Solar and Stellar Astrophysics
Observational Insights into Post-Main-Sequence Rotation and Magnetism: 20 Years of Science, from the Subgiant to the White Dwarf Phase
Emily Hatt
Rotation and magnetism are important phenomena to consider when trying to understand
stars. By shaping the structure and chemical mixing in the interior of
stars, they can induce significant changes in both their fundamental and observable properties. Despite this, our theoretical picture of how rotation and magnetism evolve throughout the
stellar lifecycle remains incomplete. Gaps in our understanding are particularly stark for evolved
stars, where slow rotation rates and weak
magnetic fields make observational studies difficult. In response to this issue, the last 20 years have seen instrumentation to probe magnetism and rotation progressing in leaps and bounds. Through these new tools we have found that evolved
stars rotate in ways that we cannot predict and have
magnetic fields with properties that we did not expect. In the following, I will provide a brief overview of some recent observational results and their implications for our understanding of
stars. I will focus on advances in asteroseismology and high-precision spectropolarimetry, as these techniques have advanced significantly over the last few decades. In keeping with the theme of Cool
Stars, I'll cover
stars with low to intermediate masses and their
evolution from the subgiant to the white
dwarf phase.
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Solar and Stellar Astrophysics
Spectroscopic Binary Detection as Agent-Callable Tools: Detecting 40,000+ Main-Sequence Binary Candidates from SDSS DR19 APOGEE Spectra
Serat M. Saad and Yuan-Sen Ting
Unresolved binaries are common in spectroscopic surveys, and their blended light biases the parameters inferred for them. Methods to detect them exist, but applying one to a new data release is limited mostly by operating know-how that is rarely written down. We package the double-lined spectroscopic binary decomposition of El-Badry et al. (2018b) for reuse on APOGEE spectra, with the executable operations as nine tool servers built on the Model Context Protocol (MCP) and the operating decisions as a Skill. Run over the 238,205 DR19
dwarfs under a fixed driver script, the classifier flags 41,466 SB2 candidates with median mass ratio q = 0.91, about fifteen times the 2,645 identified in DR13, though not at matched purity. The 8.1% control false-positive rate implies that close to 40% are single
stars, so we release the sample as a candidate list. Refitting the individual visits confirms 68.5% of the multiply-visited SB2 and adds 519 single-lined velocity variables and 8,981 orbit-ready systems. We compare the eccentricities of the best-sampled binaries and find no significant difference between the close twins (q > 0.95) and matched non-twins, which excludes an eccentricity excess of the kind measured at wide separations. An ablation of two operating decisions illustrates that a fresh
agent recovers a removed decision only when its absence leaves a measurable trace in the fit. We release the tools, the Skill, and the DR19 catalog with its multi-epoch supplement.
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High Energy Astrophysical Phenomena
Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A
Felix Ryde and Asaf Pe'er
We analyze the
prompt emission of the exceptionally bright GRB 220426A observed by
Fermi/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first $\sim 5$~s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, $r_0$, to be inferred with unusual precision. We find $r_0 \sim \mathrm{few} \times 10^{10}\,\mathrm{cm}$, increasing linearly with time. These findings match the theoretical expectation for the recollimation shock, implying that this GRB shows the first clear evidence for the existence and
evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the
prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest
prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting
supernovae.
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Astrophysics of Galaxies
ATLAS. III. Dust Around Little Red Dots: Hydrogen Line Ratios beyond Dust-free Non-Case B Models
Tomokazu Kiyota, Masami Ouchi, Hiroto Yanagisawa et al. (+7 more)
We investigate broad hydrogen line ratios in little red dots (LRDs) using five high-
redshift ($z>2$) sources from JWST/NIRSpec medium/high-resolution spectra in the DAWN JWST Archive and fifteen low-
redshift sources ($z=0.1$–$0.9$) from the literature, all with broad H$\beta$ detected at $>5\sigma$. After carefully measuring the broad-line fluxes while accounting for absorption features and neighboring emission lines, we find that the broad H$\alpha$/H$\beta$ ratios are very high, ranging from 6 to 30, well above the Case B recombination value. Using plane-parallel Cloudy photoionization models with the total line emission from both sides of the slab, we investigate the physical origin of the broad hydrogen line ratios beyond Case B, jointly modeling the Balmer and Paschen line ratios for the subset of one high-$z$ and two low-$z$ LRDs with detected broad Paschen lines. We find that one low-$z$ LRD is reproduced by a high gas number density ($\log (n_{\mathrm H}/{\rm cm^{-3}})\sim10$–11) on the broad-line H$\alpha$/H$\beta$–Pa$\beta$/Pa$\gamma$ plane, whereas the remaining two LRDs additionally require substantial
dust extinction of $E(B-V)\gtrsim0.2$, even after accounting for non-Case B effects. Since the narrow H$\alpha$/H$\beta$ ratios do not indicate such large
dust extinction, these results demonstrate that the obscuring
dust is spatially associated with the broad-line region. Even without Paschen-line measurements, two and eight LRDs in the high- and low-
redshift samples, respectively, exhibit H$\alpha$/H$\beta>13$, which cannot be reproduced by the non-Case B models, suggesting that
dust obscuration might be common among LRDs, occurring in at least about half of the population. Such
dust may represent a lower-column-density counterpart of the dusty
torus in AGNs, reconciling the weak hot-
dust emission.
