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Physics Astrophysics

astro-ph Watch Astrophysics

Sunday 14 December

Solar and Stellar Astrophysics

HEL1OS – A Hard X-ray Spectrometer on Board Aditya-L1

Anuj Nandi, Manju Sudhakar, Srikar Paavan Tadepalli et al. (+33 more)

HEL1OS (High Energy L1 Orbiting X-ray Spectrometer) is one of the remote sensing payloads on board Aditya-L1 mission designed to continuously monitor and measure the time-resolved spectra of solar flares between 8 keV and 150 keV. This broad energy range has been covered by using compound semiconductor detectors: cadmium telluride (CdTe: 8 - 70 keV) and cadmium zinc telluride (CZT: 20 - 150 keV) with geometric areas of 0.5 cm$^2$ and 32 cm$^2$, respectively. A stainless steel collimator provides a field-of-view of 6$^\circ$ $\times$ 6$^\circ$ optimized to limit the off-axis response while keeping the design within the instrument mass constraints. The in-house designed low-noise digital pulse processing-based front-end electronics has achieved a spectral resolution of $\approx$ 1 keV at 14 keV (CdTe) and $\approx$ 7 keV at 60 keV (CZT). The instrument is also equipped with processing and power electronics to process the signal, drive the electronics, bias the detectors with required low and high voltages for optimal performance of the overall system. In this article, we present design aspects of the instrument, results from the pre-launch ground-based tests, and the in-orbit operations, which have indicated optimal performance in line with that expected.
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All authors Anuj Nandi, Manju Sudhakar, Srikar Paavan Tadepalli, Anand Jain, Brajpal Singh, Reenu Palawat, Ravishankar B. T., Bhuwan Joshi, Monoj Bug, Anurag Tyagi, Sumit Kumar, Mukund Kumar Thakur, Akanksha Baggan, Srikanth T., Arjun Dey, Veeresha D. R., Abhijit Avinash Adoni, Padmanabhan, Vivechana M. S., Evangelin Leeja Justin, James M. P., Kinshuk Gupta, Shalini Maiya P. R., Lakshmi A., Sajjade Faisal Mustafa, Vivek R. Subramanian, Gayatri Malhotra, Shree Niwas Sahu, Murugiah S., Medasani Thejasree, Narayan Rao G. S., T Rethika., Motamarri Srikanth, Ravi A., Nashiket Premlal Parate, Nigar Shaji

Astrophysics of Galaxies

Analysis of Orbital Dynamics of Globular Clusters in the Central Region of the Milky Way

A.T. Bajkova, A.A. Smirnov and V.V. Bobylev

The regularity/chaoticity of orbits of 45 globular clusters in the central region of the Galaxy with a radius of 3.5 kpc, which are subject to the greatest influence of the elongated rotating bar, is analyzed. Various methods of analysis are used, namely, the methods of calculating the maximum characteristic Lyapunov exponents (MCLE), MEGNO (Mean Exponential Growth factor of Nearby Orbits), the Poincaré section method, the frequency method based on calculating fundamental frequencies, and a new method is proposed based on calculating the orbit amplitude spectrum as a function of time and calculating the entropy of the amplitude spectrum as a measure of orbital chaos. Bimodality is found in the histogram of the distribution of positive Lyapunov exponents calculated in the classical version, without renormalizing the shadow orbit, which allows implementing a probabilistic method for GC classification, which is also a new approach. To construct the orbits of globular clusters, we used the gravitational potential model with a bar in the form of a triaxial ellipsoid. The following bar parameters were adopted: mass $10^{10} M_\odot$, length of the semi-major axis 5 kpc, angle of rotation of the bar axis 25$^o$, rotation velocity 40 km s$^{-1}$ kpc$^{-1}$. To form the 6D-phase space required for integrating the orbits, we used the most accurate astrometric data to date from the Gaia satellite (EDR3), as well as new refined average distances to globular clusters. Globular clusters with regular and chaotic dynamics were classified.
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High Energy Astrophysical Phenomena

Probing Periodic and Aperiodic Variability of X-ray Sources in M31, M81 and Centaurus A with Chandra

Jiachang Zhang, Zhiyuan Li, Ziqian Hua et al. (+1 more)

