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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Monday 24 August

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Neutron star masses from electron-capture supernovae under equation-of-state uncertainties

Vishal Parmar, Domenico Scordino and Ignazio Bombaci

Electron-capture supernovae (ECSNe) are a promising formation channel for low-mass neutron stars, but the minimum gravitational mass of the neutron stars they produce depends on both the progenitor core structure and the neutron-star matter equation of state (EOS). We compute the electron-capture (EC) threshold gravitational mass ($M^\star_{WD}$) of cold white-dwarf-like O–Ne–Mg cores with representative compositions and map the baryon number onto cold neutron-star configurations constructed from a Bayesian ensemble of unified crust–core EOSs. Although the EC threshold density is sensitive to the concentrations of the O–Ne–Mg mixture, the threshold baryon number of the O–Ne–Mg core varies only weakly, producing a narrow remnant-mass window. In the baseline case with no baryonic mass loss during the transition from the EC threshold mass O–Ne–Mg core to the remnant neutron star, standard ECSNe yield remnant neutron stars with gravitational masses of $1.24$–$1.265\, M_\odot$, with only a small EOS-induced spread. Small baryonic mass losses of $0.01$–$0.02\, M_\odot$ shift this range modestly downward, but the $1.174\, M_\odot$ companion of PSR J0453+1559 would require an extreme mass loss close to $0.10\, M_\odot$, which is not favored by current ECSN simulations. We find that the residual EOS dependence of the remnant mass is controlled mainly by the pressure around nuclear saturation density, while the corresponding tidal deformability remains sensitive to the remnant radius and compactness. Thus, low-mass double neutron star systems can in principle connect ECSN-like formation channels with gravitational-wave constraints on the EOS. Our results show that ECSNe naturally form low-mass neutron stars, but within a restricted mass range; the lightest observed neutron stars likely require low-mass iron-core collapse, ultra-stripped supernovae, or other nonstandard channels.
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Solar and Stellar Astrophysics

Six new doubly eclipsing quadruples in a 2+2 architecture

P. Zasche, H. Kucakova, J. Kara et al. (+5 more)

The study presents a confirmation of six quadruples with two sets of eclipses that have the 2+2 architecture. These so-called doubly eclipsing systems still present a quite rare group of stars. We collected all available photometric data and carried out a detailed analysis of them. In addition to the precise TESS photometry used to model the light curves for both inner eclipsing binaries, photometric survey data were also used, and more than 100 nights of our own dedicated observations were carried out. These were mainly used for the detection of the long-term evolution of orbital periods. Thanks to these data, we were able to derive the mutual orbits of the inner doubles via eclipse timing variations. The systems studied are: CzeV1254 (periods 0.715348 d + 0.362593 d, mutual period 15.6 yr); ASASSN-V J070838.27-171952.9 (4.300490 d + 3.767235 d, 4 yr); ASASSN-V J091951.17-593306.9 (2.668674 d + 14.342218 d, 0.95 yr); WISE J100820.07-731554.0 (0.368315 d + 7.945339 d, 3.8 yr); ASASSN-V J143536.01-721459.4 (7.353842 d + 6.368567 d, 27.4 yr); and Corot 310284765 (1.875418 d + 2.371126 d, 22.6 yr). Some of the binaries show significantly eccentric orbits. We also estimated their apsidal motion periods. With our six new systems, the number of confirmed orbits of 2+2 quadruples has been increased to 64 in total.
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All authors P. Zasche, H. Kucakova, J. Kara, J. Merc, Z. Henzl, M. Masek, L. Cervinka, V. Dienstbier

Solar and Stellar Astrophysics · Cross-listed from cs.LG

Neural Operator based Multi-Field Reconstruction of Inner Solar Boundary State

Vignesh Kumar Pandian Sathia, Reza Mansouri, Dustin J. Kempton et al. (+2 more)

The Solar wind is a continuous flow of charged particles emanating from the solar surface and governed by complex, interacting magnetohydrodynamic processes. Accurate specification of inner-boundary conditions is essential for heliospheric modeling and solar-wind prediction. In many practical applications, only a subset of interacting multi-field variables is directly available, but for a comprehensive view of solar wind prediction and downstream magnetohydrodynamic simulations, a more complete boundary state is required. In this work, we study the problem of learning the multi-field multi-scale solar magnetohydrodynamic state at 30 solar radii ($R_\odot$) using operator learning. Specifically, given the radial velocity and radial magnetic field, we aim to reconstruct the non-radial velocity and magnetic field components, radial and non-radial current density, thermodynamic density, and pressure components. This mapping is highly nonlinear, spatially coupled, and multi-scale, making it a challenging task for data-driven scientific machine learning. To address this problem, we employ a Local Neural Operator (LocalNO) that learns mappings between input and output function spaces while retaining locality and resolution-awareness. Unlike conventional regression models and autoencoder models, neural operators are better suited for learning structured field-to-field transformations arising from physical systems. The resulting predictions along with inputs are intended to serve as boundary condition variables for future inner-heliospheric modeling pipelines.
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All authors Vignesh Kumar Pandian Sathia, Reza Mansouri, Dustin J. Kempton, Pete Riley, Rafal A. Angryk

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Thermochemical constraints on a primordial-origin of gas-rich debris disks

H. Mitani, W. Ooyama, R. Nakatani et al. (+2 more)

Recent observations have revealed gas-rich debris disks around intermediate-mass stars at ages of tens of Myr. The origin of this gas remains unclear: it may be primordial, retained from the protoplanetary phase, or secondary, released from volatile-rich solids. Secondary-origin models reproduce CO emission but often overpredict neutral carbon. Recent observations and disk-evolution models suggest that primordial gas may survive longer than previously assumed, motivating thermochemical tests of the primordial-remnant scenario. We test the previously unexplored possibility that primordial-origin disks satisfy the observational constraints on gas-rich debris disks. Specifically, we determine under what conditions a disk around a $2\,M_{\odot}$ star reproduces substantial CO, low CI/CO ratios, and weak HCO+ emission consistent with current non-detections. We post-processed 20-40 Myr structures from 1D disk-evolution models with Cloudy, varying irradiation geometry, dust-to-gas mass ratio (DTG), and cosmic-ray ionisation rate. In the dust-poor models (DTG $=10^{-4}$), CO remains optically thick around $R\sim100$~au. The models yield low disk-integrated CI/CO mass ratios. Our model produces CO radial intensities of the observed order of magnitude, but its CI-emitting region is more extended than observed. The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits. These results demonstrate that a primordial origin remains chemically viable for CO-rich debris disks. The main remaining tension is the excessive radial extent of the CI emission, although it may reflect our simplified modelling. Further testing of the primordial-origin scenario will require multidimensional, self-consistent modelling, spatially resolved CI observations, and deeper searches for HCO+.
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All authors H. Mitani, W. Ooyama, R. Nakatani, R. Kuiper, T. Hosokawa