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All authors Tomokazu Kiyota, Masami Ouchi, Hiroto Yanagisawa, Makoto Ando, Yuichi Harikane, Yuta Kageura, Minami Nakane, Yurina Nakazato, Yoshiaki Ono, Yui Takeda
Solar and Stellar Astrophysics
Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind
Aakash Gupta, Yogesh, Dibyendu Chakrabarty et al. (+2 more)
The influence of collisional age $(A_c)$ on the alpha-to-
proton temperature ratio $(T_\alpha/T_p)$ has been explored in the past. However, the modulation of this ratio with respect to the
solar cycle has remained unexplored so far. We show
solar-cycle modulation of $T_\alpha/T_p$ and $A_c$ using nearly three decades of in-situ observations from Wind spacecraft across distinct
solar wind speed regimes and
solar activity phases. Our results reveal that in the slow
solar wind with velocity $<400$ km s$^{-1}$, where $A_c$ happens to be typically $>1$, the ratio $T_\alpha/T_p$ stays close to unity. This suggests frequent Coulomb collisions efficiently iron out temperature differences. In contrast, the fast wind with velocity $>500$ km s$^{-1}$, where $A_c$ happens to be typically $<1$, mass-proportional heating is most pronounced, with $T_\alpha/T_p$ often exceeding 4. The intermediate speed regime ($400$-$500$ km s$^{-1}$) represents a gradual transition between the slow and fast wind populations in terms of their
solar-cycle dependence. This behavior reflects the changing dominance of high-speed streams from polar coronal holes during minima to denser slow wind during maxima. These results suggest that mass-proportional ion heating at 1 AU is not solely governed by local collisional physics but is significantly modulated by the
solar cycle dependent variations in the
solar wind sources.
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All authors Aakash Gupta, Yogesh, Dibyendu Chakrabarty, Leon Ofman, Gregory G. Howes
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Radiative Signatures from Warp Drives Traveling Through the Earth's Atmosphere
Shaun David Brocus Fell and Abraham Loeb
We investigate the observable signatures of zero ADM mass warp drive spacetimes traversing Earth's atmosphere. Numerical simulations indicate that an aircraft-scale spacetime bubble moving at relativistic velocities would have a pronounced observational signature, where interaction with the atmosphere can produce luminosities exceeding one terawatt. The signature of a spacetime bubble at rest or moving at low velocity relative to the Earth would not generate such extreme luminosities. These results establish observational constraints on spacetime-based propulsion operating within the terrestrial environment and provide a framework for identifying potential high-velocity signatures. In particular, a warp drive traveling through the atmosphere at speeds exceeding approximately 10% of the speed of light would produce a unique brilliant glow.
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Instrumentation and Methods for Astrophysics
Wavefront Profiling via correlation of GLAO open loop telemetry
Eden A. McEwen, Ryan Dungee, Mark Chun et al. (+3 more)
Adaptive Optics (AO) used in ground based observatories can be strengthened in both design and
algorithms by a more detailed understanding of the atmosphere they seek to correct. Nowhere is this more true than on Maunakea, where a clearer profile of the atmosphere informs AO system development from the small separations of Extreme AO (ExAO) to the wide field Ground Layer AO (GLAO). Employing telemetry obtained from the 'imaka GLAO demonstrator on the University of Hawaii 2.2-meter
telescope, we apply a wind profiling method that identifies turbulent layer velocities through spatial-temporal cross correlations of multiple wavefront sensors (WFSs). We compare the derived layer velocities with nearby wind anemometer data and meteorological model predictions of the upper wind speeds and discuss similarities and differences. The strengths and limitations of this profiling method are evaluated through successful recovery of injected, simulated layers into real telemetry. We detail the profilers' results, including the percentage of data with viable estimates, on four characteristic 'imaka observing runs on open loop telemetry throughout both winter and summer targets. We report on how similar layers are to external measures, the confidence of these results, and the potential for future use of this technique on other multi conjugate AO systems.
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All authors Eden A. McEwen, Ryan Dungee, Mark Chun, Jessica R. Lu, Olivier Lai, Christoph Baranec
Cosmology and Nongalactic Astrophysics
Payaswinee Arvikar, Aseem Paranjape and Saee Dhawalikar
Neutron stars (NS) are one of the indirect detection probes for dark matter (DM). The presence of DM is known to affect the observable properties of NS. Theoretical calculations for various equations of state of a DM admixed NS lead to estimates of the DM mass in a NS of the order of $10^{-2} M_{\odot}$. On the other hand, simplistic estimates of the amount of DM that is accreted on to the NS in the
Solar neighborhood, over its age, suggest that this number is of the order $10^{-14} M_{\odot}$. Various studies have addressed this non-agreement theoretically by explaining the mechanisms leading to higher fraction than expected from smooth spherically symmetric
accretion. In this work, we attempt to assess the role of the dynamic DM environmental density in the
Galactic halo to explain possible enhancement in DM
accretion. We consider a high resolution N-body simulation and method of Voronoi tessellation to calculate local DM density around putative NS locations in a statistically representative sample of Milky Way analogues. We infer that the dynamics of substructure may enhance the DM mass in NS by about a factor 2 as compared to the baseline, spherically symmetric expectation. Environmental effects therefore cannot explain the orders of magnitude discrepancy between equation of state and
accretion based estimates of DM admixed in NS.
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Solar and Stellar Astrophysics
Bernhard-1: An Eccentric Binary Periodically Obscured by its Misaligned Circumbinary Disk
Zhecheng Hu, Wei Zhu, Ping Chen et al. (+2 more)
Bernhard-1 is a proposed KH 15D-like circumbinary disk occultation (CBO) system, but its binary nature and disk geometry have not previously been confirmed. We present new optical and near-infrared spectroscopy together with multi-band photometric monitoring of the system. The radial velocities confirm that Bernhard-1 hosts a highly eccentric binary with $e = 0.80 \pm 0.09$, confirming that the periodic photometric variability arises from occultation by a misaligned circumbinary disk. Joint modeling of the spectra and phase-dependent spectral energy distributions yields pre-main-sequence components with masses of $\sim 1.1\,M_\odot$ and $\sim 0.8\,M_\odot$. Combining
stellar isochrones with the measured lithium abundance yields a system age of $\sim$ 10 Myr. Together with the spatial, astrometric, and
metallicity properties of Bernhard-1, this suggests that Bernhard-1 is probably a member of the open cluster Dolidze 42. By combining the RV orbit with a semi-transparent occultation-screen model, we infer a disk–binary mutual inclination of roughly $50^\circ$ or $130^\circ$, with the degeneracy arising from the unknown disk rotation direction. This
geometric method can be applied to any CBO system once radial velocity monitoring yields an orbital solution. The new light curves deviate from earlier model predictions, consistent with ongoing disk precession, while the phase-dependent H$\alpha$ profiles indicate pulsed
accretion near periastron. Bernhard-1 therefore joins KH 15D and Bernhard-2 as a rare spectroscopically confirmed CBO system.
