adsabs/SciX_UAT_keywords huggingface dataset:
Unusual Hard X-Ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

The nuclear transient AT2019cuk/Tick Tock/SDSS J1430+2303 has been suggested to harbor a supermassive black hole (SMBH) binary near coalescence. We report results from high-cadence NICER X-ray monitoring with multiple visits per day from 2022 January to August, as well as continued optical monitoring during the same time period. We find no evidence of periodic/quasiperiodic modulation in the X-ray, UV, or optical bands; however, we do observe exotic hard X-ray variability that is unusual for typical active galactic nuclei (AGN). The most striking feature of the NICER light curve is repetitive hard (2-4 keV) X-ray flares that result in distinctly harder X-ray spectra compared to the nonflaring data. In its nonflaring state, AT2019cuk looks like a relatively standard AGN, but it presents the first case of day-long, hard X-ray flares in a changing-look AGN. We consider a few different models for the driving mechanism of these hard X-ray flares, including (1) corona/jet variability driven by increased magnetic activity, (2) variable obscuration, and (3) self-lensing from the potential secondary SMBH. We prefer the variable corona model, as the obscuration model requires rather contrived timescales and the self-lensing model is difficult to reconcile with a lack of clear periodicity in the flares. These findings illustrate how important high-cadence X-ray monitoring is to our understanding of the rapid variability of the X-ray corona and necessitate further high-cadence, multiwavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.

Keywords: active galactic nuclei; high energy astrophysics; supermassive black holes; x-ray transient sources
MMS Direct Observations of Kinetic-scale Shock Self-reformation

Studies of shocks have long suggested that a shock can undergo cyclical self-reformation on a timescale of ion cyclotron period. This process has been proposed as a primary mechanism for energy dissipation and energetic particle acceleration at shocks. Unambiguous observational evidence, however, has remained elusive. Here, we report direct observations for the self-reformation process of a collisionless, high Mach number, quasi-perpendicular shock using Magnetospheric Multiscale (MMS) measurements. We find that reflected ions by the old shock ramp form a clear phase-space vortex, which gives rise to a new ramp. The new ramp observed by MMS2 has not yet developed to a mature stage during the self-reformation, and is not strong enough to reflect incident ions. Consequently, these ions are only slightly slowed down and show a flat velocity profile from the new ramp all the way to the old one. The present results provide direct evidence of shock self-reformation, and also shed light on energy dissipation and energetic particle acceleration at collisionless shocks throughout the universe.

Keywords: shocks; solar-planetary interactions; plasma astrophysics
Quantified Plasma Heating and Energy Dissipation in the Earth's Quasi-perpendicular Bow Shock

The high-speed solar wind decelerates as it travels through the Earth's bow shock, during which a portion of the incident flow energy is converted into the plasma thermal energy. However, the energy partition of plasma heating among different species, as well as the energy dissipation mechanism, remains elusive. In this paper, we quantify the plasma heating and the energy dissipation by calculating the pressure–strain interactions in 33 quasi-perpendicular bow shocks observed by the Magnetospheric Multiscale spacecraft. Our results show that the Joule dissipation measured by ${\boldsymbol{J}}\cdot {{\boldsymbol{E}}}^{{\prime} }$ and the quasi-viscous dissipation quantified by PiD are distinct in the examined shocks. We also reveal that ions gain more energy than electrons, and the compressive effect is more important than the incompressible channel in plasma heating. PiD contributed by the gyrotropic pressure tensor is consistently positive for electrons; however, there is no discernible distinction between the gyrotropic and nongyrotropic contribution to PiD for ions.

