Solar and Stellar Astrophysics
The origin of dispersion in the wind mass-loss rate–luminosity relation of massive O stars
Jiri Krticka and Iva Krtickova
Stellar winds of hot
stars are accelerated by the radiative force that mainly comes from the line transitions of heavy elements. As a result, the wind mass-loss rate is expected to scale mostly with the stellar luminosity. This scaling clearly emerges from the observational analysis of O
stars and their winds, but with substantial scatter. The origin of this scatter is unclear. Part of the scatter of the observational relation between the hot
star wind mass-loss rate and luminosity may originate from the sensitivity of the mass-loss rate to other stellar parameters besides the luminosity, such as the effective temperature, mass, and
metallicity. We tested this possibility using our wind models. We calculated tailored wind models for stellar parameters corresponding to selected individual O
stars from the Large Magellanic Cloud as derived from a previous analysis. To model the wind, we used our global wind code that calculates the radiative force from the solution of the radiative transfer equation in the comoving frame. Wind models calculated for particular stellar parameters reproduce the observational mass-loss rates better than simple scaling relations derived from model grids. This indicates that the dependence of the mass-loss rate on the other stellar parameters except the luminosity might explain part of the dispersion in the relation between the mass-loss rate and luminosity. However, some dispersion still remains, which might indicate that wind variability is another source of the dispersion.
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High Energy Astrophysical Phenomena
Guide field effects on particle acceleration in three-dimensional relativistic magnetic reconnection
Despina Karavola, Lorenzo Sironi and Maria Petropoulou
Relativistic
magnetic reconnection is a key mechanism for particle acceleration in astrophysical
plasmas. Recent 3D particle-in-
cell simulations have shown that, for weak guide fields, particles can be accelerated at nearly the maximal rate by escaping the reconnected
plasma and gaining energy from the large-scale upstream electric field. Here, we investigate whether this fast acceleration channel persists as the guide field becomes comparable to the reconnecting field. We perform 3D simulations of relativistic pair-
plasma reconnection, complemented by corresponding 2D runs, using open boundaries along the reconnection outflows to establish a quasi-steady state. We find that increasing the guide field strength reduces the reconnection rate and causes energetic particles to become more closely aligned with the local
magnetic field. In 3D, the fast acceleration channel—absent in 2D—persists across the full range of guide fields investigated, but involves a progressively smaller fraction of particles as the guide field increases. Consequently, stronger guide fields produce steeper particle energy spectra and lower maximum Lorentz factors.
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Astrophysics of Galaxies
A multi-scale view of molecular gas and magnetic fields in the Antennae galaxies
Akhil Lasrado, Enrique Lopez-Rodriguez, Claudia Cicone et al. (+5 more)
Major mergers are transformative events in
galaxy evolution, dynamically reshaping the stellar and gas makeup of the hosts, as well as the
galaxy-scale
magnetic fields (B-fields) that permeate their interstellar medium (ISM), altering both B-field strength and configuration. In this work, we study in detail the link between the co-evolving gas and B-fields in the cold ISM phase of the nearby
Antennae galaxy merger (NGC 4038/4039), using CO(3-2) data from the LAsMA receiver at the APEX
telescope and ALMA, together with archival 154 $\mu$m dust polarization data from the SOFIA/HAWC+. We characterize trends between measurements of molecular gas intensity and velocity dispersion with polarization measurements at kpc-scales with the APEX and SOFIA data. We find anti-correlations with CO intensity ($\rho\lesssim-0.6$) and CO velocity dispersion ($\rho\lesssim-0.4$), which are signposts of
geometric depolarization. Since dust polarization measurements are a density-weighted line-of-sight integrated average of B-field vectors below the beam size, the ALMA data can serve as a proxy of density structures below the SOFIA beam on a channel-by-channel basis. Through a
geometric analysis of structures in the ALMA channels, we identify salient kinematic features in the overlap region strongly aligned with the SOFIA-inferred B-field orientation. This indicates that these kinematic features might dominantly lend their orientation to the measured field direction, assuming a correspondence between molecular gas structures and galactic B-fields, owing to their density or high polarization fraction. We also identify a modest trend ($\rho\sim0.5$) between the coherence of the molecular gas, a proxy of the line-of-sight angular dispersion, at small scales, and the SOFIA polarization fraction, showing that the polarization fraction measured at $\sim2$ kpc scales is sensitive to structures at $\sim50$ pc scales.
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All authors Akhil Lasrado, Enrique Lopez-Rodriguez, Claudia Cicone, Axel Weiss, Andreas Lundgren, Boy Lankhaar, Robert Wissing, Sijing Shen
Astrophysics of Galaxies
Discovery of thermal radio recombination line emission toward the high-mass protostar IRAS18162$-$2048
R. Fedriani, J.F. Gomez, Y. Cheng et al. (+7 more)
The physical conditions within the innermost few tens of au of high-mass protostars ($M_*>8~M_\odot$) remain largely unexplored. At these scales, ionised emission may trace photoionised gas,
accretion disks, or the base of protostellar jets. We search for ionised emission in the form of
radio recombination lines (RRLs) in high-mass
star-forming regions. We use archival data from the Atacama Millimeter/submillimeter Array (ALMA) interferometer in band 6 ($\sim$1.3 mm) with a resolution of $\theta_\mathrm{beam}\sim53~\mathrm{mas}\times42~\mathrm{mas}$ to examine the high-mass protostar IRAS18162$-$2048, driving source of the Herbig-Haro (HH) objects HH80-81. We detect, for the first time in this protostar, the thermal emission of the RRL H30$\alpha$ ($\mathrm{rest~frequency}=231.90093$ GHz). The ionised emission is spatially unresolved in our observations, but we estimate an emitting region $\lesssim50$ au at the source distance of 1.4 kpc. The H30$\alpha$ line profile is broad and spectrally resolved with a Local Standard of Rest velocity of $\sim-14~\mathrm{km~s^{-1}}$ (i.e., $\sim-26~\mathrm{km~s^{-1}}$ blue-shifted with respect to the systemic velocity of the cloud) and a full width at half maximum of $\sim82~\mathrm{km~s^{-1}}$. These observations suggest that the H30$\alpha$ line is tracing outflowing blue-shifted material in the well-known
radio jet from this object. This detection opens the possibility of detailed studies of the kinematics and physical properties of ionised jets.
