adsabs/SciX_UAT_keywords huggingface dataset:
Compact Symmetric Objects. II. Confirmation of a Distinct Population of High-luminosity Jetted Active Galaxies

Compact symmetric objects (CSOs) are compact (<1 kpc), jetted active galactic nuclei (AGN), whose jet axes are not aligned close to the line of sight, and whose observed emission is not predominantly relativistically boosted toward us. Two classes of CSOs have previously been identified: approximately one-fifth are edge dimmed and the rest are edge brightened. We designate these as CSO 1s and 2s, respectively. This paper focuses almost exclusively on CSO 2s. Using complete samples of CSO 2s we present three independent lines of evidence, based on their relative numbers, redshift distributions, and size distributions, which show conclusively that the vast majority (>99%) of CSO 2s do not evolve into larger-scale radio sources. These CSO 2s belong to a distinct population of jetted AGN, which should be characterized as "short-lived," as opposed to "young," compared to the classes of larger jetted AGN. We show that there is a sharp upper cutoff in the CSO 2 size distribution at ≈500 pc. The distinct differences between most CSO 2s and other jetted AGN provides a crucial new time domain window on the formation and evolution of relativistic jets in AGN and the supermassive black holes that drive them.

Keywords: active galactic nuclei; active galaxies; relativistic jets
A Search for Millilensing Gamma-Ray Bursts in the Observations of Fermi GBM

Millilensing of gamma-ray bursts (GRBs) is expected to manifest as multiple emission episodes in a single triggered GRB with similar light-curve patterns and similar spectrum properties. Identifying such lensed GRBs could help improve constraints on the abundance of compact dark matter. Here we present a systemic search for millilensing among 3000 GRBs observed by the Fermi GBM up to 2021 April. Eventually we find four interesting candidates by performing an autocorrelation test, hardness test, and time-integrated/resolved spectrum test. GRB 081126A and GRB 090717A are ranked as the first-class candidates based on their excellent performance in both temporal and spectrum analysis. GRB 081122A and GRB 110517B are ranked as the second-class candidates (suspected candidates), mainly because their two emission episodes show clear deviations in part of the time-resolved spectrum or in the time-integrated spectrum. Considering a point-mass model for the gravitational lens, our results suggest that the density parameter of lens objects with mass M L ~ 106 M ⊙ is larger than 1.5 × 10-3.

Keywords: gamma-ray bursts
The Breakthrough Listen Search For Intelligent Life Near the Galactic Center. I.

A line of sight toward the Galactic Center (GC) offers the largest number of potentially habitable systems of any direction in the sky. The Breakthrough Listen program is undertaking the most sensitive and deepest targeted SETI surveys toward the GC. Here, we outline our observing strategies with Robert C. Byrd Green Bank Telescope (GBT) and Parkes telescope to conduct 600 hr of deep observations across 0.7-93 GHz. We report preliminary results from our survey for extraterrestrial intelligence (ETI) beacons across 1-8 GHz with 7.0 and 11.2 hr of observations with Parkes and GBT, respectively. With our narrowband drifting signal search, we were able to place meaningful constraints on ETI transmitters across 1-4 GHz and 3.9-8 GHz with EIRP limits of ≥4 × 1018 W among 60 million stars and ≥5 × 1017 W among half a million stars, respectively. For the first time, we were able to constrain the existence of artificially dispersed transient signals across 3.9-8 GHz with EIRP ≥1 × 1014 W/Hz with a repetition period ≤4.3 hr. We also searched our 11.2 hr of deep observations of the GC and its surrounding region for Fast Radio Burst-like magnetars with the DM up to 5000 pc cm-3 with maximum pulse widths up to 90 ms at 6 GHz. We detected several hundred transient bursts from SGR J1745-2900, but did not detect any new transient bursts with the peak luminosity limit across our observed band of ≥1031 erg s-1 and burst rate of ≥0.23 burst hr-1. These limits are comparable to bright transient emission seen from other Galactic radio-loud magnetars, constraining their presence at the GC.