Based on archival Chandra observations, we present a systematic timing survey of several hundred X-ray sources in M31, M81, and Centaurus A, mostly low-mass X-ray binaries (LXMBs), focusing on searching and characterizing aperiodic and periodic variability within single observation. We identify flares in 24 sources in M31, 5 in M81, and 26 in Cen~A; several display recurrent events. Flare durations span from tens of seconds to a few $10^{4}$ s, with peak luminosities of $10^{37}$-$10^{40}\ \mathrm{erg\ s^{-1}}$ and low flare duty cycles of $4.9\times10^{-6}$-$3.5\times10^{-2}$. Dipping events are found in 8 sources in M31, 1 in M81, and 5 in Cen A, including two repeaters. On multi-epoch baselines, the standard deviation of the source luminosity correlates linearly with the mean luminosity, with a coefficient of 0.49 (M31), 0.30 (M81), and 0.67 (Cen A), indicating galaxy-to-galaxy diversity. No statistically significant periodic signals are detected in M81 or Cen A, which, along with several periodic signals previously found among the M31 sources, can be understood considering a joint effect of our detection sensitivity and intrinsic distributions of the orbital period and X-ray luminosity of LMXBs. The ensemble of short-duty-cycle flares, a mix of recurrent and isolated dips, and galaxy-dependent rms–flux factor, supports a picture in which stochastic accretion-rate fluctuations modulate luminosity on $\sim$10-$10^{4}$ s. Conducted at known distances and across distinct host environments, this extragalactic survey provides uniform flare/dip samples and rms-flux scalings for bulge-dominated fields, offering empirical constraints for accretion physics and illustrating the promise of timing analyses in external galaxies using the Chandra archive.
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All authors Jiachang Zhang, Zhiyuan Li, Ziqian Hua, Tong Bao

Cosmology and Nongalactic Astrophysics

Comments on "Little ado about everything" by A. Lapi et al. and on cosmological back-reaction

Julian Adamek

In two papers, A. Lapi et al. introduce and discuss what they call the $\eta$CDM model, a stochastic framework in which they claim that fluctuations in the density field at the scale of tens of Mpc due to structure formation would effectively drive the accelerated expansion of the Universe. They claim that this qualitative behaviour would emerge from the dynamics of standard cold dark matter alone, without introducing any new physics. In this short comment, I argue that such a proposition is implausible. Some of my remarks are relevant more generally to frameworks that try to describe cosmological back-reaction.
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Instrumentation and Methods for Astrophysics

CUBE2: A Parallel $N$-Body Simulation Code for Scalability, Accuracy, and Memory Efficiency

Hao-Ran Yu, Bing-Hang Chen, Kun Xu et al. (+4 more)

$N$-body simulation serves as a critical method for modeling cosmic evolution and poses a significant challenge in high-performance computing. We present CUBE2, an open-source cosmological $N$-body code emphasizing memory efficiency, computational performance, scalability and precision. The core of its algorithm utilizes multi-level Particle-Mesh (PM) method to solve the Poisson equation for matter distribution, leveraging the well-optimized Fast Fourier Transform (FFT) for computational efficiency. Precision is ensured by the optimized Green's function that seamlessly bridges gravitational interactions between multi-level PM and Particle-Particle (PP) calculations. The program design enhances per-core/node efficiency in processing $N$-body particles, while the Information Optimized Storage (IOS) addresses memory constraints for large particle counts. Using CUBE2, we run two cosmological simulations with particle counts of $6144^3$ on the Advanced Computing East China Sub-center (ACECS) to test performance and accuracy.
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All authors Hao-Ran Yu, Bing-Hang Chen, Kun Xu, Ming-Jie Sheng, Jiaxin Han, Yipeng Jing, Huahua Cui

