adsabs/SciX_UAT_keywords huggingface dataset:
X-Ray Spectra and Multiwavelength Machine Learning Classification for Likely Counterparts to Fermi 3FGL Unassociated Sources

We conduct X-ray spectral fits on 184 likely counterparts to Fermi-LAT 3FGL unassociated sources. Characterization and classification of these sources allows for more complete population studies of the high-energy sky. Most of these X-ray spectra are well fit by an absorbed power-law model, as expected for a population dominated by blazars and pulsars. A small subset of seven X-ray sources have spectra unlike the power law expected from a blazar or pulsar and may be linked to coincident stars or background emission. We develop a multiwavelength machine learning classifier to categorize unassociated sources into pulsars and blazars using gamma-ray and X-ray observations. Training a random forest (RF) procedure with known pulsars and blazars, we achieve a cross-validated classification accuracy of 98.6%. Applying the RF routine to the unassociated sources returned 126 likely blazar candidates (defined as Pbzr ≥ 90%) and five likely pulsar candidates (Pbzr ≤ 10%). Our new X-ray spectral analysis does not drastically alter the RF classifications of these sources compared to previous works, but it builds a more robust classification scheme and highlights the importance of X-ray spectral fitting. Our procedure can be further expanded with UV, visual, or radio spectral parameters or by measuring flux variability.

Keywords: gamma-ray sources; x-ray sources; active galactic nuclei; pulsars
Infrared Spectra and Vapor Pressures of Crystalline C2N2, with Comparisons to Crystalline HCN

In a continuation of our work on nitriles, we have examined cyanogen (C2N2) as a crystalline solid, comparing the results to our recent work on crystalline hydrogen cyanide (HCN). A density and refractive index for C2N2 ice were measured and used to prepare solid samples from which infrared (IR) spectra, band strengths, and optical constants were measured. The vapor pressures (sublimation pressures) of both C2N2 and HCN ices were determined with a quartz-crystal microbalance at temperatures relevant to Titan's atmosphere and much lower than those in the literature. Comparisons of the newly measured low-temperature vapor pressures to values extrapolated from higher temperatures revealed differences on the order of 45 and 130%. The enthalpies of sublimation of C2N2 and HCN in the 120 K region were measured and found to be similar to those at higher temperatures. Comparisons were made to previous spectroscopic work, but hindered in the case of IR intensities by the lack of published details.

Keywords: titan; astrochemistry; experimental techniques
Multiwavelength Emission from Magnetically Arrested Disks around Isolated Black Holes

We discuss the prospects for identifying the nearest isolated black holes (IBHs) in our Galaxy. IBHs accreting gas from the interstellar medium likely form magnetically arrested disks (MADs). We show that thermal electrons in the MADs emit optical signals through the thermal synchrotron process while nonthermal electrons accelerated via magnetic reconnections emit a flat-spectrum synchrotron radiation in the X-ray to MeV gamma-ray ranges. The Gaia catalog will include at most a thousand IBHs within ≲1 kpc that are distributed on and around the cooling sequence of white dwarfs (WDs) in the Hertzsprung-Russell diagram. These IBH candidates should also be detected by eROSITA, with which they can be distinguished from isolated WDs and neutron stars. Follow-up observations with hard X-ray and MeV gamma-ray satellites will be useful to unambiguously identify IBHs.

Keywords: stellar mass black holes; compact radiation sources; non-thermal radiation sources; accretion; plasma astrophysics
Whistler Waves as a Signature of Converging Magnetic Holes in Space Plasmas

Magnetic holes are plasma structures that trap a large number of particles in a magnetic field that is weaker than the field in its surroundings. The unprecedented high time-resolution observations by NASA's Magnetospheric Multiscale Mission enable us to study the particle dynamics in magnetic holes in the Earth's magnetosheath in great detail. We reveal the local generation mechanism of whistler waves by a combination of Landau-resonant and cyclotron-resonant wave-particle interactions of electrons in response to the large-scale evolution of a magnetic hole. As the magnetic hole converges, a pair of counter-streaming electron beams form near the hole's center as a consequence of the combined action of betatron and Fermi effects. The beams trigger the generation of slightly oblique whistler waves. Our conceptual prediction is supported by a remarkable agreement between our observations and numerical predictions from the Arbitrary Linear Plasma Solver. Our study shows that wave-particle interactions are fundamental to the evolution of magnetic holes in space and astrophysical plasmas.