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All authors R. Fedriani, J.F. Gomez, Y. Cheng, M. Osorio, G. Anglada, C-Y. Law, J.M. Masque, A.F. Placinta-Mitrea, G. Blazquez-Calero, G. Cosentino
Astrophysics of Galaxies
Merger remnant in TXS 1033+026 - I: $\rm H\alpha$ and HI 21-cm observations suggest counter-rotating gas
J. N. H. S. Aditya, Hyein Yoon, L. G. Dahmer-Hahn et al. (+5 more)
We conducted eMERLIN and Gemini integral field unit (IFU) observations towards TXS 1033+026, a post-merger ultraluminous infrared
galaxy, to study the
radio continuum and $\rm H\alpha$ and HI 21 cm lines at sub-kiloparsec scales. We find a symmetric, two-sided core- jet structure in both L-($\approx 1.4$ GHz) and C-($\approx 4.8$ GHz) band continuum images. The active galactic
nucleus likely has a double-double
radio morphology, indicating possible multiple episodes of activity. The Gemini IFU observations provided sub-arcsecond resolution. Results from these observations suggest the presence of a kiloparsec-scale circumnuclear, warped $\rm H\alpha$ disc that rotates around the core. The HI 21 cm maps reveal a complex kinematic structure, with indications of two counter-rotating gas components orbiting the core-jet axis; one of these components counter-rotates relative to the kiloparsec-scale $\rm H\alpha$ disc. However, the true morphology of this structure may be more complex than our interpretation suggests. We estimate a HI 21 cm velocity-integrated optical depth of $\rm 90 \pm 3 \ km \ s^{-1}$. We note that these are the first results to probe $\rm H\alpha$ and cold neutral hydrogen gas in TXS 1033+026 at sub-kiloparsec scales.
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All authors J. N. H. S. Aditya, Hyein Yoon, L. G. Dahmer-Hahn, Renzhi Su, Rajeshwari Dutta, Xiaolong Yang, Elaine Sadler, Matthew T. Whiting
Solar and Stellar Astrophysics · Cross-listed from nucl-ex
First strength measurements of low-energy resonances in the $^{45}$Sc($p,\gamma$)$^{46}$Ti reaction and its astrophysical implications
R. S. Sidhu, R. J. deBoer, J. Görres et al. (+13 more)
The $^{45}$Sc($p,\gamma$)$^{46}$Ti reaction plays an important role in both hydrostatic and explosive nucleosynthesis. During hydrostatic oxygen and silicon burning in massive
stars, including dynamic processes such as convective carbon-oxygen (C-O) shell mergers, it acts as a bottleneck that regulates the reaction flow in the Sc-Ti mass region. In oxygen-neon (ONe) novae, the ejecta indicate the production of elements up to the Fe group, as observed in V1974 Cygni, and the $^{45}$Sc($p,\gamma$)$^{46}$Ti reaction links
nuclei above Ca to the Fe region. In core-collapse
supernovae (CCSNe), it influences the production of the radioactive isotope $^{47}$Sc. At present, the thermonuclear reaction rate is based on statistical model calculations. In this work, we report on measurements of the $^{45}$Sc($p,\gamma$)$^{46}$Ti reaction performed using the 5~MV accelerator at the Nuclear Science Laboratory, University of Notre Dame, over a laboratory
proton energy range of 526–1275~keV. We present the first direct measurements of the resonance strengths for six resonances at $E^{\rm lab}_{\rm R} = 917.4$, 1026.0, 1031.1, 1049.7, 1059.4 and 1257.5~keV, together with integrated resonance strengths at $E^{\rm lab} = 555$, 630, 700, 761 and 871~keV. For resonance energies below the lowest directly measured energy, available transfer-reaction data were used to estimate the resonance strengths. An updated thermonuclear reaction rate is derived, and its astrophysical implications are discussed.
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All authors R. S. Sidhu, R. J. deBoer, J. Görres, M. Wiescher, W. N. Catford, C. Dembski, C. R. Jones, Ó. E. López-López, G. Lotay, K. Manukyan, M. Matney, J. O'Neill, J. Rufino, A. T. Sanchez, M. Williams, L. Zimmer
Astrophysics of Galaxies
A two-stage settling of the Milky Way disk revealed by precise ChronoGal ages
David Mirabal, Carme Gallart, Tomás Ruiz-Lara et al. (+9 more)
The spatial distribution of Galactic disk
stars of different ages and
metallicities encodes its dynamical
evolution history and formation pathways. The lack of precise ages for large, representative samples of
stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk (e.g. upside-down formation versus secular dynamical heating). In this Letter, we use CMD-fitting of Gaia data to derive very precise age-
metallicity distributions for complete stellar samples in the solar neighborhood. We identify quasi-mono-age-
metallicity stellar populations with an age resolution better than $5 \%$ ($\sim 0.5$ Gyr at old age), and study their absolute vertical density profile. We observe a clear correlation between scale height and stellar age, with younger populations consistently more confined to the plane. We find evidence that the scale height decrease rate changed drastically $10.85^{+0.13}_{-0.12}$ Gyr ago: at older ages, the
evolution is very steep, with $h_Z$ dropping from over $1.26$ kpc to $\sim 0.5$ kpc in merely $2$ Gyr; subsequently, the thinning is very gradual, with $h_Z \sim 0.1$ kpc for the youngest populations. The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed. We thus strongly support an evolutionary scenario defined by two distinct temporal phases: an initial, highly turbulent epoch that gave birth to the thick disk, and a subsequent more quiescent period of secular
evolution and gradual gas settling which formed the thin disk.
