Sazuna/UAT_keywords huggingface dataset:
A White-light Flare Powered by Magnetic Reconnection in the Lower Solar Atmosphere. White-light flares (WLFs), first observed in 1859, refer to a type of solar flare showing an obvious enhancement of the visible continuum emission. This type of enhancement often occurs in most energetic flares, and is usually interpreted as a consequence of efficient heating in the lower solar atmosphere through nonthermal electrons propagating downward from the energy release site in the corona. However, this coronal-reconnection model has difficulty in explaining the recently discovered small WLFs. Here we report a C2.3 WLF, which is associated with several observational phenomena: a fast decrease in opposite-polarity photospheric magnetic fluxes, the disappearance of two adjacent pores, significant heating of the lower chromosphere, a negligible increase of the hard X-ray flux, and an associated U-shaped magnetic field configuration. All these suggest that this WLF is powered by magnetic reconnection in the lower part of the solar atmosphere rather than by reconnection higher up in the corona.

Keywords: Solar active regions, Solar flares, Solar magnetic fields, Solar magnetic reconnection
Gamma-Ray Burst Spectrum with a Time-dependent Injection Rate of High-energy Electrons. Although the physical origin of prompt emission in gamma-ray bursts (GRBs) remains inconclusive, previous studies have considered the synchrotron radiation of relativistic electrons as a promising mechanism. These works usually adopted an invariable injection rate of electrons (Q) which may be discordant with that in a Poynting-flux-dominated jet. In a Poynting-flux-dominated jet (e.g., internal-collision-induced magnetic reconnection and turbulence model), the number of magnetic reconnections occurring simultaneously may grow rapidly with time and result in an increase of Q with time. This paper is dedicated to studying the synchrotron radiation spectrum in this scenario. It is found that the radiation spectrum would obviously get harder if an increasing Q is adopted and a Band-like radiation spectrum can be obtained if the increase of Q is fast enough. The latter is related to the fact that a bump shape rather than a power-law spectrum appears in the low-energy regime of the obtained electron spectrum. This effect can strongly harden the low-energy radiation spectrum. It indicates that an increasing Q can help to alleviate the "fast-cooling problem" of synchrotron radiation for GRBs. Our studies also reveal that a Poynting-flux dominated jet with a large emission radius, a short magnetic reconnection region length, or an injected electron with low minimum energy would prefer to form a Band-like radiation spectrum. We suggest that the Band spectrum found in GRBs may be the synchrotron emission of the electrons with a bump-shape distribution in its low-energy regime.

Keywords: Gamma-ray bursts
A Shining Death of Unequal Supermassive Black Hole Binaries. In the ΛCDM scenario, small galaxies merge to produce larger entities. Since supermassive black holes (SMBHs) are found in galaxies of all sizes, SMBH binaries (SMBHBs) are generally expected to form during the amalgamation of galaxies. It is unclear what fraction of these binaries could eventually merge, but a general consensus is that initially the orbital decay is mediated by the surrounding gas and stars. In this Letter, we show that in active galactic nuclei (AGNs) the radiation field also causes the orbits of the accreting SMBHs to shrink. The corresponding mechanism, known as the "Poynting-Robertson drag" (PR drag), takes effect on a well-defined timescale CTSal, where TSal is the Salpeter timescale of the AGN, presumably coinciding with the primary SMBH, and $C=4{\XI }^{-1}{\epsilon }^{-1}{q}^{1/3}{(1+q)}^{2/3}(1-\epsilon )$ is a constant determined by the radiative efficiency ɛ, the mass ratio q of the two black holes, and a parameter ξ characterizing the size of the circumsecondary accretion disk. We find that when q ≲ a few × 10-5, the PR drag is more efficient in shrinking the binary than many other mechanisms, such as dynamical friction and type-I migration. Our finding points to a possible new channel for the coalescence of unequal SMBHBs and the clearing of intermediate-massive black holes in AGNs.

