adsabs/SciX_UAT_keywords huggingface dataset:
A CHIME/FRB Study of Burst Rate and Morphological Evolution of the Periodically Repeating FRB 20180916B

FRB 20180916B is a repeating fast radio burst (FRB) with a 16.3 day periodicity in its activity. In this study, we present morphological properties of 60 FRB 20180916B bursts detected by CHIME/FRB between 2018 August and 2021 December. We recorded raw voltage data for 45 of these bursts, enabling microseconds time resolution in some cases. We studied variation of spectro-temporal properties with time and activity phase. We find that the variation in dispersion measure (DM) is ≲1 pc cm-3 and that there is burst-to-burst variation in scattering time estimates ranging from ~0.16 to over 2 ms, with no discernible trend with activity phase for either property. Furthermore, we find no DM and scattering variability corresponding to the recent change in rotation measure from the source, which has implications for the immediate environment of the source. We find that FRB 20180916B has thus far shown no epochs of heightened activity as have been seen in other active repeaters by CHIME/FRB, with its burst count consistent with originating from a Poissonian process. We also observe no change in the value of the activity period over the duration of our observations and set a 1σ upper limit of 1.5 × 10-4 day day-1 on the absolute period derivative. Finally, we discuss constraints on progenitor models yielded by our results, noting that our upper limits on changes in scattering and DM as a function of phase do not support models invoking a massive binary companion star as the origin of the 16.3 day periodicity.

Keywords: radio transient sources; radio bursts
SDSS J1058+5443: A Blue Quasar without Optical/NUV Broad Emission Lines

In this paper, the blue quasar SDSS J105816.19+544310.2 (=SDSS J1058+5443) at redshift 0.479 has been reported as the best true type 2 quasar candidate with the disappearance of central broad-line regions. There are no definite conclusions on the very existence of true type 2 active galactic nuclei (AGN), mainly due to detected optical broad emission lines in high-quality spectra of some previously classified true type 2 AGN candidates. Here, unlike previously reported true type 2 AGN candidates among narrow emission-line galaxies with weak AGN activities but strong stellar lights, the definitely blue quasar SDSS J1058+5443 can be well confirmed as a true type 2 quasar due to apparent quasar-shape blue continuum emissions but an apparent loss of both the optical broad Balmer emission lines and the near-UV (NUV) broad Mg II emission line. Based on different model functions and the F-test statistical technique, after considering blueshifted optical and UV Fe II emissions, there are no apparent broad optical Balmer emission lines and/or broad NUV Mg II lines, and the confidence level is smaller than 1σ in support of broad optical and NUV emission lines. Moreover, assuming the virialization assumption to broad-line emission clouds, the reconstructed broad emission lines strongly indicate that the probable intrinsic broad emission lines, if they exist, cannot be hidden or overwhelmed in the noise of the Sloan Digital Sky Survey spectrum of SDSS J1058+5443. Therefore, SDSS J1058+5443 is so far the best and most robust true type 2 quasar candidate, leading to the clear conclusion of the very existence of true type 2 AGN.

Keywords: active galaxies; active galactic nuclei; quasars; supermassive black holes
Cascade Radiations of e ± from γγ-annihilation Process as an Extra Component of the Early Optical/X-Ray Afterglows of Gamma-Ray Bursts

Chromatic break and/or plateau observed in the early optical and X-ray afterglow lightcurves challenge the conventional external shock models of gamma-ray bursts (GRBs). Detection of TeV gamma-ray afterglows indicates strong gamma-ray production within the afterglow jets. We investigate the cascade radiations of the e ± production via the γ γ interaction in the jets. Our numerical calculations show that the cascade synchrotron emission can make a significant contribution to the early optical/X-ray afterglows. The combination of the primary and cascade emission fluxes can shape a chromatic break and/or plateau in the early optical/X-ray lightcurves, depending on the jet properties. Applying our model to GRBs 050801 and 080310, we found that their optical plateaus and the late X-ray/optical lightcurves can be explained with our model in reasonable parameter values. We suggest that such a chromatic optical plateau could be a signature of strong e ± production in GRB afterglow jets. The TeV gamma-ray flux of such GRBs should be significantly reduced and hence tends to be detectable for those GRBs that have a single power-law decaying optical afterglow lightcurve.

