Sazuna/UAT_keywords huggingface dataset:
Formation of Power-law Electron Energy Spectra in Three-dimensional Low-β Magnetic Reconnection. While observations have suggested that power-law electron energy spectra are a common outcome of strong energy release during magnetic reconnection, e.g., in solar flares, kinetic simulations have not been able to provide definite evidence of power-laws in energy spectra of nonrelativistic reconnection. By means of 3D large-scale fully kinetic simulations, we study the formation of power-law electron energy spectra in nonrelativistic low-β reconnection. We find that both the global spectrum integrated over the entire domain and local spectra within individual regions of the reconnection layer have power-law tails with a spectral index p ∼ 4 in the 3D simulation, which persist throughout the nonlinear reconnection phase until saturation. In contrast, the spectrum in the 2D simulation rapidly evolves and quickly becomes soft. We show that 3D effects such as self-generated turbulence and chaotic magnetic field lines enable the transport of high-energy electrons across the reconnection layer and allow them to access several main acceleration regions. This leads to a sustained and nearly constant acceleration rate for electrons at different energies. We construct a model that explains the observed power-law spectral index in terms of the dynamical balance between particle acceleration and escape from main acceleration regions, which are defined based upon a threshold for the curvature drift acceleration term. This result could be important for explaining the formation of power-law energy spectrum in solar flares.

Keywords: Space plasmas, Solar flares, Solar magnetic reconnection, Plasma astrophysics, Interplanetary particle acceleration, Solar corona
A Reverse Shock in GRB 181201A. We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from ≈150 s to ≈163 days after the burst, comprising the first joint ALMA-VLA-GMRT observations of a gamma-ray burst (GRB) afterglow. The radio and millimeter-band data reveal a distinct signature at ≈3.9 days, which we interpret as reverse-shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all discovered degeneracies in the model parameters. The joint RS-FS modeling reveals a weakly magnetized (σ ≈ 3 × 10-3), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of Γ0 ≈ 103 for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections and find a likely observational bias selecting for longer lasting, nonrelativistic RSs. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multifrequency data sets.

Keywords: Gamma-ray bursts, Astronomy data modeling, High energy astrophysics, Relativistic jets, Transient sources, X-ray transient sources, Interstellar synchrotron emission, Markov chain Monte Carlo, Shocks, Gamma-ray transient sources, Relativistic fluid dynamics, Radio transient sources
Anisotropic Radio-wave Scattering and the Interpretation of Solar Radio Emission Observations. The observed properties (i.e., source size, source position, time duration, and decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker-Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor of ∼0.3 for sources observed at around 30 MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half width at half maximum of about 40° near 30 MHz. The results are applicable to various solar radio bursts produced via plasma emission.

Keywords: Solar coronal radio emission, Solar radio flares, Radio bursts, Solar radio emission
Reanalysis of the Pulsar Scenario to Explain the Cosmic Positron Excess Considering the Recent Developments. The slow diffusion of cosmic rays could be common around pulsars as indicated by the recent observations of HAWC, which can significantly change the pulsar interpretation of the well-known positron excess. Meanwhile, the latest measurement by AMS-02 shows a clear high-energy cutoff in the positron spectrum. Here, we check all the identified pulsars under the two-zone diffusion model to explain the new AMS-02 data. We find that the candidates must be nearby and middle-aged. Geminga, which was generally believed to be a very likely candidate, has recently been disfavored by Fermi-LAT observations of the GeV γ-ray flux. Following recent studies which indicate that PSR B1055-52 is much closer to the Earth than previously assumed, we propose for the first time that PSR B1055-52 is the most promising source of the positron excess. PSR B1055-52 can well reproduce both the intensity and the high-energy cutoff of the AMS-02 positron spectrum.

Keywords: Pulsars, Cosmic ray sources, Cosmic rays
X-Ray Intraday Variability of the TeV Blazar Mrk 421 with Suzaku. We present X-ray flux and spectral analyses of the three pointed Suzaku observations of the TeV high synchrotron peak blazar Mrk 421 taken throughout its complete operational duration. The observation taken on 2008 May 5 is, at 364.6 ks (i.e., 101.3 hr), the longest and most evenly sampled continuous observation of this source, or any blazar, in the X-ray energy 0.8-60 keV until now. We found large amplitude intraday variability in all soft and hard bands in all the light curves. The discrete correction function analysis of the light curves in soft and hard bands peaks on zero lag, showing that the emission in hard and soft bands are cospatial and emitted from the same population of leptons. The hardness ratio plots imply that the source is more variable in the harder bands compared to the softer bands. The source is harder when brighter, following the general behavior of high synchrotron peak blazars. Power spectral densities of all three light curves are red noise dominated, with a range of power spectra slopes. If one assumes that the emission originates very close to the central super massive black hole, a crude estimate for its mass, of ∼4 × 108 M ⊙, can be made; but if the variability is due to perturbations arising there that are advected into the jet and are thus Doppler boosted, substantially higher masses are consistent with the quickest seen variations. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux and spectral variability.