Keywords: Accretion, Astrodynamics, Quasars, Active galactic nuclei, Gravitational waves
Very Slow Rotators from Tidally Synchronized Binaries. A recent examination of K2 lightcurves indicates that ∼15% of Jupiter Trojans have very slow rotation (spin periods Ps &gt; 100 hr). Here we consider the possibility that these bodies formed as equal-size binaries in the massive outer disk at ∼20-30 au. Prior to their implantation as Jupiter Trojans, tight binaries tidally evolved toward a synchronous state with Ps ∼ Pb, where Pb is the binary orbit period. They may have been subsequently dissociated by impacts and planetary encounters with at least one binary component retaining its slow rotation. Surviving binaries on Trojan orbits would continue to evolve by tides and spin-changing impacts over 4.5 Gyr. To explain the observed fraction of slow rotators, we find that at least ∼15%-20% of outer disk bodies with diameters 15 &lt; D &lt; 50 km would have to form as equal-size binaries with 12 ≲ ab/R ≲ 30, where ab is the binary semimajor axis and R = D/2. The mechanism proposed here could also explain very slow rotators found in other small-body populations.

Keywords: Tidal interaction, Kuiper belt, Jupiter trojans
Observations of a Quasi-periodic Pulsation in the Coronal Loop and Microwave Flux during a Solar Preflare Phase. We report a quasi-periodic pulsation (QPP) event simultaneously detected from the spatial displacements of the coronal loop at both EUV images and microwave emission during the preflare phase of a C1.1 flare on 2016 March 23. Using the motion magnification technique, a low-amplitude transverse oscillation with the growing period is discovered in a diffuse coronal loop in Atmospheric Imaging Assembly (AIA) image sequences at wavelength of 171 Å, and the initial oscillation period is estimated to be ∼397 s with a slow growth rate of 0.045. At the same time, a QPP with growing periods from roughly 300 s to nearly 500 s is discovered in the microwave flux in the same active region. Based on the imaging observations measured at EUV wavelengths by the AIA and at microwave 17 GHz by Nobeyama Radioheliograph, the diffuse coronal loop and the microwave radiation source are found to be connected through a hot loop seen in AIA images at wavelength of 94 Å. The growing period of the QPP should be related to the modulation of LRC-circuit oscillating process in a current-carrying plasma loop. The existence of electric currents may imply the non-potentialities in the source region during the preflare phase.

Keywords: Solar oscillations, Solar radio emission, Solar flares, Solar ultraviolet emission
Formation of Macroscale Flux Transfer Events at Mercury. Flux transfer events (FTEs) are magnetic flux ropes that are produced via magnetic reconnection at the planetary magnetopause where the solar wind directly interacts with the magnetosphere. Previous observations show that FTEs with a duration of several seconds, corresponding to a spatial scale of ∼0.5-1 RM, can occur at Mercury. However, the formation of these macroscale FTEs at a small dimensional magnetopause with a radius of ∼1.5 RM remains unclear. Here, we report the observations of active magnetic reconnection events at Mercury's magnetopause by the MESSENGER spacecraft. The reconnection process is dominated by the formation of a series of multi-scale FTEs. Ion-scale flux ropes, typically with durations of ∼1 s or less, may be produced by the tearing instability in the thin current sheet near the subsolar position. Moreover, the commonly observed macroscale FTEs consist of three to tens of successive small-scale FTEs. We propose that macroscale FTEs at Mercury are generated by the interaction and merging of multiple ion-scale flux ropes, probably through two or more steps. This is distinct from the formation of typical FTEs, mainly between a pair of X-lines, at Earth's magnetopause. Thus, the formation and evolution of FTEs may differ among planetary magnetospheres with a vast range of scale sizes. We further conclude that Mercury's magnetopause is a natural plasma laboratory to study flux rope dynamics and evolution for the upcoming Bepi-Colombo mission.

Keywords: Solar magnetic reconnection, Planetary boundary layers, Solar-planetary interactions, Mercury (planet), Planetary magnetospheres
The Changing-look Optical Wind of the Flaring X-Ray Transient Swift J1858.6-0814. We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4 m Gran Telescopio Canarias telescope over eight different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He I at 5876 Å and Hα. These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on timescales as short as 5 minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ∼2400 $\mathrm{km}\,{{\rm{s}}}^{-1}$ . In agreement with previous reports, our observations are characterized by the presence of frequent flares, although the relation between the continuum flux variability and the presence/absence of wind features is not evident. The reported high activity of the system at radio waves indicates that the optical wind of Swift J1858.6-0814 is contemporaneous with the radio jet, as is the case for the handful of X-ray binary transients that have shown so far optical P-Cyg profiles. Finally, we compare our results with those of other sources showing optical accretion disk winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong and variable optical wind features as well as similar flaring behavior.

