Sazuna/UAT_keywords huggingface dataset:
Metallicity Dependence of the H/H<SUB>2</SUB> and C<SUP>+</SUP>/C/CO Distributions in a Resolved Self-regulating Interstellar Medium. We study the metallicity dependence of the H/H2 and C+/C/CO distributions in a self-regulated interstellar medium (ISM) across a broad range of metallicities (0.1 &lt; Z/Z⊙ &lt; 3). To this end, we conduct high-resolution (particle mass of 1 M⊙) hydrodynamical simulations coupled with a time-dependent H2 chemistry network. The results are then post-processed with an accurate chemistry network to model the associated C+/C/CO abundances, based on the time-dependent non-steady-state ("nonequilibrium") H2 abundances. We find that the time-averaged star formation rate and the ISM structure are insensitive to metallicity. The column densities relevant for molecular shielding appear correlated with the volume densities in gravitationally unstable gas. As metallicity decreases, H2 progressively deviates from steady state ("equilibrium") and shows shallow abundance profiles until they sharply truncate at the photodissociation fronts. In contrast, the CO profile is sharp and controlled by photodissociation as CO quickly reaches steady state. We construct effective 1D cloud models that successfully capture the time-averaged chemical distributions in simulations. At low metallicities, the steady-state model significantly overestimates the abundance of H2 in the diffuse medium. The overestimated H2, however, has little impact on CO. Consequently, the mass fraction of CO-dark H2 gas is significantly lower than what a fully steady-state model predicts. The mass ratios of H2/C+ and H2/C both show a weaker dependence on {Z}^{{\prime} }$ than H2/CO, which potentially indicates that C+ and C could be alternative tracers for H2 at low {Z}^{{\prime} }$ in terms of mass budget. Our chemistry code for post-processing is publicly available.

Keywords: Hydrodynamical simulations, Interstellar medium, Astrochemistry
Discovery of ASKAP J173608.2-321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder. We report the discovery of a highly polarized, highly variable, steep-spectrum radio source, ASKAP J173608.2-321635, located ~4° from the Galactic Center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888 MHz. It exhibited a high degree (~25%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2-4 week cadence. The source was not detected with MeerKAT before 2021 February 7 when it appeared and reached a peak flux density of 5.6 mJy. The source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from -11.8 ± 0.8 rad m-2 to -64.0 ± 1.5 rad m-2 over three days. No X-ray counterpart was found in follow-up Swift or Chandra observations about a week after the first MeerKAT detection, with upper limits of ~5.0 × 1031 erg s-1 (0.3-8 keV, assuming a distance ~10 kpc). No counterpart is seen in new or archival near-infrared observations down to J = 20.8 mag. We discuss possible identifications for ASKAP J173608.2-321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center radio transient: none of these fully explains the observations, which suggests that ASKAP J173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys.

Keywords: Neutron stars, Galactic center, Galactic radio sources, Magnetars, Radio transient sources
On the Circular Polarization of Repeating Fast Radio Bursts. Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarized emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarization-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarization for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarization can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarization in current observations of known repeating FRBs. We suggest that the circular polarization of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.

Keywords: Neutron stars, Radio bursts, Non-thermal radiation sources, Magnetars, Radio transient sources
Velocity Difference of Ions and Neutrals in Solar Prominences. Marked velocity excesses of ions relative to neutrals are obtained from two time series of the neighboring emission lines He I 5015 Å and Fe II 5018 Å in a quiescent prominence. Their Doppler shifts show time variations of quasi-periodic character where the ions are faster than the neutrals, 1.0 ≤ Vmacro(Fe II)/Vmacro(He I) ≤ 1.35 in series A and ≤1.25 in series B. This "ratio excess" confirms our earlier findings of a 1.22 ion velocity excess, but the present study shows a restriction in space and time of typically 5 Mm and 5 minutes. The ratio excess is superposed by a time- and velocity-independent "difference excess" of -0.3 ≤ Vmacro(Fe II)-Vmacro(He I) ≤ +0.7 km s-1 in series A (also indicated in series B). The high repetition rate of 3.9 s enables the detection of high-frequency oscillations with several damped 22 s periods in series A. These show a ratio excess with a maximum of 1.7. We confirm the absence of a significant phase delay of He neutrals with respect to the Fe ions.