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All authors David Mirabal, Carme Gallart, Tomás Ruiz-Lara, Emma Fernández-Alvar, Anna B. A. Queiroz, Guillem Aznar-Menargues, Chris B. A. Brook, Santi Cassisi, Yllari K. González-Koda, Julio F. Navarro, Pedro A. Palicio, Francisco Surot
High Energy Astrophysical Phenomena
Twins in the Shadow: Phase Transitions in Proto-Neutron Stars
Jannik Milthaler, Jan-Erik Christian, Pia Jakobus et al. (+1 more)
We study the impact of a first-order phase transition from hadronic to deconfined
quark matter on the
evolution of proto-
neutron stars. To maximize the applicability of our results, we construct an equation of state that is highly parameterizable, where we use a relativistic mean field approach to model hadronic matter and a constant speed of sound approach to model
quark matter. We approximate the cooling behavior of proto-
neutron stars as a sequence of decreasing
entropy at constant baryonic mass. Some parameterizations lead to so-called "twin-
stars", where two
neutron stars have the same (gravitational) mass but different radii. Detecting them is regarded as a smoking gun signal for phase transition. We find, however, that proto-
neutron star evolution suggests that gravitational twin
stars are only possible if the hybrid
star does not have, in addition, a baryonic twin. This rules out most twin
star pairs, especially if the phase transition occurs at large densities. In a mass-radius diagram the unpopulated hybrid branch appears to be "in the shadow" of the hadronic branch.
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All authors Jannik Milthaler, Jan-Erik Christian, Pia Jakobus, Stephan Rosswog
High Energy Astrophysical Phenomena
The 2015-2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-Dependent Viewing Geometry
Wanchaloem Khwammai, Minchul Kam, Motoki Kino et al. (+11 more)
We present high-cadence, multi-frequency monitoring of the blazar OJ 287 using KVN (22–129 GHz), ALMA (91.5–343.5 GHz), and Metsähovi (37 GHz), covering 2012–2023, together with published optical polarimetry. Within this decade-long
dataset, an exceptionally large and smooth EVPA rotation is observed only during 2015–2017. The millimeter-band EVPA rotates by more than $\sim$300 deg over $\sim$1.5 yr, with a comparably large and more rapidly evolving rotation observed at optical wavelengths, while remaining comparatively stable at other epochs. The rotation coincides with a sequence of strong
radio flares from 37 to 343 GHz whose peak amplitudes increase toward 2017 March, when the rotation ends. Modeling the ALMA 91.5 GHz light curve yields variability Doppler factors of $\delta_{\rm var} \sim 8$–$11$ during the EVPA-rotation interval, with no systematic increase across the
flare sequence. The absence of a monotonic change in $\delta_{\rm var}$ indicates that progressively enhanced relativistic beaming is unlikely to be the primary driver of the rising
flare envelope. The confinement of the large EVPA rotation to this interval, together with the nearly constant $\delta_{\rm var}$, indicates that the 2015–2017 event occurred when a single newly ejected disturbance temporarily dominated the polarized emission while propagating through structured inner-jet regions threaded by a helical
magnetic field under modest jet-direction changes. Large EVPA rotations therefore arise intermittently as the jet orientation evolves, and are realized only when emission dominance and viewing geometry align favorably.
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All authors Wanchaloem Khwammai, Minchul Kam, Motoki Kino, Sascha Trippe, Hiroshi Nagai, Seiji Kameno, Ilje Cho, Ioannis Myserlis, Jeffrey A. Hodgson, Do-Young Byun, Sang-Sung Lee, Taehyun Jung, Anne Lahteenmaki, Merja Tornikoski
Astrophysics of Galaxies
No Convincing Evidence for Tidal Binary Clusters in the Galaxy
Juan Casado and Zahra Al
Context. The existence of tidal binary clusters (TBCs) in the Milky Way has been repeatedly suggested using Gaia DR3 astrometry and recent catalogs of binary clusters and groups. However, the dynamical feasibility of forming truly bound binary clusters through tidal capture remains uncertain. Aims. We perform a systematic reassessment of twenty
star clusters, currently claimed as members of TBCs from the catalog of binary and grouped clusters of Liu et al. (2025) to ascertain their classification. Methods. For each candidate pair, we use Gaia DR3 astrometry and photometry to perform a simultaneous and self-consistent opti mization of age, distance modulus, reddening, and
metallicity. Results. Our analysis shows that all studied TBCs fail in the required physical condition that ages are compatible with independent formation. After applying strict evolutionary criteria, these systems are more naturally explained as co-eval members of primordial cluster pairs or groups rather than products of tidal capture. Conclusions. We conclude that genuine tidal binary clusters, if present in the Milky Way, must be extremely rare. This work un derscores the importance of robust age determinations and physically motivated selection criteria in future searches for dynamically assembled cluster pairs.
