Sazuna/UAT_keywords huggingface dataset:
The Role of Magnetic Shear in Reconnection-driven Flare Energy Release. Using observations from the Solar Dynamics Observatory's Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present novel measurements of the shear of post-reconnection flare loops (PRFLs) in SOL20141218T21:40 and study its evolution with respect to magnetic reconnection and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. We measure the magnetic reconnection rate from the ribbon evolution, and also the shear angle of a large number of PRFLs observed in extreme ultraviolet passbands (≲1 MK). For the first time, the shear angle measurements are conducted using several complementary techniques allowing for cross validation of the results. In this flare, the total reconnection rate is much enhanced before a sharp increase in the hard X-ray emission, and the median shear decreases from 60°-70° to 20°, on a timescale of 10 minutes. We find a correlation between the shear-modulated total reconnection rate and the nonthermal electron flux. These results confirm the strong-to-weak shear evolution suggested in previous observational studies and reproduced in numerical models, and also confirm that, in this flare, reconnection is not an efficient producer of energetic nonthermal electrons during the first 10 minutes when the strongly sheared PRFLs are formed. We conclude that an intermediate shear angle, ≤40°, is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.

Keywords: Solar ultraviolet emission, Solar flares, Solar magnetic reconnection, Solar x-ray emission
The Temporal Symmetrical and Translational Structure in Gamma-Ray Burst Light Curves. Tremendous information is hidden in the light curve of a gamma-ray burst (GRB). Based on Compton Gamma Ray Observatory/Burst And Transient Source Experiment (BATSE) data, Hakkila found a majority of GRBs can be characterized by a smooth, single-peaked component superposed with a temporally symmetrical residual structure, i.e., a mirror feature for the fast-varying component. In this study, we conduct a similar analysis on the same data, as well as on Fermi/Gamma-Ray Burst Monitor data. We obtained a similar conclusion, which is that most GRBs have this symmetrical fast-varying component. Furthermore, we chose an alternative model to characterize the smooth component and used a three-parameter model to identify the residual, i.e., the fast component. By choosing 226 BATSE GRBs based on a few criteria, we checked the time-symmetrical feature and time-translational feature for the fast components and found the ratio is roughly 1:1. We propose that both features could come from the structure of the ejected shells. In the future, the Square Kilometre Array might be able to observe the early radio emission from the collision of the shells.

Keywords: Gamma-ray bursts, Relativistic jets, Astronomy data analysis
Change of Rotation Measure during the Eclipse of a Black Widow PSR J2051-0827. Black widows are millisecond pulsars ablating their companions. The material blown from the companion blocks the radio emission, resulting in radio eclipses. The properties of the eclipse medium are poorly understood. Here, we present direct evidence of the existence of magnetic fields in the eclipse medium of the black widow PSR J2051-0827 using observations made with the Five-hundred-meter Aperture Spherical radio Telescope. We detect a regular decrease in rotation measure (RM) in the egress of the eclipse, changing from 60 to -28.7 rad m-2. The RM gradually changes back to normal when the line of sight moves away from the eclipse. The estimated line-of-sight magnetic field strength in the eclipse medium is ~0.1 G. The RM reversal could be caused by a change in the magnetic field strength along the line of sight due to binary orbital motion. The RM reversal phenomenon has also been observed in some repeating fast radio bursts (FRBs), and the study of spider pulsars may provide additional information about the origin of FRBs.