Keywords: space plasmas; planetary magnetospheres; solar wind; interplanetary turbulence
The ELM Survey. VIII. Ninety-eight Double White Dwarf Binaries

We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremely Low Mass (ELM) Survey, a spectroscopic survey targeting <0.3 M⊙ He-core WDs completed in the Sloan Digital Sky Survey footprint. Over the course of the survey we observed ancillary low-mass WD candidates like GD 278, which we show is a P = 0.19 day double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too high to be WDs given their Gaia parallaxes. Here, we define a clean sample of ELM WDs that is complete within our target selection and magnitude range 15 < g0 < 20 mag. The measurements are consistent with 100% of ELM WDs being 0.0089 < P < 1.5 day double WD binaries, 35% of which belong to the Galactic halo. We infer that these are mostly He+CO WD binaries given the measurement constraints. The merger rate of the observed He+CO WD binaries exceeds the formation rate of stable mass-transfer AM CVn binaries by a factor of 25, and so the majority of He+CO WD binaries must experience unstable mass transfer and merge. The systems with the shortest periods, such as J0651+2844, are signature LISA verification binaries that can be studied with gravitational waves and light.

Keywords: white dwarf stars; compact binary stars; galaxy stellar content
An Ultra-deep Multiband Very Large Array (VLA) Survey of the Faint Radio Sky (COSMOS-XS): New Constraints on the Cosmic Star Formation History

We make use of ultra-deep 3 GHz Karl G. Jansky Very Large Array (VLA) observations of the COSMOS field from the multiband COSMOS-XS survey to infer radio luminosity functions (LFs) of star-forming galaxies (SFGs). Using ~1300 SFGs with redshifts out to z ~ 4.6, and fixing the faint and bright end shape of the radio LF to the local values, we find a strong redshift trend that can be fitted by pure luminosity evolution with the luminosity parameter given by α L ∝ (3.40 ± 0.11) - (0.48 ± 0.06)z. We then combine the ultra-deep COSMOS-XS data set with the shallower VLA-COSMOS 3 GHz large project data set over the wider COSMOS field in order to fit for joint density+luminosity evolution, finding evidence for significant density evolution. By comparing the radio LFs to the observed far-infrared and ultraviolet (UV) LFs, we find evidence of a significant underestimation of the UV LF by 22% ± 14% at high redshift (3.3 < z < 4.6, integrated down to $0.03\,{L}_{z=3}^{\star }$ ). We derive the cosmic star formation rate density (SFRD) by integrating the fitted radio LFs and find that the SFRD rises up to z ~ 1.8 and then declines more rapidly than previous radio-based estimates. A direct comparison between the radio SFRD and a recent UV-based SFRD, where we integrate both LFs down to a consistent limit ( $0.038\,{L}_{z=3}^{\star }$ ), reveals that the discrepancy between the radio and UV LFs translates to a significant (~1 dex) discrepancy in the derived SFRD at z > 3, even assuming the latest dust corrections and without accounting for optically dark sources.

Keywords: star formation; radio continuum emission; galaxies
Analysis of Previously Classified White Dwarf-Main-sequence Binaries Using Data from the APOGEE Survey