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High Energy Astrophysical Phenomena
Comparing the size evolution of GRB 221009A with afterglow models
S. Giarratana, O. S. Salafia, B. Marcote et al. (+10 more)
We present very long baseline interferometry (VLBI) observations of GRB221009A obtained with the global-VLBI network at 5 GHz between 231 and 750 days after the burst. The quality of our observations allowed us to measure the sub-milliarcsecond full-width at half maximum of the source surface brightness distribution with a one-sigma equivalent error of 13% and 30% at 231 and 391 days post-burst, respectively. By fitting the temporal
evolution of these new size measurements alongside those previously published in the literature at 5 and 8 GHz with a single power law $\propto t^a$, we find a slope of $a = 0.52^{+0.14}_{-0.12}$. Using the public code VegasAfterglow, we model the 5 GHz size
evolution considering the afterglow emission of the forward shock produced by a jet decelerating in either a homogeneous interstellar medium (ISM) or into a surrounding medium with a wind-like density profile. The model constrains the collimation-corrected Sedov ratio, that is, the ratio of the jet kinetic energy $E_\mathrm{jet}$ to the density of the surrounding medium ($n$ in the ISM scenario, or $A_\star$ in the wind scenario), in the range $\log[(E_\mathrm{jet}/n)\,\mathrm{erg}^{-1}\,\mathrm{cm}^{-3}] = 54.2^{+0.4}_{-0.5}$ for the ISM scenario and $\log[(E_\mathrm{jet}/A_\star)\,\mathrm{erg}^{-1}] = 51.7^{+0.4}_{-0.2}$ for the wind scenario. Among the works in the literature that fit the light curves and spectra of GRB221009A based on the standard afterglow model, and which can be reproduced with VegasAfterglow, none has best-fitting parameters that are compatible with these constraints within $2\sigma$. Moreover, we show that their predicted size
evolution is not compatible with the observed one. Our results highlight the importance of size measurements of GRB afterglows as an independent probe of their dynamics, structure, and emission mechanisms.
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All authors S. Giarratana, O. S. Salafia, B. Marcote, Z. Paragi, M. Giroletti, G. Ghirlanda, L. Nava, A. van der Horst, J. K. Leung, L. Rhodes, P. G. Edwards, M. Pérez-Torres, J. Yang
High Energy Astrophysical Phenomena
XMM-Newton Resolves Parsec-scale X-ray Jets in the PeVatron Microquasar V4641 Sgr
Naomi Tsuji, Caterina Tresoldi, Kaya Mori et al. (+34 more)
The microquasar V4641 Sagittarii (V4641 Sgr) has recently been established as one of the most powerful Galactic PeV
cosmic-ray accelerators (PeVatrons) through the detection of degree-scale gamma-ray emission to 0.8 PeV. Using XMM-Newton, we performed an X-ray survey of its gamma-ray-emitting regions during the quiescent phase. We discovered extended X-ray emission to the north and south of V4641 Sgr, aligned with the
radio-jet axis, revealing X-ray jet structures extending over several tens of parsecs for the first time. The northern and southern
knot-like features, dubbed n1 and s1, are located approximately 20 pc from the central binary and spatially coincident with the gamma-ray emission. The X-ray emission from each
knot, characterized by a
photon index of $\Gamma_X = 1.9 \pm 0.1_{\rm stat} \pm 0.5_{\rm syst}$ and an energy flux of $\sim$(2–4)$\times10^{-13}$~erg~cm$^{-2}$~s$^{-1}$ in the 2–10 keV band, is likely non-thermal in origin, indicating synchrotron radiation from accelerated electrons. The morphological coincidence between the X-ray and gamma-ray emission favors a leptonic scenario for the gamma rays, although the X-ray emission is more compact than the gamma-ray-emitting region and the hadronic scenario cannot be excluded. Through X-ray to gamma-ray modeling, we obtain a weak
magnetic field strength of 3–5~$\mu$G. We propose that the newly detected X-ray
knots represent particle-acceleration sites, such as jet termination or (re-)collimation regions powered by V4641 Sgr, or downstream structures analogous to the
knots of the outer jets in SS~433/W50. This discovery suggests that extended X-ray emission along jets would be a key signature, yet previously overlooked, in PeVatron microquasars.
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All authors Naomi Tsuji, Caterina Tresoldi, Kaya Mori, Hiromasa Suzuki, Matthew Lundy, Dmitry Khangulyan, Samar Safi-Harb, Laura Olivera-Nieto, Felix Aharonian, Shotaro Abe, Zhen Cao, Sabrina Casanova, Maria Chernyakova, Yi-Heng Chi, Stephane Corbel, E. de la Fuente, Hua Feng, Tatsuki Fujiwara, Noa Grollimund, Dezhi Huang, Yoshiyuki Inoue, Ryohei Kanazashi, Shigeo S. Kimura, Marianne Lemoine-Goumard, Jonathan Mackey, Alessio Marino, Samaresh Mondal, Kai Morita, Maxime Parra, Gabriele Ponti, Brian Reville, Megumi Shidatsu, Shunya Takekawa, Michelle Tsirou, Xiaojie Wang, Heshou Zhang, Ping Zhou
High Energy Astrophysical Phenomena
Candidate X-ray Counterparts of Ultra-High-Energy Emission of the Galactic Microquasar V4641 Sgr Revealed by Einstein Probe
Yi-Heng Chi, Shao-Qiang Xi, Ping Zhou et al. (+20 more)
Microquasars have emerged as promising PeVatron candidates capable of accelerating particles to PeV energies, with recent observations revealing ultra-high-energy (UHE) $\gamma$-ray emission from extended structures associated with these systems. Multi-wavelength studies of such extended structures are crucial for understanding the acceleration, transport, and radiation processes of UHE
cosmic rays (CRs), yet observational evidence remains limited. Here, we report the discovery of diffuse X-ray structures spatially coincident with the TeV $\gamma$-ray emission regions to the north and south of the microquasar V4641 Sgr, using the Follow-up X-ray
Telescope (FXT) onboard the Einstein Probe. We identify X-ray structures located $\sim20$-$40$ pc from V4641 Sgr, extending over several arcminutes. These structures exhibit X-ray fluxes of $\sim1$-$3\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ and
photon indices of $\Gamma\sim2$-3 in 0.5-8 keV per individual
knot. Under the one-zone leptonic interpretation, the weak X-ray emission implies a
magnetic field of order $1-2 \mu$G. Stronger fields remain possible if only part of the TeV-PeV emission is produced by inverse Compton scattering, with the remaining emission arising from hadronic interactions. The EP-FXT observations support the presence of relativistic electrons on tens-of-parsec scales, while the origin of the UHE $\gamma$-ray emission remains open.