Keywords: Neutron stars, X-ray binary stars, Stellar mass black holes, Stellar winds, Stellar accretion disks
Proton Synchrotron Gamma-Rays and the Energy Crisis in Blazars. The origin of high-energy emission in blazars jets (I.e., leptonic versus hadronic) has been a longstanding matter of debate. Here, we focus on one variant of hadronic models where proton synchrotron radiation accounts for the observed steady γ-ray blazar emission. Using analytical methods, we derive the minimum jet power ( ${P}_{j,\min }$ ) for the largest blazar sample analyzed to date (145 sources), taking into account uncertainties of observables and jet's physical parameters. We compare ${P}_{j,\min }$ against three characteristic energy estimators for accreting systems, I.e., the Eddington luminosity, the accretion disk luminosity, and the power of the Blandford-Znajek process, and find that ${P}_{j,\min }$ is about 2 orders of magnitude higher than all energetic estimators for the majority of our sample. The derived magnetic field strengths in the emission region require either large amplification of the jet's magnetic field (factor of 30) or place the γ-ray production site at sub-pc scales. The expected neutrino emission peaks at ∼0.1-10 EeV, with typical peak neutrino fluxes ∼10-4 times lower than the peak γ-ray fluxes. We conclude that if relativistic hadrons are present in blazar jets, they can only produce a radiatively subdominant component of the overall spectral energy distribution of the blazar's steady emission.

Keywords: Supermassive black holes, Black hole physics, Blazars, Neutrino astronomy, Relativistic jets, Non-thermal radiation sources, Radiative processes
Constraining the Dense Matter Equation of State with Joint Analysis of NICER and LIGO/Virgo Measurements. The Neutron Star Interior Composition Explorer collaboration recently published a joint estimate of the mass and the radius of PSR J0030+0451, derived via X-ray pulse-profile modeling. Raaijmakers et al. explored the implications of this measurement for the dense matter equation of state (EOS) using two parameterizations of the high-density EOS: a piecewise-polytropic model, and a model based on the speed of sound in neutron stars (NSs). In this work we obtain further constraints on the EOS following this approach, but we also include information about the tidal deformability of NSs from the gravitational wave signal of the compact binary merger GW170817. We compare the constraints on the EOS to those set by the recent measurement of a 2.14 M⊙ pulsar, included as a likelihood function approximated by a Gaussian, and find a small increase in information gain. To show the flexibility of our method, we also explore the possibility that GW170817 was a NS-black hole merger, which yields weaker constraints on the EOS.

Keywords: X-ray astronomy, Bayesian statistics, Neutron star cores, Rotation powered pulsars, Millisecond pulsars, Nuclear astrophysics, Gravitational waves, Nuclear physics
Thermally Driven Angular Momentum Transport in Hot Jupiters. We study the angular momentum transport inside hot Jupiters under the influence of gravitational and thermal forcing. Due to the strong stellar irradiation, a radiative region develops on top of the convective region. Internal gravity waves are launched at the radiative-convective boundaries (RCBs). The thermal response is dynamical and plays an important role in the angular momentum transport. By separating the gravitational and thermal forcing terms, we identify the thermal effects of increasing the angular momentum transport. For the low-frequency (in the corotating frame with planets) prograde (retrograde) tidal frequency, the angular momentum flux is positive (negative). The tidal interactions tend to drive the planet to the synchronous state. We find that the angular momentum transport associated with the internal gravity wave is very sensitive to relative position between the RCB and the penetration depth of the thermal forcing. If the RCB is in the vicinity of the thermal forcing penetration depth, even with small amplitude thermal forcing, the thermally driven angular momentum flux could be much larger than the flux induced by gravitational forcing. The thermally enhanced torque could drive the planet to the synchronous state in as short as a few 104 yr.

