adsabs/SciX_UAT_keywords huggingface dataset:
Eliminating Primary Beam Effect in Foreground Subtraction of Neutral Hydrogen Intensity Mapping Survey with Deep Learning

In neutral hydrogen (H I) intensity mapping (IM) survey, foreground contamination on cosmological signal is extremely severe, and systematic effects caused by radio telescopes further aggravate the difficulties in subtracting foreground. We investigate whether the deep-learning method, the 3D U-Net algorithm, can play a crucial role in foreground subtraction when considering the systematic effect caused by the telescope's primary beam. We consider two beam models, i.e., the Gaussian beam and Cosine beam models. The traditional principal component analysis (PCA) method is employed as a preprocessing step for the U-Net method to reduce the map dynamic range. We find that in the case of the Gaussian beam, the PCA method can effectively clean the foreground. However, the PCA method cannot handle the systematic effect induced by the Cosine beam, and the additional U-Net method can improve the result significantly. To show how well the PCA and U-Net methods can recover the H I signal, we also derive the H I angular power spectrum and H I 2D power spectrum after performing foreground subtraction. It is found that in the case of Gaussian beam, the concordance with the original H I map using U-Net is better than that using PCA by 27.4%, and in the case of Cosine beam, the concordance using U-Net is better than that using PCA by 144.8%. Therefore, the U-Net-based foreground subtraction can efficiently eliminate the telescope primary beam effect and shed new light on recovering H I power spectrum for future H I IM experiments.

Keywords: h i line emission; sky surveys; radio telescopes; neural networks; large-scale structure of the universe; observational cosmology
Atomic Transition Probabilities for Transitions of Si I and Si II and the Silicon Abundances of Several Very Metal-poor Stars

We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si I) originating in the 3s 23p4s 3Po 1,2,1Po 1, and 3s3p 3 1Do 1,2 upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the 4P1/2-2Po 1/2,3/2 intercombination lines in the spectrum of singly ionized silicon (Si II). The weak spin-forbidden lines of Si I and Si II provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars. ∗ Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Other data have been obtained from the European Southern Observatory (ESO) Science Archive Facility; and the Keck Observatory Archive, which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute, under contract with NASA. These data are associated with HST programs GO-7402, GO-14161, and GO-14232; ESO programs 66.D-0636(A), 073.D-0024(A), and 095.D-0504(A); and Keck program H41aH.

Keywords: transition probabilities; atomic spectroscopy; stellar abundances
Magnetic Fields in Solar Plage Regions: Insights from High-sensitivity Spectropolarimetry

Plage regions are patches of concentrated magnetic field in the Sun's atmosphere where hot coronal loops are rooted. While previous studies have shed light on the properties of plage magnetic fields in the photosphere, there are still challenges in measuring the overlying chromospheric magnetic fields, which are crucial to understanding the overall heating and dynamics. Here, we utilize high-sensitivity, spectropolarimetric data obtained by the 4 meter Daniel K. Inouye Solar Telescope to investigate the dynamic environment and magnetic field stratification of an extended, decaying plage region. The data show strong circular polarization signals in both plage cores and surrounding fibrils. Notably, weak linear polarization signals clearly differentiate between plage patches and the fibril canopy, where they are relatively stronger. Inversions of the Ca II 8542 Å spectra show an imprint of the fibrils in the chromospheric magnetic field, with typical field strength values ranging from ~200 to 300 G in fibrils. We confirm the weak correlation between field strength and cooling rates in the lower chromosphere. Additionally, we observe supersonic downflows and strong velocity gradients in the plage periphery, indicating dynamical processes occurring in the chromosphere. These findings contribute to our understanding of the magnetic field and dynamics within plages, emphasizing the need for further research to explore the expansion of magnetic fields with height and the three-dimensional distribution of heating rates in the lower chromosphere.

Keywords: plages; spectropolarimetry; solar atmosphere; solar chromosphere; solar magnetic fields
A Large-scale, Regular Intergalactic Magnetic Field Associated with Stephan's Quintet?