Keywords: Solar prominences
Tying the Geometrical Traits of Massive Young Stellar Objects and Their Disks to a Potential Evolutionary Sequence Using Infrared Observations. Young massive stars influence their surroundings from local to galactic scales, but the observational challenges associated with their distance and embedded nature has, until the recent decade, made high-resolution studies of these objects difficult. In particular, comparative analyses of massive young stellar object (MYSO) disks are currently lacking and our understanding of their evolution is limited. Here, we combine the results of two studies with the aim to attribute geometrical features to an evolutionary sequence for a sample of seven MYSOs. The time evolution is based on near-infrared spectral features, while the geometry is determined from a multi-size-scale study of MYSOs. We find that MYSO disks with determined geometrical substructure turn out to be also spectroscopically more evolved. This implies that disk evolution and dispersal occur within MYSOs similarly to low-mass young stellar object (YSO) disk evolution, despite their faster formation timescales.

Keywords: Observational astronomy, Massive stars, Infrared astronomy, Star formation, Circumstellar disks, Interferometry
The Quasi-biennial-oscillation-type Signals in the Subsurface Flow Fields during Solar Cycles 23 and 24. We studied the presence and spatiotemporal evolution of quasi-biennial oscillations (QBOs) in the rotation-rate residuals at target depths of 0.90R⊙, 0.95R⊙, and 0.99R⊙ and at low- (0°-30°), mid- (30°-50°), and high-latitudinal (50°-70°) bands. To achieve these objectives we used data from the Michelson Doppler Imager on the Solar and Heliospheric Observatory and the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory, covering solar cycles 23 and 24, respectively. The results show that there are QBO-like signals in each latitudinal band and depth; however, they are affected by higher-amplitude and longer-timescale variations. The QBO-like signals found in each target depth and latitudinal bands show different spatiotemporal evolution. The amplitudes of variations of the rotation-rate residuals in the QBO timescale increase with increasing depth.

Keywords: Solar rotation
Search for Neutrinos from the Tidal Disruption Events AT2019dsg and AT2019fdr with the ANTARES Telescope. On 2019 October 1, the IceCube Collaboration detected a muon track neutrino with a high probability of being of astrophysical origin, IC191001A. After a few hours, the tidal disruption event (TDE) AT2019dsg, observed by the Zwicky Transient Facility (ZTF), was indicated as the most likely counterpart of the IceCube track. More recently, the follow-up campaign of the IceCube alerts by ZTF suggested a second TDE, AT2019fdr, as a promising counterpart of another IceCube muon track candidate, IC200530A, detected on 2020 May 30. Here, these intriguing associations are followed-up by searching for neutrinos in the ANTARES detector from the directions of AT2019dsg and AT2019fdr using a time-integrated approach. As no significant evidence for space clustering is found in the ANTARES data, upper limits on the one-flavor neutrino flux and fluence are set.

Keywords: Neutrino astronomy
Evidence of a Dwarf Galaxy Stream Populating the Inner Milky Way Halo. Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 data set using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. We detect LMS-1 as a 60° long stream to the north of the galactic bulge, at a distance of ~20 kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, the Sloan Digital Sky Survey, and the Apache Point Observatory Galactic Evolution Experiment, we infer that the stream is very metal-poor (⟨[Fe/H]⟩ = -2.1) with a significant metallicity dispersion (σ[Fe/H] = 0.4), and it possesses a large radial velocity dispersion (σv = 20 ± 4 km s-1). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of 75° to the galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024, and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.

Keywords: Milky Way formation, Stellar streams, Surveys, Galaxy structure, Galaxy stellar content, Milky Way stellar halo
Energy Transfer Across the Magnetopause Under Radial IMF Conditions. A global MHD model is used to study the energy transfer from solar wind to magnetosphere through magnetopause under radial interplanetary magnetic fields (IMFs). We use the streamline method to determine the smooth surface of the magnetopause by searching the inner boundary of the solar-wind streamline and discuss the roles of magnetic reconnection and viscous interaction under radial IMFs, which we compare with cases of north-south IMFs. We find that (1) the energy transfer across the magnetopause is asymmetric between the northern and southern hemispheres due to different reconnection locations, particularly for electromagnetic energy; (2) for sunward IMF, the most significant area of the net input of mechanical energy occurs on the day side and near-Earth magnetotail, and the electromagnetic energy input in the northern hemisphere is much larger than in the southern hemisphere on the night side; (3) the mechanical and electromagnetic energy-transfer distribution in the northern (southern) hemisphere for earthward IMF is the same as that in the southern (northern) hemisphere for sunward IMF; (4) the electromagnetic energy input for radial IMF is two times larger than for northward IMF, but three times smaller than for southward IMF, the viscous effect is smaller than for northward IMF but comparable to that for southward IMF, the rate of energy transfer is 2.22% for radial IMF, which is lower than 4.95% for southward IMF, but higher than 1.7% for northward IMF; and (5) the Akasofu-type energy-coupling formula, ϵ, is not suitable for the solar-wind events dominated by IMF Bx.

