adsabs/SciX_UAT_keywords huggingface dataset:
First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ∼15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication.

Keywords: polarimetry; radio interferometry; very long baseline interferometry; supermassive black holes; active galactic nuclei; low-luminosity active galactic nuclei; astronomy data modeling; galaxy accretion disks
On the Nature of the Three-part Structure of Solar Coronal Mass Ejections

Coronal mass ejections (CMEs) result from eruptions of magnetic flux ropes (MFRs) and can possess a three-part structure in white-light coronagraphs, including a bright front, dark cavity, and bright core. In traditional opinion, the bright front forms due to the plasma pileup along the MFR border, the cavity represents the cross section of the MFR, and the bright core corresponds to the erupted prominence. However, this explanation on the nature of the three-part structure is being challenged. In this paper, we report an intriguing event that occurred on 2014 June 14 that was recorded by multiple space- and ground-based instruments seamlessly, clearly showing that the CME front originates from the plasma pileup along the magnetic arcades overlying the MFR, and the core corresponds to a hot-channel MFR. Thus the dark cavity is not an MFR; instead it is a low-density zone between the CME front and a trailing MFR. These observations are consistent with a new explanation on the CME structure. If the new explanation is correct, most (if not all) CMEs should exhibit the three-part appearance in their early eruption stage. To examine this prediction, we make a survey of all CMEs in 2011 and find that all limb events have the three-part feature in the low corona, regardless of their appearances in the high corona. Our studies suggest that the three-part structure is the intrinsic structure of CMEs, which has fundamental importance for understanding CMEs.

Keywords: solar coronal mass ejections; solar activity; solar coronal loops; solar flares
Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares

Young solar-type stars frequently produce superflares, serving as a unique window into the young Sun-Earth environments. Large solar flares are closely linked to coronal mass ejections (CMEs) associated with filament/prominence eruptions, but observational evidence for stellar superflares remains scarce. Here, we present a 12-day, multiwavelength campaign observation of young solar-type star EK Draconis (G1.5V, 50-120 Myr age) utilizing the Transiting Exoplanet Survey Satellite, the Neutron star Interior Composition ExploreR, and the Seimei telescope. The star has previously exhibited blueshifted Hα absorptions as evidence for a filament eruption associated with a superflare. Our simultaneous optical and X-ray observations identified three superflares of 1.5 × 1033-1.2 × 1034 erg. We report the first discovery of two prominence eruptions on a solar-type star, observed as blueshifted Hα emissions at speeds of 690 and 430 km s-1 and masses of 1.1 × 1019 and 3.2 × 1017 g, respectively. The faster, massive event shows a candidate of post-flare X-ray dimming with the amplitude of up to ~10%. Several observational aspects consistently point to the occurrence of a fast CME associated with this event. The comparative analysis of the estimated length scales of flare loops, prominences, possible dimming region, and starspots provides the overall picture of the eruptive phenomena. Furthermore, the energy partition of the observed superflares in the optical and X-ray bands is consistent with flares from the Sun, M-dwarfs, and close binaries, yielding the unified empirical relations. These discoveries provide profound implications of the impact of these eruptive events on early Venus, Earth, and Mars and young exoplanets.

Keywords: stellar flares; stellar coronal mass ejections; stellar coronal dimming; solar prominences; solar filament eruptions; optical flares; stellar x-ray flares; flare stars; g dwarf stars; solar analogs
SOFIA/HAWC+ Far-infrared Polarimetric Large-area CMZ Exploration Survey. III. Full Survey Data Set

We present the second data release (DR2) of the Far-InfraREd Polarimetric Large-Area CMZ Exploration (FIREPLACE) survey. This survey utilized the Stratospheric Observatory for Infrared Astronomy High-resolution Airborne Wideband Camera plus instrument at 214 μm (E band) at a resolution of 19.″6 to observe thermal polarized dust emission throughout the Central Molecular Zone (CMZ). DR2 consists of observations obtained in 2022 covering the region of the CMZ extending from the Brick to the Sgr C molecular clouds (corresponding to a 1° × 0.°75 region of the sky). We combine DR2 with the first FIREPLACE data release (DR1) to obtain full coverage of the CMZ (a 1.°5 × 0.°75 region of the sky). After applying total and polarized intensity significance cuts on the full FIREPLACE data set, we obtain ∼64,000 Nyquist-sampled polarization pseudovectors. The distribution of polarization pseudovectors confirms a bimodal distribution in the CMZ magnetic field orientations, recovering field components that are oriented predominantly parallel or perpendicular to the Galactic plane. This distribution of orientations is similar to what was observed in DR1 and other studies. We also inspect the magnetic fields toward a set of prominent CMZ molecular clouds (the Brick, Three Little Pigs, 50 and 20 km s‑1 clouds, circumnuclear disk, CO 0.02-0.02, and Sgr C), revealing spatially varying magnetic fields having orientations that generally trace the total intensity morphologies of the clouds. We find evidence that compression from stellar winds and shear from tidal forces are prominent mechanisms influencing the structure of the magnetic fields.