Solar and Stellar Astrophysics

Where Do Periodic Density Structures Acquire Coherence? A Low-Coronal Resonator Candidate and a 12-Event STEREO/SECCHI Transfer Test

Olena Podladchikova

Periodic density structures (PDS) are trains of plasma enhancements carried into the heliosphere, but their repeated timing does not by itself identify where the cadence is set. We test whether PDS selected independently in the outer corona can be traced back to a low-coronal clock, and whether that organization survives through the EUVI–COR1–COR2 observing chain. We analyze 12 STEREO-A/SECCHI events on 2008 January 11–14 along a nonradial path. EUVI 171 A at 1.10–1.20 R_sun shows an unusual concentration of power in the predeclared 80–130 minute band relative to filter-matched red-noise controls, while the ensemble is most strongly organized in upper COR1 at 2.5–3.0 R_sun. These signatures identify a low-coronal modulation candidate and an intermediate-height organization domain, but they do not form a unique phase-preserving clock extending into COR2. Instead, the more persistent observable is spatial order. Events 9 and 12 retain expansion-stable outward ordering. In polar r–theta maps, intersections of oppositely inclined, expansion-aware matched-filter ridge supports form repeated X-/diamond-like patterns. The morphology is compatible with stationary or quasi-stationary shock-cell processing, although the brightness diagnostics are insufficient to establish a stationary MHD shock branch. The observations therefore favor an intermittent source–gate–transfer picture: a low-coronal cadence may modulate plasma release, while event-dependent propagation progressively destroys exact phase coherence. The outward ordering of the density structures can survive, providing a more persistent signature of the source-to-wind transfer than phase locking itself.
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Sunday 23 August

Solar and Stellar Astrophysics

On the formation of subdwarf B stars via engulfment of substellar companions I. Conditions for and occurrence of envelope ejection events

Antoine Thuillier, Lionel Siess and Valérie Van Grootel

The canonical scenario for the formation of subdwarf B (sdB) stars involves the ejection of the progenitor's envelope near the tip of the Red Giant Branch (RGB), concurrent with the onset of core He-burning. While binary interactions are known to dominate sdB formation, the origin of apparently single sdB stars remains uncertain. We investigate the conditions under which an sdB progenitor can eject its envelope through the engulfment of a substellar companion, using an energy balance approach. We simulate the orbital evolution of substellar companions during the subgiant and RGB phases of their host stars (1.2-2.0 M_Sun) to determine the onset of engulfment and calculate the energy released during inspiral within the stellar envelope. By comparing this energy with the envelope binding energy and exploring different efficiencies for its deposition, we identify the conditions required for envelope ejection. We then estimate the occurrence of such events using observed population of substellar companions. The engulfment of substellar objects can lead to envelope ejection within a limited region of the mass-semi-major axis parameter space, whose extent depends on the host star's properties and energy transfer efficiency. The minimum companion mass required for ejection increases with decreasing efficiency and increasing stellar mass. As a result, the frequency of envelope ejection is strongly influenced by the distribution of substellar companions, particularly by the paucity of objects in the brown-dwarf desert. The engulfment of low-mass brown dwarfs and massive planets during the late RGB can provide sufficient energy to eject the stellar envelope, which could ultimately lead to the formation of sdB stars. Our results define the region of parameter space where this mechanism is energetically possible and provide a guide for future multidimensional hydrodynamical simulations.
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Solar and Stellar Astrophysics

A superflare of BP Tau simultaneously caught by EP X-ray and TESS optical observations

Xingyu Zhou, Mingjun Liu, Gregory J. Herczeg et al. (+10 more)

Multiwavelength observations of stellar flares trace the activity of different components of the stars' outer atmosphere, providing insight into their interactions. In the present paper, we report a superflare from BP Tau, simultaneously observed with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) satellite and TESS. While we attribute the X-ray flux increase to a magnetically powered flare, the optical light curve likely results from the superposition of the flare and an accretion burst. The X-ray flare has a mean flux of $(1.5^{+0.3}_{-0.4})\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ in the WXT energy band (0.5-4.0 keV), with e-folding times of $1.7\pm1.0$ ks and $14\pm5$ ks for the rise and decay phase, respectively. The corresponding time-integrated flare energy is $(1.0\pm 0.2)\times 10^{36}$ erg. The optical flare has an e-folding time of $0.33\pm0.04$ ks for the rise phase, but the data do not constrain the decay timescale. Assuming a decay phase equal to the rise phase, the resulting optical flare energy is $(2.8\pm0.4)\times10^{34}$ erg in the TESS band ($\sim6,000$-$\sim10,000$ Å), corresponding to a bolometric energy of $(1.9\pm0.3)\times10^{35}$ erg (assuming a blackbody at 11000 K). The Follow-up X-ray Telescope (FXT) on EP triggered an observation $\sim1.5$ day after the flare, with a flux of $(4.6^{+0.2}_{-0.5})\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ (0.5-10.0 keV), indicating that BP Tau had returned to quiescence. This work demonstrates the potential of jointly analyzing EP and TESS data for superflares. WXT is expected to detect $\sim800$ superflares per year, with FXT capable of slewing to the flaring star within $\sim3$-5 minutes. The large field of view of both missions offers us the opportunity to study multiwavelength variability during energetic flares.
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All authors Xingyu Zhou, Mingjun Liu, Gregory J. Herczeg, P. Christian Schneider, Fabio Favata, Chenwei Yang, Chichuan Jin, Dongyue Li, Xuan Mao, Yi-Han Iris Yin, Minghao Zhang, Weimin Yuan, Hongyan Zhou

Solar and Stellar Astrophysics · Cross-listed from physics.plasm-ph

Plasmon-polaritons in a rarefied nonrelativistic neutron gas

P.O. Kazinski and A.M. Trushchuk

The electromagnetic properties of a dilute unpolarized nonrelativistic neutron gas are described in the case when neutron-by-neutron scattering is negligible. The Gaussian (Maxwell-Boltzmann) and Fermi-Dirac one-particle density matrices for the neutron gas are considered, the typical space scale of variations of these density matrices being assumed to be much larger than the the typical space scale of variations of the electromagnetic field. The Green functions for the static effective Maxwell equations are obtained. The parameters of a nonequilibrium neutron gas are found where this gas possesses a ferromagnetic instability. The analog of Friedel oscillations is described. The properties of plasmon-polaritons in the neutron gas are revealed. It turns out that the dispersion law of longitudinal and transverse plasmon-polaritons has an infinite number of branches at a given momentum. The region of parameters where the plasmon-polaritons can be regarded as quasiparticles is found. The plasmon-polaritons on a Gaussian wave packet of a single electron are described. Their dispersion law proves to have an infinite number of branches at a given momentum. It is shown that there exist stable longitudinal plasmon-polaritons even on a single electron.
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Saturday 22 August