Keywords: space plasmas; plasma physics; shocks; planetary bow shocks
The Common Envelope Origins of the Fast Jet in the Planetary Nebula M 3-38

We present the analysis of Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía (MEGARA) high-dispersion integral field spectroscopic observations of the bipolar planetary nebula (PN) M 3-38. These observations unveil the presence of a fast outflow aligned with the symmetry axis of M 3-38 that expands with a velocity of up to ±225 km s-1. The deprojected space velocity of this feature can be estimated to be ≈320 ${}_{-60}^{+130}$ km s-1, which together with its highly collimated morphology suggests that it is one of the fastest jets detected in a PN. We have also used Kepler observations of the central star of M 3-38 to unveil variability associated with a dominant period of 17.7 days. We attribute this to the presence of a low-mass star with an orbital separation of ≈0.12-0.16 au. The fast and collimated ejection and the close binary system point toward a common envelope formation scenario for M 3-38.

Keywords: planetary nebulae; stellar jets; stellar winds; low mass stars
The Outflow of The Protostar in B335. I.

The isolated globule B335 contains a single, low-luminosity Class 0 protostar associated with a bipolar nebula and outflow system seen nearly perpendicular to its axis. We observed the innermost regions of this outflow as part of JWST/NIRCam Guaranteed Time Observations program 1187, primarily intended for wide-field slitless spectroscopy of background stars behind the globule. We find a system of expanding shock fronts with kinematic ages of only a few decades emerging symmetrically from the position of the embedded protostar, which is not directly detected at NIRCam wavelengths. The innermost and youngest of the shock fronts studied here shows strong emission from CO. The next older shock front shows less CO and the third shock front shows only H2 emission in our data. This third and most distant of these inner shock fronts shows substantial evolution of its shape since it was last observed with high spatial resolution in 1996 with Keck/NIRC. This may be evidence of a faster internal shock catching up with a slower one and of the two shocks merging.

Keywords: young stellar objects; protostars; cometary globules; stellar jets
Interpretation of Flat Energy Spectra Upstream of Fast Interplanetary Shocks

Interplanetary shocks are large-scale heliospheric structures often caused by eruptive phenomena at the Sun, and represent one of the main sources of energetic particles. Several interplanetary (IP) shock crossings by spacecraft at 1 au have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. Surprisingly, in some shock events ion fluxes with energies between 100 keV and about 2 MeV acquire similar values (which we refer to as "overlapped" fluxes), corresponding to flat energy spectra in that range. In contrast, closer to the shock the fluxes are observed to depend on energy. In this work, we analyze three IP-shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity-filter mechanism for particles in a given energy range. In particular, ions with velocity parallel to the local magnetic field larger than the speed of the upstream plasma, in the reference frame of the shock, can easily propagate back upstream, while lower-energy ions tend to be confined to the shock front, thus reducing their fluxes far upstream and giving rise to flat energy spectra. The velocity-filter mechanism has been corroborated from observations of particle flux anisotropy by the Solid-State Telescope of Wind/3DP.

Keywords: interplanetary shocks; interplanetary turbulence; solar energetic particles
On the Maximum Mass of Neutron Stars and GW190814

Motivated by the recent discovery of a compact object with mass in the range 2.5-2.67 M⊙ in the binary merger GW190814, we revisit the question of the maximum mass of neutron stars (NSs). We use a Markov Chain Monte Carlo approach to generate about two million phenomenological equations of state with and without first-order phase transitions. We fix the crust equation of state and only assume causality at higher densities. We show how a strict upper bound on the maximum NS mass can be inferred from upcoming observation of NS radii and masses. The derived upper bounds depend only on relativity and causality, so it is not affected by nuclear physics uncertainties. We show how a lower limit on the maximum mass of NSs, in combination with upcoming measurements of NS radii by LIGO/Virgo and NICER, would constrain the equation of state of dense matter. Finally, we discuss the implications for GW190814.