Keywords: Eclipsing binary stars, Millisecond pulsars, Radio pulsars
The RS Oph Outburst of 2021 Monitored in X-Rays with NICER. The 2021 outburst of the symbiotic recurrent nova RS Oph was monitored with the Neutron Star Interior Composition Explorer Mission (NICER) in the 0.2-12 keV range from day one after the optical maximum, until day 88, producing an unprecedented, detailed view of the outburst development. The X-ray flux preceding the supersoft X-ray phase peaked almost 5 days after optical maximum and originated only in shocked ejecta for 21-25 days. The emission was thermal; in the first 5 days, only a non-collisional-ionization equilibrium model fits the spectrum, and a transition to equilibrium occurred between days 6 and 12. The ratio of peak X-ray flux measured in the NICER range to that measured with Fermi in the 60 MeV-500 GeV range was about 0.1, and the ratio to the peak flux measured with H.E.S.S. in the 250 GeV-2.5 TeV range was about 100. The central supersoft X-ray source (SSS), namely the shell hydrogen burning white dwarf (WD), became visible in the fourth week, initially with short flares. A huge increase in flux occurred on day 41, but the SSS flux remained variable. A quasi-periodic oscillation every ≃35 s was always observed during the SSS phase, with variations in amplitude and a period drift that appeared to decrease in the end. The SSS has characteristics of a WD of mass &gt;1 M ⊙. Thermonuclear burning switched off shortly after day 75, earlier than in the 2006 outburst. We discuss implications for the nova physics.

Keywords: X-ray binary stars, High energy astrophysics, Transient sources, Novae, X-ray transient sources, Recurrent novae
Orbital Support and Evolution of Flat Profiles of Bars (Shoulders). Many barred galaxies exhibit upturns (shoulders) in their bar-major-axis density profile. Simulation studies have suggested that shoulders are supported by looped x 1 orbits, occur in growing bars, and can appear after bar buckling. We investigate the orbital support and evolution of shoulders via frequency analyses of orbits in simulations. We confirm that looped orbits are shoulder-supporting, and can remain so, to a lesser extent, after being vertically thickened. We show that looped orbits appear at the resonance ( Ω φ  - ΩP)/Ω R  = 1/2 (analogous to the classical inner Lindblad resonance, and here called ILR) with vertical-to-radial frequency ratios 1 ≲ Ω z /Ω R  ≲ 3/2 (vertically warm orbits). Cool orbits at the ILR (those with Ω z /Ω R  &gt; 3/2) are vertically thin and have no loops, contributing negligibly to shoulders. As bars slow and thicken, either secularly or by buckling, they populate warm orbits at the ILR. Further thickening carries these orbits toward crossing the vertical ILR [vILR, ( Ω φ  - ΩP)/Ω z  = 1/2], where they convert in-plane motion to vertical motion, become chaotic, kinematically hotter, and less shoulder-supporting. Hence, persistent shoulders require bars to trap new stars, consistent with the need for a growing bar. Since buckling speeds up trapping on warm orbits at the ILR, it can be followed by shoulder formation, as seen in simulations. This sequence supports the recent observational finding that shoulders likely precede the emergence of BP-bulges. The python module for the frequency analysis, naif, is made available.

Keywords: Galaxy bars, Orbits, Galaxy dynamics, N-body simulations, Galactic bar
Multidimensional Radiation Hydrodynamics Simulations of Pulsational Pair-instability Supernovae. Stars with masses of 80-130 M ☉ can encounter pulsational pair-instability at the end of their lives, which triggers consecutive episodes of explosive burning that eject multiple massive shells. Collisions between these shells produce bright transients known as pulsational pair-instability supernovae (PPI SNe) that may explain some extreme supernovae. In this paper, we present the first 2D and 3D radiation hydrodynamics simulations of PPI SNe with the CASTRO code. Radiative cooling causes the collided shells to evolve into thin, dense structures with hot spots that can enhance the peak luminosity of the SN by factors of 2-3. The light curve peaks at 1.9-2.1 × 1043 erg s-1 for 50 days and then plateaus at 2-3 × 1042 erg s-1 for 200 days, depending on the viewing angle. The presence of 12C and 16O and the absence of 28Si and 56Fe in its spectra can uniquely identify this transient as a PPI SN in follow-up observations. Our models suggest that multidimensional radiation hydrodynamics is required to model the evolution and light curves of all shell-collision SNe, such as Type IIne, not just PPI SNe.