We present analyses of near-infrared spectroscopic data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey for 45 previously confirmed or candidate white dwarf-main-sequence (WDMS) binaries identified by the optical Sloan Digital Sky Survey (SDSS) and LAMOST surveys. Among these 45 systems, we classify three as having red giant primaries in the LAMOST sample and 14 as young stellar object contaminants in the photometrically identified SDSS sample. From among the subsample of 28 systems that we confirm to have MS primaries, we derive and place limits on orbital periods and velocity amplitudes for 14. Seven systems have significant velocity variations that warrant a post-common-envelope (PCE) binary classification, four of which are newly classified, three of which are newly confirmed, and five for which we can derive full orbital parameters. If confirmed, one of these newly discovered systems (2M14544500+4626456) will have the second-longest orbital period reported for a typical compact PCE WDMS binary (P = 15.1 days). In addition to the seven above, we also recover and characterize with APOGEE data the well-known PCE WDMS systems EG UMa and HZ 9. We also investigate the overall metallicity distribution of the WDMS sample, which is a parameter space not often explored for these systems. Of note, we find one system (2M14244053+4929580) to be extremely metal-poor ([Fe/H] = - 1.42) relative to the rest of the near-solar sample. Additionally, the PCE systems in our sample are found to be, on average, higher in metallicity than their wide-binary counterparts, though we caution that with this small number of systems, the sample may not be representative of the overall distribution of WDMS systems.

Keywords: close binary stars; spectroscopic binary stars; white dwarf stars; low mass stars
Relativistic Effects on Circumbinary Disk Evolution: Breaking the Polar Alignment around Eccentric Black Hole Binary Systems

We study the effects of general relativity (GR) on the evolution and alignment of circumbinary disks around binaries on all scales. We implement relativistic apsidal precession of the binary into the hydrodynamics code PHANTOM. We find that the effects of GR can suppress the stable polar alignment of a circumbinary disk, depending on how the relativistic binary apsidal precession timescale compares to the disk nodal precession timescale. Studies of circumbinary disk evolution typically ignore the effects of GR, which is an appropriate simplification for low-mass or widely separated binary systems. In this case, polar alignment occurs, provided that the disks initial misalignment is sufficiently large. However, systems with a very short relativistic precession timescale cannot polar align and instead move toward coplanar alignment. In the intermediate regime where the timescales are similar, the outcome depends upon the properties of the disk. Polar alignment is more likely in the wavelike disk regime (where the disk viscosity parameter is less than the aspect ratio, α < H/r), since the disk is in good radial communication. In the viscous disk regime, disk breaking is more likely. Multiple rings can destructively interact with one another, resulting in short disk lifetimes and the disk moving toward coplanar alignment. Around main-sequence star or stellar mass black hole binaries, polar alignment may be suppressed far from the binary, but in general, the inner parts of the disk can align to polar. Polar alignment may be completely suppressed for disks around supermassive black holes for close binary separations.

Keywords: binary stars; compact binary stars; accretion; stellar accretion disks; hydrodynamics; black hole physics; relativistic binary stars; relativistic disks; relativity
Investigating the Globally Collapsing Hub-Filament Cloud G326.611+0.811

We present a dynamics study toward the G326.611+0.811 (G326) hub-filament system (HFS) cloud using new APEX observations of both 13CO and C18O (J = 2-1). The G326 HFS cloud constitutes a central hub and at least four hub-composing filaments that are divided into a major branch of filaments (F1 and F2) and a side branch (F3-F5). The cloud holds ongoing high-mass star formation as characterized by three massive dense clumps (i.e., 370-1100 M ⊙ and 0.14-0.16 g cm-2 for C1-C3) with high clump-averaged mass infalling rates (>10-3 M ⊙ yr-1) within the major filament branch, and the associated point sources bright at 70 μm, typical of young protostars. Along the five filaments, velocity gradients are found in both 13CO and C18O (J = 2-1) emission, suggesting that filament-aligned gravitational collapse toward the central hub (i.e., C2) is responsible for the high-mass star formation therein. Moreover, a periodic velocity oscillation along the major filament branch is revealed in both 13CO and C18O (J = 2-1) emission with a characteristic wavelength of ~3.5 pc and an amplitude of ~0.31-0.38 km s-1. We suggest that this pattern of velocity oscillation in G326 could arise from clump-forming gas motion induced by gravitational instabilities. The prevalent velocity gradients, fragmentation of the major branch of filaments, and the ongoing collapse of the three massive dense clumps are indicative that G326 is an HFS undergoing global collapse.