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All authors Zhecheng Hu, Wei Zhu, Ping Chen, Richard Post, Weicheng Zang
Instrumentation and Methods for Astrophysics
Application of continuous corrections in widefield imaging obtained with the IDGCAL calibration method
Sebastiaan van der Tol
Image Domain Gridding (IDG) is an efficient method to evaluate A-projection, a method for correcting for direction dependent effects (DDEs) in
radio astronomical imaging. The corrections i.e. the A-terms need to be estimated from the observed data through calibration. IDG-CAL is a calibration
algorithm that employs the IDG
algorithm in an iterative fashion to estimate A-terms directly. Initial results for low order A-terms are promising, however for deeper images higher order A-terms are needed. Here two improvements to IDG-CAL are proposed to 1) reduce the computational cost of A-terms described by more parameters and 2) optimize the set of basis functions describing the A-terms based on a
stochastic model of the gain variations and the model image.
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Solar and Stellar Astrophysics
Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars
Arijit Maiti, Ruchi Pandey, Sube Singh Gurjar et al. (+1 more)
Measuring polarization and its temporal variability in the continuum and across emission features provides a powerful probe of the small-scale circumstellar environments of astrophysical sources, on spatial scales otherwise inaccessible to direct imaging techniques. However, the
photon-hungry nature of spectro-polarimetry has significantly limited the availability of such
datasets in the literature. Here we present the results of a multi-epoch spectro-polarimetric survey of a sample of symbiotic and red
giant stars. These observations were first conducted during the performance verification phase of a recently developed medium-resolution echelle spectro-polarimeter, named ProtoPol, mounted on the Physical Research Laboratory (PRL) 2.5m
telescope, Mt Abu, India. Considering the scarcity of such observing samples in the literature, the targets were re-observed over a period exceeding 26 months, from March 2024 to May 2026, in order to explore the time variability of their polarimetric measurements. Our sample comprises 6 symbiotic
stars and 18 red
giant stars. Our observations found that, unlike the Raman-scattered $\lambda \lambda$ 6830, 7088 angstrom features, the H$\alpha$ emission in most of the symbiotics did not exhibit any notable polarization signature. However, the continuum polarization of most symbiotics and red
giants in the sample showed significant variation between observation epochs. The observations presented here constitute one of the rare multi-epoch spectro-polarimetric
datasets spanning more than two years and should be of considerable interest to the broader astronomical community. This paper is Part-II of a two-part series of sample studies; corresponding results for Herbig Ae/Be and classical Be
stars are presented in Part-I.
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All authors Arijit Maiti, Ruchi Pandey, Sube Singh Gurjar, Mudit K. Srivastava
Astrophysics of Galaxies
SIRIUS Project: Dynamical Evolution of Primordial Binaries during Star Cluster Formation
Naoto Harada, Michiko S. Fujii, Takayuki R. Saitoh et al. (+1 more)
Binary populations are closely linked to the
star formation process; however, their
primordial properties can be changed by subsequent dynamical interactions within their natal clusters. The aim of this study is to clarify how different
primordial binary populations affect the
evolution of multiplicity and the global structure of forming
star clusters. We investigate the dynamical
evolution of
primordial binaries during
star cluster formation using self-consistent Nbody/smoothed particle hydrodynamics simulations that follow the collapse of a molecular cloud to a
star cluster. We systematically compare three
star formation models: close binary (CB), wide binary (WB), and single
star (SS) formation model. In CB and WB models, the multiplicity fraction decreases with time due to dynamical interactions. In particular, the fraction in the WB model drops to a level comparable to that in the SS model. The multiplicity fraction of high-mass
stars is similarly high in all models, whereas only the CB model shows a relatively high fraction for low-mass
stars. Due to the assumption of equal-mass binary formation, the CB and WB models exhibit an excess at q= 1 in the mass-ratio distribution, while the SS model has no clear trend. Frequent few-body interactions generate distinct
stellar populations inside and outside the cluster: the multiplicity fraction within the cluster is systematically higher, while mass functions in the outside have a shallower slope. Finally,
stellar density profiles in the clusters are broadly similar among all models. The
primordial binary population significantly affects the final binary properties, while having only a limited impact on their host cluster structures. Our results suggest that close binaries need to form at the
star formation stage to reproduce the observed multiplicity fraction of low-mass
stars and the excess of equal-mass binaries.
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All authors Naoto Harada, Michiko S. Fujii, Takayuki R. Saitoh, Yutaka Hirai
High Energy Astrophysical Phenomena
Synchrotron-Regulated Relativistic Magnetohydrodynamic Turbulence: Emission, Polarization, and Faraday Rotation
Xiaochen Sun, Luca Comisso, Lorenzo Sironi et al. (+2 more)
Relativistic magnetized
plasmas in many high-energy astrophysical systems are both turbulent and strongly radiative, yet their nonlinear dynamics and radiative outcomes remain poorly understood. Here we present results from three-dimensional driven
turbulence simulations in relativistic magnetohydrodynamics with synchrotron cooling. We compute Faraday rotation measures, synthetic synchrotron spectra and linear polarization maps from the simulated
turbulence. The balance between energy injection from turbulent driving and synchrotron cooling keeps the
plasma, on average, relativistically hot, thereby influencing the rotation measure. Synchrotron cooling triggers the thermal instability and drives the
plasma into hot dilute and cold dense phases, which enhances the spatial and temporal variability of synchrotron emission, especially at high frequencies. These diagnostics show qualitative similarities to observations of fast
radio bursts,
pulsar wind nebulae, and blazars, suggesting that
turbulence may play an important role in shaping emission and propagation effects around high-energy sources.
