adsabs/SciX_UAT_keywords huggingface dataset:
A Late-time Radio Flare Following a Possible Transition in Accretion State in the Tidal Disruption Event AT 2019azh

We report here radio follow-up observations of the optical tidal disruption event (TDE) AT 2019azh. Previously reported X-ray observations of this TDE showed variability at early times and a dramatic increase in luminosity, by a factor of ~10, about 8 months after optical discovery. The X-ray emission is mainly dominated by intermediate hard-soft X-rays and is exceptionally soft around the X-ray peak, which is L X ~ 1043 erg s-1. The high cadence 15.5 GHz observations reported here show an early rise in radio emission followed by an approximately constant light curve, and a late-time flare. This flare starts roughly at the time of the observed X-ray peak luminosity and reaches its peak about 110 days after the peak in the X-ray, and a year after optical discovery. The radio flare peaks at ν L ν ~ 1038 erg s-1, a factor of two higher than the emission preceding the flare. In light of the late-time radio and X-ray flares, and the X-ray spectral evolution, we speculate a possible transition in the accretion state of this TDE, similar to the observed behavior in black hole X-ray binaries. We compare the radio properties of AT 2019azh to other known TDEs, and focus on the similarities to the late-time radio flare of the TDE ASASSN-15oi.

Keywords: radio transient sources; tidal disruption; time domain astronomy; transient sources; black holes; high energy astrophysics
Revealing a New Black Widow Binary 4FGL J0336.0+7502

We report on the discovery of a promising candidate for a black widow (BW) millisecond pulsar binary, 4FGL J0336.0+7502, which shows many pulsar-like properties in the 4FGL-DR2 catalog. Within the 95% error region of the LAT source, we identified an optical counterpart with a clear periodicity at Porb = 3.718178(9) hr using the Bohyunsan 1.8 m Telescope, the Lulin One-meter Telescope, the Canada-France-Hawaii Telescope, and Gemini-North. At the optical position, an X-ray source was marginally detected in the Swift/X-Ray Telescope archival data, and the detection was confirmed by our Chandra/ACIS DDT observation. The spectrum of the X-ray source can be described by a power-law model of Γx = 1.6 ± 0.7 and ${F}_{0.3\mbox{--}7\mathrm{keV}}={3.5}_{-1.0}^{+1.2}\times {10}^{-14}$ erg cm-2 s-1. The X-ray photon index and the low X-ray-to-γ-ray flux ratio (i.e., <1%) are both consistent with that of many known BW pulsars. There is also a hint of an X-ray orbital modulation in the Chandra data, although the significance is very low (1.3σ). If the pulsar identity and the X-ray modulation are confirmed, it would be the fifth BW millisecond pulsar binary that showed an orbitally modulated emission in X-rays.

Keywords: high energy astrophysics; gamma-ray sources; compact binary stars; millisecond pulsars; x-ray sources
Recent Extragalactic Nova Rate Determinations and their Implications

Comparing the rates of novae in galaxies of differing Hubble types reveals the sensitivity of nova eruptions to the properties (age, star formation history, metallicity) of the underlying stellar population. Recent studies using the Hubble Space Telescope have suggested that earlier ground-based surveys may have underestimated extragalactic nova rates by factors of five or more. Here we discuss the implications of such high specific nova rates on nova theory in general, and on the chemical evolution of galaxies, including the Milky Way, in particular.