Solar and Stellar Astrophysics

Discovering Dual-Origin Slow Wind from Solar Orbiter with Self-Supervised Contrastive Learning

Henry Han and Jorge Yero Salazar

Whether the slow solar wind originates from one coronal source or two distinct channels remains a central open question in heliophysics. Resolving this requires unsupervised separation of two populations that arrive at nearly the same bulk speed and differ mainly in heavy-ion composition. We present Solar-CDC, a self-supervised contrastive deep clustering (CDC) framework that maps plasma observables to a latent space via a Transformer encoder, optimizes a triplet margin loss, and updates pseudo-labels via $k$-means. Theoretically, we prove that neighborhood-preserving embeddings such as t-SNE and UMAP are fundamentally constrained. Preserving the neighbor graph leaves the cross-cluster cut fraction unchanged, and preserving all but a fraction $\varepsilon$ of its links moves that fraction by at most $\varepsilon$. Neither bound depends on the target dimension. A margin objective rewrites the graph and drives the cut fraction to zero. Empirically, on 30,602 Solar Orbiter observations, thirty combinations of dimensionality reduction and clustering peak at a silhouette of $0.454$, whereas Solar-CDC reaches $0.869$. Escaping the geometric bound alone does not guarantee physical validity: TriMap also optimizes triplets and reaches $0.824$, yet its clusters score below chance against the published composition taxonomy. Solar-CDC instead recovers clusters with mean charge-state ratios of $0.080$, $0.160$, and $0.400$, placing the intermediate population inside the window associated with coronal-hole boundaries. Even when the defining charge-state ratio is withheld from the inputs entirely, the model still recovers the taxonomy defined on it. Solar-CDC thus connects self-supervised representation learning to coronal source diagnostics. Importantly, a learning loss recovers physical populations only when driven by dynamically updated physically-aware clusters rather than distances.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Particle acceleration and non-thermal broadband spectrum of extended shocked outflows in black hole binary V4641 Sgr - a galactic pevatron

Andrei Bykov, Sergei Osipov, Vadim Romansky et al. (+2 more)

Stellar mass black holes accreting at a super-Eddington rate produce strong outflows and non-thermal radiation up to PeV energy. Recently, very high energy (VHE) emission was discovered from the environment of the galactic microquasar V4641 Sgr, which is known to experience super-Eddington outbursts. We present MHD simulations supplemented with a nonlinear kinetic Monte Carlo model that reproduces the observed radio, X-ray, and gamma-ray emission from the region over tens of parsecs surrounding V4641 Sgr. The reported multi-wavelength non-thermal emission can be explained as produced by PeV electrons accelerated by a system of strong shock waves created by the interaction between a narrow trans-relativistic polar jet and a quasi-spherical accretion-disk wind. The shock positions are associated with bright spots of the gamma-ray and X-ray radiation, whose spatial profiles can be tested with dedicated observations. The model predicts that a sizable fraction of the jet's kinetic energy is channeled into TeV-PeV protons, suggesting that such systems might be powerful sources of PeV-range cosmic rays during the accelerator activity cycle. In this model, the bow-tie shape structure discovered by MeerKAT in V4641 Sgr is associated with the radio emission from the termination surface of the quasi-spherical accretion-disk wind. The newly presented (from the SRG/eROSITA all-sky survey) and archival (INTEGRAL and Swift BAT) X-ray data are quantitatively consistent with the model, and simultaneously reproduce VHE measurements with H.E.S.S., HAWC, and LHAASO. Deeper X-ray observations will provide a test of the intermittent operation of the accelerator, in contrast with more persistent sources such as SS433/W50.
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All authors Andrei Bykov, Sergei Osipov, Vadim Romansky, Eugene Churazov, Ildar Khabibullin

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

MIAO-ALMA: Shocks and Protostellar Outflows in 70 $\mu$m-dark clumps with $L/M$ $<$ 1 $L_{\odot}$/$M_{\odot}$

Shuting Lin, Siyi Feng, Junzhi Wang et al. (+16 more)

To investigate the initial conditions of high-mass star-forming regions, we use SiO (2-1) emission to trace early shock-related kinematics toward sixteen 70 $\mu$m-dark and massive clumps with luminosity-to-mass ratios ($L/M$) $< 1\,L_{\odot}/M_{\odot}$, as part of the Multiwavelength Line-Imaging Survey of the 70 $\mu$m-dark and bright clouds (MIAO) project. Using ALMA observations at a spatial resolution of $\sim$0.06 pc and a velocity resolution of 0.21 km s$^{-1}$, we identify a total of thirty-seven outflows with a variety of morphologies. Outflow parameters were derived by integrating the HCO$^+$ (1-0) line wings, excluding the quiescent dense core component traced by H$^{13}$CO$^+$ (1-0). We find that outflow masses and velocities show moderate positive correlations with the masses of their driving cores. Owing to the high sensitivity of our observations, which yield longer projected outflow lengths compared to previous studies, the derived outflow dynamical ages span $\sim10^{3}$-$10^{5}$ yr. We detect six narrow-linewidth (0.6-1.4 km s$^{-1}$) and three broad ($>$ 2 km s$^{-1}$) SiO (2-1) features not associated with outflows driven by clearly identified protostars. Lacking coincident 3 mm dust continuum cores, their origins may be young outflows from undetected low-mass protostars, dissipating shocks, cloud-cloud collisions, or projection effects when the outflows lie close to the plane of the sky. The detection of these shocks and outflows in such extremely young environments demonstrates that protostellar activity has already begun.
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All authors Shuting Lin, Siyi Feng, Junzhi Wang, Shanghuo Li, Dan Miao, Zhi-Yu Zhang, Sheng-Yuan Liu, Nami Sakai, Fengwei Xu, Hauyu Baobab Liu, Henrik Beuther, Di Li, Qizhou Zhang, Tie Liu, Patricio Sanhueza, Olli Sipilä, Ken'ichi Tatematsu, Jaime E. Pineda, Xing Lu

