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Physics Astrophysics

astro-ph Watch Astrophysics

Monday 9 March

Astrophysics of Galaxies

LeMMINGs VII: 5 GHz, 50 mas e-MERLIN observations of a statistically complete sample of nearby AGN

D. R. A. Williams-Baldwin, R. D. Baldi, R. J. Beswick et al. (+36 more)

We present 5 GHz e-MERLIN radio images at 50 mas resolution of the nuclear regions of the Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies survey (LeMMINGs), the deepest statistically complete radio-band survey of the local Universe (<120 Mpc), consisting of 280 galaxies spanning all morphological and nuclear types. We detect nuclear radio emission above a median 5 sigma threshold of 0.33 mJy beam^-1 in 68 of 280 sources (24 percent), with core luminosities in the range 10^35 to 10^41.9 erg s^-1. The radio emission is attributed to active galactic nuclei, circumnuclear star formation, or, in the case of NGC 3690, a tidal disruption event. The brightest radio nuclei, with brightness temperatures >=10^6 K, reside in optically active galaxies such as LINERs and Seyferts. The detection rate for inactive systems (H II and absorption-line galaxies), which may host low-luminosity active galactic nuclei, is 8 percent. Most detections (78 percent) are compact (<10 pc), while the remaining 22 percent show extended jet-like features up to 380 pc. Compared to the 1.5 GHz LeMMINGs data, the 5 GHz observations provide superior resolution and spatial filtering, resolving out large-scale structures and isolating genuine nuclear emission. Our results suggest that low-luminosity active galactic nuclei are the primary manifestation of black hole activity in the local Universe in the form of compact jets and cores, with a preference for early-type hosts. The two LeMMINGs campaigns indicate that up to 30 percent of the local galaxy population hosts a radio-active nucleus, highlighting the necessity of high-resolution, high-sensitivity imaging for uncovering nuclear emission at the lowest luminosities.
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All authors D. R. A. Williams-Baldwin, R. D. Baldi, R. J. Beswick, I. M. McHardy, E. Carver, J. Clifford, B. T. Dullo, N. Kill, B. Krishnamoorthi, I. M. Mutie, O. Woodcock, M. K. Argo, P. Boorman, E. Brinks, D. M. Fenech, J. H. Knapen, S. Mathur, J. Moldon, T. W. B. Muxlow, M. Pahari, N. H. Wrigley, A. Alberdi, W. Baan, A. Beri, X. Cheng, D. A. Green, J. Healy, P. Kharb, E. Körding, G. Lucatelli, F. Panessa, M. Puig-Subirà, C. Romero-Cañizales, D. J. Saikia, P. Saikia, F. Shankar, S. Sharma, I. R. Stevens, E. Varenius

High Energy Astrophysical Phenomena

Multi-epoch afterglow rebrightenings in GRB 250129A: Evidence for successive shock interactions

D. Akl, S. Antier, H. Koehn et al. (+105 more)

Most long gamma-ray bursts (GRBs) exhibit afterglows broadly consistent with external forward-shock emission, typically described by smooth broken power-law decays in the multiband light curve. However, a minority of well-sampled GRBs deviate from this behavior, including GRB 250129A. This object shows multiple late-time rebrightenings at X-ray and optical wavelengths. Rebrightenings are often attributed to energy injection from prolonged central engine activity, refreshed shocks from delayed shell collisions, density jumps in the ambient medium, or angular jet structure and viewing-angle effects. After analysing the prompt emission observed in gamma-rays and the near-infrared, we investigate the origin of X-ray and optical flaring episodes in GRB 250129A. Physical processes in the afterglow light curves were investigated using methods ranging from empirical fitting to Bayesian inference. The well-sampled flares and the connection between the prompt and afterglow emission allow us to test the consistency of the fireball model and alternative scenarios. Conducting the prompt and time-resolved analyses, we obtained an isotropic-equivalent energy of E_iso,gamma = (1.35 +/- 0.12) x 10^53 erg. By modeling the afterglow using an agnostic Bayesian framework (NMMA), we rule out both a single external-shock evolution and a one-time energy-injection scenario. Numerical calculations show that the rebrightening episodes are consistent with refreshed shocks from delayed collisions between relativistic shells. Based on the consistency between our analyses of the prompt and afterglow GRB 250129A data, we find that two statistically significant rebrightening episodes occur within 1.1 days post trigger and can be explained by a sequence of refreshed shocks. Temporally and spectrally rich GRB datasets such as the one presented in this work, provide a powerful means to test current modeling frameworks.
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All authors D. Akl, S. Antier, H. Koehn, P.T.H. Pang, J.J. Geng, R. Gill, E. Abdikamalov, C. Adami, V. Aivazyan, L. Almeida, S. Alshamsi, C. Andrade, Q. André, C. Angulo-Valdez, J.-L. Atteia, K. Barkaoui, S. Basa, R.L. Becerra, P. Bendjoya, D. Berdikhan, E. Bernaud, S. Boissier, S. Brunier, A.Y. Burdanov, N.R. Butler, J. Chen, F. Colas, W. Corradi, M.W. Coughlin, D. Darson, T. Dietrich, D. Dornic, C. Douzet, C. Dubois, J.-G. Ducoin, T. du Laz, A. Durroux, D. Dutton, P.-A. Duverne, F. Dux, E.G. Elhosseiny, A. Esamdin, A.V. Filippenko, F. Fortin, M. Freeberg, L. García-García, M. Gillon, N. Globus, P. Gokuldass, N. Guessoum, P. Hello, R. Hellot, Y.H.M. Hendy, Y.L. Hua, T. Hussenot-Desenonges, R. Inasaridze, A. Iskandar, M. Jelínek, S. Karpov, A. Klotz, N. Kochiashvili, T. Laskar, A. Le Calloch, W.H. Lee, S. Leonini, X.Y. Li, A. Lien, C. Limonta, J. Liu, D. López-Cámara, F. Magnani, J. Mao, M. Mašek, E. Moreno Méndez, L.C. Menegazzi, W. Mercier, B.M. Mihov, M. Molham, S. Oates, M. Odeh, H. Peng, M. Pereyra, M. Pillas, T. Pradier, N.A. Rakotondrainibe, D. Reichart, J.-P. Rivet, F.D. Romanov, F. Sánchez-Álvarez, N. Sasaki, D. Schlekat, B. Schneider, A. Simon, L. Slavcheva-Mihova, R. Strausbaugh, T.R. Sun, A. Takey, M. Tanasan, D. Turpin, A. de Ugarte Postigo, L.T. Wang, X.F. Wang, Z.M. Wang, A.M. Watson, J. de Wit, Y.S. Yan, W. Zheng, S. Zúñiga-Fernández

