adsabs/SciX_UAT_keywords huggingface dataset:
Revealing the Variation Mechanism of ON 231 via the Two-component Shock-in-jet Model

The variation mechanism of blazars is a long-standing unresolved problem. In this work, we present a scenario to explain diverse variation phenomena for ON 231, where the jet emissions are composed of the flaring and the less variable components (most probably from the post-flaring blobs), and the variation is dominated by shock-in-jet instead of the Doppler effect. We perform correlation analysis for the multiwavelength light curves and find no significant correlations. For the optical band, ON 231 exhibits a harder when brighter (HWB) trend, and the trend seems to shift at different periods. Correspondingly, the correlation between the degree of polarization and flux exhibits a V-shaped behavior, and a similar translation relation during different periods is also found. These phenomena could be understood via the superposition of the flaring component and slowly varying background component. We also find that the slopes of the HWB trend become smaller at higher flux levels, which indicates the energy-dependent acceleration processes of the radiative particles. For the X-ray band, we discover a trend transition from HWB to softer when brighter (SWB) to HWB. We consider that the X-ray emission is composed of both the synchrotron tail and the synchrotron self-Compton components, which could be described by two log-parabolic functions. By varying the peak frequency, we reproduce the observed trend transition in a quantitative manner. For the γ-ray band, we find the SWB trend, which could be explained naturally if a very-high-energy γ-ray background component exists. Our study elucidates the variation mechanism of intermediate synchrotron-peaked BL Lac objects.

Keywords: blazars; relativistic jets; bl lacertae objects; spectropolarimetry
Identifying Plasma Fractionation Processes in the Chromosphere Using IRIS

The composition of the solar corona differs from that of the photosphere, with the plasma thought to fractionate in the solar chromosphere according to the first ionization potential (FIP) of the different elements. This produces a FIP bias, wherein elements with a low FIP are preferentially enhanced in the corona compared to their photospheric abundance, but direct observations of this process remain elusive. Here, we use a series of spectroscopic observations of active region AR 12759 as it transited the solar disk over a period of 6 days from 2020 April 2–7 taken using the Hinode Extreme ultraviolet Imaging Spectrometer and Interface Region Imaging Spectrograph (IRIS) instruments to look for signatures of plasma fractionation in the solar chromosphere. Using the Si X/S X and Ca XIV/Ar XIV diagnostics, we find distinct differences between the FIP bias of the leading and following polarities of the active region. The widths of the IRIS Si IV lines exhibited clear differences between the leading and following polarity regions, indicating increased unresolved wave activity in the following polarity region compared to the leading polarity region, with the chromospheric velocities derived using the Mg II lines exhibiting comparable, albeit much weaker, behavior. These results are consistent with plasma fractionation via resonant/nonresonant waves at different locations in the solar chromosphere following the ponderomotive force model, and indicate that IRIS could be used to further study this fundamental physical process.

Keywords: solar physics; the sun; solar corona; solar chromosphere
A White-light Flare Powered by Magnetic Reconnection in the Lower Solar Atmosphere

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

Keywords: solar flares; solar magnetic fields; solar magnetic reconnection; solar active regions
Probing Jets from Young Embedded Sources: Clues from HST Near-IR [Fe II] Images

We present near-infrared [Fe II] images of four Class 0/I jets (HH 1/2, HH 34, HH 111, HH 46/47) observed with the Hubble Space Telescope (HST) Wide Field Camera 3. The unprecedented angular resolution allows us to measure proper motions, jet widths and trajectories, and extinction along the jets. In all cases, we detect the counterjet, which was barely visible or invisible at shorter wavelengths. We measure tangential velocities of a few hundred kilometers per second, consistent with previous HST measurements over 10 years ago. We measure the jet width as close as a few tens of au from the star, revealing high collimations of about 2° for HH 1, HH 34, and HH 111 and about 8° for HH 46, all of which are preserved up to large distances. For HH 34, we find evidence of a larger initial opening angle of about 7°. Measurement of knot positions reveals deviations in trajectory of both the jet and counterjet of all sources. Analysis of asymmetries in the inner knot positions for HH 111 suggests the presence of a low mass stellar companion at separation 20-30 au. Finally, we find extinction values of 15-20 mag near the source, which gradually decrease moving downstream along the jet. These observations have allowed us to study the counterjet at unprecedentedly high angular resolution and will be a valuable reference for planning future JWST mid-infrared observations that will peer even closer into the jet engine.