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All authors Xiaochen Sun, Luca Comisso, Lorenzo Sironi, Anatoly Spitkovsky, Alexander Philippov
Solar and Stellar Astrophysics
Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be stars
Arijit Maiti and Mudit K. Srivastava
Polarization signatures across emission line features, together with their temporal
evolution, offer a powerful probe of the circumstellar environments of astrophysical sources, on spatial scales otherwise inaccessible to direct imaging techniques. However, the
photon-hungry nature of spectro-polarimetry has significantly limited the availability of such
datasets in the literature. This work reports a multi-epoch spectro-polarimetric monitoring campaign targeting a sample of Herbig Ae/Be and classical Be
stars. The initial observations were obtained during the commissioning and performance-verification phase of ProtoPol, a recently developed medium-resolution echelle spectro-polarimeter mounted on the Physical Research Laboratory (PRL) 2.5m
telescope, Mt Abu, India. Given the limited number of comparable
datasets available for this class of objects, follow-up observations of the same targets were carried out repeatedly over more than 28 months (December 2023-March 2026), enabling an investigation of the temporal behavior of their polarimetric properties. Our sample comprises 11 Herbig Ae/Be
stars and 10 classical Be
stars. Our observations found that, while the H$\alpha$ polarization remained relatively constant for the classical Be
stars, they showed significant variability for most of the Herbig
stars in the sample. The observations presented here constitute one of the rare multi-epoch spectro-polarimetric
datasets spanning more than two years and should be of considerable interest to the broader astronomical community. This paper is Part-I of a two-part series of sample studies; corresponding results for symbiotic and red
giant stars are presented in Part-II.
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High Energy Astrophysical Phenomena
Real-time observations of the transition to the quiescent state in an accreting magnetised neutron star: No propeller required?
Sergey S. Tsygankov, Galina Lipunova, Valery F. Suleimanov et al. (+6 more)
The final stages of outbursts in transient X-ray
pulsars (XRPs), which are characterised by a significant decline in the mass
accretion rate, provide valuable insight into the physics of the
accretion disc and its interaction with the strong
magnetic field of the
neutron star (NS). In particular, the `propeller effect', or centrifugal inhibition of
accretion, has been proposed as a key mechanism governing both the onset luminosity and the timescale of the rapid transition to the quiescent state. In addition, it offers an independent method for estimating the
magnetic field strength of the NS. On the other hand, the decrease in the mass
accretion rate itself is driven by processes occurring in the
accretion flow at larger distances from the NS. Recovering the information encoded in the light curve therefore requires sensitive high-cadence X-ray monitoring capable of capturing the rapid and often unpredictable transition from the accreting regime to the quiescent regime. In this study, we present the results of the first comprehensive monitoring campaign that tracks the entire transition to quiescence in the transient XRP 4U 0115+63 utilising observations by the NICER X-ray
telescope. We show that the observed behaviour can be explained by the thermal-viscous disc instability model (DIM), with the emission observed immediately after an outburst possibly arising from the ongoing
accretion from the recombined (`cold') disc and the subsequent quiescent emission being produced by the cooling NS. We further applied this model to a larger sample of XRPs encompassing a broad range of physical parameters. Ultimately, our findings indicate that the temporal behaviour of XRPs, including the quiescent state, can be consistently explained within the DIM framework without requiring the propeller effect as the primary mechanism governing the observed transition.
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All authors Sergey S. Tsygankov, Galina Lipunova, Valery F. Suleimanov, Alexander Salganik, Alexander A. Mushtukov, Sofia V. Forsblom, Andrey S. Tavleev, Aleksei V. Kuzin, Juri Poutanen
Astrophysics of Galaxies
A Wandering 35,000-Solar-Mass Black Hole Fed by a Gravitational Wake
Xin Li, Yong Shi, Fuyan Bian et al. (+8 more)
Intermediate-mass black holes are widely considered to be the seeds of supermassive black holes, a substantial population of which is expected to remain displaced from
galactic nuclei owing to hierarchical
galaxy assembly and inefficient dynamical friction. While several fueling channels can sustain central black holes, those pathways are largely inaccessible to off-
nuclear black holes, leaving their fuel supply uncertain. As these wandering black holes move through the interstellar medium of their host
galaxies, theory predicts that they can capture gas from the dense wake produced by gravitational focusing. However, direct observational evidence for this process has remained elusive. Here we report evidence for a wandering intermediate-mass black hole of 35,000
solar mass accreting through such a gravitational wake. Its black-hole nature is supported by broad-line emission, a compact continuum counterpart, long-term optical variability, and a power-law-like spectral energy distribution. Multi-epoch spectroscopy reveals three distinct gas components: a blueshifted, low-density upstream flow; a redshifted, dense downstream wake; and optically thick absorbers well within the capture radius that drive rapid changing-look variability in the broad-line emission. This discovery establishes a previously unobserved channel for the growth of wandering intermediate-mass black holes.