Astrophysics of Galaxies · Cross-listed from physics.plasm-ph

Spatiotemporal Properties of Compressible Magnetohydrodynamic Turbulence from Space Plasma

Siqi Zhao, Huirong Yan, Terry Z. Liu et al. (+2 more)

Previous studies have established that a weak-to-strong transition occurs in Alfvenic magnetohydrodynamic (MHD) turbulence as energy cascades from large to small scales. However, the spatiotemporal (frequency-wavenumber) properties of compressible MHD turbulence involving all eigenmodes, which encode the strength of nonlinear interactions, remain difficult to characterize observationally. Consequently, whether a similar weak-to-strong transition occurs in compressible turbulence remains elusive. Using a novel multi-spacecraft, polarization-based mode-decomposition technique with measurements from the Cluster spacecraft in Earth's magnetosheath, we obtain spatiotemporal power spectra of all MHD eigenmodes and present the first quantitative assessment of nonlinear frequency broadening. Our results show that slow modes exhibit a weak-to-strong transition, evolving from wave-like peaks to frequency-broadened spectra as nonlinearity increases, whereas fast modes remain weakly turbulent with narrow peaks near their eigenfrequencies. Both Alfvenic and compressible fluctuations contribute significantly to low-frequency, large-scale quasi-two-dimensional structures. These findings provide a comprehensive observational characterization of compressible turbulence across mode composition, spatiotemporal scales, and weak-strong turbulence regimes, with implications for energetic particle transport, turbulent dynamos, plasma heating, and solar wind-magnetosphere coupling.
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All authors Siqi Zhao, Huirong Yan, Terry Z. Liu, Chuanpeng Hou, Ka Ho Yuen

Solar and Stellar Astrophysics

Gas chemistry in the dust depleted inner regions of protoplanetary disks. I. Near-IR spectra and overtones

J. Bethlehem, Ch. Rab, I. Kamp et al. (+3 more)

The molecular composition inside the dust sublimation zones of protoplanetary disks is mostly unknown but important to understanding terrestrial planet formation. A few molecules have been observed from this region, specifically CO, H2O, OH and SiO. The small surface area makes observing this region difficult, hence modeling is required to disentangle the innermost disk from regions further out. We model a protoplanetary disk around a Herbig-type star including the dust depleted inner region (approx. 0.1-0.3 au) and aim to investigate the chemistry of this region and explain existing and future observations. Methods. We post-process the dust and gas distribution of a magnetohydrostatic model with the radiation thermochemical code ProDiMo to study the chemistry and to produce observables. We find that the dust free inner disk is a molecular rich environment, where besides CO we also find H2, H2O and SiO. The gas temperature profile is complex and fluctuates between 700 and 2000 K, which is warm enough to produce CO overtone line emission. Next to the CO overtone lines we also find strong high J-level fundamental CO lines between 4.3 and 4.6 micron. The elemental enrichment of Si due to dust sublimation leads to 2 orders of magnitude more SiO abundance. The SiO gas has average temperatures of approx. 1000 K resulting in strong SiO overtone emission in the spectral range between 4 and 4.3 micron. We predict that the gas density in the dust depleted inner disk is high enough to allow for H2 formation, resulting in an molecular rich environment. For our representative Herbig model, the dust-depleted inner disk is responsible for at least 90% of the line emission for CO and H2O between 1 and 28 micron. Next to CO overtone lines, SiO overtone lines are expected to be an important tracer of a dust free inner disk.
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All authors J. Bethlehem, Ch. Rab, I. Kamp, M. Flock, G. Bourdarot, P. Caselli

Solar and Stellar Astrophysics

Nonlinear evolution of unstable solar inertial modes: The case of viscous modes on a differentially rotating sphere

Muneeb Mushtaq, Damien Fournier, Rama Ayoub et al. (+2 more)

On the Sun, the inertial mode with the largest observed amplitude (rms velocity exceeding $10$ m/s) is the high-latitude mode with longitudinal wavenumber $m=1$. In two dimensions, on the sphere, linear theory predicts that this mode is unstable due to a shear instability associated with latitudinal differential rotation (fast equator, slower polar regions). We investigate the evolution of this instability numerically and theoretically. The nonlinear vorticity equation is solved using direct numerical simulations in the time domain. The only control parameter is the Ekman number $E$. For $10^{-3}\lesssim E< E_c \approx 1.5\times10^{-3}$, only the high-latitude $m=1$ mode is unstable. We extract its saturation amplitude as a function of $E$ and compare the results with predictions from two perturbative approaches in nonlinear stability theory. The simulations reveal a supercritical Hopf bifurcation. Near onset, the mode amplitude is well described by the Landau equation $d|A|/dt=\sigma_I |A|+\beta_I |A|^3$, with a positive linear growth rate $\sigma_I$ and a negative nonlinear coefficient $\beta_I$. The coefficient $\beta_I$ depends weakly on $E$, implying that the saturated amplitude scales approximately as $|A|\propto\sigma_I^{1/2}$. The equilibrium mode contains the $m=1$ fundamental and harmonics $m=2$ and $m=3$, whose amplitudes scale as $\sigma_I^{m/2}$. Saturation results from Reynolds stresses that smooth the latitudinal differential rotation. For $E=4\times10^{-4}$, consistent with solar-like turbulent viscosity, the saturated velocity reaches $28$ m/s, comparable to solar observations. These results should be interpreted cautiously, since in three dimensions the instability is baroclinic and involves different physics.
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All authors Muneeb Mushtaq, Damien Fournier, Rama Ayoub, Peter J. Schmid, Laurent Gizon

Astrophysics of Galaxies

ML in Astrophysical Turbulence I: Predicting Prestellar Cores in Magnetized Molecular Clouds using eXtreme Gradient Boosting

Nikhil Bisht and David C. Collins

Giant Molecular Clouds (GMCs) are dominated by supersonic turbulence and magnetic fields, creating a complex network of shocks and filaments that regulate star formation, making it challenging to predict which individual gas parcels will collapse into star-forming cores. Here, we present an XGBoost machine learning framework to forecast the 3D Lagrangian trajectories and gravitational collapse fate of prestellar gas. Models were trained on $\sim 2.1$ million tracer particles from self-gravitating, turbulent magnetohydrodynamic (MHD) simulations. By mapping instantaneous point-wise states (position, velocity, density, and magnetic fields) to future coordinates, our regression model predicts continuous 3D spatial evolution over $0.25~t_{\rm ff}$ ($\approx 0.31\text{ Myr}$) with $R^2 > 0.99$. Concurrently, our classifier accurately ranks gravitational core susceptibility ($\text{AUC} = 0.975$). Using SHapley Additive exPlanations (SHAP), we interpret the model's decision hierarchy, revealing that local density, kinetic energy, and magnetic field magnitude dominate core identification. SHAP dependence analysis shows that the model re-derives a sharp non-linear collapse threshold at $\log_{10}\rho \approx 0.8$ above which magnetic fields amplify collapse predictions. The classification task reveals a conservative false-positive rate (37% precision) at absolute binary thresholds. We demonstrate that while point-wise ML models excel at kinematic advection, they mathematically lack the spatial context required to evaluate local differential operators (such as magnetic directional derivatives, thermal pressure gradients, or field curvature) that are physically necessary to stop collapse. This data-driven approach offers a computationally efficient foundation for advective subgrid modeling while highlighting the necessity of spatial architectures to establish absolute core-formation criteria.
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Instrumentation and Methods for Astrophysics