Keywords: novae; galaxy chemical evolution
Chandra, HST/STIS, NICER, Swift, and TESS Detail the Flare Evolution of the Repeating Nuclear Transient ASASSN -14ko

ASASSN-14ko is a nuclear transient at the center of the AGN ESO 253-G003 that undergoes periodic flares. Optical flares were first observed in 2014 by the All-Sky Automated Survey for Supernovae (ASAS-SN) and their peak times are well-modeled with a period of ${115.2}_{-1.2}^{+1.3}$ days and period derivative of -0.0026 ± 0.0006. Here we present ASAS-SN, Chandra, HST/STIS, NICER, Swift, and TESS data for the flares that occurred on 2020 December, 2021 April, 2021 July, and 2021 November. These four flares represent flares 18-21 of the total number of flares observed by ASAS-SN so far since 2014. The HST/STIS UV spectra evolve from blueshifted broad absorption features to redshifted broad emission features over ~10 days. The Swift UV/optical light curves peaked as predicted by the timing model, but the peak UV luminosities that varied between flares and the UV flux in Flare 20 were roughly half the brightness of the other peaks. The X-ray luminosities consistently decreased and the spectra became harder during the UV/optical rise, but apparently without changes in absorption. Finally, two high-cadence TESS light curves from Flare 18 and Flare 12 showed that the slopes during the rising and declining phases changed over time, which indicates some stochasticity in the flare's driving mechanism. Although ASASSN-14ko remains observationally consistent with a repeating partial tidal disruption event, these rich multi-wavelength data are in need of a detailed theoretical model.

Keywords: black holes; active galactic nuclei; high energy astrophysics
Sizes of Active Galactic Nucleus Inhomogeneous Disks—Large in Microlensing, Small in Reverberation Mapping

Magnetohydrodynamic turbulence can drive significant temperature fluctuations in the accretion disk of an active galactic nucleus (AGN). As a result, the disk can be highly inhomogeneous and has a half-light radius larger than the static Shakura and Sunyaev disk (SSD), in agreement with quasar microlensing observations. Meanwhile, the accretion-disk sizes can also be determined using continuum reverberation mappings that measure interband cross-correlations and time lags. The interband time lags are often understood in the X-ray reprocessing scenario. Here we show that the interband continuum time lags of the X-ray reprocessing of an inhomogeneous disk are similar to or even smaller than those of a static SSD. Consequently, the X-ray reprocessing of an inhomogeneous disk cannot account for the recent continuum reverberation mappings of some Seyfert 1 AGNs, whose measured time lags are larger than those of a static SSD. In contrast to the tight correlation between UV/optical variations, the cross-correlation between X-ray and disk emission is rather weak in this model; this behavior is consistent with recent continuum reverberation mappings. Moreover, the time lags in this model are anticorrelated with the amplitude of disk temperature fluctuations. Our results suggest that the temperature fluctuations should be properly considered when modeling interband continuum time lags.

Keywords: accretion; active galaxies; supermassive black holes
Statistical Analysis of Stellar Flares from the First Three Years of TESS Observations

In this paper, we study stellar light curves from the Transiting Exoplanet Survey Satellite (TESS) for the presence of stellar flares. The main aim is to detect stellar flares using 2 minute cadence data and to perform a statistical analysis. To find and analyze stellar flares, we prepared the automatic software WARPFINDER. We implemented three methods described in this paper: trend, difference, and profile fitting. Automated searches for flares was accompanied by visual inspection. Using our software we analyzed the 2 minute cadence light curves of 330,000 stars located in the first 39 sectors of TESS observations. As a result, we detected over 25,000 stars showing flare activity with the total number of more than 140,000 flares. This means that about 7.7% of all the analyzed objects are flaring stars. The estimated flare energies range between 1031 and 1036 erg. We prepared a preliminary preview of the statistical distribution of parameters such as the flare duration, amplitude, and energy, and compared it with previous results. The relationship between stellar activity and spectral type, temperature, and mass was also statistically analyzed. Based on the scaling laws, we estimated the average values of the magnetic field strength and length of the flare loops. In our work, we used both single (about 60%), and double (about 40%) flare profiles to fit the observational data. The components of the double profile are supposed to be related to the direct heating of the photosphere by nonthermal electrons and back-warming processes.

