Sazuna/UAT_keywords huggingface dataset:
Nonlinear Simulation of Ionospheric Irregularities at Mars. Motivated by the observations and linear theory analysis of ionospheric irregularities at Mars, we performed numerical simulations of the nonlinear evolution of the electromagnetic gradient drift instability in the Martian ionospheric dynamo region. The seeding source of ionospheric irregularities is perturbation zonal neutral wind. We found that the perturbation electric fields induced by the gradient drift instability can convect lower density plasma into higher density plasma at higher altitudes. Then, the associated perturbation magnetic field and electric field can cause the velocity shear of the plasma, which induced the Kelvin-Helmholtz instability at higher altitude. The Kelvin-Helmholtz instabilities furthermore lead to smaller-scale irregularities in plasma density, magnetic field, and electric field in the Martial ionosphere. Key points: (1) Nonlinear simulation of small-scale ionospheric irregularities at Mars was present. (2) Ionospheric irregularities at Mars can be seeded by the perturbation neutral winds. (3) Model results are comparable to linear theory analysis and satellite observations.

Keywords: Mars, Plasma physics
Correction to the Photometric Colors of the Gaia Data Release 2 with the Stellar Color Regression Method. The second Gaia data release (DR2) delivers accurate and homogeneous photometry data of the whole sky of an exquisite quality, reaching down to the unprecedented millimagnitude (mmag) level for the G, GRP, and GBP passbands. However, the presence of magnitude-dependent systematic effects at the 10 mmag level limits its power in scientific exploitation. In this work, using about a half million stars in common with the LAMOST DR5, we apply the spectroscopy-based stellar color regression method to calibrate the Gaia G - GRP and GBP - GRP colors. With an unprecedented precision of about 1 mmag, systematic trends with G magnitude are revealed for both colors in great detail, reflecting changes in instrument configurations. Color-dependent trends are found for the GBP - GRP color and for stars brighter than G ∼ 11.5 mag. The maximum correction term of the calibration is about 20 mmag in general and varies by a few mmag/mag. A revised color-color diagram of Gaia DR2 is given, and some applications are briefly discussed.

Keywords: Stellar photometry, Astronomy data analysis, Fundamental parameters of stars
Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods. We report on the temporal properties of the ultraluminous X-ray (ULX) pulsar M51 ULX-7 inferred from the analysis of the 2018-2020 Swift/X-ray Telescope monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the neutron star is rotating near equilibrium. Alternatively, this off-state could be related to a variable superorbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge-on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars. These characteristics are similar to those seen in prototypical X-ray pulsars such as Her X-1 and SMC X-1 or other ULX pulsars such as NGC 5907 ULX1.

Keywords: Neutron stars, High mass x-ray binary stars, Pulsars
Antisymmetric Cross-correlation between H I and CO Line Intensity Maps as a New Probe of Cosmic Reionization. Intensity mapping of the H I 21 cm line and the CO 2.61 mm line from the epoch of reionization has emerged as powerful, complementary, probes of the high-redshift universe. However, both maps and their cross-correlation are dominated by foregrounds. We propose a new analysis by which the signal is unbiased by foregrounds, i.e., it can be measured without foreground mitigation. We construct the antisymmetric part of two-point cross-correlation between intensity maps of the H I 21 cm line and the CO 2.61 mm line, arising because the statistical fluctuations of two fields have different evolution in time. We show that the sign of this new signal can distinguish model independently whether inside-out reionization happens during some interval of time. More importantly, within the framework of the excursion-set model of reionization, we demonstrate that the slope of the dipole of H I-CO cross-power spectrum at large scales is linear to the rate of change of global neutral fraction of hydrogen in a manner independent of reionization parameters, until the slope levels out near the end of reionization, but this trend might possibly depend on the framework of reionization modeling. The H I-CO dipole may be a smoking-gun probe for the speed of reionization, or "standard speedometer" for cosmic reionization. Observations of this new signal will unveil the global reionization history from the midpoint to near the completion of reionization.