Non-common path aberration compensation and a dark hole loop with a pyramid adaptive optics system: Application to SAXO+

C. Goulas, R. Galicher, F. Vidal et al. (+16 more)

In ground-based high-contrast instruments, non-common path aberrations (NCPAs) limit detection performance, as they are unseen by the adaptive optics (AO) wavefront sensor but impact the astrophysical image, creating quasi-static speckles. SAXO+, the upgrade of the SAXO (SPHERE AO system) includes a second loop of AO downstream of the SAXO loop that is equipped with a near-infrared pyramid wavefront sensor whose nonlinearities, usually described with modal optical gains, might be challenging for removing quasi-static speckles. We investigated two methods of quasi-static speckle removal : NCPA compensation and a dark hole loop, behind a pyramid AO system, measuring the interest of compensating for the pyramid optical gains. We performed end-to-end numerical simulations under various astrophysical conditions. We offset the pyramid wavefront sensor operating point to apply both the speckle suppression methods, with or without optical gain calibration. We evaluated the performance by measuring the residual starlight in the coronagraph image. A by-product of our study is an on-sky calibration method of measuring the pyramid optical gains. NCPA compensation reduces the residual starlight in the coronagraph image by a factor of 20 for seeing between 0.7" and 1" for a bright star and a factor of 2 at 0.7" for a faint star. Optical gains compensation enhances the performance at poor seeing and small pyramid modulation radius with a bright star, but shows a useless or even negative impact due to estimation inaccuracies at faint targets. On the other hand, the dark hole loop reduces the residual starlight by a factor of 200. The optical gain calibration enhances the dark hole performance behind a single pyramid AO system but is useless behind the SAXO+ system. Our parametric study gives baseline values for the efficient control of the dark hole loop for the SAXO+ system.
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All authors C. Goulas, R. Galicher, F. Vidal, J. Mazoyer, F. Ferreira, A. Sevin, A. Potier, A. Boccaletti, E. Gendron, C. Béchet, M. Tallon, M. Langlois, C. Kulcsár, H-F. Raynaud, N. Galland, L. Schreiber, I. Bernardino Dinis, F. Wildi, G. Chauvin

Astrophysics of Galaxies

Ashes of FIRE: Modeling Dust Grain Size Evolution in the Local Group with FIRE

Caleb R. Choban, Samir Salim, Dušan Kereš et al. (+2 more)

We introduce a new, discretized grain size evolution model, incorporated into the GIZMO code and coupled with FIRE-3 stellar feedback and ISM physics, to investigate variations in dust abundance, chemical composition, and grain sizes observed in the Local Group. This model tracks the size evolution of specific dust species, and includes stellar production of dust, dust growth through gas-phase metal accretion, dust destruction by sputtering, SNe shocks, and astration, grain-grain collisional shattering and coagulation, and turbulent dust diffusion. Using idealized galaxy simulations, we test the dependence of MW dust properties on variations in each dust process and find that our model uniquely predicts a bimodal grain size distribution. This bimodality is due to our simulation's ability to resolve each dust process and where they occur in the ISM, unlike other works. We find that Local Group dust abundances are determined by dust growth and destruction, with little dependence on coagulation or shattering, explaining why models that do not include these processes can match abundance observations. We also find that variations in Local Group extinction curve slopes are determined by coagulation, with inefficient coagulation leading to steeper slopes. However, inefficient coagulation also results in stronger extinction curve bumps, which are not observed. We also do not predict a population of very small (${<}1$ nm) carbonaceous grains, required for MIR emission features, due to their rapid growth by accretion. These results highlight the possible necessity of “top-down” PAH formation from preexisting grains as a means to inhibit carbonaceous dust growth.
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All authors Caleb R. Choban, Samir Salim, Dušan Kereš, Julia Roman-Duval, Karin M. Sandstrom

Solar and Stellar Astrophysics

Extreme mass loss during common envelope evolution: the origin of the double low-mass white dwarf system J2102–4145

Leandro G. Althaus, Alejandro H. Corsico, Monica Zorotovic et al. (+3 more)

Eclipsing close double white dwarf (WD) systems provide a unique opportunity to directly constrain hydrogen-envelope retention and test common-envelope (CE) evolution in low-mass stars, since they allow precise determinations of stellar masses and radii. We analyze J2102-4145, an eclipsing binary composed of two low-mass helium-core white dwarfs in a 2.4-hour orbit. By comparing the observed radii and effective temperatures with updated evolutionary models for CE evolution and stable Roche-lobe overflow (SRLOF), we confirm that both stars are helium-core white dwarfs. The primary, with a mass of 0.375 solar masses, is consistent with SRLOF models that retain thick hydrogen envelopes and sustain residual nuclear burning, whereas the secondary, with a mass of 0.314 solar masses, can only be reproduced by CE models in which the hydrogen envelope is almost completely removed. The inferred cooling ages (approximately 220 Myr for the secondary and between about 260 and 510 Myr for the primary, depending on the contribution of residual nuclear burning) support a formation sequence in which the primary formed first through SRLOF, followed by a CE phase that produced the compact secondary. Reconstruction of the CE energy budget yields progenitor and orbital parameters consistent with this evolutionary picture. The unusually small radius of the secondary requires an extremely thin hydrogen envelope, with a mass below about 10e-7 solar masses, well below the values predicted by standard bifurcation criteria. J2102-4145 therefore provides one of the strongest observational constraints on the hydrogen-envelope mass of post-CE low-mass white dwarfs and represents a benchmark challenge for current prescriptions of envelope ejection.
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All authors Leandro G. Althaus, Alejandro H. Corsico, Monica Zorotovic, Maja Vuckovic, Alberto Rebassa-Mansergas, Santiago Torres

High Energy Astrophysical Phenomena

How interacting winds shape the mechanical feedback of massive star clusters over millions of years

