Sazuna/UAT_keywords huggingface dataset:
Magnetically Driven Neutron-rich Ejecta Unleashed: Global 3D Neutrino─General Relativistic Magnetohydrodynamic Simulations of Collapsars Probe the Conditions for r-process Nucleosynthesis. Collapsars—rapidly rotating stellar cores that form black holes—can power gamma-ray bursts and are proposed to be key contributors to the production of heavy elements in the Universe via the rapid neutron capture process (r-process). Previous neutrino-transport collapsar simulations have been unable to unbind neutron-rich material from the disk. However, these simulations have not included sufficiently strong magnetic fields and the black hole (BH), both of which are essential for launching mass outflows. We present νH-AMR, a novel neutrino-transport general relativistic magnetohydrodynamic (νGRMHD) code, which we use to perform the first 3D global νGRMHD collapsar simulations. We find a self-consistent formation of a weakly magnetized dense accretion disk, which has sufficient time to neutronize. Eventually, substantial magnetic flux accumulates near the BH, becomes dynamically important, leads to a magnetically arrested disk (MAD), and unbinds some of the neutron-rich material. However, the strong flux also hinders accretion, lowers density, and increases neutrino-cooling timescale, which prevents further disk neutronization. Typical collapsar progenitors with mass accretion rates,  Ṁ∼0.1−1M⊙s-1 , do not produce significant neutron-rich (Ye &lt; 0.25) ejecta. However, we find that MADs at higher mass accretion rates,  Ṁ≳fewM⊙s-1  (e.g., for more centrally concentrated progenitors), can unbind Mej ≲ M⊙ of neutron-rich ejecta. The outflows inflate a shocked cocoon that mixes with the infalling neutron-poor stellar gas and raises the final outflow Ye; however, the final r-process yield may be determined earlier at the point of neutron capture freeze-out. Future work will explore under what conditions more typical collapsar engines become r-process factories.

Keywords: Jets, Black holes, Nucleosynthesis, High energy astrophysics, Core-collapse supernovae, R-process, Relativistic jets, Magnetohydrodynamical simulations
First Detection of Low-frequency Striae in Interplanetary Type III Radio Bursts. We report the first detection of type III solar radio burst striae in the 30–80 kHz range, observed by the Cluster-4 spacecraft during an exceptionally quiet solar period. These low-frequency fine structures, which drift slowly in frequency and exhibit narrow bandwidths, provide a novel diagnostic of plasma processes in the inner heliosphere. The detected striae, interpreted as fundamental plasma emission, exhibit a frequency drift rate of 0.328 Hz s‑1 and a bandwidth of 1.3 kHz. By combining high-resolution radio observations with well-calibrated in situ electron velocity distribution function data from the Wind spacecraft, we characterized the plasma properties of the burst source region near 0.32 au. Our analysis estimates relative density fluctuations, at the effective turbulence scale length, as approximately 3.4% (inferred from striae bandwidths), 0.62% (from intensity fluctuations), and 3.5% (from a heliocentric distance-based empirical model). These findings offer critical insights into small-scale density inhomogeneities and turbulence that affect electron beam propagation. This study underscores the potential of combining well-calibrated in situ electron data with radio burst measurements to probe the physical conditions of the solar wind and to refine our understanding of solar radio bursts across a broad frequency range.