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

The post-inflationary cosmology of the VISH$\nu$ axion-majoron model

Alexei H. Sopov, Carlos Tamarit and Raymond R. Volkas

It was previously shown how several explanatory deficiencies of the Standard Model (including the origin of dark matter, matter-antimatter asymmetry, small active neutrino mass, strong CP-conservation and the seeds for large-scale structure formation) may be economically resolved when an experimentally-accessible QCD axion also plays the role of the majoron, and the scalar partner of the axion is dynamical during inflation. In this paper, we complete this general study of the cosmological history for a unit domain-wall number option of the DFSZ-type, dubbed VISH$\nu$, by performing a detailed lattice-informed analysis of the reheating era. In doing so, we make inflationary and leptogenesis predictions more precise through estimates of the reheating temperature and the expansion history. The viable reheating scenarios, which at the same time satisfy strict conditions for naturalness (radiative stability), are also shown to respect dark radiation bounds. We also characterise the high-frequency spectrum of gravitational waves, and mention other phenomenological implications that distinguish VISH$\nu$ from alternative proposals.
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High Energy Astrophysical Phenomena

A Search for Hard X-ray/Soft $\gamma$-ray Emission from SPT-CL J2012-5649 (Abell 3667) Using INTEGRAL/ISGRI

Siddhant Manna, Shantanu Desai and Roman A. Krivonos

We present a search for hard X-ray/soft $\gamma$-ray emission from the merging galaxy cluster SPT-CL J2012-5649 (Abell~3667) using archival INTEGRAL/ISGRI observations. This cluster located at $z=0.0556$ hosts prominent radio relics associated with merger-driven shocks, suggesting the presence of relativistic electrons capable of producing inverse-Compton (IC) emission in the hard X-ray to soft $\gamma$-ray regime. We searched for emission in the 30–300~keV energy range using the INTEGRAL Off-line scientific analysis software with a total effective exposure of 2817~s. No significant emission was detected at the cluster position in the aforementioned energy interval. The extracted ISGRI spectrum is consistent with pure background, and no physically meaningful model parameters can be constrained. From the mosaic variance maps, we derive a $3\sigma$ upper limit of $F_{30-300\,\mathrm{keV}} < 3.63 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. This limit rules out bright IC scenarios and constrains the efficiency of merger-driven particle acceleration in SPT-CL J2012-5649. Our results provide the most stringent soft $\gamma$-ray constraint on this system to date and highlight the need for next-generation hard X-ray missions, such as HEX-P or eXTP, to probe IC emission at theoretically predicted levels in merging clusters.
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High Energy Astrophysical Phenomena · Cross-listed from physics.space-ph

What are Switchbacks?

Zesen Huang, Marco Velli and Yuliang Ding

We present a solitary Alfvén wave model that exhibits nontrivial three-dimensional twisting of open magnetic field lines while preserving constant $|B|$. Embedded rotational discontinuities sharply deflect the otherwise uniform field lines, producing localized, large-amplitude field reversals in one-dimensional profiles that closely resemble the “switchbacks” observed by the Parker Solar Probe in the inner heliosphere. This indicates that switchbacks, as seen in one-dimensional spacecraft time series, arise from traversals through strongly curved segments of open magnetic field lines.
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High Energy Astrophysical Phenomena

Constraint on magnetized black bounce spacetime from HFQPOs data and the selection of resonance models via information criterion

Shining Yang, Jianbo Lu, Mou Xu et al. (+1 more)

Black bounce spacetime, proposed by Simpson and Visser (SV), can describe the Schwarzschild solution, regular black holes, and traversable wormholes in a unified manner, depending on the value of the regularization parameter $a$. This paper primarily explores the dynamics of charged particle in the magnetized SV spacetime, and constrains the parameters of the SV spacetime along with its surrounding magnetic fields. The constraints are given by using $\chi^2$ analysis combined with high-frequency quasi-periodic oscillation (HFQPO) data observed from three microquasars: GRS 1915+105, XTE 1550-564, and GRO 1655-40. The results indicate that the magnetic field significantly influences the position of the innermost stable circular orbit of charged particle and frequency distribution of epicyclic motion, which excites more resonance model variants, enhancing observational effects. Additionally, we employ the Akaike Information Criterion (AIC) to evaluate resonance model and its various variants. The support for different models from observational data shows significant variation: $E R_8$ as the best model is supported strongly, $ER_3$ model has moderate evidence of support, $ER_6$ and $ER_7$ models are considerably less support, while other resonance models have essentially no support. The result from AIC is broadly consistent with the results obtained from the Bayesian Information Criterion (BIC) and the Bayes factor. For models more supported by the observational data, the allowed ranges of the regularization parameter: $0\leq a/M<0.736$ ($68\%$ confidence level) suggests that HFQPOs data could support the magnetized black bounce spacetime as a regular black hole, and the smaller value of the regularization parameter indicates a possibility of the presence of quantum effects.
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All authors Shining Yang, Jianbo Lu, Mou Xu, Yu Liu