Thibault Vieu, Lucia Härer and Brian Reville

In recent years, massive star cluster environments have proved to be bright sources of very-high energy gamma-rays, in particular young clusters which are powered by the winds interacting in their cores. In order to understand how these winds can accelerate particles up to very-high energies, it is necessary to model their interactions from small (sub-pc) to large (10s of pc) scales over several millions of years. A key open question concerns the structure and properties of the resulting wind termination shock. By performing 3D magnetohydrodynamic simulations of clustered winds embedded in a superbubble cavity, we demonstrate that the dynamics of stellar wind interactions and the resulting shock structure solely depends on the density and pressure of the cavity. This implies that the initial conditions of the simulation can be tuned in order to simulate star clusters of arbitrary age at a reduced computational cost. This novel method is validated using a toy cluster hosting 30 identical stars. We discuss the properties of the resulting cluster-wind termination shock under various assumptions. In particular, we are able for the first time to obtain a fully decoupled spherical wind termination shock for a 5 Myr old cluster. We further show that radiative cooling increases the sphericity of the shock. In general, the morphology of the outflow depends on the number of dominant stars, on the power of the stars sitting at the edge of the cluster core, and on the compactness of the cluster. We additionally show how a semi-analytical model can be used in order to estimate key morphological properties of the outflow without relying on large-scale simulations.
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Astrophysics of Galaxies

Robust ellipticity measurements of 29 Galactic globular clusters

Laurane Fréour, Ellen Leitinger, Elena Pancino et al. (+2 more)

Globular clusters (GCs) exhibit varying degrees of flattening (ellipticity), which may provide insight into their internal dynamics and evolution histories. Commonly used methods to measure ellipticity, such as ellipse fitting of density contours and principal component analysis, often produce biased results, especially for clusters that are nearly round or have few observable stars. Using a combination of ground-based and space-based photometry, we investigate the shapes of 29 Galactic GCs. To that end, we test two commonly used methods: an ellipse fit to a kernel density profile and a principal component analysis. We find that both methods suffer from bias arising when the number of stars is small or the cluster is close to round. To solve this issue, we develop a robust method to measure the ellipticity of GCs, test it extensively on mock data, and apply it to the 29 Milky Way GCs in our sample. Using the $V/\sigma$ diagram used in the isotropic oblate rotator framework, we examine potential causes for the flattening, including rotation and velocity anisotropy. For ten clusters: NGC~104, NGC~1261, NGC~2808, NGC 3201, NGC 5286, NGC 5904, NGC 5986, NGC 6205, NGC 6341, and NGC 7078 we identify a very good agreement between the rotation angle and semi-minor axis of the ellipse, further corroborating the findings that rotation is the main driver of the ellipticity. The $V/\sigma$ diagram reveals that velocity anisotropy or tides could also be important in shaping the GCs. The robust method developed provides reliable measurements of the ellipticity of GCs, emphasising the importance of taking into account the flattening in theoretical models and simulations. It also offers a promising way to investigate the shapes of multiple stellar populations within GCs, where only small samples are usually available.
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All authors Laurane Fréour, Ellen Leitinger, Elena Pancino, Alice Zocchi, Glenn van de Ven

High Energy Astrophysical Phenomena

Double White Dwarf Mergers as Progenitors of Long-Period Transients

Manuel Malheiro, Sarah V. Borges, Jaziel G. Coelho et al. (+6 more)

There is an ongoing discussion in the literature on the nature of long-period transients (LPTs), radio-emitting sources with periods ranging from hundreds to tens of thousands of seconds. Although some of these objects have been identified as white dwarf (WD) + M-dwarf binaries, this description currently does not fit the entire class. An example is GLEAM-X J162759.5-523504.3 (hereafter GLEAM-X J1627-5235), with a period of 1091 s, for which the lack of an optical counterpart disfavors the presence of such a binary system. In this case, GLEAM-X J1627-5235 could be interpreted as an isolated, massive, fast-rotating, and highly magnetized (~ 1e+9 G) WD pulsar. Its properties are consistent with a carbon-oxygen WD of mass ~1.3 Msun and radius ~2500 km, possibly supported by small-scale multipolar magnetosphere structures that keep it above the death line for WD-pulsars. We assess a double WD merger origin, modeling the post-merger rotational evolution under accretion, propeller, and magnetic braking torques. We find rotational age of ~572 Myr for GLEAM-X J1627-5235, i.e., the post-merger time required to reach its observed period. This result is consistent with current optical upper limits for GLEAM-X J1627-5235 and support the WD pulsar interpretation for this source. We also discuss how the same model can apply to other LPTs.
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All authors Manuel Malheiro, Sarah V. Borges, Jaziel G. Coelho, Khashayar Kianfar, Ronaldo V. Lobato, Edson Otoniel, Jorge A. Rueda, Manoel F. Sousa, Fridolin Weber

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Local Origin of Hidden Symmetry in Rotating Spacetimes

Hyeong-Chan Kim

We show that, within a broad stationary-axisymmetric class, Kerr-type separability and hidden symmetry arise as a local consequence of the Einstein equations. Without assuming separability, algebraic speciality, Killing–Yano symmetry, or global boundary conditions, we analyze stationary and axisymmetric geometries in a locally non-rotating orthonormal frame and impose a minimal local equilibrium condition, namely the absence of mixed energy-momentum fluxes. We find that the mixed Einstein equations enforce a rigid projective alignment between radial and angular sectors, uniquely characterized by a constant-Schwarzian constraint. This constraint yields a universal classification of local solutions into Möbius, exponential, and trigonometric branches, of which global regularity selects precisely the Kerr-type sector. In this sense, the kinematical core of Kerr geometry is already fixed locally, and the Schwarzian structure provides the local origin of Kerr rigidity.
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Solar and Stellar Astrophysics

Homogenization of the Stetson Photometry with the BEST Database

Zhirui Li, Bowen Huang, Kai Xiao et al. (+5 more)

As one of the most widely recognized high-quality standard stars, the Stetson standards have been extensively used as a photometric reference for calibrating other surveys. In this work, we present an independent validation and re-calibration of the Stetson standard star photometry using the BEST database. Based on typically 30,000-70,000 calibration stars per band, we find that the original Stetson photometry achieves field-to-field zero-point precisions of approximately 10–40\,mmag in the $UBVRI$-band. In addition, significant spatially dependent magnitude offsets are detected within individual Stetson fields for all bands, with magnitudes exceeding 1%, probably caused by the calibration errors in the Stetson photometry. After correcting those systematic errors, the agreement between the Stetson and BEST photometry is improved to $\sim$5\,mmag for individual fields for $BVRI$-band. The re-calibrated photometry is further validated using the SCR standards, yielding agreement better than 10\,mmag for individual stars in the $BVRI$ bands and confirming zero-point precisions of 2–4\,mmag in the $BVI$ band. The precisions is further confirmed by checks using Gaia DR3 broad band colors. These results highlight the power of the BEST database for improving photometric calibration and suggest that, if feasible, it be incorporated into the calibration process of future releases of the Stetson standard catalog.
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All authors Zhirui Li, Bowen Huang, Kai Xiao, Haibo Yuan, Yang Huang, Dongwei Fan, Mingyang Ma, Tao Wang