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All authors Xin Li, Yong Shi, Fuyan Bian, Junfeng Wang, Shude Mao, Qiusheng Gu, Yifei Jin, Yanmei Chen, Zhiyuan Zheng, Qinwei Yuan, Xiaoling Yu
Earth and Planetary Astrophysics
Toward a unified framework for helium observations and interpretation of atmospheric escape
Carteret Yann and Bourrier Vincent
Atmospheric escape is considered a key process in shaping
exoplanet demographics and
evolution. The near-infrared metastable helium triplet (HeI at ~10833Å) is one of the most powerful tracers of hydrodynamical
outflows. In recent years, a large variety of instruments have been used to detect and characterize expanding upper atmospheres. High-resolution (HR) spectrographs, which can spectrally resolve the lines of the triplet, have been mostly used to probe the dynamics of atmospheric escape. More recently, JWST (low-resolution, LR) detected extended
outflows that were missed by previous ground-based observations due to night-length constraints. Since the observed transmission spectrum is computed from the ratio of
stellar spectra degraded by instrumental convolution, we demonstrate that directly convolving a theoretical transmission spectrum with the instrument's profile leads to inference biases as resolution decreases. This conclusion is particularly important for atmospheric escape studies but also holds in the more general context of LR atmospheric
retrievals. Following the proper comparison methodology, we then investigated the complementarity of HR and LR observations for atmospheric escape measurements, comparing the capability of three instruments (NIRPS, JWST/NIRSpec, and JWST/NIRISS) to detect and characterize
outflows. We find that NIRPS and JWST/NIRISS are mostly sensitive to the same helium signatures, while JWST/NIRSpec improves the detection limit of excess absorption for faint targets. Studying different
outflow configurations, we show that LR space-based measurements cannot be used to infer the dynamics of the upper atmosphere close to the
planet, and are best employed to measure the spatial extent of outflowing tails. We thus highlight the complementarity between HR and LR observations of atmospheric escape, the latter improving baseline reconstruction.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Baryogenesis and CMB spectral distortion from Axions
Zhenhao Zhang, Mingqiu Li and Sichun Sun
We discuss a mechanism for generating the
baryon asymmetry in the early universe. We show that an
axion-like particle can modify the related gauge field configurations in the Standard Model, thereby altering their dispersion relations. This change in the Chern-Simons number can source a violation of
baryon number. We derive the relationship between the resulting
baryon number and the
evolution of the
axion background. We estimate the
baryon asymmetry produced via this mechanism and show that the observed value can be naturally achieved. We also show that
axion photon coupling produces
Cosmic Microwave Background spectral distortion. Our results show that the resulting distortion approaches a constant at low frequencies, unlike the conventional y-type and $\mu$-type distortions.
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High Energy Astrophysical Phenomena
Transmutation Timescales for Dark Matter Induced Collapse of Compact Stars into Black Holes
H. A. Adarsha, Chandrachur Chakraborty and Sudip Bhattacharyya
Ultra-heavy asymmetric dark matter (DM) particles captured by compact
stars can thermalize, self-gravitate, and collapse to form an endoparasitic black hole (EBH), whose subsequent growth may transmute the host
star into a black hole. The continued existence of old millisecond
pulsars (MSPs) and white
dwarfs (WDs) thus places powerful constraints on the DM particle mass $m_\chi$ and the DM-nucleon scattering cross-section $\sigma_{\rm n\chi}$. We derive an analytical expression for the transmutation timescale by solving the EBH growth equation, consistently accounting for Bondi
accretion of
stellar matter, Hawking evaporation, and sustained DM feeding of the EBH in a steady-state capture regime. We also incorporate
quantum effects in baryonic
accretion when the hydrodynamic description breaks down, providing a unified treatment of EBH growth across both particle and fluid regimes. Adopting a physically transparent collapse criterion for fermionic and bosonic asymmetric DM, we compute EBH transmutation timescales for representative MSPs and WDs in environments with different DM densities. Although the physical ingredients are broadly similar to previous studies, this work derives updated constraints through a closed-form analytical treatment of EBH growth and an adopted prescription for the EBH formation timescale, yielding a lower critical EBH mass for sustained growth and revised transmutation timescales. Requiring the transmutation time to exceed $\sim 1$ Gyr for MSPs and $\sim 10$ Gyr for WDs, we derive revised constraints on $\sigma_{\rm n\chi}$ over $m_\chi \sim 10^{6}$–$10^{14}$ GeV, and show that EBHs with initial masses as small as $\sim 4\times10^{4}$ kg can undergo sustained growth, extending the region of DM parameter space probed by compact
stars.
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Astrophysics of Galaxies
Mapping the Inner Milky Way with Infrared-Derived Distances to AGB Stars
Rajorshi Bhattacharya, Subhajit Kar, Loránt O. Sjouwerman et al. (+1 more)
The structure and
evolution of the Milky Way (MW) can be traced with distance estimates to the evolved
stellar populations in the inner
galactic region. However, direct astrometric distances remain unavailable or highly uncertain for the majority of these sources due to instrumental limitations, large angular diameters, and complex variability. In this work, we develop a supervised machine-learning model to estimate statistical distances to oxygen-rich asymptotic
giant branch (AGB)
stars selected from the AKARI mid-infrared survey. We build an XGBoost regression model that maps multi-band IR photometry to distance using a training set of AGB
stars with previously derived SED-based distances achieving a mean absolute percentage error (MAPE) of 6% on an independent test set. Distance estimates for over 36,000 AGB sources are obtained within a 36% total error margin, greatly expanding distance coverage (0.5-20 kpc) for
dust-obscured AGB populations in the
Galactic plane. We find good agreement with reported distances for
Galactic Mira variables and independent period-luminosity relations. Utilizing the expanded distance set, we further investigate the spatial distribution of Mira variables in the
Galactic bulge and disk. Longer-period Miras preferentially trace the bulge's barred morphology compared to their shorter-period counterparts that populate the disk. We also find roughly constant, but differing, relative scale heights for the bulge and disk, with the bulge vertical dispersion about 30% larger. These findings show that IR photometry-derived statistical distances can recover large-scale
Galactic structures and establish long-period Mira variables as efficient tracers of
stellar populations in heavily obscured regions of the MW.
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All authors Rajorshi Bhattacharya, Subhajit Kar, Loránt O. Sjouwerman, Anupam Bhardwaj
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Two-Fluid Schwarzschild Solution
Rico Zöllner and Burkhard Kämpfer
We consider the interior two-fluid Schwarzschild solution. That is a model of compact (
neutron)
stars with admixed dark matter. The two constant energy densities generalize the interior Schwarschild solution to the case of two fluids (representing standard model matter and dark matter, for instance), interacting mutually solely by the common gravitational field. In general, the two-fluid core is surrounded by a one-fluid corona (envelope). Only for special parameters (the model depends on energy densities $e_1,e_2$ and central pressures $p_{\mathrm{c}1},p_{\mathrm{c}2}$ of the fluids), both fluids occupy the same region. Despite the sum of two energy densities in that region, the compactness bound of 8/9 is not exceeded. A conceivable object is one with a stark leakage of one fluid (e.g. dark matter) beyond the two-fluid core.