Keywords: star formation; stellar jets; jets; stellar winds; protostars; young stellar objects
A Magnetically Driven Disk Wind in the Inner Disk of PDS 70

PDS 70 is so far the only young disk where multiple planets have been detected by direct imaging. The disk has a large cavity when seen at submillimeter and near-infrared wavelengths, which hosts two massive planets. This makes PDS 70 the ideal target to study the physical conditions in a strongly depleted inner disk shaped by two giant planets, and in particular to test whether disk winds can play a significant role in its evolution. Using X-Shooter and HARPS spectra, we detected for the first time the wind-tracing [O I] 6300 Å line, and confirm the low-moderate value of mass-accretion rate in the literature. The [O I] line luminosity is high with respect to the accretion luminosity when compared to a large sample of disks with cavities in nearby star-forming regions. The FWHM and blueshifted peak of the [O I] line suggest an emission in a region very close to the star, favoring a magnetically driven wind as the origin. We also detect wind emission and high variability in the He I 10830 Å line, which is unusual for low accretors. We discuss that, although the cavity of PDS 70 was clearly carved out by the giant planets, the substantial inner-disk wind could also have had a significant contribution to clearing the inner disk. *Based on observations collected at the European Southern Observatory under ESO programs 098.C-0739, 0104.C-0418, 105.205R, 106.20Z8, and 1101.C-0557.

Keywords: protoplanetary disks; classical t tauri stars; weak-line t tauri stars; stellar accretion disks; variable stars; high resolution spectroscopy
Isomers of Hydrogenated Polycyclic Aromatic Hydrocarbons Explain the Presence of Infrared Bands in the 3 μm Region

A large number of hydrogenated coronene, circumcoronene, and circumcircumcoronene isomers have been investigated by means of density functional theory calculations. The computation of their IR spectra puts forward significant differences between the different isomers in the 3 μm region and shows that hydrogenated polycyclic aromatic hydrocarbons can account for the aromatic infrared bands resulting from the absorption of light by the interstellar medium. In particular, the intense 3.3 μm band, as well as the weak features observed at 3.40, 3.46, 3.51, and 3.56 μm are reasonably reproduced by the present calculations. The latter two are only observed when hydrogenation takes place in tertiary carbon atoms, showing that the frequencies at which these IR bands appear are a consequence of their position and chemical environment within the molecule.

Keywords: polycyclic aromatic hydrocarbons; interstellar molecules; infrared sources; cosmochemistry; molecular spectroscopy; line positions; spectral line identification
Gamma-Ray Urgent Archiver for Novel Opportunities (GUANO): Swift/BAT Event Data Dumps on Demand to Enable Sensitive Subthreshold GRB Searches

We introduce a new capability of the Neil Gehrels Swift Observatory to provide event-level data from the Burst Alert Telescope (BAT) on demand in response to transients detected by other instruments. We show that the availability of these data can effectively increase the rate of detections and arcminute localizations of gamma-ray bursts (GRB) like GRB 170817 by >400%. We describe an autonomous spacecraft-commanding pipeline purpose built to enable this science; to our knowledge, this is the first fully autonomous extremely low-latency commanding of a space telescope for scientific purposes. This pipeline has been successfully run in its complete form since 2020 January, and has resulted in the recovery of BAT event data for >800 externally triggered events to date (gravitational waves, GWs; neutrinos; GRBs triggered by other facilities; fast radio bursts; and very high-energy detections), now running with a success rate of ∼90%. We exemplify the utility of this new capability by using the resultant data to (1) set the most sensitive upper limits on prompt 1 s duration short GRB-like emission within ±15 s around the unmodeled GW burst candidate S200114f, and (2) provide an arcminute localization for short GRB 200325A and other bursts. We also show that using data from GUANO to localize GRBs discovered by other instruments, we can increase the net rate of arcminute-localized GRBs by 10%-20% per year. Along with the scientific yield of more sensitive searches for subthreshold GRBs, the new capabilities designed for this project will serve as the foundation for further automation and rapid target of opportunity capabilities for the Swift mission, and have implications for the design of future rapid-response space telescopes.