Keywords: X-ray astronomy, Jets, BL Lacertae objects, Quasars, Blazars
The Recent Burstiness of Star Formation in Galaxies at z ∼ 4.5 from Hα Measurements. The redshift range z = 4-6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understanding early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star formation history (SFH) of 221z ∼ 4.5 main-sequence galaxies at {log}(M/{M}☉ )&gt; 9.7 by comparing their ultra-violet (UV) continuum, Hα luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6 μm]-[4.5 μm] broadband color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between Hα- and UV-derived luminosities and star formation rates (SFRs). About half of the galaxies show a significant excess in Hα compared to expectations from a constant smooth SFH. We also find a tentative anticorrelation between Hα EW and stellar mass, ranging from 1000 Å at {log}(M/{M}☉ )&lt; 10 to below 100 Å at {log}(M/{M}☉ )&gt; 11. Consulting models suggests that most z ∼ 4.5 galaxies had a burst of star formation within the last 50 Myr, increasing their SFRs by a factor of &gt;5. The most massive galaxies on the other hand might decrease their SFRs and may be transitioning to a quiescent stage by z = 4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing Hα EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z &gt; 4, consistent with the extrapolation of measurements out to z = 2.

Keywords: Galaxy formation, Emission line galaxies, Star formation, Galaxy evolution, High-redshift galaxies
Variable Hard X-Ray Emission from the Central Star of the Eskimo Nebula. The central star of NGC 2392 shows the hardest X-ray emission among central stars of planetary nebulae (CSPNe). The recent discovery of a spectroscopic companion with an orbital period of 1.9 days could provide an explanation for its hard X-ray emission, as well as for the collimation of its fast outflow. Here, we analyze the available Chandra and XMM-Newton X-ray observations to determine accurately the spectral and temporal variation properties of the CSPN of NGC 2392. The X-ray emission can be described by an absorbed thermal plasma model with temperature {26}-5+8 MK and X-ray luminosity (8.7 ± 1.0) × 1030 erg s-1. No long-term variability is detected in the X-ray emission level, but the Chandra light curve is suggestive of short-term variations with a period ∼0.26 days. The possible origins of this X-ray emission are discussed. X-ray emission from the coronal activity of a companion or shocks in the stellar wind can be ruled out. Accretion of material from an unseen main-sequence companion onto the CSPN or from the CSPN wind onto a white dwarf companion are the most plausible origins for its hard X-ray emission, although the mismatch between the rotational period of the CSPN and the modulation timescale of the X-ray emission seems to preclude the former possibility.

Keywords: Planetary nebulae nuclei, Stellar winds, Planetary nebulae, X-ray stars
An Assessment of Contamination in the Thermal-SZ Map Using Cross-correlations. We search for potential galactic and extragalactic dust contamination in thermal Sunyaev-Zeldovich maps derived from the Planck data. To test for contamination, we apply a variety of galactic dust and cosmic infrared background (CIB) models to the data as part of the y map reconstruction process. We evaluate the level of contamination by cross-correlating these y maps with mass tracers based on weak lensing data. The lensing data we use are the convergence map, κ, from the Red Sequence Cluster Lensing survey, and the cosmic microwave background (CMB) lensing potential map, ϕ, from the Planck Collaboration. We make a CIB-subtracted y map and measure the cross-correlation between it and the lensing data. By comparing it with CIB-contaminated cross-correlation, we find that the cross-correlation between κ and y is only slightly contaminated by CIB signal, at the level of 6.8 ± 3.5%, which implies that previous detections of κ × y are robust to CIB contamination. However, we find that ϕ × y is more significantly contaminated, by 16.7 ± 3.5%, because the CMB lensing potential probes higher redshift sources that overlap more with the CIB sources. We find that Galactic dust does not significantly contaminate either cross-correlation signal.

Keywords: Cosmology, Weak gravitational lensing, Large-scale structure of the universe, Gravitational lensing, Dark matter
The Slowly Varying Corona. II. The Components of F <SUB>10.7</SUB> and Their Use in EUV Proxies. Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017), we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F 10.7, the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F 10.7 is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F 10.7 variability and that coronal bremsstrahlung emission accounts for 14.2 ± 2.1 sfu (∼20%) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F 10.7 at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F 10.7. We find that the bremsstrahlung component fit, along with the Mg II index, correlates better with the observed 5-37 nm spectrum than the common 81 day averaged F 10.7 proxy. The bremsstrahlung component of F 10.7 is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models.

Keywords: Solar coronal radio emission, Solar atmosphere, Solar extreme ultraviolet emission, Solar abundances, Solar radio emission, Quiet solar corona, Solar corona, Solar-terrestrial interactions, Solar activity, Active solar corona, Solar chromosphere
Escape of Flare-accelerated Particles in Solar Eruptive Events. Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensional (3D) geometries. We report the results of magnetohydrodynamic simulations of a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the coronal mass ejection (CME) eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. All our results compare favorably with observations.

Keywords: Solar coronal mass ejections, Solar magnetic reconnection
Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038. Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind-disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180° apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind-disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar.