Keywords: gamma-ray bursts; high energy astrophysics; non-thermal radiation sources
SN 2018gk Revisited: the Photosphere, the Central Engine, and the Putative Dust

We perform a comprehensive study of the physical properties of SN 2018gk, which is a luminous Type IIb supernova (SN). We find that the early-time photospheric velocity varies from a larger value to a smaller one before the photosphere reaches a temperature floor. We generalize the photosphere modulus and fit the multiband light curves (LCs) of SN 2018gk. We find that the 56Ni mass model requires ~0.90 M ⊙ of 56Ni, which is larger than the derived ejecta mass (~0.10 M ⊙). Alternatively, we use the magnetar plus 56Ni and the fallback plus 56Ni models to fit the LCs of SN 2018gk, finding that the two models can fit the LCs. We favor the magnetar plus 56Ni model since the parameters are rather reasonable (M ej = 1.65 M ⊙, M Ni = 0.05 M ⊙, which is smaller than the upper limit of the 56Ni mass that can by synthesized by neutrino-powered core-collapse SNe, B = 6.52 × 1014 G, which is comparable to magnetic fields in luminous and superluminous SNe studied in the literature, and P 0 = 10.42 ms, which is comparable to initial periods for luminous SNe), while the validity of the fallback plus 56Ni model depends on the accretion efficiency (η). Therefore, we suggest that SN 2018gk might be an SN IIb powered mainly by a central engine. Finally, we confirm the near-IR excesses of the spectral energy distributions of SN 2018gk at some epochs and constrain the physical properties of the putative dust using the blackbody plus dust emission model.

Keywords: supernovae; type ii supernovae; magnetars; circumstellar dust
Three Long-period Transiting Giant Planets from TESS

We report the discovery and orbital characterization of three new transiting warm giant planets. These systems were initially identified as presenting single-transit events in the light curves generated from the full-frame images of the Transiting Exoplanet Survey Satellite. Follow-up radial velocity measurements and additional light curves were used to determine the orbital periods and confirm the planetary nature of the candidates. The planets orbit slightly metal-rich late F- and early G-type stars. We find that TOI 4406b has a mass of M P = 0.30 ± 0.04 M J, a radius of R P = 1.00 ± 0.02 R J, and a low-eccentricity orbit (e = 0.15 ± 0.05) with a period of P = 30.08364 ±0.00005 days. TOI 2338b has a mass of M P = 5.98 ± 0.20 M J, a radius of R P = 1.00 ± 0.01 R J, and a highly eccentric orbit (e = 0.676 ± 0.002) with a period of P = 22.65398 ± 0.00002 days. Finally, TOI 2589b has a mass of M P = 3.50 ± 0.10 M J, a radius of R P = 1.08 ± 0.03 R J, and an eccentric orbit (e = 0.522 ± 0.006) with a period of P = 61.6277 ± 0.0002 days. TOI 4406b and TOI 2338b are enriched in metals compared to their host stars, while the structure of TOI 2589b is consistent with having similar metal enrichment to its host star. *Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere under ESO programmes 0104.C-0413, 106.21ER.001, and 108.22A8.001 and MPG programmes 0103.A-9008 and 0104.A-9007.

Keywords: exoplanets; extrasolar gaseous giant planets; radial velocity; transit photometry
False Planets around Giant Stars: A Case Study of Sanders 364 in M67

Discovering planets in sparsely populated regions of parameter space is crucial to improving our understanding of planetary formation and evolution. One such region is the subset of planets that orbit giant, evolved stars. However, some of these evolved stars are known to exhibit long-period quasiperiodic radial velocity signals, which can masquerade as signals from orbital motion due to planetary companions. In this paper, we investigate the case of Sanders 364, a K giant star in the old open cluster M67. A paper by Brucalassi et al. reports the discovery of a giant planet with a period of 121 days orbiting Sanders 364. From our analysis of a large set of independent radial velocities, we find no convincing evidence for the giant planet reported by Brucalassi et al. We did identify six long-period radial velocity signals of unclear origin, including the 121 days signal reported by Brucalassi et al., but based on our analysis, we speculate that these are quasiperiodic signals that arise from nonplanetary origins, such as stellar variability or aliasing. The results from our study of Sanders 364 suggest that the detection of true orbital motion from a long-period planetary companion requires extra care when the host star is highly evolved. We conclude by offering recommendations for future study of planetary companions around evolved host stars.