Regular magnetic fields are frequently found within and in the outskirts of galaxies, but their presence, properties, and origin has not yet been established for galaxy groups. On the basis of broadband radio polarimetric imaging with the Westerbork Synthesis Radio Telescope, we made use of rotation measure synthesis to disentangle contributions from magnetic fields on various scales for several polarized radio sources inside, behind, or in the vicinity of Stephan's Quintet (HCG 92, SQ). We recognize the signature of a large-scale, genuinely regular, magnetized screen, seemingly constrained to the Quintet itself. Although we cannot exclude a contribution from the Milky Way, our analysis favors a magnetic structure within the SQ system. If indeed associated with the galaxy group in question, it would span a volume of at least 60 × 40 × 20 kpc and have a strength at least as high as that previously detected within large spiral galaxies. This field would then surpass the extent of any other known galactic, regular magnetic fields; have a considerable strength of a few microgauss; and would be the first known example of such a structure in a galaxy system other than a galaxy pair. Several other explanations are also presented and evaluated.

Keywords: galaxy groups; hickson compact group; extragalactic magnetic fields; spectropolarimetry; polarimetry
The Low-redshift Lyman Continuum Survey. I. New, Diverse Local Lyman Continuum Emitters

The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a z = 0.2-0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.

Keywords: reionization; galactic and extragalactic astronomy; ultraviolet astronomy; hubble space telescope
Evaluation of Scale-dependent Kurtosis with HelioSwarm

Plasma turbulence involves complex, nonlinear interactions of electromagnetic fields and charged particles across multiple scales. Studying these phenomena in space plasmas, like the solar wind, is facilitated by the intrinsic scale separations and the availability of in situ spacecraft observations. However, the single-point or single-scale configurations of current spacecraft limit our understanding of many properties of the turbulent solar wind. To overcome these limitations, multipoint measurements spanning a range of characteristic scales are essential. This Letter prepares for the enhanced measurement capabilities of upcoming multispacecraft missions by demonstrating that higher-order statistics, specifically kurtosis, as a baseline for intermittency can be accurately measured. Using synthetic turbulent fields with adjustable intermittency levels, we achieve scale separations analogous to those in the solar wind and apply these techniques to the planned trajectories of the HelioSwarm mission. This approach promises significant advancements in our understanding of plasma turbulence.

Keywords: solar wind; interplanetary turbulence; theoretical techniques
Arp 220: A Post-starburst Galaxy with Little Current Star Formation outside of Its Nuclear Disks

The ultraluminous infrared galaxy Arp 220 is a late-stage merger with several tidal structures in the outskirts and two very compact, dusty nuclei that show evidence for extreme star formation and host at least one active galactic nucleus (AGN). New and archival high-resolution images taken by the Hubble Space Telescope provide a state-of-the-art view of the structures, dust, and stellar clusters in Arp 220. These images cover the near-ultraviolet, optical, and near-infrared in both broad- and narrowband filters. We find that ~90% of the Hα emission arises from a shock-ionized bubble emanating from the AGN in the western nucleus, while the nuclear disks dominate the Paβ emission. Four very young (~3-6 Myr) but lower-mass (≲104 M ⊙) clusters are detected in Hα within a few arcseconds of the nuclei, but they produce less than 1% of the line emission. We see little evidence for a population of massive clusters younger than 100 Myr anywhere in Arp 220, unlike previous reports in the literature. From the masses and ages of the detected clusters, we find that star formation took place more or less continuously starting approximately a few gigayears ago with a moderate rate between ≈3 and 12 M ⊙ yr-1. Approximately 100 Myr ago, star formation shut off suddenly everywhere (possibly due to a merging event), except in the nuclear disks. A very recent flicker of weak star formation produced the four young, low-mass clusters, while the rest of the galaxy appears to have remained in a post-starburst state. Cluster ages indicate that the tidal structures on the west side of the galaxy are older than those on the east side, but all appear to predate the shutoff of star formation. Arp 220 has many of the characteristics expected of a "shocked post-starburst galaxy," since most of the system has been in a post-starburst state for the past ~100 Myr and the detected Hα emission arises from shocked rather than photoionized gas.

Keywords: galaxy evolution
Systematics-insensitive Periodogram for Finding Periods in TESS Observations of Long-period Rotators

NASA's TESS mission \citep{tess} has produced high precision photometry of millions of stars to the community. The majority of TESS observations have a duration of $\approx$27 days, corresponding to a single observation during a TESS sector. A small subset of TESS targets are observed for multiple sectors, with approximately 1-2\% of targets falling in the Continuous Viewing Zone (CVZ) during the prime mission \citep{yield}, where targets are observed continuously for a year. These targets are highly valuable for extracting long period rotation rates, which can be linked to stellar ages. We present a pip installable Python tool for extracting long period rotation rates in the TESS CVZ, while simultaneously mitigating instrument systematics.