Keywords: infrared dark clouds; star forming regions; interstellar filaments
How Binaries Accrete: Hydrodynamic Simulations with Passive Tracer Particles

Linear analysis of gas flows around orbiting binaries suggests that a centrifugal barrier ought to clear a low-density cavity around the binary and inhibit mass transfer onto it. Modern hydrodynamics simulations have confirmed the low-density cavity, but show that any mass flowing from large scales into the circumbinary disk is eventually transferred onto the binary components. Even though many numerical studies confirm this picture, it is still not understood precisely how gas parcels overcome the centrifugal barrier and ultimately accrete. We present a detailed analysis of the binary accretion process, using an accurate prescription for evolving grid-based hydrodynamics with Lagrangian tracer particles that track the trajectories of individual gas parcels. We find that binary accretion can be described in four phases: (1) gas is viscously transported through the circumbinary disk up to the centrifugal barrier at the cavity wall, (2) the cavity wall is tidally distorted into accretion streams consisting of near-ballistic gas parcels on eccentric orbits, (3) the portion of each stream moving inwards of an accretion horizon radius $\bar{r}\simeq a$ -the radius beyond which no material is returned to the cavity wall-becomes bound to a minidisk orbiting an individual binary component, and (4) the minidisk gas accretes onto the binary component through the combined effect of viscous and tidal stresses.

Keywords: accretion; hydrodynamical simulations; binary stars; black holes
The Relationships between Emission Geometry and Jitter Noise in Millisecond Pulsars

The relationships between several parameters of an emission geometry and jitter noise in 17 millisecond pulsars (MSPs) are investigated. By assuming the jitter noise is due only to a pulse variation in phase, the former can be modeled as changes in the plasma flow rate leading to variation in the measured pulse arrival time relative to the predicted time. In the model for pulsar magnetospheres with multiple emission states, the plasma flow is associated with the emission states, and a change in the emission state corresponds to a change in the plasma flow causing variation in the pulse arrival time. These can be specified in an emission geometry defined by the obliquity and viewing angles, measured from the rotation axis to the magnetic axis and to the line of sight, respectively. We calculate the maximum change in the emission state based on the reported jitter noise for each of the MSPs. Using the results, we show that the MSPs possess relatively large obliquity angles, which is consistent with observation, and the jitter noise exhibits dependency on frequency. We find that the jitter noise in our sample displays an exponential decay as a function that combines the obliquity angle and the rotation period, revealing the correlation among the three parameters. This suggests that the magnitude of the jitter noise is likely specific to an MSP. We discuss how jitter noise may be related to the evolution of an MSP.

Keywords: millisecond pulsars
Mapping of Ice Storage Processes on the Moon with Time-dependent Temperatures

Lunar cold traps are defined by extremely low time-integrated sublimation loss, so that water ice is expected to accumulate in them. Due to the strong dependence of the sublimation rate on temperature, they have heretofore been delineated by the peak rather than the time-averaged sublimation rate. Here, time-averaged sublimation rates are calculated for the south polar region of the Moon based on 11 yr of Diviner surface temperature measurements. Data for each spatial pixel are binned according to subsolar (diurnal) and ecliptic (seasonal) longitude. Frequency-domain filtering of temperature time series is applied to interpolate and smooth the data set. The cold trap area is 17,000 km2 from 80°S to the south pole, about 24% larger than defined by peak temperature. In addition, subsurface ice stability is mapped based on solutions of the heat equation with Diviner surface temperatures as the upper boundary condition. Even a thin layer of dust reduces the sublimation loss dramatically. A third potential mechanism for ice storage, vapor pumping by temperature cycles, is also mapped, based on a model for the time-variable population of adsorbed water molecules on the lunar surface. The area of significant pumping is estimated to be 96,000 km2 poleward of 80°S. In summary, a new processing technique has been developed that exploits information about the time variability of lunar surface temperatures measured by Diviner, and it results in new maps for the geographic distribution of potential water-ice deposits in the south polar region of the Moon.