Keywords: k giant stars; exoplanets; radial velocity
Two-zone Spatially Dependent Model of the Nonthermal Emission from Plerionic Supernova Remnant G21.5-0.9

Observations of plerionic supernova remnant (SNR) G21.5-0.9 indicate an extended X-ray halo surrounding the bright central pulsar wind nebula (PWN), and that the photon index and the surface brightness of the SNR have been discovered to change with the increase of radial distance in the X-ray band. To explain the observational features of this source, a two-zone spatially dependent model with a slow diffusion in the PWN and a fast diffusion in the extended region is proposed in this paper. In the model, the evolution of electron inside the entire remnant is described with a particle transport equation under the assumption of a spherically symmetric system with dynamical evolution. The observed photon spectral energy distribution of the central PWN, as well as the radial profiles of the photon index and surface brightness observed in the X-ray band of the entire remnant, can be well reproduced in the framework of the model. The modeling results reveal that the X-ray halo of SNR G21.5-0.9 is mainly due to the synchrotron radiation of the accelerated electrons derived from the PWN shock, and the current diffusion coefficient in the extended region has a value of 1.7 × 1028 cm2 s-1 at an electron energy of 1 TeV, which is much larger than the obtained spatial averaged diffusion coefficient of 2.5 × 1025 cm2 s-1 in the central nebula.

Keywords: interstellar medium; supernova remnants; pulsars
X-Ray Spectral Correlations in a Sample of Low-mass Black Hole X-Ray Binaries in the Hard State

The power-law emission and reflection component provide valuable insights into the accretion process around a black hole. In this work, thanks to the broadband spectra coverage of the Nuclear Spectroscopic Telescope Array, we study the spectral properties for a sample of low-mass black hole X-ray binaries (BHXRBs). We find that there is a positive correlation between the photon index Γ and the reflection fraction R (the ratio of the coronal intensity that illuminates the disk to the coronal intensity that reaches the observer), consistent with previous studies, but except for MAXI J1820+070. It is quite interesting that this source also deviates from the well-known "V"-shaped correlation between the photon index Γ and the X-ray luminosity $\mathrm{log}{L}_{{\rm{X}}}$ , when it is in the bright hard state. More specifically, the Λ-shaped correlation between Γ and logL X is observed, as the luminosity decreases by a factor of 3 in a narrow range from ~1038 to 1037.5 erg s-1. Furthermore, we discover a strong positive correlation between R and the X-ray luminosity for BHXRBs in the hard state, which puts a constraint on the disk-corona coupling and the evolution.

Keywords: black hole physics; x-ray transient sources; accretion; stellar accretion disks
The Extraordinary Outburst in the Massive Protostellar System NGC 6334 I-MM1: Strong Increase in Mid-Infrared Continuum Emission

In recent years, dramatic outbursts have been identified toward massive protostars via infrared and millimeter dust continuum and molecular maser emission. The longest lived outburst (>6 yr) persists in NGC 6334 I-MM1, a deeply embedded object with no near-IR counterpart. Using FORCAST and HAWC+ on SOFIA, we have obtained the first mid-IR images of this field since the outburst began. Despite being undetected in pre-outburst ground-based 18 μm images, MM1 is now the brightest region at all three wavelengths (25, 37, and 53 μm), exceeding the UCHII region MM3 (NGC 6334 F). Combining the SOFIA data with ALMA imaging at four wavelengths, we construct a spectral energy distribution of the combination of MM1 and the nearby hot core MM2. The best-fit Robitaille radiative transfer model yields a luminosity of (4.9 ± 0.8) × 104 L⊙. Accounting for an estimated pre-outburst luminosity ratio MM1:MM2 = 2.1 ± 0.4, the luminosity of MM1 has increased by a factor of 16.3 ± 4.4. The pre-outburst luminosity implies a protostar of mass 6.7 M⊙, which can produce the ionizing photon rate required to power the pre-outburst HCHII region surrounding the likely outbursting protostar MM1B. The total energy and duration of the outburst exceed the S255IR-NIRS3 outburst by a factor of ≳3, suggesting a different scale of event involving expansion of the protostellar photosphere (to ≳20 R⊙), thereby supporting a higher accretion rate (≳0.0023 M⊙ yr-1) and reducing the ionizing photon rate. In the grid of hydrodynamic models of Meyer et al., the combination of outburst luminosity and magnitude (3) places the NGC 6334 I-MM1 event in the region of moderate total accretion (∼0.1-0.3 M⊙) and hence long duration (∼40-130 yr).