Keywords: astronomy data reduction; time series analysis; stellar rotation
Secular Extragalactic Parallax: Measurement Methods and Predictions for Gaia

Secular extragalactic parallax caused by the solar system's velocity relative to the cosmic microwave background rest frame may be observable as a dipole proper motion field with amplitude 78 μas yr-1 Mpc. Nearby galaxies also exhibit proper motions caused by their transverse peculiar velocities that prevent detection of secular parallax for any single galaxy, although a statistical detection may be made instead. Such a detection could constrain the local Hubble parameter. We present methods to measure secular parallax using correlated extragalactic proper motions and find a first limit on the secular parallax amplitude using proper motions of 232 nearby galaxies from Gaia Data Release 2. The recovered dipole has an insignificant upper limit of 3500 μas yr-1 Mpc. This measurement will be improved by a larger sample size and reduced proper motion uncertainties in future data releases. Using the local peculiar velocity field derived from Cosmicflows-3, we simulate galaxy proper motions and predict that a significant detection (5-10σ) of the secular parallax amplitude will be possible by Gaia's end of mission. The detection does not correspond to a constraint on the Hubble parameter because it depends on nearby (<5 Mpc), bright (G < 15 mag) galaxies and the underlying peculiar motion dipole. We further investigate the implications of our simulations for the study of transverse peculiar velocities. The peculiar velocity field additionally results in low multipole-correlated proper motions on the order of 0.3 μas yr-1 that may be confounded with other cosmological proper motion measurements, such as limits on the gravitational-wave background and the anisotropy of the Hubble expansion.

Keywords: astrometry; proper motions; astronomical methods; observational cosmology; large-scale structure of the universe
Processing and Analysis for Radio Science Experiments (PARSE): Graphical Interface for Bistatic Radar

Opportunistic bistatic radar (BSR) observations of planetary surfaces can probe the textural and electrical properties of several solar system bodies without needing a dedicated instrument or additional mission requirements, providing unique insights into volatile enrichment and supporting future landing, anchoring, and in situ sampling. Given their opportunistic nature, complex observation geometries, and required radiometric knowledge of the received radio signal, these data are particularly challenging to process, analyze, and interpret for most planetary science data users, who can be unfamiliar with link budget analysis of received echoes. The above impedes real-time use of BSR data to support mission operations, such as identifying safe landing locations on small bodies, as was the case for the Rosetta mission. To address this deficiency, we develop an open-source graphical user interface-Processing and Analysis for Radio Science Experiments (PARSE)-that assesses the feasibility of performing BSR observations and automates radiometric signal processing, power spectral analysis, and visualization of DSN planetary radio science data sets acquired during mission operations or archived on NASA's Planetary Data System. In this first release, PARSE automates the processing chain developed for Dawn at Asteroid Vesta, streamlining the detection of DSN-received surface-scatter echoes generated as the spacecraft enters/exits occultations behind the target. Future releases will include support for existing Arecibo data sets and other Earth-based radio observatories. Our tool enables the broader planetary science community, beyond planetary radar signal processing experts, to utilize BSR data sets to characterize electrical and textural properties of planetary surfaces. Such tools are becoming increasingly important as the number of space missions-and subsequent opportunities for orbital radio science observations-continue to grow.

Keywords: doppler shift; radar astronomy; small solar system bodies; solar system; astronomy software
Weakened Magnetic Braking in the Exoplanet Host Star 51 Peg

The consistently low activity level of the old solar analog 51 Peg not only facilitated the discovery of the first hot Jupiter, but also led to the suggestion that the star could be experiencing a magnetic grand minimum. However, the 50 yr time series showing minimal chromospheric variability could also be associated with the onset of weakened magnetic braking (WMB), where sufficiently slow rotation disrupts cycling activity and the production of large-scale magnetic fields by the stellar dynamo, thereby shrinking the Alfvén radius and inhibiting the efficient loss of angular momentum to magnetized stellar winds. In this Letter, we evaluate the magnetic evolutionary state of 51 Peg by estimating its wind braking torque. We use new spectropolarimetric measurements from the Large Binocular Telescope to reconstruct the large-scale magnetic morphology, we reanalyze archival X-ray measurements to estimate the mass-loss rate, and we detect solar-like oscillations in photometry from the Transiting Exoplanet Survey Satellite, yielding precise stellar properties from asteroseismology. Our estimate of the wind braking torque for 51 Peg clearly places it in the WMB regime, driven by changes in the mass-loss rate and the magnetic field strength and morphology that substantially exceed theoretical expectations. Although our revised stellar properties have minimal consequences for the characterization of the exoplanet, they have interesting implications for the current space weather environment of the system.

