adsabs/SciX_UAT_keywords huggingface dataset:
NuSTAR Observations of Abell 665 and 2146: Constraints on Nonthermal Emission

Observations from past missions such as RXTE and Beppo-SAX suggested the presence of inverse Compton (IC) scattering at hard X-ray energies within the intracluster medium of some massive galaxy clusters. In subsequent years, observations by, e.g., Suzaku, and now NuSTAR, have not been able to confirm these detections. We report on NuSTAR hard X-ray searches for IC emission in two massive galaxy clusters, A665 and A2146. To constrain the global IC flux in these two clusters, we fit global NuSTAR spectra with three models: single (1T) and two-temperature (2T) models, and a 1T plus power-law component (T+IC). The temperature components are meant to characterize the thermal ICM emission, while the power law represents the IC emission. We find that the 3-30 keV A665 and 3-20 keV A2146 spectra are best described by thermal emission alone, with average global temperatures of kT = (9.15 ± 0.1) keV for A665 and kT = (8.29 ± 0.1) keV for A 2146. We constrain the IC flux to F NT <0.60 × 10-12 erg s-1 cm-2 and F NT < 0.85 × 10-12 erg s-1 cm-2 (20-80 keV) for A665 and A2146, respectively both at the 90% confidence level. When we couple the IC flux limits with 1.4 GHz diffuse radio data from the VLA, we set lower limits on the average magnetic field strengths of >0.14 μG and >0.011 μG for A665 and A2146, respectively.

Keywords: galaxy clusters; abell clusters; intracluster medium; non-thermal radiation sources; astronomical radiation sources; x-ray astronomy
The Ionizing Photon Production Efficiency (ξion) of Lensed Dwarf Galaxies at z ∼ 2

We measure the ionizing photon production efficiency (ξion) of low-mass galaxies (107.8-109.8 M⊙) at 1.4 < z < 2.7 to better understand the contribution of dwarf galaxies to the ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of Hα-to-UV luminosity ratios (LHα/LUV) of galaxies to determine the standard log(ξion). Second, we take the logarithm of the total LHα over the total LUV. We prefer the latter, as it provides the total ionizing UV luminosity density of galaxies when multiplied by the nonionizing UV luminosity density. log(ξion) calculated from the second method is ∼0.2 dex higher than the first method. We do not find any strong dependence between log(ξion) and stellar mass, far-UV magnitude (MUV), or UV spectral slope (β). We report a value of log(ξion) ∼ 25.47 ± 0.09 for our UV-complete sample ( $-22\lt {M}_{\mathrm{UV}}\lt -17.3$ ) and ∼25.37 ± 0.11 for our mass-complete sample (7.8 < log(M*) < 9.8). These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log(ξion) is 0.2-0.3 dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity or age. We also find a correlation between log(ξion) and the equivalent widths of Hα and [O III] λ5007 fluxes, confirming that these equivalent widths can be used to estimate ξion. * Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 12201, 12931, 13389, and 14209.

Keywords: dwarf galaxies; galaxy evolution; high-redshift galaxies
Living with Neighbors. III. The Origin of the Spin-Orbit Alignment of Galaxy Pairs: A Neighbor versus the Large-scale Structure

Recent observations revealed a coherence between the spin vector of a galaxy and the orbital motion of its neighbors. We refer to the phenomenon as "the spin-orbit alignment (SOA)" and explore its physical origin via the IllustrisTNG simulation. This is the first study to utilize a cosmological hydrodynamic simulation to investigate the SOA of galaxy pairs. In particular, we identify paired galaxies at z = 0 having the nearest neighbor with mass ratios from 1/10 to 10 and calculate the spin-orbit angle for each pair. Our results are as follows. (a) There exists a clear preference for prograde orientations (i.e., SOA) for galaxy pairs, qualitatively consistent with observations. (b) The SOA is significant for both baryonic and dark matter spins, being the strongest for gas and the weakest for dark matter. (c) The SOA is stronger for less massive targets and for targets having closer neighbors. (d) The SOA strengthens for galaxies in low-density regions, and the signal is dominated by central-satellite pairs in low-mass halos. (e) There is an explicit dependence of the SOA on the duration of interaction with its current neighbor. Taken together, we propose that the SOA witnessed at z = 0 has been developed mainly by interactions with a neighbor for an extended period of time, rather than tidal torque from the ambient large-scale structure.

