Sazuna/UAT_keywords huggingface dataset:
Asymmetric Structure of the Solar Wind and Heliosphere from IBEX Observations. Our heliosphere is formed by the interaction between the dynamic solar wind (SW) and partially ionized, local interstellar medium. "Pickup ions" (PUIs) are produced by charge exchange between SW ions and interstellar neutral atoms and propagate with the SW into the outer heliosphere. Neutralization of these PUIs in the heliosheath creates energetic neutral atoms (ENAs) that are observed near Earth by the Interstellar Boundary Explorer (IBEX). In this study, we analyze the evolution of ENAs observed by IBEX between ∼0.5 and 6 keV from 2009 to 2018, presenting the evolution of ENA spectral indices as a function of latitude in different regions of the sky. Our results show that IBEX ENA spectra reflect unique differences in the northern and southern polar coronal holes (PCHs) as a function of latitude and time. ENA spectra reflect a delayed closure of the southern PCH compared to the north by ∼1-2 yr in solar cycle 24, but then a rapid opening of the southern PCH beginning ∼1-2 yr later, in agreement with evolving SW speeds emanating from the PCHs. While ENA spectra from the front and flanks of the heliosphere exhibit similar behavior, ENA spectra from the back behave differently, suggesting that the heliotail and ENA source structures are significantly different. The mean ENA spectral index observed by IBEX evolves gradually and quasi-periodically over the ∼11 yr solar cycle, reflecting large-scale changes in the PCHs and SW speed. The standard deviation of the spectral index exhibits bumps at times when the SW dynamic pressure changes abruptly.

Keywords: Solar cycle, Heliosphere, Solar wind, Pickup ions, Interstellar medium, Solar activity, Heliosheath
The Effects of Tilt on the Images of Black Hole Accretion Flows. We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent difference is that the central shadow in the image is noticeably eccentric in tilted models, with the ring of emission relatively unchanged. Applying this procedure to M87 with a viewing angle based on the large-scale jet, we find that adding tilt increases the angular size of the ring for fixed black hole mass and distance, while at the same time increasing the number of bright spots in the image. Our findings illustrate observable features that can distinguish tilted from aligned flows. They also show that tilted models can be viable for M87, and that not accounting for tilt can bias inferences of physical parameters. Future modeling of horizon-scale observations should account for potential angular momentum misalignment, which is likely generic at the low accretion rates appropriate for EHT targets.

Keywords: Accretion, Supermassive black holes, Magnetohydrodynamics, Low-luminosity active galactic nuclei, General relativity, Relativistic fluid dynamics, Relativistic disks, Magnetohydrodynamical simulations
Compact-object Mergers in the Galactic Center: Evolution in Triaxial Clusters. There is significant observational evidence that a large fraction of galactic centers, including those in the Milky Way and M31, host a supermassive black hole (SMBH) embedded in a triaxial nuclear star cluster. In this work, we study the secular orbital evolution of binaries in these environments and characterize the regions and morphological properties of nuclear star clusters that lead to gravitational wave mergers and/or tidal captures. We show that even a modest level of triaxiality in the density distribution of a cluster (an ellipsoid with axis ratios of 0.7 and 0.95) dramatically enhances the merger rates in the central parsecs of the Galaxy by a factor of up to ∼10-30 relative to a spherical density distribution. Moreover, we show that the merger fraction of binaries with semimajor axes in the range 10-100 au remains above 10% for the entire central parsec of the cluster, reaching values close to unity at a distance of ∼0.2-0.4 pc from the SMBH. We understand this large merger efficiency in terms of two distinct mechanisms: (I) eccentricity oscillations driven by the dominant axisymmetric part of the cluster potential that are enhanced by the slow modulation of a binary's angular momentum from the triaxial contribution, similar to the well-known octupole-level dynamics in three-body systems; and (II) chaotic diffusion of eccentricities arising when the nodal precession timescale of a binary's orbit about the SMBH becomes comparable to its characteristic secular timescale. Overall, our results indicate that galactic centers are significantly more collisional than previously thought, with mergers taking place up to the effective radii of their nuclear star clusters.