Solar and Stellar Astrophysics

WISE J005128.08+645651.7: An enigmatic R Coronae Borealis star

J. Budaj, K. Bernhard and I. Hubeny

Context. R Coronae Borealis (RCB) stars are hydrogen-deficient and carbon-rich (HdC) supergiants that likely originate from a merger of two white dwarfs. As such they are not expected to be members of close binary systems, in agreement with current observations. Aims. We test the above hypothesis and search for eclipses of RCB stars that could be due to a companion. Methods. We exploited old archival photometric data from Harvard photographic plates (DASCH) as well as modern-era photometric surveys such as Gaia, ASAS-SN, and ZTF to search for eclipses, and conducted follow-up spectroscopic observations. Results. We found one RCB star (WISE J005128.08+645651.7) that features two 3.5 yr long dimming events. We argue that these may be eclipses separated by an orbital period of 66 yr and that the star is an eclipsing binary candidate. The next eclipse is predicted for 2080. If confirmed, this would be the first RCB star in a binary system, the second-longest orbital period of any known eclipsing binary, and the longest duration of an eclipse. Light curve simulations support the idea that the eclipses could be caused by a dusty disk around the companion. The star shows unusually large pulsations. Our follow-up spectroscopy indicates a stellar temperature of about 11000 K, the presence of sharp emission lines (including forbidden lines) during the normal brightness, and extremely strong diffuse interstellar bands. The star is likely surrounded by a hotter chromosphere and an extended low-density nebula. We calculated atmosphere models of the star. Models feature a density inversion and the derived chemical composition is compatible with some cool extreme He stars (EHe). The star constitutes a bridge between the classical RCB, hot RCB, and EHe stars. Conclusions. Future research is encouraged to confirm or refute the binary nature of the star and search for other RCB binaries.
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Solar and Stellar Astrophysics

CPD-82 291: Multi-sector TESS Photometry and SED Analysis of an Active Eclipsing Binary

Volkan Bak{\i}ş and Gökhan Yücel

We present a multi-sector $TESS$ photometric and spectral energy distribution analysis of the eclipsing binary CPD–82~291 (RX~J0853.1–8244). Eight $TESS$ sectors reveal well-defined eclipses and substantial, changing out-of-eclipse modulation. The asymmetries are reproduced by different surface-brightness configurations at different epochs, consistent with strong and evolving stellar activity, although the fitted circular regions are not unique maps of individual starspots. A detached-binary light-curve model yields an orbital inclination of $i\simeq82.1^\circ$ and a photometric mass ratio of $q_{\rm phot}\simeq0.464$. Without a spectroscopic radial-velocity orbit, the mass ratio and absolute parameters are model-dependent rather than dynamical measurements. Combining the light-curve geometry with the SED radius scale and Gaia distance gives $R_1\simeq1.25\,R_\odot$ and $R_2\simeq3.70\,R_\odot$; the corresponding model-inferred masses remain substantially less secure than the dimensionless light-curve parameters. The stellar photospheres reproduce the observed SED from the optical region through WISE W4, with no statistically significant mid-infrared excess. The only positive long-wavelength deviation is a low-significance $60\,\mu$m measurement, while the $90\,\mu$m result is an upper limit. A cool circumbinary-dust model can reproduce this tentative signal, but such a component is not required by the photometry, and its inferred mass and radial extent are non-unique and strongly model dependent. Evolutionary tracks place the primary close to the zero-age main sequence and the secondary well above the main-sequence locus. However, the absence of radial velocities and spectroscopic youth indicators, together with $Gaia$ astrometry inconsistent with Chamaeleon~I membership, prevents a secure pre-main-sequence classification.
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Friday 21 August

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes

Alexandra G. Guerrero, Michael Zevin, Duncan B. Maclean et al. (+4 more)

Binary population synthesis (BPS) codes are valuable tools for investigating the end-to-end lives of binary stars as well as a myriad of observed astrophysical phenomena. Many “rapid" BPS codes rely on semi-analytical single-star evolutionary tracks and disjointed prescriptions for binary physics. Updated “hybrid" or “detailed" BPS codes incorporate improved methodologies, but at a steeper computational cost that may limit broad exploration of physical uncertainties. Given the widespread use of BPS in modern astrophysical research, it is imperative to systematically compare BPS codes across the spectrum of computational efficiency, flexibility, and physical realism to gauge their consistency and robustness. In this work, we perform BPS on three single-metallicity populations of identical initial binaries using three modern codes — the rapid code $\texttt{COSMIC}$, the hybrid code $\texttt{METISSE}$ integrated into $\texttt{COSMIC}$, and the detailed code $\texttt{POSYDON}$ — ensuring consistent physical parameterizations where possible. We find stark differences in the properties, formation pathways, and progenitors of binary black hole (BBH) merger populations across the three codes. In a population of one million binaries at $0.01 Z_\odot$, each code results in $\sim 5,000\mbox{--}9,000$ BBHs that merge within a Hubble time. However, $\lesssim 14\%$ of progenitors overlap between two codes, and only $\textit{one}$ initial binary becomes a BBH merger across all three codes. Identical progenitors often follow different evolutionary pathways in each code and result in inconsistent BBH properties. Our results highlight the need for a deeper understanding of physical and computational differences between BPS codes and caution against over-interpreting results from any individual BPS code.
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All authors Alexandra G. Guerrero, Michael Zevin, Duncan B. Maclean, Katelyn Breivik, Carl L. Rodriguez, Max M. Briel, Daniel E. Holz

Solar and Stellar Astrophysics · Cross-listed from gr-qc

Resonance crossings as entire functions of the Koopman operator

Priscilla Canizares

Astrophysical binaries formed by a stellar-mass compact object and a massive black hole are among the most promising sources for future space-based gravitational-wave detectors. These extreme mass-ratio inspiral (EMRI) systems are tracked coherently over more than $10^{5}$ orbits, encoding detailed information about strong-field gravity. Translating a detection into physical parameters requires models of comparable precision. One outstanding difficulty is the treatment of transient orbital resonances. In this work we address the problem at the level of the Koopman operator, where the waveform is promoted to an observable and the evolution over one radial cycle is a linear operator. Removing the fast timescale without a singularity at a crossing is then a matter of choosing a function of that operator. We demonstrate the approach on a Kerr geodesic driven through the 3:2 crossing by a modelled forcing term. There the coefficient of the standard near-identity transformation diverges, while the finite-window generator attains its analytic bound to better than one part in $10^{6}$. Averaging over twice the crossing duration recovers the resonant jump with its predicted magnitude and phase dependence.
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Solar and Stellar Astrophysics

alpha_theta waves in the tachoclines of Sun-like stars

T. V. Zaqarashvili, M. Dikpati and P. A. Gilman

Vertical gradient of mean turbulent electromotive force (expressed by the dynamo coefficient, alpha, caused by the penetrating convection into the tachoclines of Sun-like stars leads to the non-propagating alpha-mode patterns, which result in the periodic variations of magnetic field horizontal components. The aim of the paper is to study the influence of the latitudinal variation of the dynamo coefficient on the linear dynamics of large-scale waves in the overshoot layers of the tachoclines in Sun-like stars. We use the linear magnetohydrodynamic (MHD) equations with the dynamo coefficient in a simple rectangular geometry. It is shown that the vertical gradient of the alpha coefficient has the same appearance in the induction equation as the Coriolis force in the momentum equation. Therefore, the latitudinal variation of alpha parameter excites new large-scale waves similar to the Rossby waves on a rotating sphere. The dispersion properties of the waves depend only on vertical and latitudinal gradients of the alpha parameter, therefore the modes are basically different from the ordinary dynamo waves. The alpha_theta waves may have either prograde or retrograde propagation depending on the signs of the gradients. The time scales of the waves may vary from hundreds of days to tens of years in various parameters of the solar tachocline. These waves are coupled with the Rossby waves on a rotating sphere in the existence of the large scale magnetic field, which may lead to the mutual transformation of convective and rotation energies.
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Solar and Stellar Astrophysics · Cross-listed from physics.soc-ph

From Timbuktu to SKA: Who owns the Astronomy knowledge Africa produces?