Keywords: Internal waves, Hydrodynamics, Exoplanet tides, Exoplanet structure, Exoplanet atmospheres
SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020. In 2019 October Betelgeuse began a decline in V-band brightness that went beyond the minimum expected from its quasi-periodic ∼420 day cycle, becoming the faintest in recorded photometric history. Observations obtained in 2019 December with Very Large Telescope/SPHERE have shown that the southern half of the star has become markedly fainter than in 2019 January, indicating that a major change has occurred in, or near, the photosphere. We present Stratospheric Observatory for Infrared Astronomy (SOFIA) Echelon Cross Echelle Spectrograph (EXES) high spectral-resolution observations of [Fe II] $25.99\,\mu {\rm{m}}$ and [S I] $25.25\,\mu {\rm{m}}$ emission lines from Betelgeuse obtained during the unprecedented 2020 February V-band brightness minimum to investigate potential changes in the circumstellar flow. These spectra are compared to observations obtained in 2015 and 2017 when the V magnitude was typical of brighter phases. We find only very small changes in the gas velocities reflected by either of the line profiles, no significant changes in the flux to continuum ratios, and hence no significant changes in the [Fe II]/[S I] flux ratios. There is evidence that absorption features have appeared in the 2020 continuum. The Alfvén wave-crossing time from the upper photosphere is sufficiently long that one would not expect a change in the large-scale magnetic field to reach the circumstellar [Fe II] and [S I] line-forming regions, 3 &lt; R (R*) &lt; 20. However, the light-crossing time is of order a few hours and a reduction in luminosity may reduce the dust-gas heating rate and [O I] $63.19\,\mu {\rm{m}}$ emission, which has contributions from R &gt; 20R*, where significant circumstellar oxygen-rich dust is observed.

Keywords: Circumstellar envelopes, Stellar mass loss, Stellar spectral lines, Supergiant stars
EAGLE and Illustris-TNG Predictions for Resolved eROSITA X-Ray Observations of the Circumgalactic Medium around Normal Galaxies. We simulate stacked observations of nearby hot X-ray coronae associated with galaxies in the EAGLE and Illustris-TNG hydrodynamic simulations. A forward modeling pipeline is developed to predict 4 yr eROSITA observations and stacked image analysis, including the effects of instrumental and astrophysical backgrounds. We propose an experiment to stack z ≈ 0.01 galaxies separated by specific star formation rate (sSFR) to examine how the hot (T ≥ 106 K) circumgalactic medium (CGM) differs for high- and low-sSFR galaxies. The simulations indicate that the hot CGM of low-mass ( ${M}_{* }\approx {10}^{10.5}\ {M}_{\odot }$ ), high-sSFR (defined as the top one-third ranked by sSFR) central galaxies will be detectable to a galactocentric radius r ≈ 30-50 kpc. Both simulations predict lower luminosities at fixed stellar mass for the low-sSFR galaxies (the lower third of sSFR) with Illustris-TNG predicting 3× brighter coronae around high-sSFR galaxies than EAGLE. Both simulations predict detectable emission out to r ≈ 150-200 kpc for stacks centered on high-mass ( ${M}_{* }\approx {10}^{11.0}\ {M}_{\odot }$ ) galaxies, with EAGLE predicting brighter X-ray halos. The extended soft X-ray luminosity correlates strongly and positively with the mass of circumgalactic gas within the virial radius (fCGM). Prior analyses of both simulations have established that fCGM is reduced by expulsive feedback driven mainly by black hole growth, which quenches galaxy growth by inhibiting replenishment of the interstellar medium. Both simulations predict that eROSITA stacks should not only conclusively detect and resolve the hot CGM around L* galaxies for the first time, but provide a powerful probe of how the baryon cycle operates, for which there remains an absence of consensus between state-of-the-art simulations.