Keywords: technosignatures; galactic center; magnetars; neutron stars; radio transient sources; search for extraterrestrial intelligence
Pulsar-candidate Selection Using a Generative Adversarial Network and ResNeXt

Pulsar research has been a hot topic in the area of astronomy since they were first discovered. Pulsar discovery is fundamental for pulsar research. While pulsars are now visible across the electromagnetic spectrum, pulsar searches with modern radio telescopes are most promising. As the performance of astronomical instruments improves, the number of pulsar candidates detected by modern radio telescopes grows at an exponential rate. The application of artificial intelligence to the field of pulsar-candidate identification can automatically and efficiently address the identification problem with enormous amounts of data. However, there are still significant challenges in enhancing the accuracy of deep-learning-based pulsar-candidate identification. These problems result primarily from the fact that real pulsar data is scarce: the number of candidates that can be successfully identified as real pulsars (positive samples) is much smaller than those candidates that turn out to not be pulsars but instead radio-frequency interference or noise (negative samples). This makes it difficult to train a machine-learning model that can accurately select those candidates that are real pulsars. Therefore a novel pulsar-candidate identification framework is proposed that combines a deep convolutional generative adversarial neural network (DCGAN) and a deep aggregation residual network (ResNeXt). To overcome sample imbalance, the DCGAN is utilized to generate images that approximate real pulsars, while observed and generated candidates are employed together to train the pulsar-candidate identification model ResNeXt. Experiments on the HTRU Medlat data set back up the framework's performance. The precision, recall, and F1-score of the framework are 100%.

Keywords: pulsars; astronomy data analysis
Electron Surfing Acceleration at Rippled Reconnection Fronts

The reconnection front (RF), one of the most efficient accelerators of particles in the terrestrial magnetosphere, is a sharp plasma boundary resulting from transient magnetic reconnection. It has been both theoretically predicted and observationally confirmed that electron-scale substructures can develop at the RFs. How such electron-scale structures modulate the electron energization and transport has not been fully explored. Based on high-resolution data from MMS spacecraft and particle tracing simulations, we investigate and compare the electron acceleration across two typical RFs with or without rippled electron-scale structures. Both observations and simulations reveal that high-energy electron flux behind the RF increases more dramatically if the electrons encounter a rippled RF surface, as compared to a smooth RF surface. The main acceleration mechanism is electron surfing acceleration, in which electrons are trapped by the ripples, due to the large local magnetic field gradient, and therefore undergo surfing motion along the motional electric field.

Keywords: particle physics; space plasmas; interplanetary particle acceleration
Simulation of Alfvén Wave Propagation in the Magnetic Chromosphere with Radiative Loss: Effects of Nonlinear Mode Coupling on Chromospheric Heating

We perform magnetohydrodynamic simulations to investigate the propagation of Alfvén waves in the magnetic chromosphere. We use the 1.5D expanding flux tube geometry setting and transverse perturbation at the bottom to generate the Alfvén wave. Compared with previous studies, our expansion is that we include the radiative loss term introduced by Carlsson & Leenaarts. We find that when an observation-based transverse wave generator is applied, the spatial distribution of the time-averaged radiative loss profile in our simulation is consistent with that in the classic atmospheric model. In addition, the energy flux in the corona is larger than the required value for coronal heating in the quiet region. Our study shows that the Alfvén wave-driven model has the potential to simultaneously explain chromospheric heating and how energy is transported to the corona.

Keywords: solar chromosphere; solar chromospheric heating
Observing the Earliest Stages of Star Formation in Galaxies: 8 μm Cores in Three Edge-on Disks