Rhea Koch and Amare Abebe

Africa has deep and diverse traditions of astronomical knowledge, ranging from archaeological astronomy, pharaonic stellar timekeeping, and manuscript astronomy in Timbuktu and the Sahel, to ecological seasonal astronomy in southern Africa and calendrical computation in the Ethiopian tradition. These multiple epistemic traditions form part of a long intellectual history that precedes and intersects with contemporary astronomy on the continent. In the context of this long-standing intellectual heritage, an important question arises regarding the contemporary circulation of African astronomical knowledge within the global scientific system. This study therefore examines where African astronomical knowledge is produced, validated, and circulated in modern scholarly communication. Using bibliometric data from the Web of Science Core Collection, our analysis investigates publication and citation patterns in the research area Astronomy and Astrophysics between 2000 and 2025, with particular attention to the publication venues of African scholars. The findings highlight a critical paradox: while African researchers are active contributors to global astronomical discovery, the intellectual capital generated through this work is largely stored, validated, and circulated through publication systems located outside the continent. This pattern reflects broader asymmetries within global scholarly communication, where dominant publishing infrastructures shape visibility, citation impact, and authority. By applying a decolonial lens to metrics such as citation impact and ownership, the paper calls for a critical reassessment of the academic practices that sustain epistemic coloniality. It concludes that achieving scientific equity requires a strategic shift in publication choices to build and fortify a sovereign African knowledge archive.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Gas-phase and Surface Chemistry in the Massive Star-Forming Region RCW\,120

K. V. Plakitina, M. S. Kirsanova, D. S. Wiebe et al. (+1 more)

We analysed broadband emission spectra of a dense molecular clump in RCW 120, obtained with the APEX telescope in the 200–260 GHz range, in order to investigate molecular formation pathways in regions of massive star formation at an early evolutionary stage. We examined the correlations between the molecular column densities derived under the LTE assumption. An excess of methanol was found in the southern part of the dense clump relative to its northern part, while the abundances of other molecules, such as CH$_3$CN and CH$_3$CCH, remain comparable. The methanol abundance is also elevated relative to that of other oxygen-bearing molecules, such as OCS and SO. To identify possible causes of the enhanced methanol abundance in the southern part of the clump, we carried out simulations with the astrochemical model Presta in a two-phase approximation, accounting for chemical processes both in the gas phase and in the mantles of dust grains. The modelling shows that the enhanced gas-phase methanol abundance may be due to photodesorption from icy mantles. At Av values between $4^{m}$ and $6^{m}$, methanol desorbs efficiently from the ice mantles of dust grains upon interaction with photons, but is not yet destroyed by UV radiation in the gas phase. A strong linear correlation between molecular column densities indicates that the molecules form in the same phase — either in the gas phase or on dust. Their integrated intensity maps may nevertheless differ, as is the case for CCH and CH$_3$CN. If two molecules form in different phases — one in the gas phase and the other in dust mantles — no correlation is observed, as for CCH and CH$_3$OH. The weak correlation between methanol and the oxygen-bearing molecules that form on dust suggests that only the upper part of the dust mantles, rich in CO ice, is destroyed in the southern part of the clump.
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All authors K. V. Plakitina, M. S. Kirsanova, D. S. Wiebe, O. V. Kochina

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Nucleosynthesis in the Accretion-induced Collapse of magnetised, Rotating White Dwarfs

Laurenz Thümmler, Takami Kuroda and Masaru Shibata

Accretion-induced collapse (AIC) of a rotating oxygen-neon-magnesium white dwarf (WD) is an alternative channel of neutron-star formation, in which a mass-accreting star near the Chandrasekhar mass collapses instead of being disrupted thermonuclearly. If the progenitor WD is sufficiently magnetised, it can drive magnetorotational outflows analogous to those of magnetorotational supernovae and may serve as a site for rapid neutron-capture ($r$-process) nucleosynthesis. We post-process tracer particles from five three-dimensional general-relativistic neutrino-(magneto)hydrodynamic AIC simulations, spanning four initial rotation rates, with the WinNet nuclear reaction network. Ejecta mass, the fraction of matter reaching nuclear statistical equilibrium, and the mass ejected beyond the iron group all increase monotonically with rotation rate, with magnetic fields amplifying these trends. The slowest-rotating model synthesises essentially only iron-group material, while the most rapidly rotating and magnetised model reaches the second $r$-process abundance peak at mass number $A \approx 130$, with a trace, non-robust signal up to $A \approx 244$. The corresponding $^{56}$Ni mass rises from $0.007$ to $0.014 M_\odot$, lower than representative yields for core-collapse supernovae and hypernovae by factors of five to more than forty, implying a correspondingly faint radioactively powered component. Contrary to the picture established for magnetorotationally driven supernovae, the heaviest ejecta do not track the polar outflow: they instead reside in equatorial-to-mid-latitude, moderate-entropy lobes, while the polar column is itself a local minimum in some heavy elements. We conclude that rapidly rotating, magnetised AIC events could contribute to the Galactic yields of trans-iron and weak $r$-process nuclei, although none of our models produces a robust third $r$-process peak.
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Solar and Stellar Astrophysics · Cross-listed from nucl-th

Neutron star matter with hyperons: Bayesian comparison of nucleonic and SU(6)/SU(3) hyperonic models

Athira S., Vishal Parmar, Monika Sinha et al. (+1 more)

We investigate neutron star matter with hyperons within a density-dependent relativistic mean-field framework using Bayesian inference, considering three composition scenarios: purely nucleonic matter, hyperonic matter under SU(6) flavor symmetry, and hyperonic matter under SU(3) symmetry with free vector-sector parameters. The analysis incorporates constraints from empirical nuclear matter properties, theoretical inputs at low densities, and multimessenger observations of neutron stars. We find that the SU(6) scheme, grounded in the quark model and isospin counting rule, leads to a significantly softer equation of state. In contrast, the additional flexibility of the SU(3) framework enhances vector repulsion and yields a comparatively stiffer equation of state consistent with observational bounds across the explored parameter space; in particular, the posterior distributions favor values of the vector coupling ratio $\alpha_v$ lower than the SU(6) limit $\alpha_v = 1$. These differences are reflected in neutron star observables, including mass–radius relations, tidal deformabilities, direct Urca thresholds, and oscillation properties, all of which remain compatible with current constraints within the SU(3) scenario. We further examine structural signatures through the curvature of the mass–radius relation and find that, although hyperon-rich configurations can induce noticeable variations, such features depend sensitively on the stiffness of the equation of state and are therefore not universally robust indicators. Bayesian model comparison further shows that present constraints do not meaningfully discriminate between the purely nucleonic and SU(3) hyperonic scenarios, while providing positive, but not decisive, evidence against the more restrictive SU(6) framework.
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All authors Athira S., Vishal Parmar, Monika Sinha, Ignazio Bombaci