Keywords: Large-scale structure of the universe, Astrostatistics techniques, H I line emission, CO line emission, Reionization, Line intensities, Two-point correlation function
Gamma-Ray Absorption by the Cosmic Lyman Continuum from Star-forming Galaxies. Motivated by the discovery of the ultra-strong emission-line starburst galaxies (EELGs) known as "green pea galaxies," in this work we consider their contribution to the intergalactic flux of ionizing UV at high redshifts. Most galaxies that have been observed show a precipitous drop in the flux blueward of their Lyman limit. However, recent observations of EELGs have discovered that many more Lyman continuum photons escape from them into intergalactic space than previously suspected. We calculate their contribution to the extragalactic background light. We also calculate the effect of these photons on the absorption of high-energy γ-rays. For the more distant γ-ray sources, particularly at z ≥ 3, an intergalactic opacity above a few GeV is significantly higher than previous estimates which ignored the Lyman continuum photons. We calculate the results of this increased opacity on observed γ-ray spectra, which produce a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.

Keywords: Gamma-ray sources, Compact dwarf galaxies, Emission line galaxies, Cool intergalactic medium, Primordial galaxies, Galaxy evolution, Intergalactic medium, Lyman-alpha galaxies, Gamma-ray observatories, High-redshift galaxies
Spectroscopic and Photometric Redshift Estimation by Neural Networks for the China Space Station Optical Survey (CSS-OS). The estimation of spectroscopic and photometric redshifts (spec-z and photo-z) is crucial for future cosmological surveys. It can directly affect several powerful measurements of the universe, such as weak lensing and galaxy clustering. In this work, we explore the accuracies of spec-z and photo-z that can be obtained by the China Space Station Optical Surveys, which is a next-generation space survey, using a neural network. The one-dimensional Convolutional Neural Networks and Multi-Layer Perceptron (MLP, the simplest form of an artificial neural network) are employed to derive spec-z and photo-z, respectively. The mock spectral and photometric data used for training and testing the networks are generated based on the COSMOS catalog. The networks have been trained with noisy data by creating Gaussian random realizations to reduce the statistical effects, resulting in a similar redshift accuracy for data with both high and low signal-to-noise ratios. The probability distribution functions of the predicted redshifts are also derived via Gaussian random realizations of the testing data, and then the best-fit redshifts and 1σ errors also can be obtained. We find that our networks can provide excellent redshift estimates with accuracies of ∼0.001 and 0.01 on spec-z and photo-z, respectively. Compared to existing photo-z codes, our MLP has a similar accuracy but is more efficient in the training process. The fractions of catastrophic redshifts or outliers can be dramatically suppressed compared to the ordinary template-fitting method. This indicates that the neural network method is feasible and powerful for spec-z and photo-z estimations in future cosmological surveys.

Keywords: Cosmology, Large-scale structure of the universe
A Model for the Coupled Eruption of a Pseudostreamer and Helmet Streamer. A highly important aspect of solar activity is the coupling between eruptions and the surrounding coronal magnetic field topology, which determines the trajectory and morphology of the event and can even lead to sympathetic eruptions from multiple sources. In this paper, we report on a numerical simulation of a new type of coupled eruption, in which a coronal jet initiated by a large pseudostreamer filament eruption triggers a streamer-blowout coronal mass ejection (CME) from the neighboring helmet streamer. Our configuration has a large opposite-polarity region positioned between the polar coronal hole and a small equatorial coronal hole, forming a pseudostreamer flanked by the coronal holes and the helmet streamer. Further out, the pseudostreamer stalk takes the shape of an extended arc in the heliosphere. We energize the system by applying photospheric shear along a section of the polarity inversion line within the pseudostreamer. The resulting sheared-arcade filament channel develops a flux rope that eventually erupts as a classic coronal-hole-type jet. However, the enhanced breakout reconnection above the channel as the jet is launched progresses into the neighboring helmet streamer, partially launching the jet along closed helmet streamer field lines and blowing out the streamer top to produce a classic bubble-like CME. This CME is strongly deflected from the jet's initial trajectory and contains a mixture of open and closed magnetic field lines. We present the detailed dynamics of this new type of coupled eruption, its underlying mechanisms, and the implications of this work for the interpretation of in situ and remote-sensing observations.

