Sazuna/UAT_keywords huggingface dataset:
Alfvén Number for the Richtmyer-Meshkov Instability in Magnetized Plasmas. The magnetohydrodynamical (MHD) evolution of the Richtmyer-Meshkov instability (RMI) is investigated using two-dimensional MHD simulations. The RMI is suppressed by a strong magnetic field, whereas the RMI amplifies an ambient magnetic field by many orders of magnitude if the seed field is weak. We have found that the suppression and amplification processes can be evaluated continuously along with the amplitude of the Alfvén number RA, which is defined as the ratio of the linear growth velocity of the RMI to the Alfvén speed at the interface. When the Alfvén number is less than unity, the Lorentz force acting on the fluid mitigates the unstable motion of the RMI significantly, and the interface oscillates stably in this limit. If RA ≳ 1, on the other hand, the surface modulation increases due to the growth of the RMI. The maximum strength of the magnetic field is enhanced up to by a factor of RA. This critical feature is universal and independent of the initial Mach number of the incident shock, the Atwood number, corrugation amplitude, and even the direction of the initial magnetic field.

Keywords: PG 1159 stars, Interstellar magnetic fields, Interstellar medium, Shocks, Magnetohydrodynamical simulations
The Picture of BLR in 2.5D FRADO: Dynamics and Geometry. The dynamics of the broad line region (BLR) in active galaxies is an open question; direct observational constraints suggest a predominantly Keplerian motion, with possible traces of inflow or outflow. In this paper we study in detail the physically motivated BLR model of Czerny &amp; Hryniewicz based on the radiation pressure acting on dust at the surface layers of the accretion disk (AD). We consider here a nonhydrodynamical approach to the dynamics of the dusty cloud under the influence of radiation coming from the entire AD. We use here a realistic description of the dust opacity, and we introduce two simple geometrical models of the local shielding of the dusty cloud. We show that the radiation pressure acting on dusty clouds is strong enough to lead to dynamical outflow from the AD surface, so the BLR has a dynamical character of a (mostly failed) outflow. The dynamics strongly depends on the Eddington ratio of the source. Large Eddington ratio sources show a complex velocity field and large vertical velocities with respect to the AD surface, while for lower Eddington ratio sources vertical velocities are small and most of the emission originates close to the AD surface. Cloud dynamics thus determines the 3D geometry of the BLR.

Keywords: Gas-to-dust ratio, Supermassive black holes, Quasars, Broad-absorption line quasar, Reverberation mapping, Galaxy accretion disks, Active galaxies, Astrophysical dust processes, Active galactic nuclei, Galaxy winds, Dust continuum emission, Radiative processes
Solar Anti-Hale Bipolar Magnetic Regions: A Distinct Population with Systematic Properties. Besides their causal connection with long and short-term magnetic variability, solar bipolar magnetic regions are our chief source of insight into the location, size, and properties of large-scale toroidal magnetic structures in the solar interior. The great majority of these regions (≈95%) follow a systematic east-west polarity orientation (Hale's law) that reverses in opposite hemispheres and across even and odd cycles. These regions also present a systematic north-south polarity orientation (Joy's law) that helps build the poloidal field that seeds the new cycle. Exceptions to Hale's law are rare and difficult to study due to their low numbers. Here, we present a statistical analysis of the inclination (tilt) with respect to the equator of Hale versus anti-Hale regions spanning four solar cycles, considering two complementary tilt definitions adopted in previous studies. Our results show that anti-Hale regions belong to a separate population than Hale regions, suggesting a different originating mechanism. However, we find that anti-Hale region tilts present similar systematic tilt properties and similar latitudinal distributions to Hale regions, implying a strong connection between the two. We see this as evidence that they belong to a common toroidal flux system. We speculate that anti-Hale regions originate from poloidal field sheared and strengthened on the spot after the emergence of Hale regions with very strong poloidal contribution. Thus, they are not in contradiction with the idea of largely coherent toroidal flux systems inside the solar interior.