Keywords: Hydrodynamical simulations, Stellar structures, Population III stars, Type II supernovae, Massive stars, Radiative transfer simulations, Eruptive phenomena, Circumstellar shells, Shocks, Astrophysical fluid dynamics, Supernovae
Large Photospheric Doppler Shift in Solar Active Region 12673. I. Field-aligned Flows. Delta (δ) sunspots sometimes host fast photospheric flows along the central magnetic polarity inversion line (PIL). Here we study the strong Doppler shift signature in the central penumbral light bridge of solar active region NOAA 12673. Observations from the Helioseismic and Magnetic Imager (HMI) indicate highly sheared and strong magnetic fields. Large Doppler shifts up to 3.2 km s‑1 appeared during the formation of the light bridge and persisted for about 16 hr. A new velocity estimator, called DAVE4VMwDV, reveals fast converging and shearing motion along the PIL from HMI vector magnetograms, and recovers the observed Doppler signal much better than an old version of the algorithm. The inferred velocity vectors are largely (anti-)parallel to the inclined magnetic fields, suggesting that the observed Doppler shift contains a significant contribution from the projected field-aligned flows. High-resolution observations from the Hinode/Spectro-Polarimeter further exhibit a clear correlation between the Doppler velocity and the cosine of the magnetic inclination, which is in agreement with HMI results and consistent with a field-aligned flow of about 9.6 km s‑1. The complex Stokes profiles suggest significant gradients of physical variables along the line of sight. We discuss the implications on the δ-spot magnetic structure and the flow-driving mechanism.

Keywords: Solar active region velocity fields, Solar active region magnetic fields, Delta sunspots
Ion Loss within a Reconnection Exhaust near Mars: MAVEN Observations. We present observations from Mars Atmosphere and Volatile EvolutioN of a reconnection exhaust, which was probably not ongoing, in the Martian magnetosheath. The reconnection jet with abundant heavy ions is directed outward to space. The magnetic field lines within the exhaust are found to be draped, suggesting that the reconnection exhaust was detached from the ionosphere and the heavy ions within it escaped from Mars. The dimensionless reconnection rate estimated from proton measurements is about 5.56%. The total oxygen loss rate via the reconnection exhaust is calculated to be 1.3 × 1021 s-1. This much lower ion loss rate than that estimated in active magnetic reconnection at the ionopause suggests that the reconnection associated with the reconnection exhaust is not ongoing, so most of the heavy ions have already escaped. Our results demonstrate that magnetic reconnection can directly contribute to the ion loss of Mars. Due to the participation of multiple heavy ions, the characteristics of magnetic reconnection could be very different.

Keywords: Mars, Solar wind, Solar-planetary interactions, Planetary ionospheres
Late-time Hubble Space Telescope Observations of AT 2018cow. I. Further Constraints on the Fading Prompt Emission and Thermal Properties 50-60 days Post-discovery. The exact nature of the luminous fast blue optical transient AT 2018cow is still debated. In this first of a two-paper series, we present a detailed analysis of three Hubble Space Telescope (HST) observations of AT 2018cow covering ~50-60 days post-discovery in combination with other observations throughout the first two months and derive significantly improved constraints of the late thermal properties. By modeling the spectral energy distributions (SEDs), we confirm that the UV-optical emission over 50-60 days was still a smooth blackbody (i.e., optically thick) with a high temperature (T BB ~ 15,000 K) and small radius (R BB ≲ 1000 R ⊙). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of L BB ∝ t -2.40 but faded much faster at t -3.06 after day 13. Reexamining possible late-time power sources, we disfavor significant contributions from radioactive decay based on the required 56Ni mass and lack of UV line blanketing in the HST SEDs. We argue that the commonly proposed interaction with circumstellar material may face significant challenges in explaining the late thermal properties, particularly the effects of the optical depth. Alternatively, we find that continuous outflow/wind driven by a central engine can still reasonably explain the combination of a receding photosphere, optically thick and rapidly fading emission, and intermediate-width lines. However, the rapid fading may have further implications on the power output and structure of the system. Our findings may support the hypothesis that AT 2018cow and other "Cow-like transients" are powered mainly by accretion onto a central engine.