Keywords: Supermassive black holes, Galactic center, Stellar dynamics, Gravitational wave sources, Compact objects, Low-mass x-ray binary stars, Star clusters
Energy Budget of High-speed Plasma Flows in the Terrestrial Magnetotail. Significant advances have been presented for the theoretical model and quantitative investigation of the energy input from the solar wind and its subsequent release into the ring current, the Joule heating, and particle precipitation energy flux that are closely related to high-speed flows in the plasma sheet of the terrestrial magnetotail during both isolated and storm-time substorms. Here we further determine the plasma flow vorticity/shear and heating observed in the terrestrial magnetotail by the Magnetospheric Multiscale (MMS) mission by deducing that other energy may be dissipated along with the above-mentioned three energy releases. The energy transported by the flow is also estimated here. Three-dimensional observations have shown new detailed information that can be compared with quasi-2D simulations. In this investigation, we calculate vorticity/shear directly from simultaneous observation of four MMS satellites. Our results generally indicate that plasma heating or temperature enhancements are related to both the flow vorticity/shear and current density, but more strongly with flow vorticity/shear. This research suggests that the ubiquitous energy input from the solar wind can be used to estimate plasma heating or temperature enhancements in the absence of any process or phenomenon relating to explosive energy release in planetary magnetospheres.

Keywords: Space plasmas, Plasma astrophysics
NuSTAR Detection of Quiescent Hard X-Ray Emission from SGR 0526-66 in the Large Magellanic Cloud. The soft γ-ray repeater (SGR) 0526-66 is the first-identified magnetar, and is projected within the supernova remnant N49 in the Large Magellanic Cloud. Based on our ∼50 ks NuSTAR observation, we detect the quiescent-state 0526-66 for the first time in the 10-40 keV band. Based on the joint analysis of our NuSTAR and the archival Chandra ACIS data, we firmly establish the presence of the nonthermal component in the X-ray spectrum of 0526-66 in addition to the thermal emission. In the best-fit blackbody (BB) plus power-law (PL) model, the slope of the PL component (photon index Γ = 2.1) is steeper than those (Γ ≲ 1.5) for other magnetars. The soft part of the X-ray spectrum can be described with a BB component with the temperature of kT = 0.43 keV. The best-fit radius (R = 6.5 km) of the X-ray-emitting area is smaller than the canonical size of a neutron star. If we assume an underlying cool BB component with the canonical radius of R = 10 km for the neutron star in addition to the hot BB component (2BB + PL model), a lower BB temperature of kT = 0.24 keV is obtained for the passively cooling neutron star's surface, while the hot spot emission with kT = 0.46 keV dominates the thermal spectrum (∼85% of the thermal luminosity in the 0.5-5 keV band). The nonthermal component (Γ ∼ 1.8) is still required.

Keywords: Neutron stars, Magnetars, Soft gamma-ray repeaters, X-ray astronomy
Stellar Inclination Angles from Be-star Hα Emission Line Profiles. We demonstrate that the angle between a star's rotation axis and the observer's line of sight, usually called the inclination angle, can be reliably determined for Be stars via Hα emission line profile fitting. We test our method on a sample of 11 Be stars with available long-baseline interferometric data from the Navy Precision Optical Interferometer. We fit the Hα emission line profile of each star to obtain a spectroscopic inclination angle IHα. We then obtain an independent inclination angle estimate, ${I}_{{{\rm{V}}}^{2}}$ , by fitting the observed interferometric visibilities with model visibilities based on a purely geometric representation of the light distribution on the sky. The sample differences, ${\rm{\Delta }}I\equiv {I}_{{\rm{H}}\alpha }-{I}_{{{\rm{V}}}^{2}}$ , are normally distributed with a mean of zero and a standard deviation of 6°7, and the linear correlation coefficient between IHα and ${I}_{{{\rm{V}}}^{2}}$ is r = 0.93. As Be stars comprise upwards of one-fifth of all main-sequence B-type stars, this Hα line profile fitting technique has the potential to provide an efficient method for detecting correlated stellar spin axes in young open clusters. Furthermore, if the orientation of the Be star's circumstellar disk on the plane of the sky can be constrained by polarization measurements, it is possible to determine the full 3D stellar rotation vector of each Be star.