Keywords: galaxy interactions; galaxy pairs; galaxy encounters; galaxy kinematics; hydrodynamical simulations
Global Mapping of Surface Composition on an Exo-Earth Using Sparse Modeling

The time series of light reflected from exoplanets by future direct imaging can provide spatial information with respect to the planetary surface. We apply sparse modeling to the retrieval method that disentangles the spatial and spectral information from multiband reflected light curves termed as spin-orbit unmixing. We use the ℓ 1-norm and the total squared variation norm as regularization terms for the surface distribution. Applying our technique to a toy model of cloudless Earth, we show that our method can infer sparse and continuous surface distributions and also unmixed spectra without prior knowledge of the planet surface. We also apply the technique to the real Earth data as observed by DSCOVR/EPIC. We determined the representative components that can be interpreted as cloud and ocean. Additionally, we found two components that resembled the distribution of land. One of the components captures the Sahara Desert, and the other roughly corresponds to vegetation, although their spectra are still contaminated by clouds. Sparse modeling significantly improves the geographic retrieval, in particular, of clouds and leads to higher resolutions for other components when compared with spin-orbit unmixing using Tikhonov regularization.

Keywords: astrobiology; astroinformatics; exoplanet astronomy; direct imaging; exoplanet surfaces; exoplanet surface characteristics; exoplanet surface composition
Change-point Detection and Image Segmentation for Time Series of Astrophysical Images

Many astrophysical phenomena are time-varying, in the sense that their intensity, energy spectrum, and/or the spatial distribution of the emission suddenly change. This paper develops a method for modeling a time series of images. Under the assumption that the arrival times of the photons follow a Poisson process, the data are binned into 4D grids of voxels (time, energy band, and x-y coordinates), and viewed as a time series of non-homogeneous Poisson images. The method assumes that at each time point, the corresponding multiband image stack is an unknown 3D piecewise constant function including Poisson noise. It also assumes that all image stacks between any two adjacent change points (in time domain) share the same unknown piecewise constant function. The proposed method is designed to estimate the number and the locations of all of the change points (in time domain), as well as all of the unknown piecewise constant functions between any pairs of the change points. The method applies the minimum description length principle to perform this task. A practical algorithm is also developed to solve the corresponding complicated optimization problem. Simulation experiments and applications to real data sets show that the proposed method enjoys very promising empirical properties. Applications to two real data sets, the XMM observation of a flaring star and an emerging solar coronal loop, illustrate the usage of the proposed method and the scientific insight gained from it.

Keywords: time series analysis; poisson distribution; maximum likelihood estimation; detection; spatial point processes; astronomy data analysis
Preliminary Discussion on the Current Sheet

The current sheet is a characteristic structure of magnetic energy dissipation during the magnetic reconnection process. So far, the width and depth of the current sheet are still indefinite. Here we investigate 64 current sheets observed by four telescopes from 1999 to 2022, and all of them have been well identified in the previous literature. In each current sheet, three width values are obtained at the quartering points. Based on these investigated cases, we obtain 192 values, which are in a wide range from hundreds to tens of thousands of kilometers. By calculating the pixel width (PW: the ratio of the current sheet width to the pixel resolution of corresponding observed data) of these current sheets, we find that more than 80% of the PW values concentrate on 2–4 pixels, indicating that the widths of the current sheets are dependent strongly on the instrument resolutions and all the sheets have no observable three-dimensional information. To interpret this result, we suggest that there are two probabilities. One is that the width of the current sheet is smaller than the instrument resolution, and the other is that the detected current sheet is only a small segment of the real one. Furthermore, there is another possible scenario. The so-called current sheet is just an emission-enhanced region.

Keywords: solar magnetic fields; solar magnetic reconnection; solar activity
The Young Stellar Population, Distance, and Cloud-Cloud Collision Induced Star Formation Scenario of the Trifid Nebula

The Trifid Nebula is a young, nearby star-forming region where star formation is proposed to have been triggered by cloud-cloud collision (CCC), based on observations of molecular clouds. It offers a unique opportunity to test whether the CCC hypothesis is supported by the spatial distribution and star formation chronology of young stars. We present the first study of the optically visible pre-main sequence (PMS) population of the region using riHα imaging and Gaia astrometry. Combined with an analysis of young stellar objects (YSOs) using infrared imaging, we capture the spatial distribution and star formation chronology of the young stellar population. From the analysis, 15 Flat/Class I YSOs, 46 Class II YSOs, and 41 accreting PMS stars are identified (diskless/non-accreting sources are not included in the analysis). The distance based on Gaia parallaxes is ~1250 pc, significantly closer than previously reported. The Class II YSOs and PMS stars (~1.5 Myr old) are spread toward the edge of the molecular clouds. They are slightly younger than the estimated crossing time of ~2.7 Myr and closer to the estimated dynamical age ~0.85 Myr. Younger Class I YSOs are more concentrated spatially. There exists a cavity devoid of young stars where the two clouds overlap. This evidence suggests that the current generation of stars formed after the collision of two clouds ~1 Myr ago, and this result can be corroborated using future spectroscopic studies.