Keywords: Solar cycle, Solar active region magnetic fields, Sunspots, Solar dynamo, Solar active regions, Solar physics, Solar magnetic fields
The GOGREEN Survey: Evidence of an Excess of Quiescent Disks in Clusters at 1.0. We present the results of the measured shapes of 832 galaxies in 11 galaxy clusters at 1.0 &lt; z &lt; 1.4 from the GOGREEN survey. We measure the axis ratio (q), the ratio of the minor to the major axis, of the cluster galaxies from near-infrared Hubble Space Telescope imaging using Sérsic profile fitting and compare them with a field sample. We find that the median q of both star-forming and quiescent galaxies in clusters increases with stellar mass, similar to the field. Comparing the axis ratio distributions between clusters and the field in four mass bins, the distributions for star-forming galaxies in clusters are consistent with those in the field. Conversely, the distributions for quiescent galaxies in the two environments are distinct, most remarkably in 10.1\leqslant \mathrm{log}(M/{M}_{\odot })\lt 10.5$ where clusters show a flatter distribution, with an excess at low q. Modelling the distribution with oblate and triaxial components, we find that the cluster and field sample difference is consistent with an excess of flattened oblate quiescent galaxies in clusters. The oblate population contribution drops at high masses, resulting in a narrower q distribution in the massive population than at lower masses. Using a simple accretion model, we show that the observed q distributions and quenched fractions are consistent with a scenario where no morphological transformation occurs for the environmentally quenched population in the two intermediate-mass bins. Our results suggest that environmental quenching mechanism(s) likely produce a population that has a different morphological mix than those resulting from the dominant quenching mechanism in the field.

Keywords: Galaxy clusters, Galaxy evolution, High-redshift galaxy clusters
Metallicity Estimation of RR Lyrae Stars From Their I-Band Light Curves. We have revisited the problem of metallicity prediction of RR Lyrae stars from their near-infrared light curves in the Cousins I wave band. Our study is based on high-quality time series photometry and state-of-the-art high-resolution spectroscopic abundance measurements of 80 fundamental-mode (RRab) and 24 first-overtone (RRc) stars, spanning ~[-2.7, +0.18] dex and ~[-3, -0.5] dex ranges, respectively. Employing machine-learning methods, we investigated various light-curve representations and regression models to identify their optimal form for our objective. Accurate new empirical relations between the [Fe/H] iron abundance and the light-curve parameters have been obtained using Bayesian regression for both RRab and RRc stars with mean absolute prediction errors of 0.16 dex and 0.18 dex, respectively. We found that earlier I-band [Fe/H] estimates had a systematic positive bias of up to ~0.4 dex in the metal-poor regime. Our new predictive models were deployed on large ensembles of RR Lyrae stars to obtain photometric metallicity distribution functions (MDFs) for various old stellar populations in and around the Milky Way. We find that the mode of the old bulge component's MDF is approximately -1.4 dex, in remarkable agreement with the latest spectroscopic result. Furthermore, we derive MDF modes of -1.83, -2.13, and -1.77 dex for the Large and Small Magellanic Clouds and the Sagittarius dwarf galaxy, respectively.

Keywords: Light curves, RR Lyrae variable stars, Metallicity
Optical Transients from Fast Radio Bursts Heating Companion Stars in Close Binary Systems. Fast radio bursts (FRBs) are bright radio transients with short durations and extremely high brightness temperatures, and their physical origins are still unknown. Recently, a repeating source, FRB 20200120E, was found in a globular cluster in the very nearby M81 galaxy. The associated globular cluster has an age of ~9.13 Gyr, and hosts an old population of stars. In this work, we consider that an FRB source is in a close binary system with a low-mass main-sequence star as its companion. Due to the large burst energy of the FRB, when the companion star stops the FRB, its surface would be heated by the radiation-induced shock, and make reemission. For a binary system with a solar-like companion star and an orbital period of a few days, we find that the reemission is mainly at an optical band, with delays of a few seconds after the FRB. Its luminosity is several times larger than the solar luminosity, and the duration is about hundreds of seconds. Such a transient might be observable in a future multiwavelength follow-up observation for Galactic FRB sources.