Keywords: protostars; star formation; far infrared astronomy; submillimeter astronomy; millimeter astronomy; infrared astronomy; interstellar masers; stellar jets; dust continuum emission; radio continuum emission; radio interferometry; accretion
Stellar Properties of Observed Stars Stripped in Binaries in the Magellanic Clouds

Massive stars (~8-25 M ⊙) stripped of their hydrogen-rich envelopes via binary interaction are thought to be the main progenitors for merging neutron stars and stripped-envelope supernovae. We recently presented the discovery of the first set of such stripped stars in a companion paper. Here, we fit the spectra of 10 stars with new atmosphere models in order to constrain their stellar properties precisely. We find that the stellar properties align well with the theoretical expectations from binary evolution models for helium-core burning envelope-stripped stars. The fits confirm that the stars have high effective temperatures (T eff ~ 50-100 kK), high surface gravities ( $\mathrm{log}g\sim $ 5), and hydrogen-poor/helium-rich surfaces (X H,surf ~ 0-0.4) while showing for the first time a range of bolometric luminosities (103-105 L ⊙), small radii (~0.5-1 R ⊙), and low Eddington factors (Γ e ~ 0.006-0.4). Using these properties, we derive intermediate current masses (~1-8 M ⊙), which suggest that their progenitors were massive stars (~5-25 M ⊙) and that a subset will reach core-collapse, leaving behind neutron stars or black holes. Using the model fits, we also estimate the emission rates of ionizing photons for these stars, which agree well with previous model expectations. Further, by computing models for a range of mass-loss rates, we find that the stellar winds are weaker than predicted by any existing scheme ( ${\dot{M}}_{\mathrm{wind}}\lesssim {10}^{-9}$ M ⊙ yr-1). The properties of this first sample of intermediate-mass helium stars suggest they both contain progenitors of type Ib and IIb supernovae, and provide important benchmarks for binary evolution and population synthesis models.

Keywords: binary stars; close binary stars; interacting binary stars; early-type stars; helium-rich stars; helium burning; stellar properties; stellar spectral types; stellar spectral lines; ionization; stellar winds
Photometric Redshifts with the LSST. II. The Impact of Near-infrared and Near-ultraviolet Photometry

Accurate photometric redshift (photo-z) estimates are essential to the cosmological science goals of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). In this work, we use simulated photometry for mock galaxy catalogs to explore how LSST photo-z estimates can be improved by the addition of near-infrared (NIR) and/or ultraviolet (UV) photometry from the Euclid, Wide-Field InfrarRed Survey Telescope (WFIRST), and/or Cosmological Advanced Survey Telescope for Optical and ultraviolet Research (CASTOR) space telescopes. Generally, we find that deeper optical photometry can reduce the standard deviation of the photo-z estimates more than adding NIR or UV filters but that additional filters are the only way to significantly lower the fraction of galaxies with catastrophically under- or overestimated photo-z. For Euclid, we find that the addition of JH 5σ photometric detections can reduce the standard deviation for galaxies with z > 1 (z > 0.3) by ∼20% (∼10%), and the fraction of outliers by ∼40% (∼25%). For WFIRST, we show how the addition of deep YJHK photometry could reduce the standard deviation by ≳50% at z > 1.5 and drastically reduce the fraction of outliers to just ∼2% overall. For CASTOR, we find that the addition of its UV- and u-band photometry could reduce the standard deviation by ∼30% and the fraction of outliers by ∼50% for galaxies with z < 0.5. We also evaluate the photo-z results within sky areas that overlap with both the NIR and UV surveys and when spectroscopic training sets built from the surveys' small-area deep fields are used.