Keywords: Solar flares, Solar magnetic reconnection, Solar coronal mass ejections, Solar filament eruptions, Active solar corona, Solar coronal streamers
Observing Carbon and Oxygen Carriers in Protoplanetary Disks at Mid-infrared Wavelengths. Infrared observations probe the warm gas in the inner regions of planet-forming disks around young Sun-like T Tauri stars. In these systems, H2O, OH, CO, CO2, C2H2, and HCN have been widely observed. However, the potentially abundant carbon carrier CH4 remains largely unconstrained. The James Webb Space Telescope (JWST) will be able to characterize mid-infrared fluxes of CH4 along with several other carriers of carbon and oxygen. In anticipation of the JWST mission, we model the physical and chemical structure of a T Tauri disk to predict the abundances and mid-infrared fluxes of observable molecules. A range of compositional scenarios are explored involving the destruction of refractory carbon materials and alterations to the total elemental (volatile and refractory) C/O ratio. Photon-driven chemistry in the inner disk surface layers largely destroys the initial carbon and oxygen carriers. This causes models with the same physical structure and C/O ratio to have similar steady-state surface compositions, regardless of the initial chemical abundances. Initial disk compositions are better preserved in the shielded inner disk midplane. The degree of similarity between the surface and midplane compositions in the inner disk will depend on the characteristics of vertical mixing at these radii. Our modeled fluxes of observable molecules respond sensitively to changes in the disk gas temperature, inner radius, and total elemental C/O ratio. As a result, mid-infrared observations of disks will be useful probes of these fundamental disk parameters, including the C/O ratio, which can be compared to values determined for planetary atmospheres.

Keywords: Protoplanetary disks, Astrochemistry
CO Excitation, Molecular Gas Density, and Interstellar Radiation Field in Local and High-redshift Galaxies. We study the carbon monoxide (CO) excitation, mean molecular gas density, and interstellar radiation field (ISRF) intensity in a comprehensive sample of 76 galaxies from local to high redshift (z ∼ 0-6), selected based on detections of their CO transitions J = 2 → 1 and 5 → 4 and their optical/infrared/(sub)millimeter spectral energy distributions (SEDs). We confirm the existence of a tight correlation between CO excitation as traced by the CO (5-4)/(2-1) line ratio R52 and the mean ISRF intensity $\left\langle U\right\rangle $ as derived from infrared SED fitting using dust SED templates. By modeling the molecular gas density probability distribution function (PDF) in galaxies and predicting CO line ratios with large velocity gradient radiative transfer calculations, we present a framework linking global CO line ratios to the mean molecular hydrogen gas density $\left\langle {n}_{{{\rm{H}}}_{2}}\right\rangle $ and kinetic temperature Tkin. Mapping in this way observed R52 ratios to $\left\langle {n}_{{{\rm{H}}}_{2}}\right\rangle $ and Tkin probability distributions, we obtain positive $\left\langle U\right\rangle $ - $\left\langle {n}_{{{\rm{H}}}_{2}}\right\rangle $ and $\left\langle U\right\rangle $ -Tkin correlations, which imply a scenario in which the ISRF in galaxies is mainly regulated by Tkin and (nonlinearly) by $\left\langle {n}_{{{\rm{H}}}_{2}}\right\rangle $ . A small fraction of starburst galaxies showing enhanced $\left\langle {n}_{{{\rm{H}}}_{2}}\right\rangle $ could be due to merger-driven compaction. Our work demonstrates that ISRF and CO excitation are tightly coupled and that density-PDF modeling is a promising tool for probing detailed ISM properties inside galaxies.