Keywords: Accretion, Hubble Space Telescope, Circumstellar matter, Compact objects, Supernovae
Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV-bright Underlying Source 2-4 Yr Post-discovery. In this second of a two-paper series, we present a detailed analysis of three Hubble Space Telescope observations taken ~2-4 yr post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at ~2-3 yr post-discovery revealed an exceptionally blue (L  ν  ∝ ν 1.99) underlying source with relatively stable optical brightness, fading in the near-UV was observed at year 4, indicating flattening in the spectrum (to L  ν  ∝ ν 1.64). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation, based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with known radio-producing circumstellar material (CSM) as well as magnetar spin down with B ~ 1015 G as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., a previously ejected hydrogen envelope) or a magnetar with B ≲ 1014 G. Finally, we find that a highly inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source.

Keywords: Accretion, Black holes, Hubble Space Telescope, Circumstellar matter, Supernovae
On the Blackbody Spectrum of Kilonovae. The early spectra of the kilonova AT2017gfo have a remarkably smooth blackbody continuum, which reveals information on the thermal properties and radioactive heating within the ejecta. However, the widespread use of a single-temperature blackbody to fit kilonova data is theoretically invalid, because (1) the significant travel-time delays for a rapidly cooling surface result in a broad distribution of temperatures and (2) the relativistic Doppler correction varies across different surface elements. Thus, the observed spectrum should be a modified blackbody with a range of temperatures over the surface. In this paper we quantify the impact of these effects and illustrate the typical wavelength-dependent spectral corrections. We apply the multitemperature blackbody framework to the first-epoch X-shooter AT2017gfo spectrum, to deconvolve the underlying physical temperature at the photosphere from the relativistic Doppler shift. We show that the cooling and Doppler effects individually result in a variation of temperatures over the photosphere of up to 30%, but in combination these effects nearly cancel and produce the single-temperature blackbody observed. Finally, we show that fitting the UV, optical, or near-infrared separately yields blackbody temperatures consistent at the percent level, which puts stringent limits on any proposed modification of the spectral continuum.

Keywords: R-process, Effective temperature
Prospects for a Precise Equation of State Measurement from Advanced LIGO and Cosmic Explorer. Gravitational-wave observations of neutron star mergers can probe the nuclear equation of state by measuring the imprint of the neutron star's tidal deformability on the signal. We investigate the ability of future gravitational-wave observations to produce a precise measurement of the equation of state from binary neutron star inspirals. Because measurability of the tidal effect depends on the equation of state, we explore several equations of state that span current observational constraints. We generate a population of binary neutron stars as seen by a simulated Advanced LIGO-Virgo network, as well as by a planned Cosmic Explorer observatory. We perform Bayesian inference to measure the parameters of each signal, and we combine measurements across each population to determine R 1.4, the radius of a 1.4 M ⊙ neutron star. We find that, with 321 signals, the LIGO-Virgo network is able to measure R 1.4 to better than 2% precision for all equations of state we consider; however, we also find that achieving this precision could take decades of observation, depending on the equation of state and the merger rate. On the other hand, we find that with one year of observation, Cosmic Explorer will measure R 1.4 to better than 0.6% precision. In both cases, we find that systematic biases, such as from an incorrect mass prior, can significantly impact measurement accuracy, and efforts will be required to mitigate these effects.