Instrumentation and Methods for Astrophysics

Design and Performance of the Upgraded Prototype Schwarzschild-Couder Telescope Camera Module

Giovanni Ambrosi, Carla Aramo, Mattia Barbanera et al. (+54 more)

The Cherenkov Telescope Array Observatory (CTAO) is a ground-based observatory that will improve upon the sensitivities of the current generation of very-high-energy gamma-ray instruments. The Schwarzschild-Couder Telescope (SCT) is a dual-mirror candidate design for a CTAO Medium-Sized Telescope (MST). The prototype Schwarzschild-Couder Telescope (pSCT) was inaugurated in 2019 at Fred Lawrence Whipple Observatory (FLWO) in Arizona and observed significant gamma-ray emission from the Crab Nebula with a partially populated camera. The pSCT camera is currently being upgraded to fully instrument the focal plane with 11,328 silicon photomultiplier (SiPM) pixels split between 177 camera modules. Additionally, the modules will feature upgraded electronics designed to reduce electronics crosstalk and noise. A module calibration procedure has been developed using a preproduction test module. Following this calibration procedure, performance testing shows that the upgrade module has low noise, minimal electronics crosstalk, and excellent charge resolution. After calibration and optimization, the 177 production modules will be installed in the pSCT camera for commissioning. This will be followed by observations of known VHE gamma-ray sources for camera performance validation.
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All authors Giovanni Ambrosi, Carla Aramo, Mattia Barbanera, Chiara Bartolini, Wystan Benbow, Bruna Bertucci, Elisabetta Bissaldi, Massimiliano Bitossi, James H. Buckley, Massimo Capasso, Mirco Caprai, Davide Cerasole, Zachary Curtis-Ginsberg, Gaia De Palma, Leonardo Di Venere, Miguel Escobar Godoy, Qi Feng, Emanuele Fiandrini, Lucy Fortson, Stefan Funk, Amy Furniss, Alasdair Gent, Stefano Germani, Nicola Giglietto, Francesco Giordano, William Hanlon, Sam Heiman, Olivier Hervet, Maria Ionica, Weidong Jin, David Kieda, Francesco Licciulli, Pierpaolo Loizzo, Serena Loporchio, Giovanni Marsella, Reshmi Mukherjee, Nepomuk Otte, Francesca Romana Pantaleo, Riccardo Paoletti, Deivid Ribeiro, Luca Riitano, Emmet Roache, Duncan Ross, Lab Saha, Heiko Salzmann, Benjamin Schwab, Ruo-Yu Shang, Gianluigi Silvestre, Leonardo Stiaccini, Hiroyasu Tajima, Svanik Tandon, Giovanni Tripodo, Justin Vandenbroucke, Vladimir V. Vassiliev, Richard White, David A. Williams, Adrian Zink

Astrophysics of Galaxies

Mode Energy Partition in Partially Ionized Compressible MHD Turbulence

Yue Hu

We investigate how neutral-ion collisional damping modifies the spectral properties and energy partition of compressible MHD turbulence using a suite of 3D two-fluid simulations. By systematically varying the neutral-ion coupling strength and decomposing the turbulent velocity field into Alfvén, slow, and fast (polarization) modes, we quantify how each mode responds to the transition from strong to weak coupling. In the strong-coupling regime, the Alfvén and slow modes follow nearly Kolmogorov $k^{-5/3}$ spectra and dominate the kinetic energy budget, while fast modes exhibit a steeper spectrum and contribute $\sim$10% of the total energy. As the coupling weakens and neutral-ion damping becomes significant, all mode spectra steepen, approaching a dissipation-dominated $k^{-4}$ spectrum, except that the slope mode's spectrum parallel to the mean magnetic field has a power-law slope shallower than -4. While the total kinetic energy is reduced in the weak coupling regime, the slow-mode energy fraction increases substantially toward small scales, whereas the Alfvén-mode fraction decreases correspondingly. In contrast, the fast-mode energy fraction remains largely insensitive to coupling strength. These results demonstrate that partial ionization not only steepens the turbulent spectra but also reshapes the mode energy distribution, enhancing the relative importance of the slow mode while suppressing Alfvén mode in the damping regime. Our findings have important implications for turbulence-driven processes in the partially ionized interstellar medium, including cosmic-ray transport and acceleration.
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Astrophysics of Galaxies