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All authors Yi-Heng Chi, Shao-Qiang Xi, Ping Zhou, Hua Feng, Ruo-Yu Liu, Songzhan Chen, Naomi Tsuji, Caterina Tresoldi, Gabriele Ponti, Zhen Cao, Yong Chen, Shumei Jia, Chi-Chuan Jin, Chengkui Li, Yuan Liu, Matthew Lundy, Kaya Mori, Haiwu Pan, Lian Tao, Junfeng Wang, Weimin Yuan, Juan Zhang, Haisheng Zhao
Solar and Stellar Astrophysics
T. Merle, P. F. L. Maxted, O. L. Creevey et al. (+23 more)
The ESA PLATO mission aims to detect and characterise Earth-size
exoplanets orbiting bright, late-type
stars using high-precision photometry and asteroseismology. Precise stellar parameters of host
stars are essential for
exoplanet characterisation and model benchmarking. This work defines the PLATO
benchmark stars (PBS) catalogue, a comprehensive sample of well-characterised
stars designed to support all mission phases. We compiled data from eclipsing and spectroscopic binaries, interferometry, asteroseismology, and large spectroscopic and photometric surveys. Fundamental and atmospheric stellar properties (mass, radius, luminosity, age, effective temperature, surface
gravity, density, and limb darkening) were assigned quality levels (B1, B2, B3) reflecting their degree of model dependence. Homogenized ancillary parameters (including Gaia DR3 bias-corrected parallaxes,
metallicities, bolometric fluxes, extinction, rotation periods, and X-ray fluxes for M dwarfs) are also provided. The PBS catalogue contains 1090 FGKM main-sequence and subgiant
stars, divided into three quality tiers based on model-independent (B1) parameters: gold (15
stars with B1 mass, radius, and luminosity), silver (195 with B1 mass and radius), and bronze (416 with at least one B1 property). The remaining
stars feature well-characterised model-dependent (B2) or empirical (B3) properties. About 40% of the sample lies within the PLATO fields of view (285 in LOPN1 and 143 in LOPS2). The PBS sample represents the largest catalogue of
benchmark stars assembled to date, providing a key resource for validating automated pipelines, testing evolutionary models, and cross-survey calibration. The catalogue is available online at https://astro.ulb.ac.be/pbs and will be updated regularly.
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All authors T. Merle, P. F. L. Maxted, O. L. Creevey, U. Heiter, D. B. Palakkatharappi, K. G. Hełminiak, C. C. Aguilera Gómez, M. Bazot, S. Cassisi, N. Ebrahimkutty, R. E. Giribaldi, P. Jofré, F. Kiefer, M. N. Lund, N. Miller, T. Morel, D. Mourard, N. Nardetto, J.-D. do Nascimento, Jr., T. Olander, G. N. Pawar, A. Rebassa-Mansergas, R. Raddi, B. Rojas-Ayala, J. Southworth
High Energy Astrophysical Phenomena
Study of the Ultra-Long GRB 220627A: Exploring a Possible Blue Supergiant Collapsar Origin
Ayush Garg, Amit Shukla, Dorottya Szécsi et al. (+1 more)
Ultra-long gamma-ray bursts (ul-GRBs) challenge standard collapsar models because their extreme durations require central engines active for $\sim10^{3}-10^{4}$ s. While the
prompt spectral
evolution of typical GRBs is difficult to resolve due to their short durations, ul-GRBs provide a unique opportunity to study spectral
evolution on extended timescales. We present a time-resolved spectroscopic study of GRB 220627A, one of the most luminous ul-GRBs known ($E_{\rm iso}\sim10^{54}$ erg, $T_{90}>1000$ s), consisting of two bright emission episodes separated by $\sim600$ s quiescent interval. Using a novel HOP
algorithm-based segmentation method, we identify optimal time intervals for time-resolved spectroscopy. A Comptonized power-law (CPL) model well describes the
prompt emission ($\sim 10$ keV-40 MeV) across all timescales. Fermi-GBM spectroscopy reveals a transition from a hard, slow-cooling synchrotron spectrum ($\alpha_{\rm I}=-0.70$) during episode-I to a softer, fast-cooling synchrotron spectrum ($\alpha_{\rm II}=-1.29$) during episode-II. This
evolution closely resembles that of the magnetar-candidate GRB 091024A, highlighting a potential progenitor degeneracy based solely on spectral
evolution. However, the extreme isotropic-equivalent energy exceeds the maximum energy available from magnetar
spin-down, strongly favouring a blue supergiant (BSG) collapsar origin. The prolonged activity and multi-episode structure are naturally explained by sustained
accretion through an extended stellar envelope. Our results demonstrate that ul-GRBs can display magnetar-like spectral
evolution while being powered by BSG collapsars. GRB 220627A highlights the need for combining time-resolved spectroscopy with energetic constraints to distinguish between progenitor scenarios and understand the physical origin of ul-GRBs.
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All authors Ayush Garg, Amit Shukla, Dorottya Szécsi, G. C. Anupama
High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Kinetic Equilibrium between SIMP Dark Matter and Radiation via Internal Bremsstrahlung
Xiaoyong Chu, Ajay Kaladharan and Yassine El Mohtadi
As an alternative to standard dark matter freeze-out via two-body annihilation, strongly interacting massive particles are proposed to regulate their relic abundance through three/four-body self-annihilation. During such freeze-out, dark matter must undergo both number reduction and energy dissipation. The former requires number-reducing processes, and the latter requires an efficient energy-dissipation mechanism. In this work, we propose a novel mechanism where internal Bremsstrahlung processes establish kinetic
equilibrium between dark matter and radiation, and thus dissipate energy. More appealingly, this mechanism can be naturally realized in a class of strong dynamics models with flavor-universal
quarks and a light
axion. To demonstrate our results in a $Sp(4)_c$ dark-color model, we compute all relevant interaction terms and numerically solve the full Boltzmann equations. The same mechanism can also be applied to vector dark matter models.