To study the vertical distribution of the earliest stages of star formation in galaxies, three edge-on spirals, NGC 891, NGC 3628, and IC 5052, observed by the Spitzer Space Telescope InfraRed Array Camera (IRAC) were examined for compact 8 μm cores using an unsharp mask technique; 173, 267, and 60 cores were distinguished, respectively. Color-color distributions suggest a mixture of polycyclic aromatic hydrocarbons and highly extincted photospheric emission from young stars. The average V-band extinction is ∼20 mag, equally divided between foreground and core. IRAC magnitudes for the clumps are converted to stellar masses assuming an age of 1 Myr, which is about equal to the ratio of the total core mass to the star formation rate in each galaxy. The extinction and stellar mass suggest an intrinsic core radius of ∼18 pc for 5% star formation efficiency. The half-thickness of the disk of 8 μm cores is 105 pc for NGC 891 and 74 pc for IC 5052, varying with radius by a factor of ∼2. For NGC 3628, which is interacting, the half-thickness is 438 pc, but even with this interaction, the 8 μm disk is remarkably flat, suggesting vertical stability. Small-scale structures like shingles or spirals are seen in the core positions. Very few of the 8 μm cores have optical counterparts.

Keywords: disk galaxies; spiral galaxies; star formation; star forming regions; young star clusters; interstellar emissions
On the Nature of AGN and Star Formation Enhancement in the z = 3.1 SSA22 Protocluster: The HST WFC3 IR View

We examine possible environmental sources of the enhanced star formation and active galactic nucleus (AGN) activity in the z = 3.09 SSA22 protocluster using Hubble WFC3 F160W (~1.6 μm) observations of the SSA22 field, including new observations centered on eight X-ray selected protocluster AGN. To investigate the role of mergers in the observed AGN and star formation enhancement, we apply both quantitative (Sérsic-fit and Gini-M20) and visual morphological classifications to F160W images of protocluster Lyman-break galaxies (LBGs) in the fields of the X-ray AGN and z ~ 3 field LBGs in SSA22 and GOODS-N. We find no statistically significant differences between the morphologies and merger fractions of protocluster and field LBGs, though we are limited by small number statistics in the protocluster. We also fit the UV-to-near-IR spectral energy distributions of F160W-detected protocluster and field LBGs to characterize their stellar masses and star formation histories. We find that the mean protocluster LBG is a factor of ~2 times more massive and more attenuated than the mean z ~ 3 field LBG. We take our results to suggest that ongoing mergers are not more common among protocluster LBGs than field LBGs, though protocluster LBGs appear to be more massive. We speculate that the larger mass of the protocluster LBGs contributes to the enhancement of SMBH mass and accretion rate in the protocluster, which in turn drives the observed protocluster AGN enhancement.

Keywords: high-redshift galaxy clusters; high-redshift galaxies; active galactic nuclei; x-ray active galactic nuclei; galaxy mergers
A Component of the Smith High-velocity Cloud Now Crossing the Galactic Plane

We have identified a new structure in the Milky Way: a leading component of the Smith high-velocity cloud that is now crossing the Galactic plane near longitude 25°. Using new 21 cm H I data from the Green Bank Telescope (GBT), we measured the properties of several dozen clouds that are part of this structure. Their kinematics is consistent with that of the Smith Cloud with a V LSR exceeding that permitted by circular rotation in their direction. Most of the clouds in the leading component show evidence that they are interacting with disk gas allowing the location of the interaction to be estimated. The leading component crosses the Galactic plane at a distance from the Sun of 9.5 kpc, about 4.5 kpc from the Galactic Center. Its H I mass may be as high as 106 M ⊙, comparable to the mass of the neutral component of the Smith Cloud, but only a fraction of this is contained in clouds that are resolved in the GBT data. Like the Smith Cloud, the leading component appears to be adding mass and angular momentum to the interstellar medium in the inner Galaxy. We suggest that the Smith Cloud is not an isolated object, but rather part of a structure that stretches more than 40° (~7 kpc) across the sky, in two pieces separated by a gap of ~1 kpc.