Keywords: galactic center; interstellar medium; molecular clouds
A New Feedback Cycle in the Archetypal Cooling Flow Group NGC 5044

The fate of cooling gas in the centers of galaxy clusters and groups is still not well understood, as is also the case for the complex process of triggering active galactic nucleus (AGN) outbursts in their central dominant galaxies and the consequent reheating of the gas by the AGN jets. With the largest known reservoir of cold molecular gas of any group-dominant galaxy and three epochs of AGN activity visible as cavities in its hot gas, NGC 5044 is an ideal system in which to study the cooling/AGN feedback cycle at the group scale. We present VLBA observations of NGC 5044 to ascertain the current state of the central AGN. We find a compact core and two small jets aligned almost in the plane of the sky and in the orthogonal direction to the location of cavities. We construct the radio/submillimeter spectral energy distribution over more than 3 orders of magnitude. We find that below 5 GHz, the spectrum is best fit by a self-absorbed continuous injection model representing emission coming from the jets, while the higher frequencies show clear signs of an advection-dominated accretion flow. We derive a black hole mass and accretion rate consistent with independent measurements. We conclude that the age of the jets is much younger than the innermost cavities, marking the start of a new feedback cycle.

Keywords: active galactic nuclei; radio jets; jets; galaxy groups; accretion
Prospects of Identifying Hierarchical Triple Mergers for the Third-generation Ground-based Detectors

A hierarchical triple merger (HTM) constitutes a type of event in which two successive black hole (BH) mergers occur sequentially within the observational window of gravitational-wave (GW) detectors, which has an important role in testing general relativity and studying BH population. In this work, we conduct an analysis to determine the feasibility of identifying HTMs from a large GW event catalog using third-generation ground-based GW detectors. By comparing the Bhattacharyya coefficient that measures the overlap between the posterior distributions of the remnant and progenitor BH parameters, we find that the overlap between the event pair can serve as a preliminary filter, which balances between computational demand and the probability of false alarms. Following this initial, time-efficient, yet less accurate screening, a subset of potential HTM candidates will be retained. These candidates will subsequently be subjected to a more precise, albeit time-intensive, method of joint parameter estimation for verification. Ultimately, this process will enable us to robustly identify HTMs.

Keywords: compact objects; gravitational wave sources
Modeling of Engulfment Material in the Binary System HIP 71726/71737

A parameterized regression fit to precision differential abundances, [El/H], in the wide binary system HIP 71726/71737 accounts for the condensation temperature dependence with a high significance. This fit includes 27 elements, Carbon through Europium. We discuss conditions and the implications of a fit to [Li/H] with the same parameters. These results support the hypothesis of engulfment of earthlike material.

Keywords: chemical abundances
Fast Blast Wave and Ejecta in the Young Core-collapse Supernova Remnant MSH 15-52/RCW 89

One of the youngest known remnants of a core-collapse supernova (SN) in our Galaxy is G320.4-1.2/MSH 15-52, containing an energetic pulsar with a very short (1700 yr) spindown age and likely produced by a stripped-envelope SN Ibc. Bright X-ray and radio emission north of the pulsar overlaps with an Hα nebula RCW 89. The bright X-rays there have a highly unusual and quite puzzling morphology, consisting of both very compact thermally emitting knots and much more diffuse emission of nonthermal origin. We report new X-ray observations of RCW 89 in 2017 and 2018 with Chandra that allowed us to measure the motions of many knots and filaments on decade-long time baselines. We identify a fast blast wave with a velocity of $(4000\pm 500){d}_{5.2}$ km s-1 ( ${d}_{5.2}$ is the distance in units of 5.2 kpc) with a purely nonthermal spectrum, and without any radio counterpart. Many compact X-ray emission knots are moving vary fast, with velocities as high as 5000 km s-1, predominantly radially away from the pulsar. Their spectra show that they are Ne- and Mg-rich heavy-element SN ejecta. They have been significantly decelerated upon their recent impact with the dense ambient medium north of the pulsar. We see fast evolution in brightness and morphology of knots in just a few years. Ejecta knots in RCW 89 resemble those seen in Cas A at optical wavelengths in terms of their initial velocities and densities. They might have the same origin, still not understood but presumably related to stripped-envelope SN explosions themselves.