Keywords: Neutron stars, Gravitational waves
The Search for Thermonuclear Transients from the Tidal Disruption of a White Dwarf by an Intermediate-mass Black Hole. The close encounter of a white dwarf (WD) with a black hole (BH) could result in the tidal disruption of the WD. During this encounter, the WD can undergo a thermonuclear explosion due to its tidal compression, resulting in an optical transient similar to a Type Ia supernova, hereafter a Ia-TDE. Nevertheless, this will only be physically observable if the BH is ≲105 M ⊙. Finding a Ia-TDE would therefore imply the discovery of an intermediate-mass BH (IMBH) ≲105 M ⊙. Here, we search the entire Zwicky Transient Facility alert stream for these elusive Ia-TDEs. We restrict our search to nuclear transients in dwarf galaxies, the likely sites for IMBHs, and find a total of six possible nuclear Ia-TDE candidates. We find SN 2020lrt to be the most likely Ia-TDE candidate, thanks to its strong resemblance to light-curve and spectroscopic models of Ia-TDEs. We measure the stellar masses of the dwarf galaxies hosting these transients to be ≲109 M ⊙; if confirmed to harbor BHs, these would prove the existence of IMBHs in some of the lowest-mass galaxies known. Additionally, we searched for off-nuclear Ia-TDEs, but were unable to find more robust candidates in the outskirts of galaxies than in their nuclei. This supports the hypothesis that the nuclear Ia-TDE candidates are WDs tidally compressed by IMBHs in the cores of galaxies, as opposed to a class of transient that can occur anywhere in a galaxy. We have laid the groundwork to systematically search for Ia-TDE candidates in existing and future time-domain surveys. The rapid characterization of their nature will result in not only the confirmation of a Ia-TDE, but also the unambiguous discovery of bona fide IMBHs.

Keywords: Surveys, Black hole physics, White dwarf stars, Supernovae
Global Electron Thermodynamics in Radiatively Inefficient Accretion Flows. In the collisionless plasmas of radiatively inefficient accretion flows, heating and acceleration of ions and electrons are not well understood. Recent studies in the gyrokinetic limit revealed the importance of incorporating both the compressive and Alfvénic cascades when calculating the partition of dissipated energy between the plasma species. In this paper, we use a covariant analytic model of the accretion flow to explore the impact of compressive and Alfvénic heating, Coulomb collisions, compressional heating, and radiative cooling on the radial temperature profiles of ions and electrons. We show that, independent of the partition of heat between the plasma species, even a small fraction of turbulent energy dissipated to the electrons makes their temperature scale with a virial profile and the ion-to-electron temperature ratio smaller than in the case of pure Coulomb heating. In contrast, the presence of compressive cascades makes this ratio larger because compressive turbulent energy is channeled primarily into the ions. We calculate the ion-to-electron temperature in the inner accretion flow for a broad range of plasma properties, mass accretion rates, and black hole spins and show that it ranges between 5 ≲ T  i /T  e  ≲ 40. We provide a physically motivated expression for this ratio that can be used to calculate observables from simulations of black hole accretion flows for a wide range of conditions.

Keywords: Black hole physics, Plasma astrophysics, Astrophysical black holes, Plasma physics
Variability of Draped Interplanetary Magnetic Field in the Subsolar Martian Ionosphere. In the absence of an intrinsic magnetic field, the solar wind interacts directly with the Martian atmosphere, and the interplanetary magnetic field (IMF) drapes around the ionosphere. With the aid of multi-instrument measurements from Mars Atmosphere and Volatile Evolution (MAVEN), we investigate the altitudinal profiles of the draped magnetic field in the subsolar Martian ionosphere. Our analysis of 141 profiles found that they have a median profile with a nearly constant field strength of about 30 nT above 160 km. However, these profiles also show large variability, for example, with a standard deviation of 16 nT at 250 km. Timescale analysis found that magnetic field transport with downward ion plasma motion dominates the draping of IMF at high altitudes. At low altitudes, magnetic diffusion takes over due to increasing collisions. At even lower altitudes, approaching the ionospheric peak, the draped field is quickly damped owing to high conductivity. We used a 1D model simulation to confirm the above processes involved and further indicate that undulating magnetic fields with oscillation periods longer than tens of minutes can penetrate deeply into the ionosphere. This causes the large variability of draped field observed down to below 200 km. The diverse profiles of draped field observed by MAVEN are thus likely due to varying IMF conditions.

Keywords: Planetary ionospheres, Magnetic fields