Keywords: star formation; star forming regions; h ii regions; young stellar objects; pre-main sequence stars
GRB 221009A: Spectral Signatures Based on ALPs Candidates

GRB 221009A has posed a significant challenge to our current understanding of the mechanisms that produce TeV photons in gamma-ray bursts (GRBs). On one hand, the Klein–Nishina (KN) effect of the inverse Compton scattering leads to less efficient energy losses of high-energy electrons. On the other hand, at a redshift of 0.151, the TeV spectrum of GRB 221009A undergoes significant absorption by the extragalactic background light (EBL). Therefore, the observation of a 13 TeV photon in this event implies the presence of enormous photon fluxes at the source, which the synchrotron self-Compton mechanism in external shocks cannot easily generate. As an alternative, some authors have suggested the possibility of converting the TeV photons into axion-like particles (ALPs) at the host galaxy, in order to avoid the effects of EBL absorption, and then reconverting them into photons within the Milky Way. While this solution relaxes the requirement of very high photon fluxes, the KN effect still poses a challenge. Previously, we have shown that the injections of ALPs could explain the observation of 13 TeV photons. Here, we include the energy dependence of the probability of survival and the amount of energy carried to determine the ALP candidates, which could potentially explain the TeV photons observed by the Large High Altitude Air Shower Observatory and their hard spectrum. We found that the allowed candidates are generally clustered around masses of 10‑7 eV. We also considered different EBL models, for the one predicting larger attenuation tends to reject ALP candidates with the lowest coupling factor. For some hypotheses of the EBL model, these candidates are found below a region of the parameter space in which, if detected, ALPs could account for all of the cold dark matter in the Universe.

Keywords: dark matter; high energy astrophysics; gamma-ray bursts
Properties of Steady Sub-Alfvénic Solar Wind in Comparison with Super-Alfvénic Wind from Parker Solar Probe Measurements

We identify more than 10 steady sub-Alfvénic solar wind intervals from the measurements of the Parker Solar Probe (PSP) from encounter 8 to encounter 14. An analysis of these sub-Alfvénic intervals reveals similar properties and similar origins. In situ measurements show that these intervals feature a decreased radial Alfvén Mach number resulting from a reduced density and a relatively low velocity, and that switchbacks are suppressed in these intervals. Magnetic source tracing indicates that these sub-Alfvénic streams generally originate from the boundaries inside coronal holes or narrow/small regions of open magnetic fields. Such properties and origins suggest that these streams are mostly low Mach-number boundary layers (LMBLs), which is a special component of the pristine solar wind proposed by Liu et al. We find that the LMBL wind, the fast wind from deep inside coronal holes, and the slow streamer wind constitute three typical components of the young solar wind near the Sun. In these sub-Alfvénic intervals, the Alfvén radius varies between 15 and 25 solar radii, in contrast with a typical 12 radii for the Alfvén radius of the super-Alfvénic wind. These results give a self-consistent picture interpreting the PSP measurements in the vicinity of the Sun.

Keywords: solar wind; solar coronal holes; solar corona; alfven waves
First Combined Study on Lorentz Invariance Violation from Observations of Energy-dependent Time Delays from Multiple-type Gamma-Ray Sources. I. Motivation, Method Description, and Validation through Simulations of H.E.S.S., MAGIC, and VERITAS Data Sets

Gamma-ray astronomy has become one of the main experimental ways to test the modified dispersion relations (MDRs) of photons in vacuum, obtained in some attempts to formulate a theory of quantum gravity. The MDRs in use imply time delays that depend on the energy and that increase with distance following some function of redshift. The use of transient, or variable, distant and highly energetic sources already allows us to set stringent limits on the energy scale related to this phenomenon, usually thought to be of the order of the Planck energy, but robust conclusions on the existence of MDR-related propagation effects still require the analysis of a large population of sources. In order to gather the biggest sample of sources possible for MDR searches at teraelectronvolt energies, the H.E.S.S., MAGIC, and VERITAS collaborations enacted a joint task force to combine all their relevant data to constrain the quantum gravity energy scale. In the present article, the likelihood method used to combine the data and provide a common limit is described in detail and tested through simulations of recorded data sets for a gamma-ray burst, three flaring active galactic nuclei, and two pulsars. Statistical and systematic errors are assessed and included in the likelihood as nuisance parameters. In addition, a comparison of two different formalisms for distance dependence of the time lags is performed for the first time. In a second article, to appear later, the method will be applied to all relevant data from the three experiments.