Keywords: Low-mass x-ray binary stars, Neutron stars, Accretion, Millisecond pulsars
Termination Shock Measured by Voyagers and IBEX. Our heliosphere’s innermost boundary—the termination shock—slows and heats the supersonic solar wind and energizes anomalous cosmic rays (ACRs). We show that in addition to their termination shock crossings, the Voyager 1 and 2 spacecraft measurements identify additional points on the termination shock when they magnetically disconnect from the ACR source. These four points define a spherical approximation of the termination shock with radius 117 au, offset ∼32 au tailward, ∼27 au north, and ∼12 au to the port side of the Sun. Interstellar Boundary Explorer (IBEX) spacecraft observations independently confirm these general offsets, with the closest region of the termination shock ∼ 20° south of the interstellar inflow direction and a minimum distance ∼74 au. The maximum distance is ∼161 au, consistent with required ACR acceleration times. Thus, Voyager and IBEX spacecraft observations have directly revealed the global size and location of our heliosphere’s termination shock for the first time.

Keywords: Heliosphere, Termination shock, Solar wind, Solar wind termination, Interplanetary particle acceleration, Heliosheath, Cosmic rays
An ALMA Study of the FU Ori-type Object V900 Mon: Implications for the Progenitor. We present ALMA observations of 12CO, 13CO, and C18O J = 2-1 lines and the 230 GHz continuum for the FU Ori-type object (FUor) V900 Mon (d ∼ 1.5 kpc), for which the accretion burst was triggered between 1953 and 2009. We identified CO emission associated with a molecular bipolar outflow extending up to an ∼104 au scale and a rotating molecular envelope extending over &gt;104 au. The interaction with the hot energetic FUor wind, which was observed using optical spectroscopy, appears limited to a region within ∼400 au of the star. The envelope mass and collimation of the extended CO outflow suggest that the progenitor of this FUor is a low-mass Class I young stellar object (YSO). These parameters for V900 Mon, another FUor, and a few FUor-like stars are consistent with the idea that FUor outbursts are associated with normal YSOs. The continuum emission is marginally resolved in our observations with a 0.″2 × 0.″15 (∼300 × 225 au) beam, and a Gaussian model provides a deconvolved FWHM of ∼90 au. The emission is presumably associated with a dusty circumstellar disk, plus a possible contribution from a wind or wind cavity close to the star. The warm compact nature of the disk continuum emission could be explained with viscous heating of the disk, while gravitational fragmentation in the outer disk and/or a combination of grain growth and their inward drift may also contribute to its compact nature.

Keywords: Circumstellar envelopes, FU Orionis stars, Stellar jets, Stellar winds, Circumstellar disks, Stellar accretion disks
Tests of the Kerr Hypothesis with GRS 1915+105 Using Different RELXILL Flavors. In a previous paper, we tried to test the Kerr nature of the stellar-mass black hole in GRS 1915+105 by analyzing NuSTAR data of 2012 with our reflection model RELXILL_NK. We found that the choice of the intensity profile of the reflection component is crucial and eventually we were not able to get any constraint on the spacetime metric around the black hole in GRS 1915+105. In the present paper, we study the same source with Suzaku data of 2007. We confirm that the intensity profile plays an important role, but now we find quite stringent constraints consistent with the Kerr hypothesis. The key differences with respect to our previous study are likely the lower disk temperature in the Suzaku observation and the higher energy resolution near the iron line of the Suzaku data. We also apply different RELXILL flavors (different descriptions of the coronal spectrum and variable disk electron density) obtaining essentially the same results. We thus conclude that this choice is not very important for our tests of the Kerr hypothesis while the intensity profile does play an important role, and that with high-quality data it is possible to measure both the spacetime metric and the intensity profile.

Keywords: Accretion, X-ray astronomy, Black holes, Gravitation, General relativity
No Sign of G2's Encounter Affecting Sgr A*'s X-Ray Flaring Rate from Chandra Observations. An unusual object, G2, had its pericenter passage around Sgr A*, the 4 × 106 M ⊙ supermassive black hole in the Galactic Center, in Summer 2014. Several research teams have reported evidence that, following G2's pericenter encounter, the rate of Sgr A*'s bright X-ray flares increased significantly. Our analysis carefully treats varying flux contamination from a nearby magnetic neutron star and is free from complications induced by using data from multiple X-ray observatories with different spatial resolutions. We test the scenario of an increased bright X-ray flaring rate using a massive data set from the Chandra X-ray Observatory, the only X-ray instrument that can spatially distinguish between Sgr A* and the nearby Galactic Center magnetar throughout the full extended period encompassing G2's encounter with Sgr A*. We use X-ray data from the 3 Ms observations of the Chandra X-ray Visionary Program (XVP) in 2012, as well as an additional 1.5 Ms of observations up to 2018. We use detected flares to make distributions of flare properties. Using simulations of X-ray flares accounting for important factors such as the different Chandra instrument modes, we test the null hypothesis on Sgr A*'s bright (or any flare category) X-ray flaring rate around different potential change points. In contrast to previous studies, our results are consistent with the null hypothesis; the same model parameters produce distributions consistent with the observed ones around any plausible change point.

Keywords: X-ray astronomy, Supermassive black holes, Accretion