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Earth and Planetary Astrophysics
Joint RV-Astrometric Analysis of two-planet system HD 65216: Prospects for Gaia DR4
Alexander L. Wallace, Robert A. Wittenmyer and Jonathan Horner
We provide updated mass and orbital constraints on the planetary system around HD 65216, consisting of at least two giant
planets on eccentric orbits. We first analyse new RV data to constrain the periods and eccentricities of both
planets, before applying the Hipparcos-Gaia proper motion anomaly to constrain the inclination of HD 65216 c. From this joint analysis, we find that HD 65216 c is most likely in an eccentric orbit ($e$ = 0.42$\pm$0.05) with a longer period (7007$^{+192}_{-161}$ days) than previously determined. The proper motion anomaly is consistent with a bimodal distribution of inclinations, with peaks at sin$i\sim$0.5, producing true masses of 2.23$^{+0.96}_{-0.77}$ M$_{\mathrm{J}}$ and 3.32$^{+1.40}_{-1.16}$ M$_{\mathrm{J}}$ for HD 65216 b and c respectively, assuming the
planets are co-planar. Finally, we simulate Gaia DR4 epoch astrometry and the DR3-DR4 proper motion anomaly. We find that these methods will be sensitive enough to further constrain orbital parameters and true mass.
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Solar and Stellar Astrophysics
Formation and Eruption of a Vortex-driven Magnetic Flux Rope in the Simulated Quiet Sun
Zhizhen Ye and Feng Chen
Magnetic flux ropes (MFRs) are key structures for understanding
flares and coronal mass ejections in active regions, but their characteristics in the quiet Sun remain poorly understood. With a radiative MHD simulation spanning from the upper convection zone to the corona, we analyze the formation and eruption of a supergranular-scale flux rope. We use a clustering method to group the closed field lines by their connectivity. The clusters are gathered further into 8 persistent cluster assemblies (CAs) that correspond to a flux rope and ambient
magnetic structures. A persistent counterclockwise
vortex is maintained at the converging point of the supergranules, i.e. the positive footpoint of the flux rope. The
vortex continuously injects helicity into a
magnetic flux tube and wraps ambient
magnetic structures around the central flux tube. On a time scale of one hour, the flux tube evolves to a strongly twisted flux rope and erupts. The eruption exhibits similar current sheet structures as inferred from the flux rope eruption in active regions; however, the mass ejection and heating are much weaker and give rise to very insignificant observable features. The synthetic EUV images show mostly dimming features caused by density rarefaction in the expanding flux rope. This work helps elucidate the dynamic and complex
evolution of flux ropes formed in the quiet Sun and suggests that similar events in the real Sun may have been underestimated due to their stealth behavior.
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Instrumentation and Methods for Astrophysics
Robust Bayesian non-linear pulsar timing and noise analysis using a sequential data-tempered ensemble sampler
Abhimanyu Susobhanan
We develop a sequential ensemble MCMC scheme for
Bayesian pulsar timing and noise analysis that uses data size as the tempering parameter. The method enables gradual and reliable convergence from potentially uninformed initial points by iteratively increasing the data size, thereby increasing sensitivity and progressively narrowing the target distribution until the posterior of the full
dataset is reached. The initial iterations use sub-
datasets that can be orders of magnitude smaller than the full
dataset, substantially reducing computational cost, which is further lowered by employing approximate lower-rank models justified by the reduced sensitivity of these sub-
datasets.
Bayesian inference is performed using the final MCMC chain sampled from the true target distribution after applying appropriate burn-in. We demonstrate the method using the NANOGrav 12.5-year data of PSR B1855+09, showing progressive convergence to the target posterior and that most of the computational time is spent sampling the true target distribution rather than in the preceding tempered iterations.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Non-Gaussian timing residual skewness in pulsar timing arrays
Reginald Christian Bernardo, Rituparna Mandal and Kin-Wang Ng
Motivated by searches for weak primordial non-Gaussianity in the
cosmic microwave background, we investigate perturbative departures from Gaussian
stochastic gravitational wave background statistics using
pulsar timing arrays. We consider a quadratic ansatz that shows three-point correlations at first order in the nonlinear coupling and covariance corrections at second order. We first examine the ansatz for gravitational wave helicity amplitudes. However, the timing residual bispectrum that this produces is not manifestly invariant under local polarization basis rotations for tensor and vector modes. Then, we formulate a polarization-frame-invariant phenomenological model for detector-projected amplitudes and evaluate its one-
pulsar timing residual third moment with a simplified response. Our numerical
benchmarks have shown that this gives a very small Gaussian-rms-normalized skewness.
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Solar and Stellar Astrophysics
A New Analysis Tool for Radial Pulsations
Hiroyasu Ando
Wave-energy relation is introduced as a new analysis tool for linear radial pulsations. Wave energy consists of kinetic energy, acoustic-potential energy, dynamical-potential energy, and
entropy-potential energy. The relation describes changes in the wave energy caused by non-adiabatic processes and by leakage of wave energy. It is applied to modal analysis of adiabatic pulsations for the massive Zero Age Main Sequence models (10, 20, and 30 solar masses). The ordinary modes are well trapped in the interior cavity without any significant wave-energy leakage. On the other hand, the strange modes (S-modes) exhibit the steep increases in the eigenfunctions and kinetic energy toward the stellar surface and the significant wave-energy leakage. The kinetic energy of S-modes is no longer confined in the exterior cavity.