Keywords: Molecular gas, Starburst galaxies, Spiral galaxies, Star formation, Interstellar medium
The Missing Cool Corona in the Flat Magnetic Field around Solar Active Regions. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) images the full solar disk in several extreme-ultraviolet (EUV) bands that are each sensitive to coronal plasma emissions of one or more specific temperatures. We observe that when isolated active regions (ARs) are on the disk, full-disk images in some of the coronal EUV channels show the outskirts of the AR as a dark moat surrounding the AR. Here we present seven specific examples, selected from time periods when there was only a single AR present on the disk. Visually, we observe the moat to be most prominent in the AIA 171 Å band, which has the most sensitivity to emission from plasma at log10 T = 5.8. By examining the 1D line-of-sight emission measure temperature distribution found from six AIA EUV channels, we find the intensity of the moat to be most depressed over the temperature range log10 T ≈ 5.7-6.2 for most of the cases. We argue that the dark moat exists because the pressure from the strong magnetic field that splays out from the AR presses down on underlying magnetic loops, flattening those loops—along with the lowest of the AR's own loops over the moat—to a low altitude. Those loops, which would normally emit the bulk of the 171 Å emission, are restricted to heights above the surface that are too low to have 171 Å emitting plasmas sustained in them, according to Antiochos &amp; Noci, while hotter EUV-emitting plasmas are sustained in the overlying higher-altitude long AR-rooted coronal loops. This potentially explains the low-coronal-temperature dark moats surrounding the ARs.

Keywords: Solar active regions, Solar corona
Viewing Angle Effects in Quasar Application to Cosmology. The symmetry axes of active galactic nuclei (AGNs) are randomly distributed in space, but highly inclined sources are heavily obscured and are not seen as quasars with broad emission lines. The obscuring torus geometry determines the average viewing angle, and if the torus geometry changes with the redshift, this average viewing angle will also change. Thus, the ratio between the isotropic luminosity and observed luminosity may change systematically with redshift. Therefore, if we use quasars to measure the luminosity distance by evaluating the isotropic absolute luminosity and measuring the observed flux, we can have a redshift-dependent bias that can propagate to cosmological parameters. We propose a toy model for testing the effect of viewing angle uncertainty on the measurement of the luminosity distance. The model is based on analytical description of the obscuring torus applied to one-parameter observational data. It illustrates the possible change of the torus covering factor between the two chosen redshift ranges. We have estimated the possible errors in specific cosmological parameters (H0, Ωm) for the flat Lambda cold dark matter cosmology if a method is calibrated at low redshift and applied to the higher redshift. The errors in the cosmological parameters due to potential dependence of the viewing angle on redshift are found to be potentially significant, and the effect will have to be accommodated in the future in all quasar-based cosmological methods. A careful systematic study of AGNs means that a viewing angle across the redshift is necessary, with the use of appropriate samples and models that uniquely determine the inclination of each source.

Keywords: Cosmology, Galaxies, Quasars
The Origin of Strong Linear Polarization from Fast Radio Bursts. The detection of almost 100% linearly polarized emission from the fast radio burst source FRB 121102 implies coherent emission of relativistic electrons moving perpendicular to the ambient magnetic field. The origin of such a particle distribution is very intriguing. Given that FRB 121102 is likely driven by a neutron star, we explored orbits of charged particles trapped in a dipole magnetic field (the Störmer problem). Most previous studies focused on particles with relatively low energies so that the guiding center approximation may be applied. High-energy particles usually have chaotic orbits except those on a periodic orbit or near stable periodic orbits. Via evaluation of the maximum Lyapunov exponent of orbits of particles launched from the equatorial plane with an axial velocity (the angular velocity sets the length and energy scales of the system), we found prominent regions of quasi-periodic orbits around stable periodic orbits in the equatorial plane at high energies. Particles in these orbits oscillate around the equatorial plane and their radial distance from the dipole can vary by a factor of ∼2. Relativistic electrons in such orbits may be responsible for the almost 100% polarized emission from FRB 121102.