Keywords: milky way galaxy; milky way evolution; high-velocity clouds; neutral hydrogen clouds; circumgalactic medium
Geometry, Dissipation, Cooling, and the Dynamical Evolution of Wind-blown Bubbles

Bubbles driven by energy and mass injection from small scales are ubiquitous in astrophysical fluid systems and essential to feedback across multiple scales. In particular, O stars in young clusters produce high-velocity winds that create hot bubbles in the surrounding gas. We demonstrate that the dynamical evolution of these bubbles is critically dependent upon the geometry of their interfaces with their surroundings and the nature of heat transport across these interfaces. These factors together determine the amount of energy that can be lost from the interior through cooling at the interface, which in turn determines the ability of the bubble to do work on its surroundings. We further demonstrate that the scales relevant to physical dissipation across this interface are extremely difficult to resolve in global numerical simulations of bubbles for parameter values of interest. This means the dissipation driving evolution of these bubbles in numerical simulations is often of a numerical nature. We describe the physical and numerical principles that determine the level of dissipation in these simulations; we use this, along with a fractal model for the geometry of the interfaces, to explain differences in convergence behavior between hydrodynamical and magnetohydrodynamical simulations presented here. We additionally derive an expression for momentum as a function of bubble radius expected when the relevant dissipative scales are resolved and show that it still results in efficiently cooled solutions, as postulated in previous work.

Keywords: stellar winds; interstellar medium; star forming regions; stellar feedback; stellar wind bubbles; young star clusters
Modeling the Survival of Population III Stars to the Present Day

Recent numerical simulations have suggested the probability of a fraction of the primordial stars being ejected from the cluster of their origin. We explore the possibility that some of these can remain on the main sequence until the present epoch. We develop a semianalytical model guided by results of cosmological simulations to study the mass accretion by these protostars as a function of the original stellar mass and other parameters such as angular momentum and gravitational drag due to ambient gas. We also explore whether some of the protostars remain sufficiently low mass and long-lived to survive to the present day. This requires that the protostars are ejected from the star-forming region while their mass is less than $0.8\,{M}_{\odot }$ . Assuming that the protostars gain mass via the spherical Bondi-Hoyle accretion from the ambient medium, we show that Population III protostars that initially form within a certain range of mass and are ejected with velocity larger than the escape velocity may survive to the present day on the main sequence. Thus, they may even be found in our Milky Way or its satellites. Our calculations also reveal that protostars that do not get ejected from the parent gas clump accrete a large amount of gas. We predict that these can become massive enough to be progenitors of black holes.

Keywords: population iii stars; bondi accretion; theoretical models; dynamical evolution
A Mildly Relativistic Outflow Launched Two Years after Disruption in Tidal Disruption Event AT2018hyz

We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz, spanning 970-1300 d after optical discovery. In conjunction with earlier deeper limits, including those at ≈700 days, our observations reveal rapidly rising emission at 0.8-240 GHz, steeper than F ν ∝ t 5 relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (e.g., off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multifrequency data allow us to directly determine the radius and energy of the radio-emitting outflow, and we find from our modeling that the outflow was launched ≈750 days after optical discovery. The outflow velocity is mildly relativistic, with β ≈ 0.25 and ≈0.6 for a spherical geometry and a 10° jet geometry, respectively, and the minimum kinetic energy is E K ≈ 5.8 × 1049 and ≈6.3 × 1049 erg, respectively. This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE; a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+57. We suggest that such delayed outflows may be common in TDEs.

Keywords: black hole physics; radio transient sources; tidal disruption; jets
New Blue and Red Variable Stars in NGC 247

Images recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South are combined with archival images from other facilities to search for variable stars in the southern half of the nearby disk galaxy NGC 247. Fifteen new periodic and nonperiodic variables are identified. These include three Cepheids with periods <25 days, four semiregular variables, one of which shows light variations similar to those of R CrB stars, five variables with intrinsic visible/red brightnesses and colors that are similar to those of luminous blue variables (LBVs), and three fainter blue variables, one of which may be a noneclipsing close binary system. The S Doradus instability strip defines the upper envelope of a distinct sequence of objects on the (i, g-i) color-magnitude diagram (CMD) of NGC 247. The frequency of variability of an amplitude ≥0.1 magnitude in the part of the CMD that contains LBVs over the seven-month period when the GMOS images were recorded is ~0.2. The light curve of the B[e] supergiant J004702.18-204739.9, which is among the brightest stars in NGC 247, is also examined. Low-amplitude variations on day-to-day timescales are found, coupled with a systematic trend in mean brightness over a six-month time interval.