Solar and Stellar Astrophysics

A reaction-diffusion model for describing the ring/gap structure in disks surrounding individual young stars

Enrique Lopez-Cabarcos

The embedded disks surrounding individual Class 0 protostars are structureless. Disks surrounding Class I stars may be continuous or have a ring-gap substructure, whereas all disks around Class II stars have a ring-gap substructure that gradually disappear as the disks evolve into debris disks. This common sequence in young lone stars requires an explanation. This study aims to show that the physical model Reaction-Diffusion Systems with Moving Reaction Front can be used to describe and classify protostellar disks according to their structure. A comprehensive review of observations made with the ALMA radio telescope shows: first, that the protostar-disk system presents a geometry analogous to that of an reaction-diffusion system with two separate compartments, namely, protostar and disk. Second, that in the protostar, matter is processed at high temperature, resulting in a chemical composition different from that of the disk. Third, that the equatorial outflow emitted by the protostar, rich in highly reactive trihydrogen cation, acts as a moving reaction front, MRF, that triggers the formation of molecules and nuclei in the disk. The time lag of nucleation with respect to the passage of the MRf would be the cause of the formation of the gaps between the rings of particles that form in the disk. The MRF is a transient phenomenon and its passage causes the transformation of a continuous disk, Class 0, into a disk with a ring-gap structure, Class II, whose temporal evolution begins at the interface of the star and moves outwards.
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High Energy Astrophysical Phenomena

Compression-Driven Kinetic Instabilities in Magnetically Arrested Disks

Vedant Dhruv, Lorenzo Sironi, Jordy Davelaar et al. (+1 more)

Event horizon-scale observations of low-luminosity black hole accretion flows favor magnetically arrested disks, characterized by dynamically important magnetic fields ($\beta\lesssim1$, where $\beta$ is the ratio of plasma thermal pressure to magnetic pressure) and a two-temperature transrelativistic plasma. Motivated by plasma conditions in the synchrotron-emitting regions of these models, we perform 2D particle-in-cell simulations of electron-ion plasmas with a realistic mass ratio, subject to continuous compression perpendicular to the mean magnetic field $\boldsymbol{B}_0$. Conservation of particle magnetic moments drives pressure anisotropy $P_{\perp}>P_{\parallel}$, triggering anisotropy-driven instabilities. For ion plasma beta $\beta_{i0}=0.5$ and ion temperature $k_{\text{B}}T_{i0}/m_i c^2=0.05$, the ion pressure anisotropy is regulated by the ion cyclotron instability, while the mirror mode influences the late-time electron anisotropy. Both species develop nonthermal components at high energies, consistent with stochastic acceleration by cyclotron-scale fluctuations. We characterize how the onset and time evolution of the plasma instabilities, as well as the resulting ion and electron anisotropies and energy spectra, vary with $\beta_{i0}$, $k_{\text{B}}T_{i0}/m_i c^2$, electron-to-ion temperature ratio $T_{e0}/T_{i0}$, and the compression rate. Increasing the thermal energy toward relativistic values raises the anisotropy thresholds for all instabilities observed in our simulations, allowing larger anisotropies to develop. For $T_{e0}/T_{i0}<1$, as expected in collisionless two-temperature accretion flows, the growth of mirror and whistler instabilities is delayed or suppressed, leading to increasingly adiabatic evolution of the electrons. Our findings can be used to inform global fluid models of black hole accretion.
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All authors Vedant Dhruv, Lorenzo Sironi, Jordy Davelaar, Aaron Tran

High Energy Astrophysical Phenomena

Disc accretion onto a binary black hole in a hierarchical triple system as an origin of the most luminous hyper-soft sources

Sergei B. Popov and Galina V. Lipunova

We propose that the recently discovered luminous hypersoft X-ray sources can be explained by accretion onto a binary black hole in a hierarchical triple system. For black hole masses $\sim 15 M_\odot$, the orbital separation of the internal binary might be $\sim 0.01 $~AU. If the donor provides $\gtrsim 10^{-8} M_\odot$ yr$^{-1}$, then the circumbinary accretion disc can explain the observed properties of the most luminous supersoft sources.
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Solar and Stellar Astrophysics

A probable inside-out dwarf nova outburst from the period bouncer candidate ASASSN-25dc

Yusuke Tampo, Naoto Kojiguchi, Mariko Kimura et al. (+13 more)

We report optical time-resolved photometric observations of a newly-discovered outbursting system, ASASSN-25dc. Its 8-mag amplitude, 40-day duration, 1-mag dip in the outburst plateau, and positive superhumps are characteristic of a dwarf nova superoutburst in a non-magnetic cataclysmic variable. We establish its stage-A and stage-B superhump periods as 0.059387(5) d and 0.058864(3) d, respectively. The negative superhump period derivative ($-$1.4(2)$\times10^{-5}$ cycle$^{-1}$) during the stage-B superhumps and the empirical relation indicate the mass ratio is 0.054(7), below the period bounce range. The long outburst decline timescale (35.2(1) d mag$^{-1}$) and small superhump amplitude ($\simeq$0.08 mag) observed in ASASSN-25dc are also seen in some period bouncer systems, but not seen in systems well before the period bounce. Despite its short superhump period and indicated small mass ratio, we find no evidence of the excitement of the 2:1 tidal resonance. Moreover, its outburst rise timescale (1.62(9) d mag$^{-1}$) is significantly longer than those measured at less than 0.4 d mag$^{-1}$ in other dwarf nova outbursts around the period minimum. Overall, an inside-out dwarf nova outburst from a massive disc in a system with a mass ratio around or even below the period minimum, but lacking the 2:1 tidal resonance, may explain all these observations. However, this challenges the existing models of dwarf nova superoutbursts, which do not predict these outburst properties in low-mass-ratio systems.
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All authors Yusuke Tampo, Naoto Kojiguchi, Mariko Kimura, Keisuke Isogai, David. A. H. Buckley, Nikita Rawat, Stephen B. Potter, Anke van Dyk, Patrick Woudt, Paul J. Groot, Franz-Josef Hambsch, Berto Monard, Peter Starr, William Goltz, Daisaku Nogami, Taichi Kato

Earth and Planetary Astrophysics

Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres

Marrick Braam, Ellery Gopaoco, Shang-Min Tsai et al. (+4 more)