Keywords: supernova remnants; core-collapse supernovae; ejecta; rotation powered pulsars; x-ray astronomy
Magnetically Regulated Disk Formation in the Inner 100 au Region of the Class 0 Young Stellar Object OMC-3/MMS 6 Resolved by JVLA and ALMA

We have carried out polarization calibration for archival Jansky Very Large Array (JVLA) (~9 mm) full polarization observations toward the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared the results with the archival Atacama Large Millimeter Array (ALMA) 1.2 mm observations. The resolved spectral indices show that the innermost ~100 au region of OMC-3/MMS 6 is marginally optically thin (e.g., τ ≲ 1) at ~9 mm wavelength, such that the JVLA observations can directly probe the linearly polarized emission from nonspherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape. The geometry of this system is consistent with a magnetically regulated dense (pseudo)disk, although this "hourglass" appears to be ~40° inclined with respect to the previously reported outflow axis. In contrast, the inner ~100 au region of this YSO is likely very optically thick (e.g., τ ≫ 1) at ~1 mm wavelength. The electric field position angles resolved by JVLA and ALMA present ~90° offsets on this region, which indicate that the dominant polarization mechanism at 1 mm wavelength is dichroic extinction. This is the second case where the (sub)millimeter dichroic extinction is demonstrated by the direct comparison between the JVLA and ALMA polarization observations.

Keywords: young stellar objects; interstellar magnetic fields; polarimetry; circumstellar grains
A Data-driven Technique Using Millisecond Transients to Measure the Milky Way Halo

We introduce a new technique to constrain the line-of-sight integrated electron density of our Galactic halo ${\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}$ through analysis of the observed dispersion measure distributions of pulsars ${\mathrm{DM}}_{\mathrm{pulsar}}$ and fast radio bursts (FRBs) ${\mathrm{DM}}_{\mathrm{FRB}}$ . We model these distributions, correcting for the Galactic interstellar medium, with kernel density estimation—well-suited to the small data regime—to find lower/upper bounds to the corrected ${\mathrm{DM}}_{\mathrm{pulsar}}$ / ${\mathrm{DM}}_{\mathrm{FRB}}$ distributions: $\max \left[{\mathrm{DM}}_{\mathrm{pulsar}}\right]\approx 7\pm 2\,(\mathrm{stat})\pm 9\,(\mathrm{sys})\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ and $\min \left[{\mathrm{DM}}_{\mathrm{FRB}}\right]\approx {63}_{-21}^{+27}\,(\mathrm{stat})\pm 9\,(\mathrm{sys})\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ . Using bootstrap resampling to estimate uncertainties, we set conservative limits on the Galactic halo dispersion measure $-2\lt {\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}\lt 123\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ (95% c.l.). The upper limit is especially conservative because it may include a nonnegligible contribution from the FRB host galaxies and a nonzero contribution from the cosmic web. It strongly disfavors models where the Galaxy has retained the majority of its baryons with a density profile tracking the presumed dark matter density profile. Last, we perform Monte Carlo simulations of larger FRB samples to validate our technique and assess the sensitivity of ongoing and future surveys. We recover bounds of several tens of $\mathrm{pc}\,{\mathrm{cm}}^{-3}$ that may be sufficient to test whether the Galaxy has retained a majority of its baryonic mass. We estimate that a sample of several thousand FRBs will significantly tighten constraints on ${\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}$ and offer a valuable complement to other analyses.