Keywords: gamma-ray bursts; gravitational wave sources; space telescopes; x-ray telescopes
Estimation of the Eclipse Solar Radius by Flash Spectrum Video Analysis

The value of the eclipse solar radius during the 2017 August 21 total solar eclipse was estimated to be S⊙ = (959"95 ± 0"05) at 1 au with no significant dependence on wavelength. The measurement was obtained from analysis of a video of the eclipse flash spectrum recorded at the southern limit of the umbral shadow path. Our analysis was conducted by extracting light curves from the flash spectrum and comparing them to simulated light curves. The simulations were performed by integrating the limb darkening function over the exposed area of the photosphere. These numerical integrations relied on very precise computations of the relative movement of the lunar and solar limbs.

Keywords: astronomical simulations; light curves; flash spectra; solar eclipses; solar radius
The Eighteenth Data Release of the Sloan Digital Sky Surveys: Targeting and First Spectra from SDSS-V

The eighteenth data release (DR18) of the Sloan Digital Sky Survey (SDSS) is the first one for SDSS-V, the fifth generation of the survey. SDSS-V comprises three primary scientific programs or "Mappers": the Milky Way Mapper (MWM), the Black Hole Mapper (BHM), and the Local Volume Mapper. This data release contains extensive targeting information for the two multiobject spectroscopy programs (MWM and BHM), including input catalogs and selection functions for their numerous scientific objectives. We describe the production of the targeting databases and their calibration and scientifically focused components. DR18 also includes ~25,000 new SDSS spectra and supplemental information for X-ray sources identified by eROSITA in its eFEDS field. We present updates to some of the SDSS software pipelines and preview changes anticipated for DR19. We also describe three value-added catalogs (VACs) based on SDSS-IV data that have been published since DR17, and one VAC based on the SDSS-V data in the eFEDS field.

Keywords: surveys; astronomy databases; astronomy data acquisition; astronomy software
Charge-injection Device Imaging of Sirius with Contrast Ratios Greater than 1:26 Million

The intrinsic nature of many astronomical objects, such as binary and multiple systems, exoplanets, circumstellar and debris disks, and quasar host galaxies, introduces challenging requirements for observational instrumentation and techniques. In each case, we encounter situations where the light from bright sources hampers our ability to detect surrounding fainter targets. To explore all features of such astronomical scenes, we must perform observations at the maximum possible contrast ratios. Charge-injection devices (CIDs) are capable of potentially exceeding contrast ratios of ${\mathrm{log}}_{10}(\mathrm{CR})\gt 9$ (i.e., 1 part in 1 billion) due to their unique readout architectures and inherent anti-blooming abilities. An on-sky testing of a commercially available CID, SpectraCAM XDR (SXDR), demonstrated raw contrast ratios from sub-optimal ground-based astronomical observations that imposed practical limits on the maximum achievable contrast ratios using CIDs. Here, we demonstrate the extreme contrast ratio imaging capabilities of the SXDR using observations of Sirius with the 1.0 m Jacobus Kapteyn Telescope, La Palma, Spain. Based on wavelet-based analysis and precise photometric and astrometric calibrations, we report a direct contrast ratio of Δm r = 18.54, ${\mathrm{log}}_{10}(\mathrm{CR})=7.41\pm 0.08$ , or 1 part in 26 million. This is an order of magnitude higher compared to the previous CID results.