Keywords: exoplanet astronomy
Subproton-scale Intermittency in Near-Sun Solar Wind Turbulence Observed by the Parker Solar Probe

High time-resolution solar wind magnetic field data are employed to study statistics describing intermittency near the first perihelion (∼35.6 R⊙) of the Parker Solar Probe mission. A merged data set employing two instruments on the FIELDS suite enables broadband estimation of higher-order moments of magnetic field increments, with five orders established with reliable accuracy. The duration, cadence, and low noise level of the data permit evaluation of scale dependence of the observed intermittency from the inertial range to deep subproton scales. The results support multifractal scaling in the inertial range, and monofractal but non-Gaussian scaling in the subproton range, thus clarifying suggestions based on data near Earth that had remained ambiguous due to possible interference of the terrestrial foreshock. The physics of the transition to monofractality remains unclear but we suggest that it is due to a scale-invariant population of current sheets between ion and electron inertial scales; the previous suggestion of incoherent kinetic-scale wave activity is disfavored as it presumably leads re-Gaussianization that is not observed.

Keywords: solar wind; interplanetary turbulence; magnetohydrodynamics; space plasmas
Deep HST Imaging Favors the Bulgeless Edge-on Galaxy Explanation for the Hypothetical Stellar Wake Created by a Runaway Supermassive Black Hole

A long linear structure recently discovered could be the stellar wake produced by the passage of a runaway supermassive black hole (SMBH) or, alternatively, a bulgeless edge-on galaxy. We report on new very deep Hubble Space Telescope imaging that seems to be in tension with the SMBH runaway scenario but is consistent with the bulgeless edge-on galaxy scenario. The new observations were aimed at detecting two key features expected in the SMBH scenario, namely, the bow shock formed where the SMBH meets the surrounding medium, and a counter stellar wake created by another binary SMBH hypothesized as part of the ejection mechanism. Neither of these two features appears to be present in the new images, as would be expected in the edge-on galaxy scenario.

Keywords: disk galaxies; black holes
Discovery of Candidate X-Ray Jets in High-redshift Quasars

We present Chandra X-ray observations of 14 radio-loud quasars at redshifts 3 < z < 4, selected from a well-defined sample. All quasars are detected in the 0.5-7.0 keV energy band, and resolved X-ray features are detected in five of the objects at distances of 1″-12″ from the quasar core. The X-ray features are spatially coincident with known radio features for four of the five quasars. This indicates that these systems contain X-ray jets. X-ray fluxes and luminosities are measured, and jet-to-core X-ray flux ratios are estimated. The flux ratios are consistent with those observed for nearby jet systems, suggesting that the observed X-ray emission mechanism is independent of redshift. For quasars with undetected jets, an upper limit on the average X-ray jet intensity is estimated using a stacked image analysis. Emission spectra of the quasar cores are extracted and modeled to obtain best-fit photon indices, and an Fe K emission line is detected from one quasar in our sample. We compare X-ray spectral properties with optical and radio emission in the context of both our sample and other quasar surveys.

Keywords: active galactic nuclei; x-ray active galactic nuclei; high-redshift galaxies; jets; high energy astrophysics
A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating. III. 3D Simulations and Atmospheric Response

Inspired by recent observations suggesting that photospheric magnetic flux cancellation occurs much more frequently than previously thought, we analytically estimated the energy released from reconnection driven by photospheric flux cancellation, and propose that it can act as a mechanism for chromospheric and coronal heating. Using two-dimensional simulations we validated the analytical estimates and studied the resulting atmospheric response. In the present work, we set up 3D resistive MHD simulations of two canceling polarities in a stratified atmosphere with a horizontal external field to further validate and improve upon the analytical estimates. The computational evaluation of the parameters associated with the energy release are in good qualitative agreement with the analytical estimates. The computational Poynting energy flux into the current sheet is in good qualitative agreement with the analytical estimates, after correcting the analytical expression to better account for the horizontal extent of the current sheet. The atmospheric response to the cancellation is the formation of hot ejections, cool ejections, or a combination of both hot and cool ejections, which can appear with a time difference and/or be spatially offset, depending on the properties of the canceling region and the resulting height of the reconnection. Therefore, during the cancellation, a wide spectrum of ejections can be formed, which can account for the variety of multi-thermal ejections associated with Ellerman bombs, UV bursts, and IRIS bombs, and also other ejections associated with small-scale canceling regions and spicules.