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

The impact of tidal locking on Earth-like planetary dynamos

J. P. Hidalgo and D. R. G Schleicher

We investigate how tidal locking affects the planetary dynamo and the magnetospheres of Earth-like planets orbiting the habitable zones of M dwarfs. We couple an empirical stellar wind model with two distinct planetary dynamo paradigms, direct rotational scaling and energy-flux scaling, using a Constant Time Lag (CTL) model to simulate continuous tidal spin-down until synchronization. We model two hypothetical planets based on a highly dissipative Modern Earth and a less dissipative, rapidly rotating Early Earth representative of the Paleoarchean. Our results show that tidal locking acts as a severe negative feedback on planetary magnetism across both paradigms, where the dipolar field frequently collapses before full tidal synchronization is reached. Essentially, a shielded atmospheric environment around M dwarfs requires a host massive enough ($\geq 0.32 M_\odot$) to place the HZ at wider orbital distances, sparing the planet from the sub-Alfvénic environment, and the internal tidal dissipation of the planet must be sufficiently low to prevent rapid rotational decay. For mid-to-late M dwarfs, the inevitable tidal locking and the extreme stellar wind pressures result in a total collapse of the magnetic shield, reducing the likelihood of atmospheric protection to essentially zero across the habitable zone.
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Solar and Stellar Astrophysics

The DIRECD coronal mass ejection direction catalog: three-dimensional propagation inferred from coronal dimmings

Shantanu Jain, Tatiana Podladchikova, Karin Dissauer et al. (+2 more)

Coronal mass ejections (CMEs) are among the primary drivers of space weather disturbances at Earth, yet their early propagation in the low corona remains poorly constrained owing to occultation and projection effects inherent to coronagraph observations. Coronal dimmings offer an alternative diagnostic to infer CME propagation direction directly from the low corona. We present the DIRECD CME Direction Catalog, a unified dataset of three-dimensional CME propagation directions derived via the DIRECD (Dimming Inferred Estimation of CME Direction) method, which reconstructs CME cone geometry from SDO/AIA 211 Å\ coronal dimming observations. The catalog comprises 64 events spanning 2010-2026, combining 31 Solar Cycle 25 (2021-2026) events with 33 events from prior studies. For each event we report the 3D propagation direction, 2D inclination angles, angular width, and cone height. Statistical analysis reveals a systematic asymmetry in low-coronal CME propagation: meridional inclinations exhibit a systematic tendency toward latitude-dependent deflection, with poleward deflections more pronounced at higher source latitudes, while equatorial inclinations show no significant dependence on source longitude. This is consistent with low-coronal CME trajectories being governed primarily by local active-region magnetic topology, with the large-scale coronal field and heliospheric current sheet assuming a progressively dominant role at greater heliocentric distances. Incorporating DIRECD-derived propagation directions into CME arrival forecasts improves predictions of geomagnetic storm intensity, raising the correlation with observed storm strength from r = 0.58 to r = 0.70 when combined with CME speed. The DIRECD software, catalog, and all associated data products are publicly available to support space weather research.
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All authors Shantanu Jain, Tatiana Podladchikova, Karin Dissauer, Astrid Veronig, Amaia Razquin

Solar and Stellar Astrophysics

BSN: Light Curve Modeling and Orbital Period Analysis of the Contact Binaries OV Leo and V339 Leo

Razieh Aliakbari, Atila Poro, Raul Michel et al. (+1 more)

We present a combined photometric and orbital period analysis of the contact binary systems OV Leo and V339 Leo using ground-based observations together with TESS photometry. The light curves were modeled with the BSN software through a Markov Chain Monte Carlo approach, providing physical estimates of the system parameters and their uncertainties. OV Leo is analyzed photometrically for the first time, while V339 Leo is reexamined using new observations and an updated modeling methodology. The light curve analysis show that both targets are W-subtype contact binaries in shallow-contact configurations. The O'Connell effect detected in V339 Leo is reproduced with a cool starspot model, whereas no spot is required for OV Leo. Three-dimensional geometric models further indicate that OV Leo undergoes total eclipses, while V339 Leo exhibits partial eclipses. Orbital period analysis based on eclipse timing extractions reveals opposite long-term period variations in the two systems, suggesting ongoing mass transfer under the assumption of conservative mass transfer. The absolute parameters were derived using the Gaia parallaxes, the photometric light curve solution, and standard astrophysical equations.
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All authors Razieh Aliakbari, Atila Poro, Raul Michel, Sabrina Baudart

Thursday 20 August

Solar and Stellar Astrophysics

Statistical Analysis of Minifilament Eruptions Using Full-disk H$\alpha$ Blue-wing Observations at Big Bear Solar Observatory

Artin Khaleghi, Qin Li, Nengyi Huang et al. (+2 more)

This paper presents a comprehensive statistical analysis of minifilament eruptions (MFEs) using high-cadence full-disk H$\alpha$ blue-wing observations from Big Bear Solar Observatory. Despite the recognized importance of MFEs for coronal dynamics and solar wind structuring, previous efforts were often limited by small sample sizes or restricted fields of view, leaving open questions about their global occurrence and characteristic properties. We developed an algorithm incorporating intensity thresholding and temporal tracking to detect sudden enhancements in H$\alpha$ blue-wing images. A total of 1986 such events were identified during a 4 hr period on 2020 June 9. These detections were cross-validated using H$\alpha$ line-center observations to confirm the presence of associated filament structures. The analysis yields an occurrence rate of $\sim$6.6$\times 10^4$ per day, an average length of $\sim$17 Mm, and a typical lifetime of $\sim$21 minutes. A power-law distribution in eruption lengths (with slope $\sim$-4.8) and a sublinear scaling between duration and length suggest that larger eruptions tend to last longer. The length distribution of MFEs was further analyzed relative to active region proximity, revealing that eruptions occurring nearby a sunspot region were, on average, larger than the global mean eruption length. Additionally, an eruption density map was used to analyze the spatial distribution of MFEs relative to coronal hole boundaries. The results indicate a mild suppression of activity inside coronal holes and intermediate eruption frequency in the boundary regions. We briefly discuss their potential role in structuring the small-scale solar wind features such as magnetic switchbacks and small-scale magnetic flux ropes.
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All authors Artin Khaleghi, Qin Li, Nengyi Huang, Jeongwoo Lee, Haimin Wang