Keywords: astronomy data analysis; astronomical instrumentation; astronomical techniques; direct imaging
Measurements of the z ∼ 6 Intergalactic Medium Optical Depth and Transmission Spikes Using a New z > 6.3 Quasar Sample

We report new measurements of the intergalactic medium (IGM) Lyα and Lyβ effective optical depth at 5.3 < z < 6.5, using a new sample of quasar sight lines including 32 quasars at 6.308 ≤ z ≤ 7.00. These quasars provide a large statistical sample to measure the IGM evolution during the transition phase of the reionization epoch. We construct a data set of deep optical spectra of these quasars using VLT, Keck, Gemini, LBT, and MMT. We measure the Lyα effective optical depth at 5.36 < z < 6.57 using the Lyα forests of both individual spectra and the stacked spectrum. The large scatter of individual measurements is consistent with previous work, suggesting an inhomogeneous reionization process. Combining our new measurements and previous results, we obtain a best fit for the Lyα effective optical depth evolution at z > 5.3, $\tau \propto {\left(1+z\right)}^{8.6\pm 1.0}$ . We then estimate the observed Lyβ effective optical depth using Lyβ forests and convert them to Lyα optical depth for comparison, which provides additional constraints on the evolution of the IGM optical depth. The Lyβ-based measurements are generally in agreement with the best-fit evolution obtained from Lyα forests. Using this new sample, we identify 389 Lyα and 50 Lyβ transmission spikes at 5.5 < z < 6.3. The upper limits of Lyα optical depth estimated using transmission spikes are well consistent with our best-fit evolution. The evolution in number density of these high-redshift transmission spikes suggests a rapid transition phase at the end of the reionization. Comparison of our optical depth measurements with hydrodynamical simulations indicates an IGM neutral hydrogen fraction $\langle {f}_{{\rm{H}}{\rm\small{I}}}\rangle \gtrsim {10}^{-4}$ at z = 6.

Keywords: intergalactic medium; quasar absorption line spectroscopy; extragalactic astronomy; reionization; early universe
Multi-band Tractor Forced Photometry and Redshifts in the CDFS and XMM-LSS Fields

We present a catalog of multi-band forced photometry in the CDFS and XMM-LSS fields. We used The Tractor image-modeling software to produce de-blended photometry across 13 to 15 optical/infrared bands and determine photometric redshifts. Our catalog, which is publicly available on IRSA, contains ~1.5 million sources and covers a total area of ~9 deg2.

Keywords: surveys; multi-color photometry; extragalactic astronomy
First Laboratory Measurement of Magnetic-field-induced Transition Effect in Fe X at Different Magnetic Fields

The magnetic field is extremely important for understanding the properties of the solar corona. However, there are still difficulties in the direct measurement of the coronal magnetic field. The magnetic-field-induced transition (MIT) in Fe X, appearing in coronal spectra, was discovered to have prospective applications in coronal magnetic field measurements. In this work, we obtained the extreme ultraviolet spectra of Fe X in the wavelength range of 174-267 Å in the Shanghai High-temperature Superconducting Electron Beam Ion Trap, and examined the effect of MIT in Fe X by measuring the line ratios between 257.262 Å and the reference line of 226.31 Å (257/226) at different magnetic field strengths for the first time. The electron density that may affect the 257/226 value was also obtained experimentally and verified by comparing the density-sensitive line ratio (175.266 Å/174.534 Å) measurements with the theoretical predictions, and there was good agreement between them. The energy separation between the two levels of 3s23p43d 4 D 5/2 and 3s23p43d 4 D 7/2, one of the most critical parameters for determining the MIT rate, was obtained by analyzing the simulated line ratios of 257/226 with the experimental values at the given electron densities and magnetic fields. Possible reasons that may have led to the difference between the obtained energy splitting and the recommended value in previous works are discussed. Magnetic field response curves for the 257/226 value were calculated and compared to the experimental results, which is necessary for future MIT diagnostics.