Keywords: neutron stars; nuclear astrophysics
Analytic Solution to the Dynamical Friction Acting on Circularly Moving Perturbers

We present an analytic approach to the dynamical friction (DF) acting on a circularly moving point mass perturber in a gaseous medium. We demonstrate that, when the perturber is turned on at t = 0, steady state (infinite time perturbation) is achieved after exactly one sound-crossing time. At low Mach numbers ${ \mathcal M }\,\ll 1$ , the circular-motion steady-state DF converges to the linear-motion, finite time perturbation expression. The analytic results describe both the radial and tangential forces on the perturbers caused by the backreaction of the wake propagating in the medium. The radial force is directed inward, toward the motion center, and is dominant at large Mach numbers. For subsonic motion, this component is negligible. For moderate and low Mach numbers, the tangential force is stronger and opposes the motion of the perturber. The analytic solution to the circular-orbit DF suffers from a logarithmic divergence in the supersonic regime. This divergence appears at short distances from the perturber solely (unlike the linear-motion result, which is also divergent at large distances) and can be encoded in a maximum multipole. This is helpful to assess the resolution dependence of numerical simulations implementing DF at the level of Liénard-Wiechert potentials. We also show how our approach can be generalized to calculate the DF acting on a compact circular binary.

Keywords: dynamical friction; circular orbits; compact binary stars; hydrodynamics
Longitudinally Resolved Spectral Retrieval (ReSpect) of WASP-43b

Thermal phase variations of short-period planets indicate that they are not spherical cows: day-to-night temperature contrasts range from hundreds to thousands of degrees, rivaling their vertical temperature contrasts. Nonetheless, the emergent spectra of short-period planets have typically been fit using one-dimensional (1D) spectral retrieval codes that only account for vertical temperature gradients. The popularity of 1D spectral retrieval codes is easy to understand: they are robust and have a rich legacy in solar system atmospheric studies. Exoplanet researchers have recently introduced multidimensional retrieval schemes to interpret the spectra of short-period planets, but these codes are necessarily more complex and computationally expensive than their 1D counterparts. In this paper we present an alternative: phase-dependent spectral observations are inverted to produce longitudinally resolved spectra that can then be fit using standard 1D spectral retrieval codes. We test this scheme on the iconic phase-resolved spectra of WASP-43b and on simulated observations for the James Webb Space Telescope (JWST) using the open-source PYRAT BAY 1D spectral retrieval framework. Notably, we take the model complexity of the simulations one step further from previous studies by allowing for longitudinal variations in composition in addition to temperature. We show that performing 1D spectral retrieval on longitudinally resolved spectra is more accurate than applying 1D spectral retrieval codes to disk-integrated emission spectra, even though this is identical in terms of computational load. We find that for the extant Hubble and Spitzer observations of WASP-43b, the difference between the two approaches is negligible, but JWST phase measurements should be treated with longitudinally resolved spectral retrieval (ReSpect).

Keywords: exoplanet atmospheres; radiative transfer; spectroscopy; bayesian statistics
VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B

Tension remains between the observed and modeled properties of substellar objects, but objects in binary orbits, with known dynamical masses, can provide a way forward. HD 72946 B is a recently imaged brown dwarf companion to a nearby, solar-type star. We achieve ~100 μas relative astrometry of HD 72946 B in the K band using VLTI/GRAVITY, unprecedented for a benchmark brown dwarf. We fit an ensemble of measurements of the orbit using orbitize! and derive a strong dynamical mass constraint M B = 69.5 ± 0.5 M Jup assuming a strong prior on the host star mass M A = 0.97 ± 0.01 M ⊙ from an updated stellar analysis. We fit the spectrum of the companion to a grid of self-consistent BT-Settl-CIFIST model atmospheres, and perform atmospheric retrievals using petitRADTRANS. A dynamical mass prior only marginally influences the sampled distribution of effective temperature, but has a large influence on the surface gravity and radius, as expected. The dynamical mass alone does not strongly influence retrieved pressure-temperature or cloud parameters within our current retrieval setup. Independently of the cloud prescription and prior assumptions, we find agreement within ±2σ between the C/O of the host (0.52 ± 0.05) and brown dwarf (0.43-0.63), as expected from a molecular cloud collapse formation scenario, but our retrieved metallicities are implausibly high (0.6-0.8) in light of the excellent agreement of the data with the solar-abundance model grid. Future work on our retrieval framework will seek to resolve this tension. Additional study of low surface gravity objects is necessary to assess the influence of a dynamical mass prior on atmospheric analysis.