Keywords: the moon; lunar surface; surface ices
EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-poor Galaxies: Uncovering the Origins of Strong He II and the Impact on Cosmic Reionization

Strong high-ionization lines such as He II of young galaxies are puzzling at high and low redshift. Although recent studies suggest the existence of nonthermal sources, whether their ionizing spectra can consistently explain multiple major emission lines remains a question. Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong He II λ4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and nonthermal sources. We use photoionization models for nebulae and determine seven parameters of the ionizing spectra and nebulae by Markov Chain Monte Carlo methods, carefully avoiding systematics of abundance ratios. We obtain the general shapes of ionizing spectra explaining ~10 major emission lines within observational errors with smooth connections from observed X-ray and optical continua. We find that an ionizing spectrum of one EMPG has a blackbody-dominated shape, while the others have convex downward shapes at >13.6 eV, which indicate a diversity of the ionizing spectrum shapes. We confirm that the convex downward shapes are fundamentally different from ordinary stellar spectrum shapes, and that the spectrum shapes of these galaxies are generally explained by the combination of the stellar and ultraluminous X-ray sources. Comparisons with stellar synthesis models suggest that the diversity of the spectrum shapes arises from differences in the stellar age. If galaxies at z ≳ 6 are similar to the EMPGs, high-energy (>54.4 eV) photons of the nonstellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.

Keywords: galaxy formation; dwarf galaxies; ultraluminous x-ray sources
Optimal Differential Astrometry for Multiconjugate Adaptive Optics. I. Astrometric Distortion Mapping using On-sky GeMS Observations of NGC 6723

The Extremely Large Telescope and the Thirty Meter Telescope will use state of the art multiconjugate adaptive optics (MCAO) systems to obtain the full D 4 advantage that their apertures can provide. However, to reach the full astrometric potential of these facilities for on-sky science requires understanding any residual astrometric distortions from these systems and find ways to measure and eliminate them. In this work, we use Gemini multiconjugate adaptive optic system (GeMS) observations of the core of NGC 6723 to better understand the on-sky astrometric performance of MCAO. We develop new methods to measure the astrometric distortion fields of the observing system, which probe the distortion at the highest possible spatial resolution. We also describe methods for examining the time-variable and static components of the astrometric distortion. When applied to the GeMS Gemini South Adaptive Optics Imager (GSAOI) data, we are able to see the effect of the field rotator at the subpixel level, and we are able to empirically derive the distortion due to the optical design of GeMS/GSAOI. We argue that the resulting distortion maps are a valuable tool to measure and monitor the on-sky astrometric performance of future instrumentation. Our overall astrometry pipeline produces high-quality proper motions with an uncertainty floor of ~45 μas yr-1. We measure the proper motion dispersion profile of NGC 6723 from a radius of ~10″ out to ~1' based on ~12,000 stars. We also produce a high-quality optical-near infrared color-magnitude diagram, which clearly shows the extreme horizontal branch and main-sequence knee of this cluster.

Keywords: globular star clusters; astrometry; astronomical instrumentation; proper motions; ground telescopes; astronomical seeing; astronomical optics
OSSOS. XXVII. Population Estimates for Theoretically Stable Centaurs between Uranus and Neptune

We calculate the upper bounds of the population of theoretically stable Centaur orbits between Uranus and Neptune. These small bodies are on low-eccentricity, low-inclination orbits in two specific bands of semimajor axis, centered at ~24.6 and ~25.6 au. They exhibit unusually long Gyr-stable lifetimes in previously published numerical integrations, orders of magnitude longer than that of a typical Centaur. Despite the increased breadth and depth of recent Solar System surveys, no such objects have been found. Using the Outer Solar System Origins Survey simulator to calculate the detection efficiency for these objects in an ensemble of fully characterized surveys, we determine that a population of 72 stable Centaurs with absolute magnitude H r ≤ 10 (95% confidence upper limit) could remain undetected. The upcoming Legacy Survey of Space and Time will be able to detect this entire intrinsic population, due to its complete coverage of the ecliptic plane. If detected, these objects will be interesting dynamically accessible mission targets-especially as comparison of the stable Centaur orbital phase space to the outcomes of several modern planetary migration simulations suggests that these objects could be close to primordial in nature.

Keywords: centaur group; astrostatistics; surveys; orbits; trans-neptunian objects; planetary migration; small solar system bodies