-energy cosmic rays (UHECRs) and high-energy neutrinos produced by powerful jets of active galactic nuclei (AGNs). By propagating test particles in 3D relativistic magnetohydrodynamic jet simulations, including a Monte Carlo treatment of sub-grid pitch-angle scattering and attenuation losses due to realistic photon fields, we study the spectrum and composition of the accelerated UHECRs and estimate the amount of neutrinos produced in such sources. We find that UHECRs may not be significantly affected by photodisintegration in AGN jets where the espresso mechanism efficiently accelerates particles, consistent with Auger's results that favor a heavy composition at the highest energies. Moreover, we present estimates and upper bounds for the flux of high-energy neutrinos expected from AGN jets. In particular, we find that (i) source neutrinos may account for a sizable fraction, or even dominate, the expected flux of cosmogenic neutrinos; (ii) neutrinos from the β-decay of secondary neutrons produced in nucleus photodisintegration end up in the teraelectronvolt to petaelectronvolt band observed by IceCube, but can hardly account for the observed flux; (iii) UHECRs accelerated via the espresso mechanism lead to nearly isotropic neutrino emission, which suggests that nearby radio galaxies may be more promising as potential sources. We discuss our results in light of multimessenger astronomy and current/future neutrino experiments.

Keywords: relativistic jets; radio jets; particle astrophysics; ultra-high-energy cosmic radiation; neutrino astronomy; cosmological neutrinos
Jet Properties of Compact Steep-spectrum Sources and an Eddington-ratio-driven Unification Scheme of Jet Radiation in Active Galactic Nuclei

Compact steep-spectrum sources (CSSs) likely represent a population of young radio-loud active galactic nuclei (AGNs) and have been identified as γ-ray-emitting sources. We present a comprehensive analysis of their γ-ray emission observed with Fermi/LAT and establish their broadband spectral energy distributions (SEDs). We derive their jet properties using SED fits with a two-zone leptonic model for radiation from the compact core and the large-scale extended region, and explore the possible signature of a unification picture of jet radiation among subclasses of AGNs. We show that the observed γ-rays of CSSs with significant variability are contributed by the radiation of their compact cores via the inverse-Compton process of the torus photons. The derived power-law distribution index of the radiating electrons is p1 ∼ 1.5-1.8, magnetic field strength is B ∼ 0.15-0.6 G, and Doppler-boosting factor is δ ∼ 2.8-8.9. Assuming that the jet is composed of e± pairs, the compact cores of CSSs are magnetized and have a high radiation efficiency, similar to that of flat-spectrum radio quasars. The γ-ray-emitting CSSs on average have higher Eddington ratio and black hole mass than those non-GeV-detected CSSs, and they follow the correlation between the jet power in units of Eddington luminosity ( ${P}_{\mathrm{jet}}^{{e}^{\pm }}/{L}_{\mathrm{Edd}}$ ) and Eddington ratio (REdd) with other subclasses of AGNs, ${P}_{\mathrm{jet}}^{{e}^{\pm }}/{L}_{\mathrm{Edd}}\propto {R}_{\mathrm{Edd}}^{0.52\pm 0.03}$ , indicating that REdd would be a key physical driver for the unification scheme of AGN jet radiation.

Keywords: active galactic nuclei; relativistic jets; compact galaxies; gamma-rays; non-thermal radiation sources
Signatures of Untwisting Magnetic Field in a Small Emerging Bipole in the Solar Photosphere

We perform a study of fluid motions and its temporal evolution in and around a small bipolar emerging flux region using observations made by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We employ local correlation tracking of the Doppler observations to follow horizontal fluid motions and line-of-sight magnetograms to follow the flux emergence. Changes in vertical vorticity and horizontal divergence are used to derive signatures of evolving twists in the magnetic field. Our analysis reveals that the two polarities of the magnetic flux swirl in opposite directions in the early stages of flux emergence indicating an unwinding of the pre-emergence twists in the magnetic field. We further find that during the emergence, there is an increase in swirly motions in the neighboring nonmagnetic regions. We estimate the magnetic and kinetic energies and find that magnetic energy is about a factor of 10 larger than the kinetic energy. During the evolution, when the magnetic energy decreases, an increase in the kinetic energy is observed indicating transfer of energy from the unwinding of the magnetic flux tube to the surrounding fluid motions. Our results thus demonstrate the presence of pre-emergence twists in an emerging magnetic field that is important in the context of the hemispheric helicity rule warranting a detailed statistical study in this context. Further, our observations point to a possible widespread generation of torsional waves in emerging flux regions due to the untwisting magnetic field with implications for upward energy transport to the corona.