Keywords: Solar coronal radio emission, Interplanetary turbulence, Solar wind, Plasma astrophysics, Solar radio emission
Starspot Area Coverage: Correlation with Age and Spectral Type in FGK and M Stars. Starspots, analogous to sunspots, are surface manifestations of stellar magnetic activity. Their study provides crucial insights into stellar dynamo processes and the evolution of magnetic phenomena. However, due to limited data on magnetic activity across stellar lifetimes, the relationship between starspot area coverage and stellar properties remains underexplored. This work investigates the correlation between starspot area coverage and stellar age, effective temperature, and rotation period in FGK and M stars using data from Kepler and CoRoT. We utilized the starspot transit mapping method to analyze 11 stars, calculating starspot area coverage by integrating spot areas on the stellar surface. The average starspot coverage ranged from 4% to 29%, consistent with theoretical models of stellar magnetic evolution. Our analysis revealed a strong anticorrelation with stellar age (Spearman ρ = ‑0.80), confirming a significant decline in magnetic activity over time, but a weak anticorrelation between starspot coverage and rotation period. Regarding the relationship between absolute starspot area coverage and effective temperature, we found a moderate positive correlation (Spearman ρ = 0.40), with the majority of stars with an absolute area coverage of (2–4 ± 0.6) × 1010 km2. Incorporating starspot coverage into isochrone models could significantly improve stellar age estimates, especially for young stars. Moreover, these measurements are crucial for refining stellar dynamo models and advancing our understanding of magnetic field generation.

Keywords: Starspots, Stellar ages, Effective temperature, Stellar activity, Exoplanet astronomy, Stellar rotation, Transit photometry
Polarization Analysis of Type III Langmuir/Z-mode Waves with Coherent Magnetic Component Observations by Solar Orbiter. Observations from the Solar Orbiter spacecraft provide unique insights into the interaction between electron beams and the plasma background in the source regions of type III radio emissions. We analyze this interaction by examining the high-frequency electric and magnetic components of in situ wave measurements, focusing on their polarization properties. Using electron data from onboard instruments, we model the electron velocity distribution function and numerically solve the dispersion relation. We compare the predicted polarization of the electric and magnetic components with the observations. Our findings are consistent with propagation in the Langmuir/Z-mode at an oblique wavevector. We explain the magnetic component and transverse polarization by the presence of small density fluctuations, without the need for mode conversion.

Keywords: Solar wind, Solar radio emission, Space plasmas, Plasma physics
Evolution of the Three Spectral Components in the Prompt Emission of GRB 240825A. The prompt emission of gamma-ray bursts (GRBs) could be composed of different spectral components, such as a dominant nonthermal Band component in the keV–MeV range, a subdominant quasi-thermal component, and an additional hard nonthermal component extending into the GeV range. The existence and evolutionary behaviors of these components could provide essential constraints on physical models, such as ejecta composition and dissipation processes. Although numerous GRBs have been found to exhibit one or two spectral components, reports of GRBs containing all three components remain rare. In this Letter, based on the joint observations of GRB 240825A from multiple gamma-ray telescopes, we conduct a comprehensive temporal and spectral analysis to identify the presence and evolution of all three components. The bulk Lorentz factor of this bright and relatively short-duration burst is independently obtained from the thermal and hard nonthermal components, supporting a jet penetration scenario. The multisegment broken power-law feature observed in the flux lightcurves suggests the presence of an early afterglow in the keV–MeV band and hints at a possible two-jet structure. Furthermore, the observed transition from positive to negative on the spectral lag can be interpreted as misalignment in the cross-correlation function analysis of pulses, which is caused by evolution of the soft and hard components.

Keywords: Gamma-ray bursts
Machine Learning Characterization of Intermittency in Relativistic Pair Plasma Turbulence: Single and Double Sheet Structures. The physics of turbulence in magnetized plasmas remains an unresolved problem. The most poorly understood aspect is intermittency—spatiotemporal fluctuations superimposed on the self-similar turbulent motions. We employ a novel machine learning analysis technique to segment turbulent flow structures into distinct clusters based on statistical similarities across multiple physical features. We apply this technique to kinetic simulations of decaying (freely evolving) and driven (forced) turbulence in a strongly magnetized pair-plasma environment, and find that the previously identified intermittent fluctuations consist of two distinct clusters: (i) current sheets, thin slabs of electric current between merging flux ropes, and; (ii) double sheets, pairs of oppositely polarized current slabs, possibly generated by two nonlinearly interacting Alfvén-wave packets. The distinction is crucial for the construction of realistic turbulence subgrid models.