The potential of prebiotic chemistry to propagate on an exoplanet fundamentally depends on whether the atmospheric conditions can facilitate the production of prebiotic feedstock molecules. Photochemical simulations of exoplanet atmospheres can be used to explore this potential atmospheric synthesis, but require a comprehensive chemical network. We present the implementation of the CRAHCN-O network, constructed to simulate the formation of feedstock molecules such as HCN, H$_2$CO, and simple hydrocarbons, into the VULCAN photochemical kinetics code. We investigate the production of feedstock molecules driven by M-star radiation and compare these to predictions by the N-C-H-O network in VULCAN, for N$_2$-dominated atmospheres with C/O ratios between 0.5-1.5. Predicted abundances are similar for C/O${=}$0.5. Once CH$_4$ is included (i.e., for C/O${>}$0.5), the abundance profiles diverge in the photochemical regions. By analysing the attenuation of UV radiation, we find that hydrocarbon photochemical shielding causes the diverging profiles. CRAHCN-O accumulates C$_2$H$_6$, while N-C-H-O accumulates C$_4$H$_3$ and C$_3$H$_4$. Importantly, C$_2$H$_6$ is photochemically active whereas C$_4$H$_3$ and C$_3$H$_4$ are assumed inactive. With mixing ratios up to a few percent in CRAHCN-O, C$_2$H$_6$ shields CH$_4$ and CO$_2$ from photodissociation and weakens the destruction of HCN and H$_2$CO. Maximum HCN mixing ratios reach 1000 ppm with CRAHCN-O compared to only 3 ppm with N-C-H-O. Other feedstock molecules like HC$_3$N and C$_2$H$_2$ form more efficiently in N-C-H-O. The shielding mechanism and its impact on feedstock molecules persist for radiation from distinct M-star types. These results demonstrate the crucial role of chemical kinetics in understanding prebiotic processes in exoplanet atmospheres, including important considerations for the construction and applicability of chemical networks.
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All authors Marrick Braam, Ellery Gopaoco, Shang-Min Tsai, Gergely Friss, Paul I. Palmer, Paul B. Rimmer, Skyla B. White

Earth and Planetary Astrophysics

One Hundred Years of Venus Polarimetry: PICSARR Observations of the Phase Curves

Jeremy Bailey, Daniel V. Cotton, Kimberly Bott et al. (+1 more)

We report new high-precision observations of the polarization of light scattered from the atmosphere of Venus, made 100 years after the pioneering studies by Bernard Lyot. The new observations include disk-integrated observations in a range of filters as well as imaging polarimetry. We compare the new results with past observations and models. We have reproduced the 1974 modelling of the Venus polarization by Hansen and Hovenier using modern radiative transfer codes. We show that the new models are in good agreement with the originals, and enable us to calculate the polarization for wavelengths not covered by the original study and to model the polarization distribution across the disk. The new observations are in good agreement with past determinations of the size distribution of the predominant particle mode. They agree with past studies in showing variability of the phase curve between synodic cycles and also polarization variability on short timescales, particularly at higher phase angles (crescent phases). Imaging polarimetry observations show good agreement with models for the redder wavelengths. However, observations in the ultraviolet show very different polarization behavior in the polar regions (within about 30 degrees of the north and south poles). The simplest explanation of this result is that there is a larger Rayleigh scattering component in the polar regions than in the equatorial and mid-latitudes and this could be explained by a lower cloud-top height in agreement with previous spacecraft observations. These ultraviolet polarization observations are inconsistent with horizontally homogeneous atmospheric models.
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All authors Jeremy Bailey, Daniel V. Cotton, Kimberly Bott, Ievgeniia Boiko

Solar and Stellar Astrophysics

s-process nucleosynthesis in low-mass AGB stars by the $^{13}$C($\alpha$,n)$^{16}$O neutron source

Inma Domínguez, Carlos Abia, Maurizio Busso et al. (+2 more)

In this review we outline our knowledge on slow neutron captures, concentrating on its main part occurring during the final stages of stellar evolution for low or intermediate-mass stars when they evolve during the Asymptotic Giant Branch, or AGB, stars. We focus our attention on how, in this field, studies passed from a first era of inquiries based on nuclear systematics, to numerical nucleosynthesis computations performed in stellar codes. We then discuss how these last were forced, by observational constraints, to almost abandon, for the synthesis of nuclei between Sr and Pb, the rather naturally activated Ne22 neutron source (operating efficiently at T > 30 keV, and producing a neutron density N_n > 5 10^8 cm^-3). This implied considering the alternative reaction 13C(alpha,n)16O, that can be activated locally after each of the TDU. The mentioned crucial reaction occurs at T< 8 keV, in the time intervals separating two subsequent thermal pulses (TP). The layers where 13C(alpha,n)16O operates are characterized by a radiative equilibrium and their low temperature also yields low values for the neutron density (N_n < 10^7 cm^-3).
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All authors Inma Domínguez, Carlos Abia, Maurizio Busso, Oscar Straniero, Sara Palmerini

High Energy Astrophysical Phenomena

Impact of Resonant Compton Scattering on Magnetar X-Ray Polarization with QED Vacuum Resonance

Tu Guo and Dong Lai

Recent obeservations have revealed significant soft X-ray polarizations from several quiescent magnetars, including the intriguing $90^\deg$ polarization angle (PA) swing as a function of photon energy for some sources. We present a general semi-analytical framework for calculating energy-dependent soft X-ray polarization signatures from magnetars, consistently incorporating both QED vacuum resonance in the atmosphere and resonant Compton scattering (RCS) in the magnetosphere. Starting from the polarized radiative transfer equation for RCS and treating vacuum-resonance-induced mode conversion as an input, we employ a first-order approximation in RCS optical depth to evaluate the effect of different magnetospheric plasma density (which depends on magnetic twist), drift velocity and temperature, and viewing geometry on the observed radiation. Our analysis reveals that magnetic twist and plasma drift velocity are the critical parameters controlling the impact of RCS on both the absolute polarization degree and its variation across the soft X-ray spectrum. We find that sufficiently strong RCS can wash out the PA swing caused by vacuum resonance. Furthermore, in addition to the QED vacuum resonance effect, significant relativistic signatures arising from plasma drift velocity ($\beta_0 \gtrsim 0.5$) may introduce an extra $90^\circ$ PA swing in the spectrum. Our calculation framework, based on single-scattering approximation, bypasses the need for complex, multi-dimensional Monte Carlo simulations, providing an analytical pathway for modeling full-surface emission and rotational-phase-resolved radiation from magnetic neutron stars, in support of current and future X-ray polarization missions.
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Astrophysics of Galaxies

Pre-Virialized Assembly at Cosmic Dawn: The Dynamics and Extreme Ionization of Compact Group CGG-z7 at $z\sim7.04$