Keywords: radio transient sources; circumgalactic medium; astrostatistics techniques
Discovery of a Giant Molecular Loop in the Central Region of NGC 253

NGC 253 is a starburst galaxy of SAB(s)c type with increasing interest because of its high activity at unrivaled closeness. Its energetic event is manifested as the vertical gas features in its central molecular zone, for which stellar feedback was proposed as the driving engine. In order to pursue details of the activity, we have undertaken a kinematic analysis of the ALMA archive data of 12CO(J = 3 - 2) emission at the highest resolution ~3 pc. We revealed that one of the non-rotating gas components in the central molecular zone shows a loop-like structure of ~200 pc radius. The loop-like structure is associated with a star cluster, whereas the cluster is not inside the loop-like structure and is not likely as the driver of the loop-like structure formation. Further, we find that the bar potential of NGC 253 seems to be too weak to drive the gas motion by the eccentric orbit. As an alternative, we frame a scenario that magnetic acceleration by the Parker instability is responsible for the creation of the loop-like structure. We show that the observed loop-like structure properties are similar to those in the Milky Way, and argue that recent magneto-hydrodynamics simulations lend support for the picture having the magnetic field strength of ≳100 μG. We suggest that cluster formation was triggered by the falling gas to the footpoint of the loop, which is consistent with a typical dynamical timescale of the loop ~1 Myr.

Keywords: galaxy kinematics; interstellar clouds; galactic center; starburst galaxies
Ionization and Star Formation in the Giant H II Region SMC-N66

The NGC 346 young stellar system and associated N66 giant H II region in the Small Magellanic Cloud are the nearest example of a massive star-forming event in a low metallicity (Z ≈ 0.2Z ⊙) galaxy. With an age of ≲3 Myr this system provides a unique opportunity to study relationships between massive stars and their associated H II region. Using archival data, we derive a total Hα luminosity of L(Hα) = 4.1 × 1038 erg s-1 corresponding to an H-photoionization rate of 3 × 1050 s-1. A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from Hubble Space Telescope far-ultraviolet (FUV) spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the FUV luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L(Hα) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments.

Keywords: small magellanic cloud; h ii regions; photoionization
Understanding the Lateral Drifting of an Erupting Filament with a Data-constrained Magnetohydrodynamic Simulation

Solar filaments often exhibit rotation and deflection during eruptions, which would significantly affect the geoeffectiveness of the corresponding coronal mass ejections (CMEs). Therefore, understanding the mechanisms that lead to such rotation and lateral displacement of filaments is a great concern to space weather forecasting. In this paper, we examine an intriguing filament eruption event observed by the Chinese Hα Solar Explorer and the Solar Dynamics Observatory. The filament, which eventually evolves into a CME, exhibits significant lateral drifting during its rising. Moreover, the orientation of the CME flux rope axis deviates from that of the pre-eruptive filament observed in the source region. To investigate the physical processes behind these observations, we perform a data-constrained magnetohydrodynamic simulation. Many prominent observational features in the eruption are reproduced by our numerical model, including the morphology of the eruptive filament, eruption path, and flare ribbons. The simulation results reveal that the magnetic reconnection between the flux rope leg and neighboring low-lying sheared arcades may be the primary mechanism responsible for the lateral drifting of the filament material. Such a reconnection geometry leads to flux rope footpoint migration and a reconfiguration of its morphology. As a consequence, the filament material hosted in the flux rope drifts laterally, and the CME flux rope deviates from the pre-eruptive filament. This finding underscores the importance of external magnetic reconnection in influencing the orientation of a flux rope axis during eruption.

Keywords: magnetohydrodynamical simulations; solar coronal mass ejections; solar filaments; solar magnetic fields; solar flares
Case Study of Solar Wind Suprathermal Electron Acceleration at the Earth's Bow Shock

We present a case study of the in situ acceleration of solar wind suprathermal electrons at the two quasi-perpendicular-bow-shock crossings on 2015 November 4, combining the Wind 3D Plasma and Energetic Particle measurements of ambient solar wind suprathermal electrons and Magnetospheric Multiscale mission measurements of shocked suprathermal electrons. In both cases, the omnidirectional differential fluxes of shocked suprathermal electrons in the downstream exhibit a double-power-law energy spectrum with a spectral index of ∼3 at energies below a downward break ɛbrk near 40 keV and index of ∼6 at energies above, different from the unshocked suprathermal electrons observed in the ambient solar wind. At energies below (above) ɛbrk, the observed electron flux ratio between the downstream and ambient solar wind, JD/JA, peaks near 90° PA (becomes roughly isotropic). Electrons at ɛbrk have an average electron gyrodiameter (across bow shock) comparable to the shock thickness. These suggest that the bow-shock acceleration of suprathermal electrons is likely dominated by the shock drift acceleration mechanism. For electrons at energies below (above) ɛbrk, their estimated drift time appears to be roughly energy independent (decrease with energy), leading to the formation of a double-power-law spectrum substantially steepening at a break that's determined by the shock thickness.

Keywords: shocks