Keywords: Radio bursts, Magnetic fields, Periodic orbit, High energy astrophysics
Dissecting the Supercritical Filaments Embedded in the 0.5 pc Subsonic Region of Barnard 5. We characterize in detail the two ∼0.3 pc long filamentary structures found within the subsonic region of Barnard 5. We use combined Robert C. Byrd Green Bank Telescope and Very Large Array observations of the molecular lines NH3(1,1) and (2,2) at a resolution of 1800 au, as well as James Clerk Maxwell Telescope continuum observations at 850 and 450 μm at a resolution of 4400 and 3000 au, respectively. We find that both filaments are highly supercritical with a mean mass per unit length, M/L, of ∼80 M⊙ pc-1 after background subtraction, with local increases reaching values of ∼150 M⊙ pc-1. This would require a magnetic field strength of ∼500 μG to be stable against radial collapse. We extract equidistant cuts perpendicular to the spine of the filament and fit a modified Plummer profile as well as a Gaussian to each of the cuts. The filament widths (deconvolved FWHM) range between 6500 and 7000 au (∼0.03 pc) along the filaments. This equals ∼twice the radius of the flat inner region. We find an anticorrelation between the central density and this flattening radius, suggestive of contraction. Further, we also find a strong correlation between the power-law exponent at large radii and the flattening radius. We note that the measurements of these three parameters fall in a plane and derive their empirical relation. Our high-resolution observations provide direct constraints on the distribution of the dense gas within supercritical filaments showing pre- and protostellar activity.

Keywords: Star forming regions, Star formation, Interstellar filaments
Semiempirical Modeling of the Atmospheres of the M Dwarf Exoplanet Hosts GJ 832 and GJ 581. Stellar ultraviolet (UV) radiation drives photochemistry, and extreme-ultraviolet (EUV) radiation drives mass loss in exoplanet atmospheres. However, the UV flux is partly unobservable due to interstellar absorption, particularly in the EUV range (100-912 Å). It is therefore necessary to reconstruct the unobservable spectra in order to characterize the radiation environment of exoplanets. In the present work, we use a radiative transfer code SSRPM to build one-dimensional semiempirical models of two M dwarf exoplanet hosts, GJ 832 and GJ 581, and synthesize their spectra. SSRPM is equipped with an extensive atomic and molecular database and full-NLTE capabilities. We use observations in the visible, ultraviolet, and X-ray ranges to constrain atmospheric structures of the modeled stars. The synthesized integrated EUV fluxes are found to be in good agreement with other reconstruction techniques, but the spectral energy distributions disagree significantly across the EUV range. More than two-thirds of the EUV flux is formed above 105 K. We find that the far-ultraviolet (FUV) continuum contributes 42%-54% of the entire FUV flux between 1450 and 1700 Å. The comparison of stellar structures of GJ 832 and GJ 581 suggests that GJ 832 is a more magnetically active star, which is corroborated by other activity indicators.

Keywords: M dwarf stars, Stellar chromospheres, Stellar activity, Ultraviolet astronomy, Radiative transfer, Stellar coronae
First Observed Interaction of the Circumstellar Envelope of an S-star with the Environment of Sgr A*. Several publications highlight the importance of the observations of bow shocks to learn more about the surrounding interstellar medium and radiation field. We revisit the most prominent dusty and gaseous bow shock source, X7, close to the supermassive black hole, Sgr A*, using multiwavelength analysis. For the purpose of this study, we use Spectrograph for Integral Field Observations in the Near Infrared (SINFONI) (H+K-band) and NACO L'- and M'-band) data sets between 2002 and 2018 with additional COMIC/ADONIS+RASOIR (L'-band)7 data of 1999. By analyzing the line maps of SINFONI, we identify a velocity of ∼200 km s-1 from the tip to the tail. Furthermore, a combination of the multiwavelength data of NACO and SINFONI in the H-, K-, L'-, and M'-bands results in a two-component blackbody fit that implies that X7 is a dust-enshrouded stellar object. The observed ongoing elongation and orientation of X7 in the Brγ line maps and the NACO L'-band continuum indicate a wind arising at the position of Sgr A* or at the IRS16 complex. Observations after 2010 show that the dust and the gas shell seems to be decoupled in the projection from its stellar source S50. The data also implies that the tail of X7 thermally heats up due to the presence of S50. The gas emission at the tip is excited because of the related forward scattering (Mie scattering), which will continue to influence the shape of X7 in the near future. In addition, we find excited [Fe III] lines, which together with the recently analyzed dusty sources and the Brγ-bar underline the uniqueness of this source.

Keywords: Galactic center, Stellar bow shocks, Stellar physics, Stellar winds, Star formation, Circumstellar shells, Dust shells, Star clusters