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Instrumentation and Methods for Astrophysics
Comparing realtime optical gain measurement and methods on MagAO-X
Eden McEwen, Jared R. Males, Olivier Guyon et al. (+20 more)
A lingering technical challenge for pyramid wavefront sensors (PyWFS) is their change in response between calibration and correction residuals, a quantity known as optical gain (OG). Given the prevalent use of PyWFSs in current and planned high contrast adaptive optics (AO), understanding and reliably measuring OG for realtime control unlocks advanced correction and post processing techniques. The OG quantity as an unknown inhibits a system's ability to stably correct non common path errors, reconstructing wavefronts, and PSF reconstruction. This work compares kinds of optical gain measurement techniques on MagAO-X, a visible light extreme AO instrument on the 6.5m Magellan Clay
telescope. We present a set of on-sky measurements of OG across three techniques: 1) An on-sky calibration that acquires OG per spatial mode, 2) realtime measurements of the instantaneous Strehl Ratio (SR) on the pyramid tip, and 3) realtime measurement of known, high-frequency probe signal on the WFS itself. We compare these on-sky results with performance diagnostics to asses how faithfully OG is returned. We conclude with future steps for active control of OG on MagAO-X.
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All authors Eden McEwen, Jared R. Males, Olivier Guyon, Sebastiaan Y. Haffert, Vincent Deo, Joseph D. Long, Logan A. Pearce, Laird M. Close, Warren B. Foster, Kyle Van Gorkom, Alexander D. Hedglen, Parker Johnson, Maggie Y. Kautz, Jay K. Kueny, Jialin Li, Joshua Liberman, Miles Lucas, Jennifer Lumbres, Avalon L. McLeod, Elena Tonucci, Katie Twitchell, Lauren Schatz, Alycia J. Weinberger
High Energy Astrophysical Phenomena
Electromagnetic responses driven by gravitational-wave memory in magnetar-flare outflows
Wen-Biao Han
An asymmetric relativistic
outflow from a magnetar
giant flare produces a step-like gravitational-wave (GW)
memory. In the magnetized pair
plasma surrounding the
star, such GW
memory drives transverse electromagnetic (EM) X-mode/fast-magnetosonic responses at the corresponding frequencies. We use this GW-
memory spectrum to drive a global wave equation for a cold electron–positron pair
plasma. With radiative boundary conditions excluding incoming waves, the driven response forms an outgoing EM mode that reaches the outer boundary of the source region. For central-engine energy-release timescales of $10$, $1$, and $0.15\,\mu\mathrm{s}$, 90% of the integrated response energy lies below 23.3 kHz, 233 kHz, and 1.51 MHz, respectively. The principal response occupies the 10 kHz–1 MHz frequency band; a substantial extension into the 1–3 MHz frequency band also appears when a submicrosecond asymmetric-energy component is present. The electromagnetic response can escape from the source region and potentially propagate in the interstellar medium.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
The generalized second law as a thermodynamic selection criterion for dynamical dark energy
Marcos H. Cruz, Miguel Cruz, Joel Saavedra et al. (+1 more)
The growing number of generalized horizon
entropy proposals has led to a wide variety of modified cosmological models, yet there is currently no general physical principle capable of discriminating among them. We show at the level of an homogeneous background that the generalized second law (GSL) of thermodynamics provides such a criterion. Considering a general modified Friedmann framework described by an arbitrary function $f(H)$ of the Hubble parameter, and allowing the apparent horizon and the
cosmic fluid to evolve out of thermal
equilibrium, we derive model-independent constraints on the asymptotic scaling of the horizon
entropy, $S_A\propto A^k$. We find that phantom
evolution requires $k\ge(3\omega-1)/(2\omega)$, with $\omega$ the total fluid equation of state, whereas quintessence imposes the complementary upper bound. Additionally, in the exact thermal
equilibrium limit, the dynamical coupling strictly enforces $k \le 2$, recovering the non-
equilibrium quintessence bound. The two bounds converge to the unique value $k=2$ as the phantom divide is approached, indicating that a smooth crossing of the phantom divide is thermodynamically associated with a quadratic
entropy-area scaling, where the effective theory degenerates into a logarithmic
gravity framework. Applying this criterion to representative generalized
entropy models shows that many commonly used proposals are constrained by the generalized second law, whereas multiparameter constructions are naturally compatible with the required asymptotic behavior. Finally, we demonstrate that the same thermodynamic selection principle extends to derivative-dependent cosmologies described by $f(H,\dot H)$, highlighting its robustness beyond
entropy functionals that depend only on the horizon area.