Keywords: cosmology; galaxy photometry; multi-color photometry; sky surveys
A Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?

We present the study of a Lyman limit system (LLS) at zabs = 1.5441 toward quasar J134122.50+185213.9 observed with VLT X-shooter. This is a very peculiar system with strong C I absorption seen associated with a neutral hydrogen column density of log N(H I) (cm-2) = 18.10, too small to shield the gas from any external UV flux. The low-ionization absorption lines exhibit a simple kinematic structure consistent with a single component. Using CLOUDY models to correct for ionization, we find that the ionization parameter of the gas is in the range -4.5 < log U < -4.2 and the gas density -1.5 < log n(H) (cm-3) < -1.2. The models suggest that carbon is overabundant relative to iron, [C/Fe] > +2.2 at [Fe/H] ∼-1.6. Such a metal abundance pattern is reminiscent of carbon-enhanced metal-poor stars detected in the Galaxy halo. Metal enrichment by the first generation of supernovae provide a plausible explanation for the inferred abundance pattern in this system.

Keywords: quasar absorption line spectroscopy; lyman limit systems; cemp stars; quasars; chemically peculiar stars
LIGO/Virgo Sources from Merging Black Holes in Ultradwarf Galaxies

The origins of the black hole-black hole mergers discovered through gravitational waves with the LIGO/Virgo Collaboration are a mystery. We investigate the idea that some of these black holes originate from the centers of extremely low mass ultradwarf galaxies that have merged together in the distant past at z > 1. Extrapolating the central black hole/stellar mass ratio suggests that the black holes in these mergers could arise from galaxies of masses ∼105-106 M⊙. We investigate whether these galaxies merge at a rate consistent with the observed black hole rate of ∼9.7-101 Gpc-3 yr-1 using the latest LIGO/Virgo results. We show that in the nearby universe the merger rate and number densities of ultradwarf galaxies are too low, by an order or magnitude, to produce these black hole mergers. However, by considering that the merger fraction, merger timescales, and the number densities of low-mass galaxies all conspire at z > 1-1.5 to increase the merger rate for these galaxies at higher redshifts, we argue that it is possible that these observed gravitational wave events could arise from black holes in the centers of low-mass galaxies. The major uncertainty in this calculation is the merger dynamical timescales for black holes in low-mass galaxies. Our results suggest that a very long black hole merger timescale of 4-7 Gyr is needed, consistent with an extended merger history. Further simulations of black hole merger timescales are needed to test this possibility; however, our theory can be tested by searching for host galaxies of gravitational wave events. Results from these searches will put limits on dwarf galaxy mergers and/or the presence and formation mechanisms of black holes through Population III stars in the lowest-mass galaxies.

Keywords: gravitational waves; dwarf galaxies; galaxy mergers; intermediate-mass black holes; supermassive black holes; galaxy evolution; ligo
The Frequency Drift and Fine Structures of Solar S-bursts in the High Frequency Band of LOFAR

Solar S-bursts are short duration (<1 s at decameter wavelengths) radio bursts that have been observed during periods of moderate solar activity, where S stands for short. The frequency drift of S-bursts can reflect the coronal density variation and the motion state of the electron beams. In this work, we investigate the frequency drift and the fine structure of the S-bursts with the Low Frequency Array (LOFAR). We find that the average frequency drift rate of the S-bursts within 20-180 MHz could be described by df/dt = -0.0077f1.59, combined with previous results in low frequency. With the high time and frequency resolution of LOFAR, we can resolve the fine structures of the observed solar S-bursts. A fine drift variation pattern was found in the structure of S-bursts (referred to as solar Sb-bursts in this paper) during the type-III storm on 2019 April 13, in the frequency band of 120-240 MHz. The Sb-bursts have a quasiperiodic segmented pattern, and the relative flux intensity tends to be large when the frequency drift rate is relatively large. This kind of structure exists in about 20% of the solar S-burst events within the observed frequency range. We propose that the fine structure is due to the density fluctuations of the background coronal density. We performed a simulation based on this theory that can reproduce the shape and relative flux intensity of the Sb-bursts. This work shows that the fine structure of solar radio bursts can be used to diagnose the coronal plasma.

Keywords: solar radio emission; solar corona; radio bursts; radiative transfer