Keywords: solar corona; magnetic fields; atomic spectroscopy
The Metallicity and Distance of NGC 2403 from Blue Supergiants

We present the first quantitative spectral analysis of blue supergiant stars in the nearby galaxy NGC 2403. Out of a sample of 47 targets observed with the LRIS spectrograph at the Keck I telescope we have extracted 16 B- and A-type supergiants for which we have data of sufficient quality to carry out a comparison with model spectra of evolved massive stars and infer the stellar parameters. The radial metallicity gradient of NGC 2403 that we derive has a slope of -0.14(±0.05) dex ${r}_{e}^{-1}$ , and is in accordance with the analysis of H II region oxygen abundances. We present evidence that the stellar metallicities that we obtain in extragalactic systems in general agree with the nebular abundances based on the analysis of the auroral lines, over more than 1 order of magnitude in metallicity. Adopting the known relation between stellar parameters and intrinsic luminosity we find a distance modulus μ = 27.38 ± 0.08 mag. While this can be brought into agreement with Cepheid-based determinations, it is 0.14 mag short of the value measured from the tip of the red giant branch. We update the mass-metallicity relation secured from chemical abundance studies of stars in resolved star-forming galaxies.

Keywords: galaxy abundances; galaxy stellar content; stellar abundances
A Systematic Study of Gamma-Ray Flares from the Crab Nebula with Fermi-LAT. I. Flare Detection

Significant flares of GeV γ-ray emission from the Crab Nebula were found by AGILE and Fermi-LAT years ago, indicating that extreme particle acceleration and radiation occurs in young pulsar wind nebulae. To enlarge the flare sample and to investigate their statistical properties will be very useful in understanding the nature of the γ-ray flares. In this paper, we investigate the flaring emission from the Crab Nebula with eleven year observations of the Fermi-LAT. We identify 17 significant flares in the light curve of the low-energy (synchrotron) component of the γ-ray emission. The flare rate is about 1.5 per year, without any significant change or clustering during the 11 years of the observation. We detect a special flare with an extremely long duration of nearly one month, that occurred in 2018 October, with synchrotron photons up to energies of about 1 GeV. The synchrotron component could be fitted by a steady power-law (PL) background and a variable flare component with an exponentially cutoff PL spectrum, not only for the individual flare but also for the combined data, which may favor a similar emission mechanism for all flares. However, we do not find a universal relation between the cutoff energy and the energy fluxes of the flares, which may reflect the complicated acceleration and/or cooling processes of the involved particles.

Keywords: cosmic rays; gamma-ray astronomy
Cometary Activity on Quasi-Hilda Object 2018 CZ16

We present the discovery of activity originating from quasi-Hilda Object 2018 CZ16, a finding stemming from the Citizen Science project Active Asteroids. For 2018 CZ16 we identified a broad (~60°) but short (~5″) tail in archival Blanco 4 m data from Cerro Tololo Inter-American Observatory, Chile, (CTIO) Dark Energy Camera images from UT 2018 May 15, 17 and 18. Activity occurred 2 months prior to perihelion, consistent with sublimation-driven activity.

Keywords: asteroids; hilda group; comae; comet tails
Peculiar Velocity Reconstruction from Simulations and Observations Using Deep Learning Algorithms

In this paper, we introduce a U-Net model of deep learning algorithms for reconstructions of the 3D peculiar velocity field, which simplifies the reconstruction process with enhanced precision. We test the adaptability of the U-Net model with simulation data under more realistic conditions, including the redshift space distortion effect and halo mass threshold. Our results show that the U-Net model outperforms the analytical method that runs under ideal conditions, with a 16% improvement in precision, 13% in residuals, 18% in correlation coefficient, and 27% in average coherence. The deep learning algorithm exhibits exceptional capacities to capture velocity features in nonlinear regions and substantially improve reconstruction precision in boundary regions. We then apply the U-Net model trained under Sloan Digital Sky Survey (SDSS) observational conditions to the SDSS Data Release 7 data for observational 3D peculiar velocity reconstructions.

Keywords: large-scale structure of the universe; cosmology; radial velocity; convolutional neural networks