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All authors Marcos H. Cruz, Miguel Cruz, Joel Saavedra, Emmanuel N. Saridakis
High Energy Astrophysical Phenomena
Two new highly scattered fast radio bursts: evidence for scatter broadening by the circumsource medium
Joscha N. Jahns-Schindler, Keith W. Bannister, Adam T. Deller et al. (+17 more)
We found two highly scattered Fast
Radio Bursts (FRBs) during commissioning of the Commensal Realtime ASKAP Fast Transient COherent (CRACO) backend. FRB 240210D and FRB 240312D have scattering times of $34\pm6$ and $300\pm48$ ms, respectively, when scaled to 1 GHz. FRB 240312D originates near a spiral arm of a face-on
galaxy at a
redshift of only 0.05. Scintillation from a Milky Way screen constrains the distance of the scattering screen to $\sim 10$ pc from the source. FRB 240312D is therefore the first highly scattered FRB where scattering screens in the host
galaxy centre, a background
galaxy, or intervening structures can all be excluded, leaving only the circumsource medium. Integral field spectroscopy of the host reveals a Milky Way-like
galaxy with a
star-formation region at the FRB position. We find refractive scattering in a
pulsar wind nebula as the most likely scattering origin. However, the explanation is not completely satisfactory as it requires a fine-tuned orientation. Hence, additional theoretical studies under different FRB progenitor models are needed. From the two FRBs, we calculate a total rate of $R_\mathrm{tot}=210^{+460}_{-180}\,\mathrm{events}\,\mathrm{sky}^{-1}\mathrm{day}^{-1}$ with durations between 55.2 ms and 1 s and above a fluence of 9 Jy ms consistent with the rate of shorter FRBs. This elevated rate suggests that the strong scattering seen in other FRBs likewise does not arise from chance-aligned sightlines, but is instead causally linked to the FRB sources.
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All authors Joscha N. Jahns-Schindler, Keith W. Bannister, Adam T. Deller, Xinping Deng, Marcin Glowacki, Alexa C. Gordon, Vivek Gupta, Akhil Jaini, Clancy W. James, Ilya S. Khrykin, Yu Wing Joshua Lee, J. Xavier Prochaska, Hao Qiu, Hugh Roxburgh, Stuart D. Ryder, Ryan M. Shannon, Tim Sprenger, Nicolas Tejos, Yuanming Wang, Ziteng Wang
High Energy Astrophysical Phenomena
JWST Spectroscopy of Type Ia Supernova 2025rbs from Maximum Light to the Nebular Phase
Lindsey A. Kwok, Stéphane Blondin, Adam A. Miller et al. (+53 more)
We present JWST observations of the Type Ia
supernova (SN Ia) 2025rbs ($D=$14.5 Mpc) at +1, +23, and +84 days after B-band maximum, spanning peak light through a wavelength-dependent transition toward the nebular phase. Combined with ground-based optical and near-infrared (NIR) data, our panchromatic spectra (0.4-14 $\mu$m) include the first maximum-light mid-infrared (MIR) spectrum and the earliest MIR spectroscopic sequence of an SN Ia to date. At peak light, the MIR spectrum exhibits a continuum with permitted and forbidden features, including Si II, Ni II, and early-emerging [Ni III-IV] and [Ar II-III]. By +23 days the MIR is dominated by forbidden lines with a weak continuum, and by +84 days it is fully nebular, whereas the optical/NIR spectra remain transitional. The nebular spectrum reveals strongly stratified ejecta, with stable Ni concentrated at the lowest velocities, radioactive Co at intermediate velocities but absent within ~2000 km s$^{-1}$, and Ar occupying an outer shell. We detect small-scale substructure in [Ca IV] 3.21 $\mu$m with fractional amplitudes of a few percent and a characteristic velocity scale of ~800 km s$^{-1}$, which may reflect compositional structure, ionization variations, or both. Radiative-transfer calculations substantially underpredict these MIR Mg II features despite approximately reproducing the NIR Mg II 1.0927 $\mu$m line, suggesting that the relative strengths of these transitions are sensitive to the treatment of Mg ionization and excitation. These observations demonstrate that MIR spectroscopy beginning near maximum light simultaneously probes the emerging inner ejecta and rapidly fading outer burning products, providing new constraints for explosion and radiative-transfer models.
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All authors Lindsey A. Kwok, Stéphane Blondin, Adam A. Miller, Saurabh W. Jha, Willem B. Hoogendam, Cameron M. Pfeffer, Eyouel Z. Abate, Jennifer E. Andrews, Moira Andrews, Chris Ashall, Katie Auchettl, K. Azalee Bostroem, Thomas G. Brink, Fionntan P. Callan, Collin T. Christy, Joseph R. Farah, Alexei V. Filippenko, Andreas Flörs, Ryan J. Foley, Eli Gendreau-Distler, Or Graur, Jason T. Hinkle, D. Andrew Howell, David O. Jones, Rinon Kageyama, Miho Kawabata, Cristine Koelln, Conor Larison, Chang Liu, Keiichi Maeda, Kate Maguire, Curtis McCully, Kyle Medler, Nicolas E. Meza-Retamal, Ann Mina, Rüdiger Pakmor, Riley Patlak, Jeniveve Pearson, Aravind P. Ravi, Nabeel Rehemtulla, Armin Rest, David J. Sand, Huei Sears, Benjamin J. Shappee, Manisha Shrestha, Mridweeka Singh, Tamás Szalai, Kenta Taguchi, Tea Temim, Jacco H. Terwel, Stefano Valenti, József Vinkó, J. Craig Wheeler, Kathryn Wynn, Yi Yang, WeiKang Zheng
Earth and Planetary Astrophysics
PDS 70 c and SR 12 c: Observational Constraints on Giant-Planet and Satellite Formation
Ignacio Mosqueira
PDS~70~c and SR~12~c are the only bound planetary-mass objects with secure cold submillimeter disk detections. Together, these systems constrain
giant-
planet growth and satellite formation. The PDS~70
planets exhibit remarkable parallels to the Jupiter–Saturn pair in our
Solar System. Both PDS~70
planets accrete within one shared gap, which links their final masses, the material reaching each Hill sphere, and the properties of the circumplanetary disk. Planetary torques deplete the finite interplanetary reservoir, causing circumplanetary supply to decline as the protoplanets open a circumstellar gap. SR~12~c separates the planetary-growth and satellite-formation timescales: gas and solids survive even though its current mass-growth timescale is $(1.9\pm0.6)\times10^9$~yr. For PDS~70~c, the 855-$\mu$m flux implies $0.007$–$0.031\,\Mearth$ of
dust at 26~K in the optically thin limit. A fully
dust-dominated, uniform 22–26-K optically thick emitter has an equivalent coplanar radius of $0.58$–$0.66$~au, while a fiducial radial temperature profile yields an equivalent radius of approximately 0.46~au. The continuum constraints overlap the 0.5–1.5~au circularization range estimated by ballistic calculations of late-stage gap-fed inflow. We find that the PDS~70 constraints are consistent with our \SEMM{} satellite-formation model (Mosqueira & Estrada 2003a,b, submitted in 2001). Thus the observations provide strong support for a quiescent, solids-enhanced satellite-forming environment, coupled in the early stages to planetary-gap
evolution.