Keywords: solar coronal heating; magnetohydrodynamics; solar magnetic reconnection; solar activity; magnetic fields; magnetohydrodynamical simulations; solar coronal transients
Using Bayesian Deep Learning to Infer Planet Mass from Gaps in Protoplanetary Disks

Planet-induced substructures, like annular gaps, observed in dust emission from protoplanetary disks, provide a unique probe for characterizing unseen young planets. While deep-learning-based models have an edge in characterizing a planet's properties over traditional methods, such as customized simulations and empirical relations, they lacks the ability to quantify the uncertainties associated with their predictions. In this paper, we introduce a Bayesian deep-learning network, "DPNNet-Bayesian," which can predict planet mass from disk gaps and also provides the uncertainties associated with the prediction. A unique feature of our approach is that it is able to distinguish between the uncertainty associated with the deep-learning architecture and the uncertainty inherent in the input data due to measurement noise. The model is trained on a data set generated from disk-planet simulations using the FARGO3D hydrodynamics code, with a newly implemented fixed grain size module and improved initial conditions. The Bayesian framework enables the estimation of a gauge/confidence interval over the validity of the prediction, when applied to unknown observations. As a proof of concept, we apply DPNNet-Bayesian to the dust gaps observed in HL Tau. The network predicts masses of 86.0 ± 5.5 M ⊕, 43.8 ± 3.3 M ⊕, and 92.2 ± 5.1 M ⊕, respectively, which are comparable to those from other studies based on specialized simulations.

Keywords: neural networks; exoplanet detection methods; protoplanetary disks; bayesian statistics
Statistical Characterization of Hot Jupiter Atmospheres Using Spitzer's Secondary Eclipses

We report 78 secondary eclipse depths for a sample of 36 transiting hot Jupiters observed at 3.6 and 4.5 μm using the Spitzer Space Telescope. Our eclipse results for 27 of these planets are new, and include highly irradiated worlds such as KELT-7b, WASP-87b, WASP-76b, and WASP-64b, and important targets for James Webb Space Telescope such as WASP-62b. We find that WASP-62b has a slightly eccentric orbit ( $e\cos \omega =0.00614\pm 0.00064 ), and we confirm the eccentricity of HAT-P-13b and WASP-14b. The remainder are individually consistent with circular orbits, but we find statistical evidence for eccentricity increasing with orbital period in our range from 1 to 5 days. Our day-side brightness temperatures for the planets yield information on albedo and heat redistribution, following Cowan & Agol (2011). Planets having maximum day-side temperatures exceeding ∼2200 K are consistent with having zero albedo and a distribution of stellar irradiance uniformly over the day-side hemisphere. Our most intriguing result is that we detect a systematic difference between the emergent spectra of these hot Jupiters as compared to blackbodies. The ratio of observed brightness temperatures, Tb(4.5)/Tb(3.6), increases with equilibrium temperature by 100 ± 24 parts-per-million per Kelvin, over the entire temperature range in our sample (800-2500 K). No existing model predicts this trend over such a large range of temperature. We suggest that this may be due to a structural difference in the atmospheric temperature profiles of real planetary atmospheres as compared to models.