Keywords: Early-type emission stars, Be stars, Fundamental parameters of stars, Circumstellar disks
An Isolated White Dwarf with 317 s Rotation and Magnetic Emission. We report the discovery of short-period photometric variability and modulated Zeeman-split hydrogen emission in SDSSJ125230.93-023417.72 (EPIC 228939929), a variable white dwarf star observed at long cadence in K2 Campaign 10. The behavior is associated with a magnetic (B = 5.0 MG) spot on the stellar surface, making the 317.278 s period a direct measurement of the stellar rotation rate. This object is therefore the fastest-rotating, apparently isolated (without a stellar companion) white dwarf yet discovered and the second found to exhibit chromospheric Balmer emission after GD 356, in which the emission has been attributed to a unipolar inductor mechanism driven by a possible rocky planet. We explore the properties and behavior of this object, and consider whether its evolution may hold implications for white dwarf mergers and their remnants.

Keywords: Extrasolar rocky planets, Stellar mergers, White dwarf stars, Magnetic variable stars, Stellar rotation
A New Parameter of the Photospheric Magnetic Field to Distinguish Eruptive-flare Producing Solar Active Regions. Solar flares and coronal mass ejections (CMEs) are eruptive phenomena caused by coronal magnetic fields. In particular, large eruptive events originate in active regions (AR) with strong surface magnetic fields. However, it is still unclear what determines the capability of an AR to specifically produce eruptive flares and CMEs, and this hinders our knowledge of the initiation mechanism for the eruptive component of these phenomena. In this study, we propose a new parameter rm to measure the possibility that a flare that occurs in an AR can be eruptive and produce a CME. The parameter rm is defined by the ratio of the magnetic flux of twist higher than a threshold Tc to the surrounding—and specifically, the overlying—magnetic flux. The value of rm for each AR can be estimated using nonlinear force-free field extrapolation models of the coronal magnetic field. Based on the data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we calculated the values of rm for 29 ARs at 51 times prior to flares larger than M5.0 class. We find that the footpoints of field lines with twist higher than 0.2 can represent the subsequent flare ribbons well, and field lines that overlie and "fence in" the highly twisted region will work to confine the eruption, generating confined flares. Discriminant function analysis is used to show that rm is moderately well able to distinguish ARs that have the capability of producing eruptive flares.

Keywords: Solar flares, Solar physics, Solar active regions, Solar coronal mass ejections, Solar magnetic fields
AGNs Are Not That Cool: Revisiting the Intrinsic AGN Far-infrared Spectral Energy Distribution. We investigate the intrinsic spectral energy distribution (SED) of active galactic nuclei (AGNs) at infrared (IR) bands with 42 z &lt; 0.5 optically luminous Palomar Green survey quasars through SED decomposition. We decompose the SEDs of the 42 quasars by combining an AGN IR template library that covers a wide range of the AGN parameter space with three commonly used galaxy template libraries. We determine the median AGN SED from the best-fitting results. The far-IR (FIR) contribution of our median AGN SED is significantly smaller than that of Symeonidis et al., but roughly consistent with that of Lyu &amp; Rieke. The AGN IR SED becomes cooler with increasing bolometric luminosity, which might be due to the fact that more luminous AGNs might have stronger radiative feedback to change torus structures and/or their tori might have higher metallicities. Our conclusions do not depend on the choice of galaxy template libraries. However, as the predicted polycyclic aromatic hydrocarbon (PAH) emission line flux is galaxy template dependent, cautions should be taken on deriving galaxy FIR contribution from PAH fluxes.