Keywords: brown dwarfs; l dwarfs; interferometry; long baseline interferometry; optical interferometry; orbit determination; exoplanet atmospheres; stellar atmospheres
Variable Star Classification with a Multiple-input Neural Network

In this experiment, we created a Multiple-Input Neural Network, consisting of convolutional and multilayer neural networks. With this setup the selected highest-performing neural network was able to distinguish variable stars based on the visual characteristics of their light curves, while taking also into account additional numerical information (e.g., period, reddening-free brightness) to differentiate visually similar light curves. The network was trained and tested on Optical Gravitational Lensing Experiment-III (OGLE-III) data using all OGLE-III observation fields, phase-folded light curves, and period data. The neural network yielded accuracies of 89%-99% for most of the main classes (Cepheids, δ Scutis, eclipsing binaries, RR Lyrae stars, Type-II Cepheids), only the first-overtone anomalous Cepheids had an accuracy of 45%. To counteract the large confusion between the first-overtone anomalous Cepheids and the RRab stars we added the reddening-free brightness as a new input and only stars from the LMC field were retained to have a fixed distance. With this change we improved the neural network's result for the first-overtone anomalous Cepheids to almost 80%. Overall, the Multiple-input Neural Network method developed by our team is a promising alternative to existing classification methods.

Keywords: astronomy data analysis; delta scuti variable stars; rr lyrae variable stars; eclipsing binary stars; neural networks; cepheid variable stars; classification
Synthetic Color-Magnitude Diagrams for the Globular Cluster 47 Tucanae

Globular clusters contain the largest coeval populations of stars in the Milky Way, making them prime targets for stellar evolution studies. We present new theoretical isochrones for the brightest monometallic globular cluster 47 Tucanae, based on the spectroscopic measurements of the chemical composition in literature. Calculated colors are compared to existing photometric observations with Advanced Camera for Surveys on the Hubble Space Telescope. We find that the adopted abundances are consistent with the blue tail of the observed color distribution, implying a possible systematic offset between spectroscopic and photometric abundances of globular clusters. Additionally, we make color-magnitude predictions for future observations with the Near Infrared Camera on the James Webb Space Telescope in bands corresponding to the ongoing observational campaigns.

Keywords: globular star clusters; chemical abundances; stellar evolutionary models; stellar atmospheres
A Strong-lensing Model for the WMDF JWST/GTO Very Rich Cluster A1489

We present a first strong-lensing model for the galaxy cluster RM J121218.5+273255.1 (z = 0.35; hereafter RMJ1212; also known as A1489). This cluster is among the top 0.1% richest clusters in the redMaPPer catalog; it is significantly detected in X-ray and through the Sunyaev-Zel'dovich effect in ROSAT and Planck data, respectively, and its optical luminosity distribution implies a very large lens, following mass-to-light scaling relations. Based on these properties it was chosen for the Webb Medium Deep Fields (WMDF) James Webb Space Telescope/Guaranteed Time Observations program. In preparation for this program, RMJ1212 was recently imaged with Gemini Multi-Object Spectrograph (GMOS) on Gemini North and in seven optical and near-infrared bands with the Hubble Space Telescope (HST). We use these data to map the inner mass distribution of the cluster, uncovering various sets of multiple images. We also search for high-redshift candidates in the data, as well as for transient sources. We find over a dozen high-redshift (z ≳ 6) candidates based on both photometric redshift and the dropout technique. No prominent (≳5σ) transients were found in the data between the two HST visits. Our lensing analysis reveals a relatively large lens with an effective Einstein radius of θE ≃ 32 ± 3″ (zs = 2), in broad agreement with the scaling-relation expectations. RMJ1212 demonstrates that powerful lensing clusters can be selected in a robust and automated way following the light-traces-mass assumption.