Keywords: exoplanet astronomy; exoplanet atmospheres
A New Cosmic-Ray Rejection Routine for HST WFC3/UVIS via Label-free Training of deepCR

deepCR is a deep-learning-based cosmic-ray (CR) rejection framework originally presented by Zhang & Bloom. The original approach requires a dedicated training set that consists of multiple frames of the same fields, enabling automatic CR labeling through comparison with their median coadds. Here, we present a novel training approach that circumvents the need for a dedicated training set, but instead utilizes dark frames and the science images requiring CR removal themselves. During training, CRs present in dark frames are added to the science images, which the network is then trained to identify. In turn, the trained deepCR model can then be applied to identify CRs originally present in the science images. Using this approach, we present a new deepCR model trained on a diverse set of Hubble Space Telescope images taken from resolved galaxies in the Local Group, which is universally applicable across all WFC3/UVIS filters. We introduce a robust approach to determining the threshold for generating binary cosmic-ray masks from predictions from deepCR probability maps. When applied to the Panchromatic Hubble Andromeda Southern Treasury survey, our new deepCR model added ∼7% of good-quality stars that exhibit distinct features in their color–magnitude diagrams.

Keywords: astronomy data reduction; convolutional neural networks; classification; cosmic rays; hubble space telescope; astronomical detectors; galaxy stellar content; local group; andromeda galaxy
GRB Fermi-LAT Afterglows: Explaining Flares, Breaks, and Energetic Photons

The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time (≈102 s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with an SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band, and the very energetic photons with SSC emission from the forward shock, when the outflow carries a significant magnetic field (RB ≃ 30) and it decelerates in a uniform-density medium with a very low density ( $n={4.554}_{-1.121}^{+1.128}\times {10}^{-4}\,{\mathrm{cm}}^{-3}$ ).

Keywords: gamma-ray bursts; radiative processes; magnetic fields; non-thermal radiation sources; interstellar medium
Using Single-view Observations of Cometary Plasma Tails to Infer Solar Wind Speed

A comet plasma tail is a product of the interaction between the solar wind and the comet's coma, and has long been studied as a natural probe of the solar wind condition. We previously developed a method to derive the solar wind speed from dual-view observations of comet plasma tails. Here we improve the method to use single-view observations by assuming a radially propagating solar wind and apply it to two comets, C/2011 W3 (Lovejoy) and C/2012 S1 (ISON) observed by coronagraphs on board the Solar and Heliospheric Observatory and STEREO. We compare the results to the solar wind simulations and tomography and to the results from our previous dual-view method, and find they are generally consistent, especially when the comets were far away from the Sun or far away from the ecliptic plane and when the observer is high above the comet's orbital plane. Meanwhile, we notice that this method may suffer from a large error for comets near the ecliptic plane and close to the Sun, where a nonradial component of the solar wind is significant. Using the observations from the first seven orbits of the Parker Solar Probe, we show that the solar wind deviates from a radial direction significantly within around 35 R s. We also notice that, when the nonradial solar wind component is presented, the error may be even larger if the observer is closer to the comet's orbital plane. This method provides a potentially useful tool to estimate the solar wind speed around comets from only single-view imaging observations.

Keywords: solar wind; comet tails
Neutron Star Phase Transition as the Origin for the Fast Radio Bursts and Soft Gamma-Ray Repeaters of SGR J1935 + 2154

Magnetars are believed to be neutron stars (NSs) with strong magnetic fields. X-ray flares and fast radio bursts (FRBs) have been observed from the magnetar (soft gamma-ray repeater, SGR J1935+2154). We propose that the phase transition of the NS can power the FRBs and SGRs. Based on the equation of state provided by the MIT bag model and the mean field approximation, we solve the Tolman-Oppenheimer-Volkoff equations to get the NS structure. With the spin-down of the NS, the hadronic shell gradually transfers to the quark shell. The gravitational potential energy released by one time of the phase transition can be achieved. The released energy, time interval between two successive phase transitions, and glitch are all consistent with the observations of the FRBs and the X-ray flares from SGR J1935 + 2154. We conclude that the phase transition of an NS is a plausible mechanism to power the SGRs as well as the repeating FRBs.

Keywords: radio transient sources; soft gamma-ray repeaters; neutron stars; magnetars