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High Energy Astrophysical Phenomena
The Structure in the Structure Function of Black Hole Light Curves
Daniel C. M. Palumbo, Alejandro Cárdenas-Avendaño and Lena Murchikova
Of the two black holes, M87$^\ast$ and Sgr A$^\ast$, for which event-horizon-scale imaging is currently achievable, only Sgr A$^\ast$ at our Galactic Center exhibits sub-hour variability, making light curve observations of the inner
accretion flow especially valuable for time-domain studies of the spacetime and the surrounding
plasma. Observed light curves are often sampled with variable cadence, so a structure function is often used to quantify their variability properties. In this work, we assemble a heterogeneous Sgr A$^\ast$ light curve data set from discrete observations acquired over $\sim12$ years {at 230 GHz}, with different instruments, durations, and cadences, and study its structure function in three frameworks. First, we consider analytic predictions for the features of structure functions of stationary random processes with
Fourier spectra dictated by the Matérn covariance. We then study the structure functions resulting from lensed movies of synthetic optically thin
accretion flows approximated with Matérn covariance in both space and time, and identify which features in the structure function can be attributed to the black-hole geometry. Lastly, we examine the structure functions of a library of $72$ general relativistic magnetohydrodynamic simulations. We find that, in general, magnetically arrested disk (MAD) models produce smoother structure functions with steeper slopes than standard and normal
evolution (SANE) counterparts. On time scales of $5-15$ minutes, the observed structure function slope of Sgr A$^\ast$ more closely aligns with the MAD trend, while both simulation types are generally more variable than the data on all sub-hour timescales.
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Solar and Stellar Astrophysics
Mesoscale Turbulence in Type Ia Supernova Deflagrations II. Evidence for Fuel Preheat Due to Flame Thermal Expansion from Lagrangian Analysis
Ryan Learn, Andrey Zhiglo and Tomasz Plewa
Thermonuclear flame fronts in Type Ia
supernovae develop hydrodynamic instabilities that generate turbulent mesoscale flow and substantial local thermodynamic variability. Recent work by \citet{Brooker+25} demonstrated systematic shortening of fuel ignition times near Rayleigh–Taylor unstable deflagration fronts, but the physical origin remained unclear. We investigate the thermodynamic
evolution of fuel near the flame front using multidimensional simulations and Lagrangian tracer-particle analysis, focusing on flame-driven expansion and ignition-time variability in partially degenerate carbon–oxygen
plasma. Tracer trajectories show that the most strongly preconditioned fuel parcels near the flame front, which form a minority of the fuel layer, undergo approximately pressure-constrained expansion with modest density reductions and temperature increases of $\approx 2\times10^{8}$~K to $\approx7\times10^{8}$~K ($\approx13$ to $63$ per cent above the unperturbed fuel temperature) between the low- and high-density models. Despite moderate thermodynamic perturbations, the strong temperature sensitivity of carbon ignition produces substantial shortening of ignition times. The results indicate a two-step mechanism: flame thermal expansion perturbs the fuel away from hydrostatic
equilibrium, while restoration of pressure balance by weak acoustic waves redistributes internal energy from degenerate to thermal components. This raises the fuel temperature and produces localized thermodynamic preconditioning. These results suggest that unsteady thermonuclear flames can shorten ignition times ahead of the flame, influencing fuel over distances larger than the laminar flame thickness. Whether this preconditioning leads to secondary ignition or contributes to deflagration-to-detonation transition remains untested here, as nuclear burning is disabled in the analyzed fuel.
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Cosmology and Nongalactic Astrophysics
Distinguishing the origin of cosmic birefringence: dark energy, dark matter, and neutrino asymmetry
Lu Yin and Eiichiro Komatsu
Cosmic birefringence, the isotropic rotation of the plane of linear polarization of
photons from sources at cosmological distances, can be generated by different parity-violating mechanisms. While the net rotation angle of a single polarized source carries little information about its physical origin, sources at various
redshifts $z$ can be used to distinguish between different origins. Parity-sensitive correlations of the $E$- and $B$-mode polarization fields of the
cosmic microwave background measured on large and small angular scales probe sources at $z\simeq 10$ and $1100$, respectively. Thus, the detailed shape of the $EB$ power spectrum is sensitive to the
redshift evolution of the birefringence source. In this paper, we calculate the $EB$ power spectrum due to a
neutrino-asymmetry current, and compare it to
axion dark energy and dark matter using a common Chern–Simons framework. We find that the $EB$ power spectrum due to a
neutrino asymmetry is suppressed at low multipoles, $\ell\lesssim 20$, similar to the dark matter case. This can be clearly distinguished from the dark energy case, which has no such suppression. Large-scale polarization measurements, including LiteBIRD, can therefore distinguish late-time dark energy birefringence from that generated at higher
redshifts by
axion dark matter or
neutrino asymmetry. We also find that the
evolution of the
neutrino density during recombination alters the shape of the $EB$ power spectrum at high multipoles. This signature can be used to distinguish between the
neutrino and
axion dark matter interpretations, using ground-based experiments such as the Simons Observatory.
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High Energy Astrophysical Phenomena
A Refined Analytic Oblate–Schwarzschild Model for Thermal X-Ray Pulse Profiles of Rotating Neutron Stars
Xi Yang, Tong Zhao and Renxin Xu
Thermal X-ray pulse profiles constrain
neutron-
star masses and radii through the effects of gravitational light bending and rotation on surface emission. The precision anticipated from the enhanced X-ray Timing and Polarimetry (eXTP) mission motivates more accurate analytic light-bending formulae that retain the efficiency needed for repeated model evaluations. We derive a refined analytic flux expression within the oblate–Schwarzschild (OS) approximation that combines bending, propagation-delay, and
lensing approximations with a spheroidal surface. We compare its profiles with numerical Schwarzschild and second-order Hartle–Thorne ray tracing on the same spheroidal surfaces. The comparisons use a selected two-spot configuration at 300 Hz with isotropic blackbody emission and three choices for surface eccentricity: a freely specified value, the Maclaurin relation for a uniformly rotating Newtonian fluid, and a fit to rotating-
neutron-
star surfaces. Across these cases, the refined OS profiles have RMS flux differences of $1.76-1.98\%$ relative to the Hartle–Thorne reference, compared with $6.50-21.39\%$ for the spherical Schwarzschild-plus-Doppler (S+D) model. All differences are normalized to the phase-averaged reference flux. The corresponding differences between Schwarzschild and Hartle–Thorne ray tracing remain below $1.1\%$. These comparisons separate errors in analytic
photon transfer from the effects of surface deformation and the rotating exterior spacetime. For the configurations examined, accounting for oblateness substantially improves agreement with ray tracing. The model retains the speed of an analytic calculation while making the physical origin of profile changes explicit.