Solar and Stellar Astrophysics · Cross-listed from cs.PL

Portability of Fortran's 'do concurrent' on GPUs II

Ronald M. Caplan, Miko M. Stulajter, Jon A. Linker et al. (+6 more)

There continues to be growing interest in using standard language constructs for parallel and accelerated HPC computing, avoiding the need for (sometimes vendor-specific) external APIs. For Fortran applications, language features such as 'do concurrent' loops open the door for compilers to implement multi-threaded, GPU-accelerated, and even distributed multi-node code with only the standard language. Here, we explore the current status of using 'do concurrent' for GPU-accelerated Fortran applications across three major GPU vendors (NVIDIA, AMD, and Intel). Using a production application, we test their current capabilities, showing where the standard language alone can be used, and where augmenting the code with a directive-based API (e.g., OpenMP) is still desirable or required. Multi-GPU tests are performed with GPU-aware MPI libraries. We find that the three GPU vendors can now GPU-accelerate pure Fortran (zero directives), but that manual data movement directives can help with performance and compatibility. The results show that there is rapid advancement towards making GPU-accelerated scientific HPC code performance portable using the Fortran standard language.
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All authors Ronald M. Caplan, Miko M. Stulajter, Jon A. Linker, Jeff Larkin, Nikolaos Tselepidis, Harald Servat, Shiquan Su, Giacomo Capodaglio, Johanna Potyka

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

AT 2016blu: Accretion-Powered Outbursts in a Luminous Blue Variable and Compact Object Binary

Mojgan Aghakhanloo, Núria Torres-Albà, Nathan Smith et al. (+33 more)

We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559. AT 2016blu exhibited 27 detected quasiperiodic (~113 day) outbursts between 2012 and June 2026, presumably triggered by periastron passages in an eccentric binary system in which the primary is a $\gtrsim 33~M_{\odot}$ luminous blue variable (LBV). AT 2016blu was serendipitously observed by Chandra in 2001-2002, prior to its first documented outburst in 2012, but the stacked archival data show no X-ray detection. We monitored the source's visual-wavelength variability around its predicted outburst time and triggered a Chandra Target of Opportunity program in March 2026, obtaining observations with detections over five closely spaced epochs. These data indicate an X-ray luminosity of $\log L_{\rm X} \approx 38.64 \pm 0.11$~erg~s$^{-1}$ and an accretion rate of $\dot{M} \gtrsim 8\times10^{-8}~M_{\odot}~\mathrm{yr^{-1}}$, consistent with the presence of a compact companion. We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor whose outbursts are driven by intermittent accretion onto a compact object. Given that the system consists of a massive star and a compact companion, AT 2016blu is a high-mass X-ray binary, similar to SN 2010da, although the donor star in SN 2010da is a red supergiant.
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All authors Mojgan Aghakhanloo, Núria Torres-Albà, Nathan Smith, Raphael Baer-Way, Shazrene Mohamed, Alexei V. Filippenko, WeiKang Zheng, Thomas G. Brink, D. Andrew Howell, Brian Hsu, Moira Andrews, Joseph R. Farah, Curtis McCully, Kathryn Wynn, Réka Könyves-Tóth, Conor L. Ransome, Enrique Rathmann, Tamás Tordai, Sjoerd Dufoer, Charles Cynamon, Franz-Josef Hambsch, Thomas Rutherford, Simone Leonini, Franky Dubois, Ivo Peretto, Donald F. Collins, Lorenzo Betti, Lara Fossi, Irene Parenti, Vincenzo Pettina, Giulio Giordano Chiti, Maria Hofheinz, André Steenkamp, Sauro Gaudenzi, Nikola Antonov, Giuseppe Pappaa

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Leveraging Impact Parameter to Mitigate the Transit Light Source Effect: Early Insights from TRAPPIST-1

Ana Glidden, Alexander I. Shapiro, Sara Seager et al. (+19 more)

Stellar activity complicates exoplanet transmission spectra, particularly for smaller planets around M dwarfs with JWST. The transit light source (TLS) effect, the imprinting of spectral differences between the average stellar disk and the occulted transit chord onto the transmission spectrum, makes it challenging to directly use the out-of-transit spectrum to correct for stellar contamination. Theory and observations suggest that spots may concentrate towards higher latitudes when the Coriolis force is substantial relative to buoyancy, leaving the equatorial region relatively quiet. Here, we evaluate how the latitudinal distribution of active regions shapes the strength of the TLS effect for planets spanning a range of impact parameters ($b$), using TRAPPIST-1 as a testbed. We first construct a fiducial model to illustrate two distribution regimes. With our model, the moderate-$b$ outer TRAPPIST-1 planets (f, g, h) occult a more typical region of the stellar disk than the inner planets and are thereby less affected by the TLS effect, though their bias may vary more from visit-to-visit as these active regions evolve with time. More generally, our results imply an impact parameter "sweet spot" for atmospheric characterization, independent of the sign of the active region temperature contrast, whose location depends on the distribution of active regions. The distribution may be revealed by transit residuals as multiple planets probe different latitudes, while longitudes are sampled in time, such that the variance and frequency of the correlated scatter could constrain active-region filling factors, sizes, and separations.
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All authors Ana Glidden, Alexander I. Shapiro, Sara Seager, Nadiia Kostogryz, Valeriy Vasilyev, Roeland P. van der Marel, Julien de Wit, Benjamin V. Rackham, Prajwal Niraula, Natalie H. Allen, Jingcheng Huang, Nikole K. Lewis, Zifan Lin, Jacob Lustig-Yaeger, Ryan J. MacDonald, Brett M. Morris, Elijah Mullens, Kevin B. Stevenson, Jeff A. Valenti, Daniel Valentine, Hannah R. Wakeford, C. Matt Mountain

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

ALOHA IRDCs Molecular Line Follow-up: I. Gas properties and kinematics

Jinjin Xie, Yaoting Yan, Zhiyuan Ren et al. (+109 more)