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High Energy Astrophysical Phenomena
An Optically Motivated Gamma-ray Study of Fermi-LAT Novae
Owen K. Henry, Timothy A.D. Paglione, David Zurek et al. (+1 more)
High-energy (GeV) gamma-ray emission from nova eruptions was a surprise detection by the
Fermi-LAT in 2010. Since then, it has been suggested that the gamma-rays from these systems are generated in collisionless non-relativistic shocks. In this theory, the observed correlation between optical and gamma-ray nova lightcurves naturally arises if a portion of the optical luminosity is reprocessed shock power. In this work, we investigate this scenario by analyzing
Fermi-LAT-detected novae in varied time bins and then correlating the bin size that maximizes the nova's significance (tgam) with the nova's optical eruption data. Furthermore, we investigate whether there is a common optical decay across the sources' measured tgam values in the population. We find that across our population, a nova's $t_3$ (the time it takes the nova V band brightness to decay 3 magnitudes) appears to be the favored analysis bin that optimizes the
Fermi-LAT detection significance, although there is significant spread. Additionally, we report V679 Car, a source previously noted as a marginally detected gamma-ray nova, as a $>5 \sigma$ detection in this work. We perform cross-correlation analysis of gamma-ray and optical lightcurves.
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All authors Owen K. Henry, Timothy A.D. Paglione, David Zurek, Joshua Tan
Cosmology and Nongalactic Astrophysics
Evidence for Enhancement in the Rate of Fast Radio Bursts Toward Galaxy Clusters
Mawson W. Sammons, Airene Ahuja, Matt Dobbs et al. (+25 more)
Massive
galaxy clusters can introduce gravitational
lensing and populations of suppressed
star formation member
galaxies into the line of sight, potentially changing the distribution of observable sources toward them from that of the average sky. As a result, Fast
Radio Bursts (FRBs) aligned with
galaxy clusters can provide a unique window into parameter spaces of the FRB population that other lines of sight do not afford. In this study, we demonstrate that the second CHIME/FRB baseband catalog contains FRBs emitted from within or behind clusters identified from the latest DECaLS data release; we isolate a sample of 26 FRBs where this is likely, including one repeating FRB and two FRBs that intersect the Coma cluster. Comparing our results against a range of simulated FRB populations, we conclude that the number of these associations represents a $3\sigma$ rate enhancement toward
galaxy clusters, constituting $1.4\pm0.4\%$ of the FRBs detected in the second CHIME/FRB baseband catalog. We suggest that this enhancement is caused by an equal proportion of member
galaxies hosting additional FRBs, and gravitational
lensing magnifying background sources. We demonstrate that such contributions are sensitive to alternative progenitor channels and high
redshift evolution in the FRB population, providing a future avenue for constraining these features. Considering the
redshifts and masses of the associated clusters, we identify FRB 20211113A, which is aligned with the strong gravitational lens Abell 2218 as a potential lensed candidate for further consideration.
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All authors Mawson W. Sammons, Airene Ahuja, Matt Dobbs, Zarif Kader, Evan Davies-Velie, Mohit Bhardwaj, Charanjot Brar, Amanda M. Cook, Alice P. Curtin, Bryan M. Gaensler, Affan Khadir, Victoria M. Kaspi, Afrokk Khan, Adam Lanman, Calvin Leung, Lluis Mas-Ribas, Kiyoshi W. Masui, Kyle McGregor, Daniele Michilli, Ayush Pandhi, Aaron B. Pearlman, Ziggy Pleunis, Sachin Pradeep E. T., Aniket Prasad, J. Xavier Prochaska, Paul Scholz, Kaitlyn Shin, Kendrick Smith
Astrophysics of Galaxies
Estimating the local star formation rate density from ASKAP RACS
Sonja Panjkov, O. Ivy Wong. and Rachel L. Webster
Understanding the
evolution of the
cosmic star formation rate density (SFRD) is key to uncovering how the Universe arrived at its present state. This paper presents a novel and efficient method to estimate the local SFRD, which uses supervised machine learning to first identify a population of
star-forming
galaxies (SFGs). Next,
star-formation rates (SFRs) are determined using the 1.4-GHz
radio-continuum emission detected by the Australian Square Kilometre Array Pathfinder (ASKAP). Specifically, a gradient-boosted decision tree model was implemented to classify extragalactic sources from the Beck et al. (2022, MNRAS, 515, 4711) catalogue as either
galaxies or
quasars using RACS-mid and WISE photometry. The full sample, consisting of 389,392 sources, was partitioned into a 70%-15%-15% split for training, validating, and testing. The optimised model achieved a weighted F1 score of 0.93 and an accuracy of 0.94 on the test
dataset, ultimately classifying 336,674 sources as
galaxies and 52,718 sources as
quasars. Using the resulting $z<0.1$ depth-matched
galaxy sample and the photometric
redshift predictions from Beck et al. (2022, MNRAS, 515, 4711), a modified 1.4-GHz SFR calibration was determined, yielding a local, completeness-corrected, $z<0.1$ SFRD of $(1.4 \pm 0.5) \times 10^{-2} \; \rm M_{\odot}\, yr^{-1}\,Mpc^{-3}$ using 11,293 sources. This value is consistent with previous results. Thus, this study demonstrates the feasibility of using supervised learning to identify large populations of SFGs in order to investigate the SFRD
evolution. This presents an exciting prospect for future, deeper surveys such as EMU, which will enable the
cosmic SFRD to be probed out to higher
redshifts.
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