Keywords: Astrophysical fluid dynamics, Accretion, Computational astronomy, Plasma astrophysics
X-Ray Polarimetric Observations of the Western Hotspot of Pictor A. We present the results of the analysis of a ∼2 Msec spectropolarimetric observation of the western hotspot (WHS) of the radio galaxy Pictor A. This is the brightest extragalactic radio source that allows us to carry out spatially resolved observations using the Imaging X-ray Polarimetry Explorer (IXPE) because its WHS is located at ∼250″ from the radio core. The Pictor A WHS can be detected with IXPE as a pointlike source, with very low contamination from the nuclear emission, and it is extremely polarized at both radio and optical frequencies, where its emission is well described as synchrotron radiation. We find no X-ray polarization for the Pictor A WHS. However, the derived upper limit allows us to set a first constraint on the radiative processes occurring therein, ruling out a simple synchrotron scenario where particles, with a random distribution of pitch angles, radiate in a uniform magnetic field. We also tested a scenario with a random magnetic field compressed along the jet direction.

Keywords: X-ray astronomy, Spectropolarimetry, Polarimetry
Cosmology with Binary Neutron Stars: Does Mass–Redshift Correlation Matter?. Next-generation gravitational-wave detectors are expected to detect millions of compact binary mergers across cosmological distances. The features of the mass distribution of these mergers, combined with gravitational-wave distance measurements, will enable precise cosmological inferences, even without the need for electromagnetic counterparts. However, achieving accurate results requires modeling the mass spectrum, particularly considering possible redshift evolution. Binary neutron star (BNS) mergers are thought to be less influenced by changes in metallicity compared to binary black holes or neutron star–black hole mergers. This stability in their mass spectrum over cosmic time reduces the chances of introducing biases in cosmological parameters caused by redshift evolution. In this study, we use the population synthesis code COMPAS to generate astrophysically motivated catalogs of BNS mergers and explore whether assuming a nonevolving BNS mass distribution with redshift could introduce biases in cosmological parameter inference. Our findings show that despite significant variations in the BNS mass distribution across binary physics assumptions and initial conditions in COMPAS, the joint mass–redshift population can be expressed as the product of the mass distribution marginalized over redshift and the redshift distribution marginalized over masses. This enables a 2% unbiased constraint on the Hubble constant—sufficient to address the Hubble tension. Additionally, we show that in the fiducial COMPAS setup, the bias from a nonevolving BNS mass model is less than 0.5% for the Hubble parameter measured at redshift 0.4. These results establish BNS mergers as strong candidates for spectral siren cosmology in the era of next-generation gravitational-wave detectors.

Keywords: Neutron stars, Gravitational waves, Gravitational wave sources, Cosmological parameters
IXPE Detection of Highly Polarized X-Rays from the Magnetar 1E 1841-045. The Imaging X-ray Polarimetry Explorer (IXPE) observed for the first time highly polarized X-ray emission from the magnetar 1E 1841‑045, targeted after a burst-active phase in 2024 August. To date, IXPE has observed four other magnetars during quiescent periods, highlighting substantially different polarization properties. 1E 1841‑045 exhibits a high, energy-dependent polarization degree, which increases monotonically from ≈15% at 2–3 keV up to ≈55% at 5.5–8 keV, while the polarization angle, aligned with the celestial north, remains fairly constant. The broadband spectrum (2–79 keV) obtained by combining simultaneous IXPE and NuSTAR data is well modeled by a blackbody and two power-law components. The unabsorbed 2–8 keV flux (≈2 × 10‑11 erg cm‑2 s‑1) is about 10% higher than that obtained from archival XMM-Newton and NuSTAR observations. The polarization of the soft, thermal component does not exceed ≈25%, and may be produced by a condensed surface or a bombarded atmosphere. The intermediate power law is polarized at around 30%, consistent with predictions for resonant Compton scattering in the star magnetosphere; meanwhile, the hard power law exhibits a polarization degree exceeding 65%, pointing to a synchrotron/curvature origin.