Keywords: strong gravitational lensing; high-redshift galaxies; dark matter distribution; rich galaxy clusters; galaxy clusters
The Vanishing of the Primary Emission Region in PKS 1510-089

In 2021 July, PKS 1510-089 exhibited a significant flux drop in the high-energy γ-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy γ-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.

Keywords: blazars; relativistic jets; non-thermal radiation sources; flat-spectrum radio quasars; gamma-ray sources; spectropolarimetry
Measurements and Calculations of Radiative Parameters for Odd-parity Levels in Nb II

Experimental radiative lifetimes for 34 odd-parity levels belonging to 4d 35p and 4d 25s5s configurations of Nb II were measured by the time-resolved laser-induced fluorescence method. The results range from 2.2 to 11.5 ns, and 30 of them were reported for the first time, as far as we know. The theoretical radiative lifetimes for these levels were also calculated by the pseudorelativistic Hartree-Fock method including core-polarization contributions (HFR+CPOL) and the fully relativistic multiconfiguration Dirac-Hartree-Fock method, and branching fractions for these levels were also calculated by HFR+CPOL. By combining the experimental lifetimes and the calculated branching fraction values, the semiempirical transition probabilities and oscillator strengths for 389 Nb II lines were obtained.

Keywords: atomic spectroscopy; atomic physics; transition probabilities; radiative processes
Synchrotron Cutoff in Ultraluminous X-Ray Sources

The origin of spectral curvature at energies E ≃ 10 keV in ultraluminous X-ray sources (ULXs) is not well understood. In this paper, we propose a novel mechanism based on synchrotron radiation to explain this cutoff. We show that relativistic plasma can give rise to the observed spectral curvature for neutron star magnetic fields due to the variation in the latitude of synchrotron radiation. We analyze the NuSTAR data of two bright pulsar ULXs, NGC 5907 ULX1 and NGC 7793 P13, and provide estimates of the physical parameters of these sources. We fit the data for synchrotron emission at various latitudes and show that the spectral cutoff in these cases can be explained for a large range of acceptable physical parameters, e.g., a semirelativistic plasma with γ ≃ 20 for high latitudes or a highly relativistic plasma (γ ≃ 105) for emission close to the electron's orbital plane in a typical magnetic field of B ≃ 1012 G. We also discuss how such an emission mechanism can be distinguished from other proposed models. A corollary to our study is that most ULXs might be neutron stars as they display such a spectral cutoff.

Keywords: radiative processes; neutron stars; ultraluminous x-ray sources; high energy astrophysics
Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein)

We present a high-resolution observation of the distant comet C/2014 UN271 (Bernardinelli-Bernstein) using the Hubble Space Telescope on 2022 January 8. The signal of the nucleus was successfully isolated by means of a nucleus extraction technique, with an apparent V-band magnitude measured to be 21.65 ± 0.11, corresponding to an absolute magnitude of 8.63 ± 0.11. The product of the visual geometric albedo with the effective radius squared is ${p}_{V}{R}_{{\rm{n}}}^{2}=157\pm 16$ km2. If the ALMA observation by Lellouch et al. refers to a bare nucleus, we derived a visual geometric albedo of 0.033 ± 0.009. If dust contamination of the ALMA signal is present at the maximum allowed level (24%), we found an albedo of 0.044 ± 0.012 for the nucleus having an effective diameter of 119 ± 15 km. In either case, we confirm that C/2014 UN271 is the largest long-period comet ever detected. Judging from the measured surface brightness profile of the coma, whose logarithmic gradient varies azimuthally between ~1.0 and 1.7 as a consequence of solar radiation pressure, the mass production is consistent with steady-state production but not with impulsive ejection, as would be produced by an outburst. Using aperture photometry, we estimated an enormous (albeit uncertain) mass-loss rate of ~103 kg s-1 at a heliocentric distance of ~20 au.

Keywords: comets; comet nuclei; long period comets