Infrared Dark Clouds are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope (JCMT) Large Program, has mapped nearby IRDCs with SCUBA-2. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. We performed single-pointing K-band and W-band observations towards 56 ALOHA IRDCs clumps using the Effelsberg 100-m and Yebes 40-m telescopes, respectively. We derived NH3 kinetic temperatures using the hyperfine group ratio (HFGR) method and identified infall and shock signatures from HCO+, H13CO+, SiO, and HNCO profiles. Water masers and NH2D emission were used as complementary tracers of chemical evolution and star formation. The clumps exhibit kinetic temperatures of 15-29 K. We detect NH2D emission towards 18 sources, with NH2D centroid velocities consistent with NH3, indicating both species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO+ profiles, identifying them as infall candidates. Water masers are detected in 22 sources, with prominent velocity ranges and variability. Broad SiO emission (>~20 km/s) indicates strong shocks, while narrower extents (<~6km/s) likely trace large-scale interactions or low-velocity shocks. The widespread infall signatures, shock tracers, masers, and NH2D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.
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All authors Jinjin Xie, Yaoting Yan, Zhiyuan Ren, Jarken Esimbek, Di Li, Yan Duan, Gary A. Fuller, Nicolas Peretto, Jingwen Wu, Wenjin Yang, Christian Henkel, Xuepeng Chen, Qianru He, Yongxiong Wang, Keping Qiu, Ningyu Tang, Sijia Peng, Chao-Wei Tsai, Pham Ngoc Diep, Hauyu Baobab Liu, Busaba Kramer, Kee-Tae Kim, Ken'ichi Tatematsu, Mark G. Rawlings, Maria Jesus Jimenez Donaire, Gan Luo, Xin Lyu, Jiawei Liu, Yuchen Xing, Sheng-yuan Liu, Koichiro Sugiyama, Ram K. Yadav, Willem A. Baan, Gordon Macleod, Patricio Sanhueza, Long-Fei Chen, Chang Won Lee, Yang Su, Chen Wang, Ruili Wang, Ruilin Xia, Andrej Sobolev, Dmitry A. Ladeyschikov, David Eden, Woojin Kwon, Fengwei Xu, Hongjun Ma, Daniel Harsono, Sihan Jiao, Rowan Smith, Ke Wang, Tie Liu, Guangxing Li, Xin Guan, Yuxin He, Dalei Li, Xindi Tang, Chunsheng Luo, Jianjun Zhou, Kitiyanee Asanok, Dan Bintley, Huei-Ru Vivien Chen, En Chen, Chakali Eswaraiah, Ana Duarte-Cabral, Siyi Feng, Ray S. Furuya, Tomoya Hirota, Ernar Imanaly, Xue-Jian Jiang, Qaynar Jandaolet, Abay Jengis, Weiguang Ji, Yi-Jehng Kuan, Min-Young Lee, Chong Li, Cuihuan Li, Guodong Li, Hua-bai Li, Jiasheng Li, Yujie Li, Mengting Liu, Kuan-Yu Liu, Shu Liu, Rong Liu, Yongquan Luo, Yingxiu Ma, Steve Mairs, Fumitaka Nakamura, Harriet Parsons, Jaime Pineda, Hailiang Shen, Mingke Sun, Serikbek Sailanbek, Nurzhan Shaimoldin, Ya-Wen Tang, Antneh Gashaye Tegegne, Kadirya Tursun, Glenn J. White, Gang Wu, Jifeng Xia, Yuanzhen Xiong, Naiping Yu, Nannan Yue, Xinyu Yang, Yuebin Yang, Miaomiao Zhang, Chao Zhang, Shiyu Zhang, Dongdong Zhou, Qiao Zhao, Xiang Zhao

Solar and Stellar Astrophysics

Toward Operational Solar Flare Peak Flux Nowcasting: A Strategy Combining Real-Time Data, Machine Learning, and NOAA Flare Detection Criteria

Kangwoo Yi, Qin Li, Haodi Jiang et al. (+3 more)

We present the RMN strategy (Real-time data, machine learning, and NOAA flare detection criteria) for nowcasting the peak soft X-ray flux of ongoing solar flares under operationally realistic conditions. The strategy combines real-time GOES 0.1-0.8 nm X-ray observations with an attention-based sequence-to-sequence Long Short-Term Memory model. Under the NOAA flare detection criteria, predictions are evaluated at one-minute intervals from three minutes after the cataloged onset to the observed peak using the preceding 60 minutes of X-ray observations. We apply the RMN strategy to C-, M-, and X-class flares observed by GOES-8-18 from 1997 to 2024 using four-fold cross-validation. The major results of this study are as follows. First, the model nowcasts peak soft X-ray flux with RMSE and PE values of 0.26 and 3.11% for the $\geq$C-class group, 0.45 and 5.59% for the $\geq$M-class group, and 0.87 and 12.76% for the X-class group. The higher discrepancy toward stronger flare groups indicates that peak-flux prediction is more challenging for higher-intensity flares. Second, the model performance depends on flare rise time and prediction time, with larger errors for longer rise time events and improved performance as the prediction time approaches the flare peak. Shorter rise time events approach their final peak more rapidly, providing a clearer indication of the eventual peak, whereas the larger difference for longer rise time events may partly reflect more complex temporal evolution. Third, empirical coverage based on total uncertainty remains high but decreases for stronger flares, with noise uncertainty contributing more than model uncertainty.
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All authors Kangwoo Yi, Qin Li, Haodi Jiang, Meiqi Wang, Haimin Wang, Bo Shen

Solar and Stellar Astrophysics

Chromospheric heating and magnetic topology above the shared penumbra of a delta-spot: Multi-line inversions and multi-height magnetic-field extrapolations

M. Kriginsky, J. Leenaarts, J. de la Cruz Rodríguez et al. (+1 more)

We analysed observations of the Fe I 617.3 nm, Ca II 854.2 nm, and Ca II H lines obtained with CRISP and CHROMIS at the SST. Spatially coupled non-LTE inversions constrained the chromospheric atmosphere, while the WFA provided estimates of the chromospheric line-of-sight magnetic field. We combined these photospheric and chromospheric constraints with an HMI magnetogram as input to a multi-height field extrapolation. We characterised the reconstructed topology using the twist number, squashing factor, current density, and field-line connectivity. Results. The Ca II H magnetic signal is concentrated mainly above the strongest photospheric field concentrations, whereas Ca II 854.2 nm yields stronger and more spatially extended line-of-sight fields. The chromosphere above the shared penumbra is approximately 300 K hotter than nearby quiet regions. The selected brightening follows a chromospheric loop, with enhanced temperature and a transition from blueshift to redshift along the structure. The extrapolation recovers field strengths broadly consistent with the inversions and reveals a left-handed, flux-rope-like core following the polarity inversion line. Enhanced currents and connectivity gradients occur near parts of its boundary, where field lines connect the twisted structure to overarching loops. Conclusions. The temperature and velocity patterns and magnetic topology are consistent with reconnection between the twisted polarity-inversion-line field and the surrounding loops, depositing energy in the chromosphere and driving plasma along reconfigured field lines. These signatures do not uniquely establish reconnection, but show that combining high-resolution spectropolarimetric inversions with multi-height extrapolations can relate chromospheric energy release to the local three-dimensional magnetic structure.
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All authors M. Kriginsky, J. Leenaarts, J. de la Cruz Rodríguez, A. Pastor Yabar