Keywords: Planetary magnetospheres, Magnetohydrodynamical simulations
GRB 180720B: A GRB with Interesting Spectral Characteristics. The radiation mechanism and jet composition of gamma-ray bursts (GRBs) are still two important unresolved issues, and studying its energy spectrum characteristics can provide us with evidence. In this paper, we first use the smoothly broken power-law function (Band) and the cutoff power-law function to perform Bayesian time-resolved spectrum analysis on GRB 180720B and find these spectra can be well fitted. When the thermal component is added, approximately 40% of the resolution spectrum fitting statistics are significantly improved, indicating that there is a thermal spectral component in the burst. The thermal spectra are concentrated at the beginning and peak stage of the pulses. Therefore, the burst also supports that the GRB jet component transitions from material dominance to magnetic dominance. The values of the low-energy spectrum index α of the time-resolved spectra do not exceed the so-called synchrotron line of death, regardless of whether the thermal components are added or not, which indicates that the burst can be explained by synchrotron radiation. Moreover, we find that both α and Ep track the flux, but their tracking modes are different. We use the synchrotron model to fit these time-resolved spectra and find that these spectra can also be well fitted by the model. Furthermore, we find that both the electron spectrum index p and the magnetic field strength B track the flux and are positively correlated with the flux. Our results also show that the Band function can be explained by the synchrotron model in the Gamma-ray Burst Monitor energy range.

Keywords: Gamma-ray bursts
Binary Comb Models for FRB 121102. The first repeating fast radio burst source, FRB 121102, is observed to emit bursts with a periodically modulated rate. We show that FRB 121102 can be interpreted as an interacting neutron star binary system with an orbital period of ~159 days. We develop a binary comb model by introducing an eccentricity into the orbit. Besides the original funnel mode of the binary comb model, which was applied to FRB 180916.J0158+65 by Ioka &amp; Zhang, we also identify two new modes of the binary comb model, i.e., the τ-crossing mode and the inverse funnel mode, and apply them to interpret FRB 121102. These new developments expand the applicable parameter space, allowing the companion star to be a massive star, a massive black hole, or a supermassive black hole, with the latter two having larger parameter spaces. These models are also consistent with other observations, such as the persistent bright radio counterpart associated with the source. We also argue that the observed frequency dependence of the active window does not disfavor the binary comb model, in contrast to recent claims, and propose two possible scenarios to interpret the data.

Keywords: Neutron stars, Periodic orbit, Non-thermal radiation sources, Magnetars, Radio transient sources
External Inverse-Compton Emission from Low-luminosity Gamma-Ray Bursts: Application to GRB 190829A. The detection of TeV gamma-ray bursts (GRBs) brought new opportunities for studying the physics of particle acceleration at relativistic shocks. The High Energy Stereoscopic System (H.E.S.S.) telescopes recently observed very-high-energy (VHE) emission from a nearby low-luminosity GRB, GRB 190829A. Follow-up observations with, e.g., Swift-XRT, revealed unusual flare activities at ~103 s, which can be caused by a long-lasting central engine. We show that the VHE emission during the H.E.S.S. observation time is naturally produced in the external inverse-Compton (EIC) scenario, where seed photons supplied by the flares or other late-time dissipations are upscattered to VHE energies by the nonthermal electrons accelerated at the external forward shock. Our calculations show that the EIC flare nearly coincides with the late-prompt flare, but extends ~3-4 times longer than the duration of the late-prompt flare. The preferred kinetic energy and initial Lorentz factor used in our model are ~1052 erg and ~20, respectively. Understanding the mechanisms of the VHE emission from low-luminosity GRBs will help us constrain the properties of the outflow and the central engine activities, as well as the particle acceleration mechanism.

Keywords: Stellar jets, Gamma-ray bursters, Non-thermal radiation sources, X-ray sources, Radio sources, Particle astrophysics
A Family Tree of Optical Transients from Narrow-line Seyfert 1 Galaxies. The Zwicky Transient Facility (ZTF) has discovered five events (0.01 &lt; z &lt; 0.4) belonging to an emerging class of active galactic nuclei (AGNs) undergoing smooth, large-amplitude, and rapidly rising flares. This sample consists of several transients initially classified as supernovae with narrow spectral lines. However, upon closer inspection, all of the host galaxies display Balmer lines with FWHM(Hβ) ~ 900-1400 km s-1, characteristic of a narrow-line Seyfert 1 (NLSy1) galaxy. The transient events are long lived, over 400 days on average in the observed frame. We report UV and X-ray follow-up of the flares and observe persistent UV emission, with two of the five transients detected with luminous X-ray emission, ruling out a supernova interpretation. We compare the properties of this sample to previously reported flaring NLSy1 galaxies and find that they fall into three spectroscopic categories: 1) Balmer line profiles and Fe II complexes typical of NLSy1s, 2) strong He II profiles, and 3) He II profiles including Bowen fluorescence features. The latter are members of the growing class of AGN flares attributed to enhanced accretion reported by Trakhtenbrot et al. We consider physical interpretations in the context of related transients from the literature. For example, two of the sources show high-amplitude rebrightening in the optical, ruling out a simple tidal disruption event scenario for those transients. We conclude that three of the sample belong to the Trakhtenbrot et al. class and two are tidal disruption events in NLSy1s. We also hypothesize as to why NLSy1s are preferentially the sites of such rapid enhanced flaring activity.