Keywords: Supermassive black holes, Spectral energy distribution, Active galactic nuclei, Infrared galaxies
Spectral Modeling of Charge Exchange in the Central Region of M51. Charge exchange (CX) emission reveals the significant interaction between neutral and ionized interstellar medium components of the dense, multiphase, circumnuclear region of a galaxy. We use a model including a thermal and CX components to describe the high-resolution XMM-Newton/RGS spectrum of the diffuse emission in the central region of M51. Representative signatures of CX emission—especially the prominent O VII forbidden line and the excess emission in the O VIII Lyγ lines—can be explained by the model. Combined with Chandra images in the O VIII and the O VII bands, we find soft X-ray emission is dominated by the jet-driven outflow and its interaction with the ambient neutral material. The jet-driven outflow itself is likely a thermal plasma of ∼0.59 keV, with mostly sub-solar abundances. It runs into the ambient neutral gas, and produces significant CX emission that accounts for one-fifth of the diffuse X-ray emission in the 7-28 Å band. The effective interface area in the CX process is one order of magnitude greater than the geometrical surface area of the jet-driven outflow. The tenuous outflow driven by the nuclear star formation may also contribute a small portion to both the diffuse thermal and CX emission. The photoionization by the active galactic nuclei (AGNs) and the resonance scattering by the hot gas itself are disfavored, though the effects from past AGN events may not be ruled out.

Keywords: Interstellar medium, X-ray sources, Galaxies, Plasma jets
Determining the Physical Conditions of an Extremely Young Class 0 Circumbinary Disk around VLA 1623A. We present a detailed analysis of high-resolution C18O (2-1), SO (88-77), CO (3-2), and DCO+ (3-2) data obtained by the Atacama Large Millimeter/submillimeter Array toward a class 0 Keplerian circumbinary disk around VLA 1623A, which represents one of the most complete analyses toward a class 0 source. From the dendrogram analysis, we identified several accretion flows feeding the circumbinary disk in a highly anisotropic manner. Stream-like SO emission around the circumbinary disk reveals the complicated shocks caused by the interactions between the disk, accretion flows, and outflows. A wall-like structure is discovered south of VLA 1623B. The discovery of two outflow cavity walls at the same position traveling at different velocities suggests that the two outflows from both VLA 1623A and VLA 1623B are on top of each other in the plane of the sky. Our detailed flat and flared disk modeling shows that Cycle 2 C18O J = 2-1 data are inconsistent with the combined binary mass of 0.2 M☉, as suggested by early Cycle 0 studies. The combined binary mass for VLA 1623A should be modified to 0.3-0.5 M☉.

Keywords: Accretion, Observational astronomy, Protostars, Young stellar objects, Star formation, Stellar accretion, Circumstellar disks, Stellar accretion disks, Shocks, Radio interferometry, Radio astronomy
Properties of AGN Multiband Optical Variability in the HSC SSP Transient Survey. We study variability of active galactic nuclei (AGNs) by using the deep optical multiband photometry data obtained from the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) survey in the COSMOS field. The images analyzed here were taken with 8, 10, 13, and 15 epochs over three years in the g, r, I, and z bands, respectively. We identified 491 robust variable AGN candidates, down to I = 25 mag and with redshift up to 4.26. Ninety percent of the variability-selected AGNs are individually identified with the X-ray sources detected in the Chandra COSMOS Legacy survey. We investigate their properties in variability by using structure function analysis and find that the structure function for low-luminosity AGNs (Lbol ≲ 1045 erg s-1) shows a positive correlation with luminosity, which is the opposite trend for the luminous quasars. This trend is likely to be caused by a larger contribution of the host galaxy light for lower-luminosity AGNs. Using the model templates of galaxy spectra, we evaluate the amount of host galaxy contribution to the structure function analysis and find that dominance of the young stellar population is needed to explain the observed luminosity dependence. This suggests that low-luminosity AGNs at 0.8 ≲ z ≲ 1.8 are predominantly hosted in star-forming galaxies. The X-ray stacking analysis reveals the significant emission from the individually X-ray undetected AGNs in our variability-selected sample. The stacked samples show very large hardness ratios in their stacked X-ray spectrum, which suggests that these optically variable sources have large soft X-ray absorption by dust-free gas.