Keywords: Neutron stars, Globular star clusters, Binary stars, Magnetars, Radio transient sources
Discovery of Transition from Marginally Stable Burning to Unstable Burning after a Superburst in Aql X-1. Superbursts are long duration, rare, and extremely energetic thermonuclear explosions of neutron star low-mass X-ray binaries (NS LMXBs), which are proposed to be due to unstable carbon ignition. We report the superburst properties and consequences from Aql X-1 observed by the Neutron Star Interior Composition Explorer (NICER), the Monitor of All-sky X-ray Image, Swift, and Insight Hard X-ray Modulation Telescope (Insight-HXMT) on the MJD 59130.7. We found two faint type I X-ray bursts 9.44 days after the superburst with a short recurrence time of 7.6 minutes, which is the most accurate measurement of the quenching time in all NS LMXBs with observed superbursts. We also discovered mHz quasi-periodic oscillations in the frequency range 2.7-11.3 mHz immediately after the superburst, before and after the resumption of the first type I X-ray burst from NICER, Swift, and Insight-HXMT observations. For the first time, we observed the transition from superburst, via marginally stable burning to unstable burning in NS LMXBs. We compared the quenching time and the recurrence time of type I X-ray bursts with simulations.

Keywords: Low-mass x-ray binary stars, Neutron stars, X-ray sources, X-ray bursts
Investigating Toroidal Flows in the Sun Using Normal-mode Coupling. Helioseismic observations have provided valuable data sets with which to pursue the detailed investigation of solar interior dynamics. Among various methods to analyze these data, normal-mode coupling has proven to be a powerful tool, used to study Rossby waves, differential rotation, meridional circulation, and nonaxisymmetric multiscale subsurface flows. Here, we invert mode-coupling measurements from the Helioseismic Magnetic Imager and the Michelson Doppler Imager to obtain mass-conserving toroidal convective flow as a function of depth, spatial wavenumber, and temporal frequency. To ensure that the estimates of velocity magnitudes are proper, we also evaluate correlated realization noise, caused by the limited visibility of the Sun. We benchmark the near-surface inversions against results from local correlation tracking. The convective power likely assumes greater latitudinal isotropy with a decrease in spatial scale of the flow. We note the absence of a peak in toroidal-flow power at supergranular scales, in line with observations that show that supergranulation is dominantly poloidal in nature.

Keywords: Helioseismology, Solar convective zone, Solar interior
SiS Formation in the Interstellar Medium through Si+SH Gas-phase Reactions. Silicon monosulfide is an important silicon-bearing molecule detected in circumstellar envelopes and star-forming regions. Its formation and destruction routes are not well understood, partially due to the lack of detailed knowledge on the involved reactions and their rate coefficients. In this work we have calculated and modeled the potential energy surface (PES) of the HSiS system employing highly accurate multireference electronic structure methods. After obtaining an accurate analytic representation of the PES, which includes long-range energy terms in a realistic way via the DMBE method, we have calculated rate coefficients for the Si+SH → SiS+H reaction over the temperature range of 25-1000 K. This reaction is predicted to be fast, with a rate coefficient of ~1 × 10-10 cm3 s-1 at 200 K, which substantially increases for lower temperatures (the temperature dependence can be described by a modified Arrhenius equation with α = 0.770 × 10-10 cm3 s-1, β = -0.756, and γ = 9.873 K). An astrochemical gas-grain model of a shock region similar to L1157-B1 shows that the inclusion of the Si+SH reaction increases the SiS gas-phase abundance relative to H2 from 5 × 10-10 to 1.4 × 10-8, which perfectly matches the observed abundance of ~2 × 10-8.