Xiaoyang Wei, Zheng Cai, Fujiang Yu et al. (+2 more)

We report the discovery of CGG-z7, the most compact galaxy group at $z\gtrsim7$ identified to the north of the GOODS-North field, observed by the JWST POPPIES program. The system consists of at least six members within a projected size of $7.8\times5.7$ kpc$^2$, four of which are spectroscopically confirmed via [O III] and H$\beta$ emission. The group exhibits a low line-of-sight velocity dispersion ($\approx93.7$ km s$^{-1}$) relative to its substantial stellar mass ($M_* \approx 10^{9.8} M_{\odot}$), yielding a stellar-to-dynamical mass ratio of $M_*/M_{\mathrm{vir}} \approx 0.15$. This ratio, exceeding typical values for virialized halos by a factor of $3$, indicates that the system is highly likely not in dynamical equilibrium. Instead, we interpret CGG-z7 as a pre-virialized structure, likely a major merger caught near apocenter – capturing the rapid, chaotic formation of a massive "Red Nugget". Spectroscopic analysis reveals extreme ionization conditions and low metallicity across the group. In particular, the central galaxy reaches an extraordinarily high [O III]/H$\beta$ ratio of $\sim18$, which is likely indicative of an obscured AGN. CGG-z7 thus serves as a unique laboratory for the physics of pre-virialized galaxy assembly, bridging the gap between turbulent high-$z$ assembly and the quiescent galaxies seen at cosmic noon.
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All authors Xiaoyang Wei, Zheng Cai, Fujiang Yu, Mingyu Li, Yunjing Wu

Astrophysics of Galaxies

Identifying Red Supergiants in the Local Group Using JWST Photometry. I. NGC 6822, Sextans A, NGC 300, WLM, and IC 1613

Zhiwen Li, Ming Yang, Biwei Jiang et al. (+1 more)

Red supergiants (RSGs) are crucial for studying the properties and evolution of massive stars. It is representative to conduct a census of RSGs across the Local Group, which spans a broad metallicity range. However, identifying RSGs in distant and metal-poor galaxies remains challenging mainly due to contamination of foreground dwarfs and observational limitations. In this work, we perform PSF photometry on publicly released JWST/NIRCam images of five Local Group galaxies: NGC 6822, Sextans~A, NGC 300, WLM, and IC 1613 using the DOLPHOT NIRCam module. We find an optimal color-color diagram (CCD) for metal-poor environments, that is F115W $-$ F200W versus F356W $-$ F444W, which clearly separates RSGs from foreground dwarfs. By using the CCD, we identify 208, 135, and 22 RSG candidates in NGC 6822, Sextans A, and NGC 300, respectively, free from contamination by foreground dwarfs and oxygen-rich asymptotic giant branch stars (O-AGBs). In addition, 40 and 14 RSG candidates are directly selected on the CMD in WLM and IC 1613, respectively. Compared with previous works, the number of RSG candidates within the same luminosity range and sky region increases significantly, demonstrating the advantages of JWST in constructing a more complete RSG sample in the Local Group thanks to its high spatial resolution and photometric quality. In addition, catalogs of O-AGBs and carbon-rich AGBs (C-AGBs) are provided as by-products.
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All authors Zhiwen Li, Ming Yang, Biwei Jiang, Yi Ren

Astrophysics of Galaxies

Measuring the evolution of stellar bars with the host galaxy's spin

Robin Joshi, Scott M. Croom, Stefania Barsanti et al. (+3 more)

We examine to what extent the galaxy spin parameter proxy ($\lambda_R$) is affected by bar formation and how it is related to the strong and weak classifications of stellar bars. By creating mock observations of a simulated galaxy, we show that the emergence of a stellar bar can cause mass-weighted $\lambda_R$ to decrease by up to 16%, depending on the bar's orientation. This decrease can be exaggerated if there is a burst of star formation due to the bar driving gas to the center of the galaxy. We use the SAMI galaxy survey to show that weakly barred galaxies have statistically significant younger average stellar populations, higher galaxy spin proxy and higher specific star formation rates compared to strongly barred galaxies within one effective radius. If we consider galaxies with average light-weighted stellar population age less than 3 Gyr within one effective radius, we still find weakly barred galaxies to have a higher galaxy spin proxy than strongly barred galaxies. Based on these trends found from the SAMI galaxy survey, we suggest weakly barred galaxies are rapidly forming, similar to the bar formation process seen in simulations, while strongly barred galaxies are undergoing slower (secular) evolution.
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All authors Robin Joshi, Scott M. Croom, Stefania Barsanti, Elizabeth J. Iles, Joss Bland-Hawthorn, Jesse van de Sande

Earth and Planetary Astrophysics · Cross-listed from physics.plasm-ph

MMS Observations of Kinetic Alfvén Wave Turbulence and Steep Kinetic-Range Spectra in the Outer Plasma Sheet Boundary Layer

Mani K Chettri, Rupak Mukherjee and Hemam D. Singh

Energy dissipation and particle acceleration in the collisionless magnetotail plasma remain incompletely understood. We report observations of Kinetic Alfvén Wave (KAW) turbulence and parallel electric fields ($E_{\parallel}$) in the outer Plasma Sheet Boundary Layer (PSBL) using high-resolution burst-mode data from the Magnetospheric Multiscale (MMS) mission. For a crossing on May 31, 2017, we identify broadband KAW turbulence characterized by a normalized electric-to-magnetic field ratio $\mathcal{R}=|\delta E_{\perp}|/(v_A|\delta B_{\perp}|)=2.5\pm1.2$ exceeding the MHD limit, a spectral break near ion scales, a steep kinetic-range spectral slope ($\alpha=-3.48\pm0.13$), and low magnetic compressibility ($C_{\parallel}\approx0.03$). We observe impulsive $E_{\parallel}$ structures up to 15 mV/m and ion density fluctuations up to 68% during enhanced wave activity. The steep spectrum, compared with undamped KAW predictions ($-7/3$ to $-8/3$), is consistent with substantial energy removal at kinetic scales. The near-zero correlation between the KAW-band filtered $E_{\parallel}$ waveform (0.18 to 2.0 Hz) and ion density fluctuations ($r\approx0.04$) indicates no simple linear, zero-lag correspondence under the adopted processing. Together with bursty energy-conversion proxies, the observations are consistent with localized field-particle energy exchange and collisionless damping of KAW turbulence in the intermediate-beta outer PSBL. A specific damping channel, such as Landau damping, is not uniquely constrained by the present diagnostics.
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Astrophysics of Galaxies

K-DRIFT Science Theme: New Theoretical Framework Using the Galaxy Replacement Technique for LSB studies