Keywords: solar photosphere; solar magnetic fields; solar magnetic flux emergence; solar activity; solar evolution
Investigating the Effect of Galaxy Interactions on Star Formation at 0.5 < z < 3.0

Observations and simulations of interacting galaxies and mergers in the local universe have shown that interactions can significantly enhance the star formation rates (SFRs) and fueling of active galactic nuclei (AGN). However, at higher redshift, some simulations suggest that the level of star formation enhancement induced by interactions is lower due to the higher gas fractions and already increased SFRs in these galaxies. To test this, we measure the SFR enhancement in a total of 2351 (1327) massive (M * > 1010 M ⊙) major (1 < M 1/M 2 < 4) spectroscopic galaxy pairs at 0.5 < z < 3.0 with ΔV < 5000 km s-1 (1000 km s-1) and projected separation <150 kpc selected from the extensive spectroscopic coverage in the COSMOS and CANDELS fields. We find that the highest level of SFR enhancement is a factor of ${1.23}_{-0.09}^{+0.08}$ in the closest projected separation bin (<25 kpc) relative to a stellar mass-, redshift-, and environment-matched control sample of isolated galaxies. We find that the level of SFR enhancement is a factor of ~1.5 higher at 0.5 < z < 1 than at 1 < z < 3 in the closest projected separation bin. Among a sample of visually identified mergers, we find an enhancement of a factor of ${1.86}_{-0.18}^{+0.29}$ (~3σ) for coalesced systems. For this visually identified sample, we see a clear trend of increased SFR enhancement with decreasing projected separation (2.40 ${}_{-0.37}^{+0.62}$ versus ${1.58}_{-0.20}^{+0.29}$ for 0.5 < z < 1.6 and 1.6 < z < 3.0, respectively). The SFR enhancements seen in our interactions and mergers are all lower than the level seen in local samples at the same separation, suggesting that the level of interaction-induced star formation evolves significantly over this time period.

Keywords: galaxies; galaxy interactions; galaxy mergers; galaxy evolution
Revisiting Attenuation Curves: The Case of NGC 3351

Multiwavelength images from the far-UV (∼0.15 μm) to the submillimeter of the central region of the galaxy NGC 3351 are analyzed to constrain its stellar populations and dust attenuation. Despite hosting a ∼1 kpc circumnuclear starburst ring, NGC 3351 deviates from the IRX-β relation, the relation between the infrared-to-UV luminosity ratio and the UV continuum slope β that other starburst galaxies follow. To understand the reason for the deviation, we leverage the high angular resolution of archival near-UV-to-near-IR Hubble Space Telescope images to divide the ring into ∼60-180 pc size regions and model each individually. We find that the UV slope of the combined intrinsic (dust-free) stellar populations in the central region is redder than what is expected for a young model population. This is due to the region's complex star formation history, which boosts the near-UV emission relative to the far-UV. The resulting net attenuation curve has a UV slope that lies between those of the starburst attenuation curve (Calzetti et al. 2000) and the Small Magellanic Cloud extinction curve; the total-to-selective attenuation value, $R^{\prime} $ (V) = 4.93, is larger than both. As found for other star-forming galaxies, the stellar continuum of NGC 3351 is less attenuated than the ionized gas, with E(B - V)star = 0.40 E(B - V)gas. The combination of the "red" intrinsic stellar population and the new attenuation curve fully accounts for the location of the central region of NGC 3351 on the IRX-β diagram. Thus, the observed characteristics result from the complex mixture of stellar populations and dust column densities in the circumnuclear region. Despite being a sample of one, these findings highlight the difficulty of defining attenuation curves of general applicability outside the regime of centrally concentrated starbursts. * Based on observations obtained with the NASA/ESA Hubble Space Telescope, at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Keywords: starburst galaxies; interstellar extinction; reddening law