Keywords: Reaction rates, Chemical abundances, Astrochemistry
Kinematics of Crab Giant Pulses. The Crab Pulsar's radio emission is unusual, consisting predominantly of giant pulses, with durations of about a microsecond but structure down to the nanosecond level, and brightness temperatures of up to 1037 K. It is unclear how giant pulses are produced, but they likely originate near the pulsar's light cylinder, where corotating plasma approaches the speed of light. We report observations in the 400-800 MHz frequency band, where the pulses are broadened by scattering in the surrounding Crab Nebula. We find that some pulse frequency spectra show strong bands, which vary during the scattering tail, in one case showing a smooth upward drift. While the banding may simply reflect interference between nanosecond scale pulse components, the variation is surprising, as in the scattering tail the only difference is that the source is observed via slightly longer paths, bent by about an arcsecond in the nebula. The corresponding small change in viewing angle could nevertheless reproduce the observed drift by a change in Doppler shift, if the plasma that emitted the giant pulses moved highly relativistically, with a Lorentz factor γ ~ 104 (and without much spread in γ). If so, this would support models that appeal to highly relativistic plasma to transform ambient magnetic structures to coherent gigahertz radio emission, be it for giant pulses or for potentially related sources, such as fast radio bursts.

Keywords: Neutron stars, High energy astrophysics, Plasma astrophysics, Compact objects, Radio transient sources, Radio pulsars
Characteristics of the Heliospheric Current Sheets at the Sector Boundaries: Wind Observations from 1995-2020. We report results of a statistical study of 1197 heliospheric current sheet (HCS) events associated with the sector boundaries observed by the Wind spacecraft between 1995 and 2020. The average property of the HCS events can be characterized as follows: (1) The width of the current sheets ranges between ~600 km and 1.1 × 106 km, with an average width of 1.06 ± 2.37 × 105 km; (2) The longitude (in Geocentric Solar-Ecliptic coordinate) of the HCS normal shows a large peak at ~210° (0° sunward pointing) and a longer tail at smaller angles; (3) The latitudinal angle (inclination) of the HCS normal shows a near symmetric distribution (peak and average ~0°); (4) The yearly occurrence rate is relatively constant (~46 or 3.4 events per solar rotation), without showing a clear solar cycle dependence; (5) There are solar cycle variations in the properties of the plasma and field within the current sheets and these variations follow closely with the background solar wind plasma and field; (6) A mild (~10%) proton temperature increase within the HCS, suggesting that heating of the solar wind proton can occur within the current sheet; and (7) A sudden decrease in the proton temperature anisotropy (T∥/T⊥) toward unity within ~3 hr of the HCS was identified. These results suggest that on the large scale the HCS at 1 au is a relatively stable and persistent solar wind structure throughout the solar cycle. On the small scale the HCS property is probably controlled by the dynamics of the current system, which is still poorly known.

Keywords: Heliosphere
Toward Optimal Signal Extraction for Imaging X-Ray Polarimetry. We describe an optimal signal extraction process for imaging X-ray polarimetry using an ensemble of deep neural networks. The initial photoelectron angle, used to recover the polarization, has errors following a von Mises distribution. This is complicated by events converting outside of the fiducial gas volume, whose tracks have little polarization sensitivity. We train a deep ensemble of convolutional neural networks to select against these events and to measure event angles and errors for the desired gas-conversion tracks. We show how the expected modulation amplitude from each event gives an optimal weighting to maximize signal-to-noise ratio of the recovered polarization. Applying this weighted maximum likelihood event analysis yields sensitivity (MDP99) improvements of ~10% over earlier heuristic weighting schemes and mitigates the need to adjust said weighting for the source spectrum. We apply our new technique to a selection of astrophysical spectra, including complex extreme examples, and compare the polarization recovery to the current state of the art.