Keywords: disk galaxies; peculiar variable stars; periodic variable stars; spiral galaxies
Temporal Variability in Hot Jupiter Atmospheres

Hot Jupiters receive intense incident stellar light on their daysides, which drives vigorous atmospheric circulation that attempts to erase their large dayside-to-nightside flux contrasts. Propagating waves and instabilities in hot Jupiter atmospheres can cause emergent properties of the atmosphere to be time-variable. In this work, we study such weather in hot Jupiter atmospheres using idealized cloud-free general circulation models with double-gray radiative transfer. We find that hot Jupiter atmospheres can be time-variable at the ∼0.1%-1% level in globally averaged temperature and at the ∼1%-10% level in globally averaged wind speeds. As a result, we find that observable quantities are also time-variable: the secondary eclipse depth can be variable at the ≲2% level, the phase-curve amplitude can change by ≲1%, the phase-curve offset can shift by ≲5°, and terminator-averaged wind speeds can vary by ≲2 km s-1. Additionally, we calculate how the eastern and western limb-averaged wind speeds vary with incident stellar flux and the strength of an imposed drag that parameterizes Lorentz forces in partially ionized atmospheres. We find that the eastern limb is blueshifted in models over a wide range of equilibrium temperature and drag strength, while the western limb is only redshifted if equilibrium temperatures are ≲1500 K and drag is weak. Lastly, we show that temporal variability may be observationally detectable in the infrared through secondary eclipse observations with the James Webb Space Telescope, phase-curve observations with future space telescopes (e.g., ARIEL), and/or Doppler wind speed measurements with high-resolution spectrographs.

Keywords: hydrodynamics; computational methods; exoplanets; exoplanet atmospheres; exoplanet atmospheric variability
GRB 220426A: A Thermal Radiation-Dominated Gamma-Ray Burst

The physical composition of the ejecta of gamma-ray bursts (GRBs) remains an open question. The radiation mechanism of the prompt gamma rays is also in debate. This problem can be solved for the bursts hosting distinct thermal radiation. However, the events with dominant thermal spectral components are still rare. In this work, we focus on GRB 220426A, a recent event detected by the Fermi Gamma-ray Burst Monitor. The time-resolved and time-integrated data analyses yield very hard low-energy spectral indices and rather soft high-energy spectral indices. This means that the spectra of GRB 220426A are narrowly distributed. And the Bayesian inference results are in favor of the multicolor blackbody model. The physical properties of the relativistic outflow are calculated. Assuming a redshift z = 1.4, the bulk Lorentz factors Γ of the shells are found to be between ${274}_{-18}^{+24}$ and ${827}_{-71}^{+100}$ , and the corresponding photosphere radii R ph are in the range of ${1.83}_{-0.50}^{+0.52}\times {10}^{11}$ and ${2.97}_{-0.15}^{+0.14}\times {10}^{12}$ cm. Similar to GRB 090902B, the time-resolved properties of GRB 220426A satisfy the observed Γ-L and E p -L correlations, where L is the luminosity of the prompt emission and E p is the spectral peak energy.

Keywords: gamma-ray bursts
Ion-driven Instabilities in the Inner Heliosphere. II. Classification and Multidimensional Mapping

Linear theory is a well-developed framework for characterizing instabilities in weakly collisional plasmas, such as the solar wind. In the previous installment of this series, we analyzed ~1.5M proton and α particle velocity distribution functions (VDFs) observed by Helios I and II to determine the statistical properties of the standard instability parameters such as the growth rate, frequency, the direction of wave propagation, and the power emitted or absorbed by each component, as well as to characterize their behavior with respect to the distance from the Sun and collisional processing. In this work, we use this comprehensive set of instability calculations to train a machine-learning algorithm consisting of three interlaced components that: (1) predict if an interval is unstable from observed VDF parameters; (2) predict the instability properties for a given unstable VDF; and (3) classify the type of the unstable mode. We use these methods to map the properties in multidimensional phase space to find that the parallel-propagating, proton-core-induced ion cyclotron mode dominates the young solar wind, while the oblique fast magnetosonic mode regulates the proton beam drift in the collisionally old plasma.

Keywords: plasma physics; solar wind