Keywords: Magnetars, Polarimetry, X-ray sources
X-Ray Polarization of the Magnetar 1E 1841−045. We report on IXPE and NuSTAR observations beginning 40 days after the 2024 outburst onset of magnetar 1E 1841−045, marking the first IXPE observation of a magnetar in an enhanced state. Our spectropolarimetric analysis indicates that both a blackbody (BB) plus double power-law (PL) and a double blackbody plus power-law spectral model fit the phase-averaged intensity data well, with a hard PL tail (Γ = 1.19 and 1.35, respectively) dominating above ≍5 keV. For the former model, we find the soft PL (the dominant component at soft energies) exhibits a polarization degree (PD) of ≍30% while the hard PL displays a PD of ≍40%. Similarly, the cool BB of the 2BB+PL model possesses a PD of ≍15% and a hard PL PD of ≍57%. For both models, each component has a polarization angle (PA) compatible with celestial north. Model-independent polarization analysis supports these results, wherein the PD increases from ≍15% to ≍70% in the 2─3 keV and 6─8 keV ranges, respectively, while the PA remains nearly constant. We find marginal evidence for phase-dependent variability of the polarization properties, namely a higher PD at phases coinciding with the hard X-ray pulse peak. We compare the hard X-ray PL to the expectation from resonant inverse Compton scattering (RICS) and secondary pair cascade synchrotron radiation from primary high-energy RICS photons; both present reasonable spectropolarimetric agreement with the data, albeit the latter does so more naturally. We suggest that the soft PL X-ray component may originate from a Comptonized corona in the inner magnetosphere.

Keywords: Neutron stars, Pulsars, X-ray sources, Magnetars, Polarimetry
Accurate Muonic Interactions in Neutron Star Mergers and Impact on Heavy-element Nucleosynthesis. The abundances resulting from r-process nucleosynthesis as predicted by simulations of binary neutron star (BNS) mergers remain an open question as the current state of the art is still restricted to three-species neutrino transport. We present the first BNS merger simulations employing a moment-based general-relativistic neutrino transport with five neutrino species, thus including (anti)muons and advanced muonic β-processes, and contrast them with traditional three-neutrino-species simulations. Our results show that a muonic trapped-neutrino equilibrium is established, forming a different trapped-neutrino hierarchy akin to the electronic equilibrium. The formation of (anti)muons and the muonization via muonic β-processes enhance neutrino luminosity, leading to a stronger cooling in the early postmerger phase. Since muonic processes redirect part of the energy otherwise used for protonization by electronic processes, they yield a cooler remnant and disk, together with neutrino-driven winds that are more neutron-rich. Importantly, the unbound ejected mass is smaller than in three-species simulations, and, because of its comparatively smaller temperature and proton fraction, it can enhance lanthanide production and reduce the overproduction of light r-process elements for softer equations of state. This finding underlines the importance of muonic interactions and five neutrino species in long-lived BNS remnants.

Keywords: Neutron stars, Nucleosynthesis, High energy astrophysics, Neutrino astronomy, General relativity
Properties of "Lite" Intermediate-mass Black Hole Candidates in LIGO-Virgo's Third Observing Run. Over a hundred gravitational-wave (GW) detections and candidates have been reported from the first three observing runs of the Advanced LIGO-Virgo-KAGRA (LVK) detectors. Among these, the most intriguing events are binary black hole mergers that result in a "lite" intermediate-mass black hole (IMBH) of ∼102 M⊙, such as GW170502 and GW190521. In this study, we investigate 11 GW candidates from LVK's third observing run with total detector-frame masses in the lite IMBH range. Using the Bayesian inference algorithm RIFT, we systematically analyze these candidates with three state-of-the-art waveform models that incorporate higher harmonics, which are crucial for resolving lite IMBHs in LVK data. For each candidate, we infer the premerger and postmerger black hole masses in the source frame, along with black hole spin projections across all three models. Under the assumption that these are binary black hole mergers, our analysis finds that five have a postmerger lite IMBH with masses ranging from 110 to 350 M⊙ with over 90% confidence interval. Additionally, we note that one of their premerger black holes is within the pair-instability supernova mass gap (60–120 M⊙), and two premerger black holes are above the mass gap. Furthermore, we report discrepancies among the three waveform models in intrinsic parameters, with at least three GW candidates showing deviations beyond accepted statistical limits. While the astrophysical certainty of these candidates cannot be established, our study provides a foundation to probe the lite IMBH population that emerge within the low-frequency noise spectrum of LVK detectors.