Keywords: Galactic winds, Galaxy formation, Hydrodynamical simulations, Supermassive black holes, X-ray observatories, Circumgalactic medium
In Situ Measurement of Curvature of Magnetic Field in Turbulent Space Plasmas: A Statistical Study. Using in situ data, accumulated in the turbulent magnetosheath by the Magnetospheric Multiscale Mission, we report a statistical study of magnetic field curvature and discuss its role in the turbulent space plasmas. Consistent with previous simulation results, the probability distribution function of the curvature is shown to have distinct power-law tails for both high and low value limits. We find that the magnetic-field-line curvature is intermittently distributed in space. High curvature values reside near weak magnetic-field regions, while low curvature values are correlated with small magnitude of the force acting normal to the field lines. A simple statistical treatment provides an explanation for the observed curvature distribution. This novel statistical characterization of magnetic curvature in space plasma provides a starting point for assessing, in a turbulence context, the applicability and impact of particle energization processes, such as curvature drift, that rely on this fundamental quantity.

Keywords: Space plasmas, Magnetohydrodynamics, Plasma physics
A Binary Comb Model for Periodic Fast Radio Bursts. We show that the periodic FRB 180916.J0158+65 can be interpreted by invoking an interacting neutron star binary system with an orbital period of ∼16 days. The FRBs are produced by a highly magnetized pulsar, whose magnetic field is "combed" by the strong wind from a companion star, either a massive star or a millisecond pulsar. The FRB pulsar wind retains a clear funnel in the companion's wind that is otherwise opaque to induced Compton or Raman scatterings for repeating FRB emission. The 4 day active window corresponds to the time when the funnel points toward Earth. The interaction also perturbs the magnetosphere of the FRB pulsar and may trigger emission of FRBs. We derive the physical constraints on the comb and the FRB pulsar from the observations and estimate the event rate of FRBs. In this scenario, a lower limit on the period of observable FRBs is predicted. We speculate that both the intrinsic factors (strong magnetic field and young age) and the extrinsic factor (interaction) may be needed to generate FRBs in neutron star binary systems.

Keywords: Neutron stars, Relativistic mechanics, Periodic orbit, Non-thermal radiation sources, Magnetars, Magnetic fields, Binary pulsars, Radio transient sources
A Habitable-zone Earth-sized Planet Rescued from False Positive Status. We report the discovery of an Earth-sized planet in the habitable zone of a low-mass star called Kepler-1649. The planet, Kepler-1649 c, is 1.06 ${}_{-0.10}^{+0.15}$ times the size of Earth and transits its 0.1977 ± 0.0051 ${M}_{\odot }$ "mid" M-dwarf host star every 19.5 days. It receives 74% ± 3% the incident flux of Earth, giving it an equilibrium temperature of 234 ± 20 K and placing it firmly inside the circumstellar habitable zone. Kepler-1649 also hosts a previously known inner planet that orbits every 8.7 days and is roughly equivalent to Venus in size and incident flux. Kepler-1649 c was originally classified as a false positive (FP) by the Kepler pipeline, but was rescued as part of a systematic visual inspection of all automatically dispositioned Kepler FPs. This discovery highlights the value of human inspection of planet candidates even as automated techniques improve, and hints that terrestrial planets around mid to late M-dwarfs may be more common than those around more massive stars.

Keywords: Exoplanet astronomy, Habitable planets
Optical Counterpart to the Ultraluminous X-Ray Source in the UGC 6456 Galaxy. We report the identification of the optical counterpart to the transient ultraluminous X-ray source in the blue dwarf galaxy UGC 6456 (VII Zw 403). The source is highly variable in both the X-ray (by a factor of at least 100, 0.3-10 keV) and optical (by a factor of three, V-band) ranges. The peak X-ray luminosity of UGC 6456 ultraluminous X-ray source (ULX) exceeds 1040 erg s-1; the absolute magnitude when the source is optically bright is MV = -8.24 ± 0.11, which makes this source one of the brightest ULXs in the optical range. We found a correlation between the optical and X-ray fluxes (with a coefficient of 0.9 ± 0.3), which may indicate that the optical emission is produced by re-processing of the X-rays in outer parts of the optically thick wind coming from the supercritical accretion disk. Optical spectra of UGC 6456 ULX show broad and variable hydrogen and helium emission lines, which also confirms the presence of the strong wind.

Keywords: Accretion, X-ray point sources, X-ray identification, X-ray binary stars