Kyungwon Chun, Jihye Shin, Rory Smith et al. (+13 more)

Low-surface-brightness (LSB) structures provide critical insights into the hierarchical formation of galaxies and galaxy clusters. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is designed to detect such diffuse features through deep, wide-field optical imaging with a surface brightness reaching $\sim$$30~\rm{mag}~\rm{arcsec}^{-2}$. To interpret the observation data expected from K-DRIFT, we have developed the Galaxy Replacement Technique (GRT), an $N$-body simulation framework optimized for tracing the gravitational evolution of stellar components. The GRT works by inserting high-resolution galaxy models, including a dark matter (DM) halo and stellar disk, in place of multiple low-resolution DM halos in the base $N$-body cosmological simulation. It allows us to achieve very high mass ($m_{star}=5.4\times10^4\msun\ h^{-1}$) and spatial resolution (10~$\rm{pc}~h^{-1}$) with shorter computation time compared to full hydrodynamic cosmological simulations. Therefore, this technique is particularly well-suited for studying LSB structures, with a surface brightness reaching $\sim$$31~\rm{mag}~\rm{arcsec}^{-2}$. In this paper, we present the motivation and methodology of the GRT, summarize key results from previous studies, and highlight its synergy with K-DRIFT observations. We further discuss planned science cases using the GRT, aiming to build a theoretical basis for interpreting LSB features in various environments.
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All authors Kyungwon Chun, Jihye Shin, Rory Smith, Jongwan Ko, jaewon Yoo, So-Myoung Park, Woowon Byun, Sang-Hyun Chun, Sungryong Hong, Hyowon Kim, Jae-Woo Kim, Jaehyun Lee, Hong Soo Park, Jinsu Rhee, Kwang-Il Seon, Yongmin Yoon

Instrumentation and Methods for Astrophysics

Structural Design and Performance Analysis of Laser Transmitting Telescope for Space Gravitational Wave Detection

Long Yongtao, Mo Yan, Cao Shengyi et al. (+8 more)

The spaceborne laser emission telescope is a core and critical component of the space gravitational wave detection system.Compared with ground-based telescopes, the on-orbit space environment is more complex and harsh, presenting higher technical challenges for the design of the optical system and structure - both optical design and structural design face considerable difficulties. To meet the requirements of space gravitational wave detection, this paper designs a laser emission telescope based on an off-axis four-mirror configuration, with a capture field of view of 300{\mu}rad, an optical transmission efficiency of 86.3%, and an optical path stability index of TTL<0.025 nm/{\mu}rad. During the design process, based on existing theories and engineering experience, the primary mirror thickness optimization and lightweight structural design were completed, and a flexible support scheme was adopted to achieve a primary mirror surface figure accuracy of 9.42 nm; the total mass of the entire telescope (excluding mirrors) is only 3.845 kg. Multi-dimensional finite element analysis was conducted on the telescope under actual working conditions: the strength of the telescope's support materials was verified under self-weight and 10G gravity loads; after removing the rigid body displacement of the mirrors using Zernike polynomials, the surface deformation of the primary mirror was controlled within 1/30 wavelength. In the thermal stability analysis, the structural deformation of the telescope under a temperature change of 100 degree celsius was simulated, and key indicators such as eccentricity and tilt between the mirrors all meet the optical design requirements. In the modal analysis, the first-order natural frequency of the telescope reaches 200 Hz under both self-weight and weightless conditions, demonstrating excellent dynamic stability.
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All authors Long Yongtao, Mo Yan, Cao Shengyi, Cao Jiamin, Zhao Lujia, Wang Haibo, Wang Shuangbao, Tan Hao, Liu Xiaohong, Wang Dawei, Ma Donglin

Solar and Stellar Astrophysics

Fine Structure and Formation Mechanism of a Sunspot Bipolar Light Bridge in NOAA AR 13663

Fangfang Qiao, Hao Li, Jiasheng Wang et al. (+3 more)

Bipolar Light Bridges (BLBs) are bright regions located between sunspot umbrae of opposite magnetic polarity. They are typically characterized by strong magnetic fields and intense flows, which are believed to be closely associated with major solar flares. Despite their importance, their fine structure, formation and evolution remain poorly understood. In this work, we analyze the observations of a well-defined BLB obtained by the Goode Solar Telescope at the Big Bear Solar Observatory and the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The high-resolution GST observations reveal that the BLB is composed of fine, penumbral filament-like structures with widths of approximately 100-150 km. The corresponding Doppler velocity maps present a stable pattern of spatially adjacent red- and blueshifted patches within the BLB throughout the 5.5-hour GST observation. HMI observations show that the BLB arises from the converging and shearing motions of sunspots with opposite polarities. Penumbral regions originating from different polarities gradually evolve and interact, eventually forming the BLB. The observed Doppler velocity pattern, characterized by red- and blueshifted patches, can be interpreted as a projection effect of the Evershed flow within the penumbrae. Therefore, we argue that the BLB is formed through the compression and stretching of penumbral structures from oppositely polarized sunspots.
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All authors Fangfang Qiao, Hao Li, Jiasheng Wang, Yadan Duan, Zheng Sun, Leping Li

High Energy Astrophysical Phenomena

Exploring the spin dependence on mass inclination and distance for the newly discovered black hole X-ray binary Swift J151857.0-572147

Yujia Song, James F. Steiner, Tong Zhao et al. (+7 more)

The black hole X-ray binary (BHXRB) Swift J151857.0-572147 was discovered during its first outburst in March 2024. We review the archive of NICER observations from this outburst, focusing on the soft states. We select spectra for which the disk to total flux ratio exceeds 0.8 and the coronal scattering fraction fsc is less than 25%, conditions under which the accretion disk is expected to extend to the innermost stable circular orbit (ISCO) and remains geometrically thin. Through a continuum fitting analysis, we explore the dependence of spin on mass, inclination, and distance. We constrain the spin within the parameter space typical of stellar-mass black holes (sBHs), assuming a mass around 10 solar masses, inclination angles between 20 and 60 degree, and distances between 4 and 16 kpc. For fiducial parameters: a mass of 10 solar masses, a distance of 10 kpc, and an inclination angle of 40 degree, a moderate spin of approximately 0.7 is obtained. However, precise determination of the spin will require accurate measurements of these parameters. Our analysis provides a framework to infer the spin and estimate its uncertainties once more precise measurements of mass, distance, and inclination become available. As we demonstrate, lower inclination angles, greater distances, or larger black hole masses result in higher spin values.
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All authors Yujia Song, James F. Steiner, Tong Zhao, Yuexin Zhang, Ningyue Fan, Ole König, Santiago Ubach, Josephine Wong, Lijun Gou, Javier A. Garcia