Keywords: Accretion, AGN host galaxies, High energy astrophysics, Active galaxies, Seyfert galaxies, Active galactic nuclei
Observational Constraints on the Pulsar Wind Model: The Cases of Crab and Vela. As is well known, pulsars are extremely stable rotators. However, although slowly, they spindown thanks to brake mechanisms, which are in fact still a subject of intense investigation in the literature. Since pulsars are usually modeled as highly magnetized neutron stars that emit beams of electromagnetic radiation out of their magnetic poles, it is reasonable to consider that the spindown has to do with a magnetic brake. Although an interesting and simple idea, a pure magnetic brake is not able to adequately account for the spindown rate. Thus, many alternative spindown mechanisms appear in the literature, among them the pulsar wind model, where the wind of particles coming from the pulsar itself can carry part of its rotational kinetic energy. Such a spindown mechanism depends critically on three parameters, namely, the dipole magnetic field (B), the angle between the magnetic and rotation axes (φ), and the density of primary particles (ζ) of the pulsar's magnetosphere. Differently from a series of articles in this subject, we consider for the first time in the literature a statistical modeling that includes a combination of a magnetic dipole and wind brakes. As a result, we are able to constrain the above referred parameters in particular for Crab and Vela pulsars.

Keywords: Pulsars
Unraveling the Eclipse Mechanism of a Binary Millisecond Pulsar Using Broadband Radio Spectra. The frequency dependent eclipses of the radio emission from millisecond pulsars in compact binary systems provide an opportunity to understand the eclipse mechanism and to determine the nature of the eclipsing medium. We combine multifrequency observations from the upgraded Giant Metrewave Radio Telescope and model the broadband radio spectrum in the optically thick to thin transition regime to constrain the eclipse mechanism. The best-fit model to the eclipse phase spectra favors synchrotron absorption by relativistic electrons. We are able to strongly constrain the frequency of onset of the eclipse to 345 ± 5 MHz, which is an order of magnitude more precise than previous estimates. The dependence on the magnetic field strength of synchrotron absorption allowed us to estimate the magnetic field strength of the eclipse medium to be ~13 G, which is very similar to the values obtained by considering a pressure balance between the incident pulsar wind and the stellar wind of the companion. Applying this method to other millisecond binary pulsars will enable us to determine if the eclipse mechanisms are all the same and also estimate the wind and magnetic field properties of the companion stars. The method could also be applied to other systems where pulsars interact with companion winds in binary systems and in all cases it will lead to a better understanding of the evolutionary processes.

Keywords: Radio spectroscopy, Eclipses, Millisecond pulsars, Close binary stars, Interacting binary stars, Eclipsing binary stars, Binary pulsars, Radio pulsars
On the Impact of an Intermediate Duration X-Ray Burst on the Accretion Environment in IGR J17062-6143. We report on a spectroscopic analysis of the X-ray emission from IGR J17062-6143 in the aftermath of its 2020 June intermediate duration Type I X-ray burst. Using the Neutron Star Interior Composition Explorer, we started observing the source 3 hr after the burst was detected with the Monitor of All-sky X-ray Image Gas Slit Camera, and monitored the source for the subsequent 12 days. We observed the tail end of the X-ray burst-cooling phase, and find that the X-ray flux is severely depressed relative to its historic value for a three-day period directly following the burst. We interpret this intensity dip as the inner accretion disk gradually restoring itself after being perturbed by the burst irradiation. Superimposed on this trend we observed a 1.5 day interval during which the X-ray flux is sharply lower than the wider trend. This drop in flux could be isolated to the nonthermal components in the energy spectrum, suggesting that it may be caused by an evolving corona. Additionally, we detected a 3.4 keV absorption line at 6.3σ significance in a single 472 s observation while the burst emission was still bright. We tentatively identify the line as a gravitationally redshifted absorption line from burning ashes on the stellar surface, possibly associated with 40Ca or 44Ti.

Keywords: Low-mass x-ray binary stars, X-ray bursts