Keywords: Intermediate-mass black holes, Gravitational waves
Magnetic Reconnection–driven Energization of Protons up to ∼400 keV at the Near-Sun Heliospheric Current Sheet. We report observations of direct evidence of energetic protons being accelerated above ∼400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA's Parker Solar Probe (PSP) at a distance of ∼16.25 solar radii (Rs) from the Sun. Inside the exhaust, both the reconnection-generated plasma jet and the accelerated protons up to ∼400 keV propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located antisunward of PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ∼400 keV, which is ≈1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power law with spectral index of ∼‑5. Supporting simulations using the kglobal model suggest that the trapping and acceleration of protons up to ∼400 keV in the reconnection exhaust are likely facilitated by merging magnetic islands with a guide field between ∼0.2 and 0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP's proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. Our findings of in situ particle acceleration via magnetic reconnection at the HCS provide valuable insights into this fundamental process, which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the Sun's atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.

Keywords: Solar magnetic reconnection
It's Not Just a Phase: Oblique Pulsations in Magnetic Red Giants and Other Stochastic Oscillators. Magnetic fields play a significant role in stellar evolution. In the last few years, asteroseismology has enabled the measurement of strong magnetic fields 104–106 G in the cores of dozens of red giants and is the only known way to directly measure internal stellar magnetic fields. However, current data are still interpreted assuming that these fields are too weak or too axisymmetric to affect the orientation of the pulsations (i.e., make the pulsations "oblique"), rendering stronger field strengths beyond the reach of existing asteroseismic searches. We show that, even when an oblique pulsator is also stochastic (such as in a red giant with a strong nonaxisymmetric magnetic field), geometric effects will cause the signal to contain frequency components that remain in perfect relative phase with each other. This perfect phase relationship persists even over timescales in which stochasticity erases absolute phase information. This perfect relative coherence is a distinctive observational signature of oblique pulsation that does not require a model for mode frequencies to search for. However, due to its dependence on phase, this effect will not be evident in the power spectral density alone, and phase information should be retained in order to detect it. Coherence-based searches for oblique pulsations may pave the way to measurements of magnetic fields of currently inaccessible strengths in red giants, as well as some main-sequence and compact pulsators.

Keywords: Oblique rotators, Asteroseismology, Stellar magnetic fields, Red giant stars
On the Orbital Effects of Stellar Collisions in Galactic Nuclei: Tidal Disruption Events and Ejected Stars. Dense stellar clusters surround the supermassive black holes (SMBH) in galactic nuclei. Interactions within the cluster can alter the stellar orbits, occasionally driving a star into the SMBH's tidal radius, where it becomes ruptured, or expelling a star from the nuclear cluster. This proof-of-concept study examines the orbital effects of stellar collisions using a semianalytic model. Both low- and high-speed collisions occur in the SMBH's sphere of influence. We find that collisions can place stars on nearly radial orbits. Depositing stars within the tidal radius, collisions may drive the disruption of stars with unusual masses and structures: depending on the nature of the collision, the star could be the product of a recent merger, or it could have lost its outer layers in a previous high-speed impact, appearing as a stripped star. We also find that high-speed collisions near the periapsis of an eccentric orbit can unbind stars from the SMBH. However, dissipation during these high-speed collisions can substantially reduce the number of unbound stars achieved in our simulations. We conclude that tidal disruption events and ejected stars, even in the hypervelocity regime, are plausible outcomes of stellar collisions, though their frequency in a three-dimensional nuclear star cluster is uncertain. Future work will address the rates and properties of these events.

Keywords: Galactic center, Stellar mergers, Tidal disruption, Stellar dynamics, Star clusters