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Earth and Planetary Astrophysics
Performance of AutoRegressive Planet Search methodology on simulated PLATO lightcurves
Eric D. Feigelson, Marco Montalto, Geert Jan Talens et al. (+4 more)
AIM. This study examines the PLATO mission sensitivity to small transiting
planets (radii between 0.5 and 2.0 R$_\oplus$) using Autoregressive
Planet Search (ARPS) methodology based on simulated lightcurves. Methods. Several thousand simulations of PLATO-like lightcurves from magnetically active
stars expected from the PLATO Long Observation Phase South 2 (LOPS2) field with injected transiting
planets smaller than 2.0 R$_\oplus$ radius are examined. The lightcurves are analyzed with ARPS that uses autoregressive modeling for filtering of nonplanetary signals and the Transit Comb Filter periodogram for planetary transit detection. False alarms of nonexistent periodic signals are reduced by thresholds on two ARPS parameters, peak periodogram power, and transit duration. Results. When applied to PLATO's simulated first three months observations of bright
stars (P1 and P2 samples), at least 50% of Earth-size
planets are recovered for periods shorter than 40 d. In two years of LOPS2 observation, 25% are recovered for periods less than 240 d. Methodological implications on detrending, periodograms, False Alarms, vetting, and catalog evaluation are discussed. The utility of a positive predicted value heat map after detection and vetting is demonstrated. Conclusions: While the ARPS
planet detection procedure is not entirely well-suited for PLATO analysis, this study provides methodological insight into the sensitivity and reliability of small transiting
planet detection.
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All authors Eric D. Feigelson, Marco Montalto, Geert Jan Talens, Suzanne Aigrain, Luca Malavolta, Valerio Nascimbeni, Tiziano Zingales
Solar and Stellar Astrophysics
The 200-300 GHz Survey for 18 Class II Disks in the Ophiuchus Star-Forming Complex
Chia-Ying Chung, Hauyu Baobab Liu, Chuan-Yung Lin et al. (+4 more)
We measured the 198-310 GHz flux densities of 18 Class II protoplanetary disks in the Ophiuchus
star-forming complex using the Submillimeter Array (SMA). The $\sim$200-300 GHz spectral indices ($\alpha_{200-300}$) of these disks are $2.3^{+0.2}_{-0.5}$. In this observed frequency range, emission properties of the Ophiuchus disks are very similar to the 48 spatially compact Class II disks in the Taurus-Auriga region observed in a previous SMA survey. Combining the Ophiuchus and Taurus-Auriga samples, the Pearson correlation coefficient between $\alpha_{200-300}$ and the flux density at 250 GHz ($F_{250\,\mathrm{GHz}}$) is 0.27 ($p=0.02$). We do not find a convincing correlation between $\alpha_{200-300}$ and the $R_{95\%}$ radius of the disks. Our interpretation for the $\alpha_{200-300}$ values being close to 2 in the spatially compact disks is that at $\gtrsim 200$ GHz frequencies, the continuum emission in these disks is dominated by optically thick dust thermal emission, independent of $F_{250\,\mathrm{GHz}}$ and $R_{95\%}$. As a result, the (sub)millimeter luminosity and the apparent disk size are positively correlated, which has been found in numerous resolved emission surveys of Class II disks. In young clusters with ages of $\sim$1-2 Myr, Class II disks with smaller radii or lower (sub)millimeter luminosities are not necessarily those in which the dust mass budget has been consumed or dispersed.
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All authors Chia-Ying Chung, Hauyu Baobab Liu, Chuan-Yung Lin, Linjing Feng, Sean M. Andrews, Mark A. Gurwell, Melvyn Wright
Instrumentation and Methods for Astrophysics
Do Astronomical Foundation Models Know When They Will Fail?
Vaidehi Bulusu, Mehul Goyal, Md Rabius Sany Apu et al. (+2 more)
Astronomical foundation models are increasingly used as reusable representations across surveys, but it is unclear when the same object remains consistently represented across instruments or whether a model can warn us before transfer fails. We present a conformal framework for auditing cross-survey representations using cross-matched Legacy Survey–HSC, HSC–JWST, and SDSS–HSC objects. Astronomy-specific AstroPT
embeddings show lower translation errors and tighter conformal regions than general-purpose visual backbones at comparable coverage, while different model families fail on different objects. We then use adaptive conformal prediction to test whether the source
embedding contains information about its own future error. Using a ridge difficulty model fitted to source
embeddings, we find that this warning signal is consistent across the tested backbones for Legacy$\rightarrow$HSC, backbone-dependent for HSC$\rightarrow$JWST, and largely absent for SDSS$\rightarrow$HSC. When informative, adaptation improves coverage for the hardest objects. Matched-control analysis further reveals silent failures not explained by magnitude, apparent size, or nearest-neighbor distance. Hence, we show that conformal uncertainty provides a realistic way to identify when astronomical representations transfer reliably and when they fail without warning.
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All authors Vaidehi Bulusu, Mehul Goyal, Md Rabius Sany Apu, Michael J. Smith, Shashwat Sourav