Keywords: AGN host galaxies, Supermassive black holes, Optical observation, Active galaxies, Low-luminosity active galactic nuclei, Active galactic nuclei
Effects of Radial Distances on Small-scale Magnetic Flux Ropes in the Solar Wind. Small-scale magnetic flux ropes (SFRs) in the solar wind have been studied for decades. Statistical analysis utilizing various in situ spacecraft measurements is the main observational approach to investigating the generation and evolution of these small-scale structures. Based on the Grad-Shafranov reconstruction technique, we use the automated detection algorithm to build the databases of these small-scale structures via various spacecraft measurements at different heliocentric distances. We present the SFR properties, including the magnetic field and plasma parameters at different radial distances from the Sun near the ecliptic plane. It is found that the event occurrence rate is still of the order of a few hundreds per month, the duration and scale-size distributions follow power laws, and the flux-rope axis orientations are approximately centered around the local Parker spiral directions. In general, most SFR properties exhibit radial decays. In addition, with various databases established, we derive scaling laws for the changes in average field magnitude, event counts, and SFR scale sizes, with respect to the radial distances, ranging from ∼0.3 au for Helios to ∼7 au for the Voyager spacecraft. The implications of our results for comparisons with the relevant theoretical works and for applications to the Parker Solar Probe mission are discussed.

Keywords: Magnetohydrodynamics, Astronomy databases, Interplanetary turbulence, Solar wind, Astronomy data analysis
Detailed Abundances in the Galactic Center: Evidence of a Metal-rich Alpha-enhanced Stellar Population. We present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the nuclear star cluster. As a control sample we have also observed 7 M giants in the Milky Way disk with similar stellar parameters. All 27 stars are observed using the NIRSPEC spectrograph on the KECK II telescope in the K-band at a resolving power of R = 23,000. We report the first silicon abundance trends versus [Fe/H] for stars in the Galactic center. While finding a disk/bulge-like trend at subsolar metallicities, we find that [Si/Fe] is enhanced at supersolar metallicities. We speculate on possible enrichment scenarios to explain such a trend. However, the sample size is modest and the result needs to be confirmed by additional measurements of silicon and other α-elements. We also derive a new distribution of [Fe/H] and find the most metal-rich stars at [Fe/H] = +0.5 dex, confirming our earlier conclusions that the Galactic center hosts no stars with extreme chemical compositions.

Keywords: Galactic center, Late-type stars, Galaxy chemical evolution, Chemical abundances, Chemical enrichment, Star clusters
Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey. Rapidly evolving transients form a new class of transients that show shorter timescales of light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using deep data taken with the Hyper Suprime-Cam Subaru Strategic Program Transient Survey. By measuring the timescales of the light curves of 1824 transients, we identified five rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 ≤ z ≤ 1.5) and peak absolute magnitudes (-17 ≥ MI ≥ -20). The light-curve properties are similar to those of the previously discovered rapidly evolving transients. They show a relatively blue spectral energy distribution, with the best-fit blackbody of 8000-18,000 K. We show that some of the transients require power sources other than the radioactive decays of 56Ni because of their high peak luminosities and short timescales. The host galaxies of all of the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be ∼4000 events yr-1 Gpc-3, which is about 1% of core-collapse supernovae.

Keywords: Supernovae