Keywords: spectropolarimetry; stellar evolution; stellar magnetic fields; stellar oscillations; stellar winds
Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

Periodic nuclear transients have been detected with increasing frequency, with one such system-ASASSN-14ko-exhibiting highly regular outbursts on a timescale of 114 ± 1 days. It has been postulated that the outbursts from this source are generated by the repeated partial disruption of a star, but how the star was placed onto such a tightly bound orbit about the supermassive black hole remains unclear. Here we use analytic arguments and three-body integrations to demonstrate that the Hills mechanism, where a binary system is destroyed by the tides of the black hole, can lead to the capture of a star on a ~114 days orbit and with a pericenter distance that is comparable to the tidal radius of one of the stars within the binary. Thus, Hills capture can produce stars on tightly bound orbits that undergo repeated partial disruption, leading to a viable mechanism for generating not only the outbursts detected from ASASSN-14ko but periodic nuclear transients in general. We also show that the rate of change of the period of the captured star due to gravitational-wave emission is likely too small to produce the observed value for ASASSN-14ko, indicating that in this system there must be additional effects that contribute to the decay of the orbit. In general, however, gravitational-wave emission can be important for limiting the lifetimes of these systems and could produce observable period decay rates in future events.

Keywords: astrophysical black holes; binary stars; black hole physics; supermassive black holes; tidal disruption
The Initiation and Back-reaction of the X5.4 Flare on 2012 March 7

In this paper, we study the evolution of the X5.4 flare (SOL2012-03-07T00:02) in NOAA Active Region 11429, focusing on its initiation mechanisms and back-reaction effects. To help our study, three-dimensional (3D) coronal magnetic field models are extrapolated from the photospheric magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory under the assumptions of nonlinear force-free field (NLFFF) and non-force-free field (non-FFF). We investigate the 3D magnetic structure and MHD kink instability, torus instability, and double-arc instability (DAI), and find that this flare is most likely triggered by the tether-cutting reconnection and the subsequent DAI. For the back-reactions of the flare, both NLFFF and non-FFF models clearly show an increase in horizontal magnetic field (B h ) and a decrease in inclination angle (ϕ) of the magnetic field near the polarity inversion line, from the photosphere up to a certain height (5 Mm and 8 Mm for non-FFF and NLFFF, respectively). In addition, the non-FFF model shows an enhancement of the downward Lorentz force acting on the photosphere, and the location of the enhancement spatially coincides with the location of the flare onset. The observed back-reaction is likely a consequence of magnetic reconnection.

Keywords: solar flares; solar activity; solar active region magnetic fields
High-speed Focal Plane Wave Front Sensing with an Optical Chopper

Focal plane wave front sensing and control is a critical approach to reducing noncommon path errors between a conventional astronomical adaptive optics (AO) wave front sensor (WFS) detector and a science camera. However, in addition to mitigating noncommon path errors, recent focal plane wave front sensing techniques have been developed to operate at speeds fast enough to enable "multi-WFS" AO, where residual atmospheric errors are further corrected by a focal plane WFS. Although a number of such techniques have been recently developed for coronagraphic imaging, here we present one designed for noncoronagraphic imaging. Utilizing conventional AO system components, this concept additionally requires (1) a detector imaging the focal plane of the WFS light source and (2) a pupil plane optical chopper device that is the noncommon path to the first WFS and is synchronized to the focal plane imager readout. These minimal hardware requirements enable the temporal amplitude modulation to resolve the sine ambiguity of even wave-front modes for low, mid, and high wave front spatial frequencies. Similar capabilities have been demonstrated with classical phase diversity by defocusing the detector, but such techniques are incompatible with simultaneous science observations. This optical chopping technique, however, enables science imaging at up to 50% duty cycle. We present both simulations and laboratory validation of this concept on SEAL, the Santa Cruz Extreme AO Laboratory testbed.

Keywords: astronomical instrumentation; astronomical optics