Keywords: gamma-ray astronomy; gamma-rays
The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82

We report photometric estimates of effective temperature, Teff, metallicity, [Fe/H], carbonicity, [C/Fe], and absolute carbon abundances, A(C), for over 700,000 stars from the Southern Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: (1) halo main-sequence turnoff stars and (2) K-dwarf stars of mass 0.58 < M/M⊙ < 0.75 in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (SPHINX) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperatures, 4250 < Teff(K) < 7000. The use of multiple narrow-band S-PLUS filters enables SPHINX to achieve substantially lower levels of "catastrophic failures" (i.e., large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way's K-dwarf halo metallicity distribution function (MDF), λ10,[Fe/H] = 0.85 ± 0.21, over the metallicity range -2.5 < [Fe/H] < -1.0; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters α = 2.8 and β = 4.2. S-PLUS photometry obtains absolute carbon abundances with a precision of ∼0.35 dex for stars with Teff < 6500 K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon-Beers morphological groups in the A(C)-[Fe/H] space, and we derive the CEMP frequencies.

Keywords: stellar abundances; milky way stellar halo; solar neighborhood; narrow band photometry; medium band photometry; sky surveys; milky way disk
Exploring the Temporal Variation of the Solar Quadrupole Moment J 2

Recently, Rozelot & Eren pointed out that the first solar gravitational moment (J 2) might exhibit a temporal variation. The suggested explanation is through the temporal variation of the solar rotation with latitude. This issue is deeper developed due to an accurate knowledge of the long-term variations in solar differential rotation regarding solar activity. Here we analyze solar cycles 12-24, investigating the long-term temporal variations in solar differential rotation. It is shown that J 2 exhibits a net modulation over the 13 studied cycles of ≈(89.6 ± 0.1) yr, with a peak-to-peak amplitude of ≈0.1 × 10-7 for a reference value of 2.07 × 10-7). Moreover, J 2 exhibits a positive linear trend in the period of minima solar activity (sunspot number up to around 40) and a marked declining trend in the period of maxima (sunspot number above 50). In absolute magnitude, the mean value of J 2 is more significant during periods of minimum than in periods of maximum. These findings are based on observational results that are not free of errors and can be refined further by considering torsional oscillations for example. They are comforted by identifying a periodic variation of the J 2 term evidenced through the analysis of the perihelion precession of planetary orbits either deduced from ephemerides or computed in the solar equatorial coordinate system instead of the ecliptic coordinate one usually used.

Keywords: solar physics; solar activity; solar rotation; sunspots; gravitation; fundamental parameters of stars; equatorial coordinate system; ecliptic coordinate system; solar evolution; the sun; solar motion
The Primary Proton Spectrum of the Hadronic PeVatron Candidate HAWC J1825-134

The γ-ray spectrum of the source HAWC J1825-134 measured with the High Altitude Water Cherenkov (HAWC) observatory extends beyond 200 TeV without any evidence for a steepening or cutoff. There are some indications that the γ-rays detected with HAWC were produced by cosmic-ray protons or nuclei colliding with the ambient gas. Assuming primary protons, we inquire which shape of the primary proton spectrum is compatible with the HAWC measurements. We find that the primary proton spectrum with the power-law shape of γ p = 2.2 and the cutoff energy E c-p > 500 TeV describes the data well. However, much harder spectra with γ p down to 1.3 and E c-p as low as 200 TeV also do not contradict the HAWC measurements. The former option might be realized if the accelerator is inside or very near to the γ-ray production zone. The latter option is viable for the case of a cosmic-ray source that effectively confines low-energy (E p < 10 TeV) accelerated protons. Using publicly available data of the Fermi-LAT space γ-ray telescope, we derive upper limits on the intensity of the HAWC J1825-134 source in the 1 GeV-1 TeV energy range. We show that the account of these upper limits drastically changes the interpretation: only hard (γ p < 1.7) spectra describe the combined HAWC and Fermi-LAT data sets well.

Keywords: gamma-ray astronomy; gamma-ray sources; gamma-rays; gamma-ray observatories; galactic cosmic rays; high energy astrophysics
Simultaneous Detection of Optical Flares of the Magnetically Active M-dwarf Wolf359

We present detections of stellar flares of Wolf 359, an M6.5 dwarf in the solar neighborhood (2.41 pc) known to be prone to flares due to surface magnetic activity. The observations were carried out from 2020 April 23 to 29 with a 1 m and a 0.5 m telescope separated by nearly 300 km in Xinjiang, China. In 27 hr of photometric monitoring, a total of 13 optical flares were detected, each with a total energy of ≳ 5 × 1029 erg. The measured event rate of about once every two hours is consistent with those reported previously in radio, X-ray, and optical wavelengths for this star. One such flare, detected by both telescopes on April 26, was an energetic event with a released energy of nearly 1033 erg. The two-telescope light curves of this major event sampled at different cadences and exposure timings enabled us to better estimate the intrinsic flare profile, which reached a peak of up to 1.6 times the stellar quiescent brightness, that otherwise would have been underestimated in the observed flare amplitudes of about 0.4 and 0.8, respectively, with single telescopes alone. The compromise between fast sampling so as to resolve a flare profile versus a longer integration time for a higher photometric signal-to-noise ratio provides a useful guidance in the experimental design of future flare observations.

Keywords: stellar flares