Keywords: X-ray astronomy, Convolutional neural networks, Astrostatistics, X-ray telescopes, Polarimetry
Spectral Signature of Mass Outflow in the Two Component Advective Flow Paradigm. Outflows are common in many astrophysical systems. In the Two Component Advective Flow (TCAF) paradigm, which is essentially a generalized Bondi flow including rotation, viscosity, and cooling effects, the outflow originates in the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with a high velocity. In this paper, we study the spectral properties of black holes using TCAF that includes also a jet (JeTCAF) in the vertical direction of the disk plane. Soft photons from the Keplerian disk are up-scattered by the post-shock region as well as by the base of the jet and are emitted as hard radiation. We also include the bulk motion Comptonization effect by the diverging flow of the jet. Our self-consistent accretion-ejection solution shows how the spectrum from the base of the jet varies with accretion rates, geometry of the flow, and the collimation factor of the jet. We apply the solution to a jetted candidate GS 1354-64 to estimate its mass outflow rate and the geometric configuration of the flow during the 2015 outburst observed by NuSTAR. The estimated mass outflow to mass inflow rate is {0.12}_{-0.03}^{+0.02}$ . From the model-fitted accretion rates, shock compression ratio, and the energy spectral index, we identify the presence of hard and intermediate spectral states of the outburst. Our model-fitted jet collimation factor (fcol) is found to be {0.47}_{-0.09}^{+0.09}$ .

Keywords: Accretion, Black hole physics, X-ray astronomy, Jets, Hydrodynamics, Radiative transfer, Shocks
EvryFlare. IV. Detection of Periodicity in Flare Occurrence from Cool Stars with TESS. Phased flaring, or the periodic occurrence of stellar flares, may probe electromagnetic star-planet interaction (SPI), binary interaction, or magnetic conditions in spots. For the first time, we explore flare periodograms for a large sample of flare stars to identify periodicity due to magnetic interactions with orbiting companions, magnetic reservoirs, or rotational phase. Previous large surveys have explored periodicity at the stellar rotation period, but we do not assume periods must correspond with rotation in this work. Two-minute TESS light curves of 284 cool stars are searched for periods of 1-10 days using two newly developed periodograms. Because flares are discrete events in noisy and incomplete data, typical periodograms are not well suited to detect phased flaring. We construct and test a new Bayesian likelihood periodogram and a modified Lomb-Scargle periodogram. We find six candidates with a false-alarm probability below 1%. Three targets are ≥3σ detections of flare periodicity; the others are plausible candidates that cannot be individually confirmed. Periods range from 1.35 to 6.7 days and some, but not all, correlate with the stellar rotation period or its 1/2 alias. Periodicity from two targets may persist from TESS Cycle 1 into Cycle 3. The periodicity does not appear to persist for the others. Long-term changes in periodicity may result from the spot evolution observed from each candidate, which suggests magnetic conditions play an important role in sustaining periodicity.

Keywords: Starspots, Wide-field telescopes, Stellar magnetic fields, Stellar flares, Star-planet interactions, Stellar rotation
Enhanced Localization of Transients Based on a Novel Cross-correlation Method. Locating gamma-ray transients including gamma-ray bursts (GRBs) and soft gamma repeater (SGR) bursts is of great importance in multi-messenger and multi-wavelength astronomy, such as guiding optical and radio follow-up observations and evaluating the association of a GRB with a gravitational wave or an SGR with a galaxy or fast radio burst (FRB). A time delay (or triangulation) method with multiple missions has been utilized to localize GRBs since their discovery in the 1970s. In this paper, we propose to use the Modified Cross-correlation Function (called Li-CCF hereafter) to derive the accurate time delay from the high time resolution light curves recorded by GRB detectors. This method has been extensively verified with simulation and a sample of GRBs and SGRs with precise positions, using event-by-event data (TTE or Evt data) of Insight-HXMT/HE, Fermi/GBM, Swift/BAT, and GECAM. We find that the 1σ uncertainty (annulus half-width) can be less than 0.3 degrees despite the relatively short distance between these satellites. We note that the localization error given by Li-CCF can be reduced by several times compared with the traditional CCF method or the Fermi/GBM-alone localization, depending on the timing, spectral lag and direction of the GRB, and the distance and number of satellites used. Finally, systematic errors of this time delay method, including the effect of GRB spectral lags and background variation in light curves, have been studied in detail and several improvements have been proposed to deal with those effects.

Keywords: Gamma-ray bursts, Astronomy data analysis