Reconnection-Driven Turbulent Fluctuations in the Magnetically Dominated Collisionless Regime

Yue Hu, Luca Comisso, Lorenzo Sironi et al. (+1 more)

Magnetic reconnection is a fundamental plasma process that converts magnetic energy into bulk flow energy, thermal energy, and nonthermal particle acceleration. Despite its importance, the statistical properties of the turbulent fluctuations generated by collisionless reconnection, which are essential for understanding how this energy conversion proceeds, remain poorly understood. Here, we employ large-scale 3D particle-in-cell simulations to investigate the turbulence characteristics of velocity and magnetic field fluctuations generated by collisionless reconnection in a magnetically dominated pair plasma. We characterize their statistical properties by computing structure functions along different directions within the reconnection layer. We find that the square root of the second-order velocity structure function follows a power-law scaling with a slope $\sim1/3$ at intermediate to large scales. The square root of the second-order magnetic structure function consistently exhibits a steeper slope, in the range $\sim 0.6 - 0.8$. The presence of a finite guide field does not systematically modify the slope of the velocity fluctuations, while it progressively steepens the scaling of the magnetic fluctuations in the guide-field and inflow directions. We measure higher-order structure functions, which reveal strong magnetic intermittency along the outflow direction and weaker intermittency in the inflow and guide-field directions. Additionally, the local anisotropies of both velocity and magnetic field fluctuations are greater for stronger guide fields. These results provide a systematic characterization of the multiscale nature of turbulence in collisionless and magnetically dominated reconnection layers, with important implications for plasma heating and particle acceleration.
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All authors Yue Hu, Luca Comisso, Lorenzo Sironi, Siyao Xu

Instrumentation and Methods for Astrophysics

A Practical Study of Lightweight Neural Gravitational-Wave Detection in Real LIGO Noise: Models, Ablations, and Open Software

Victoria Tiki and Eliu Huerta

Machine-learning methods offer a computationally efficient complement to conventional gravitational-wave searches, but their performance depends strongly on training data, preprocessing, inference design, and robustness to nonstationary detector noise. We present a framework for developing and evaluating lightweight neural detectors using real LIGO data. The pipeline includes strain acquisition, waveform injection, training, inference, and trigger construction. Using O3a data, we compare twelve architectures and perform ablations of normalization, whitening, label width, glitch handling, SNR curricula, and inference triggers. We find that simple temporal convolutional networks match or outperform more complex architectures, including attention-based and graph models, within our implementation. We then train four independently initialized 116k-parameter TCNs on O3b noise and evaluate them on 22.7 days of coincident data containing 13 confident GWTC-3 events. At a threshold fixed using O3a data, the four models recover 8, 9, 9, and 9 events while producing 0, 0, 1, and 0 false-positive triggers, respectively, without post hoc vetoes or inter-model coincidence filtering. In injection studies, the models achieve a median sensitive volume of $5.4\,\mathrm{Gpc}^3$ at the preselected threshold, corresponding to a median false-alarm rate of $20.5\,\mathrm{yr}^{-1}$. At a stricter threshold, the median sensitive volume remains $4.6\,\mathrm{Gpc}^3$, with three models returning 0 false alarms and one model yielding 1 false alarm, observed across one year of background per model. These results show that, within the tested implementations and training setup, careful pipeline design can matter more than architectural complexity and provide a lightweight pipeline for future studies on real interferometer data.
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Astrophysics of Galaxies

THRILS – The High-(Redshift+Ionization) Line Search: Program Description & Redshift Catalog

Taylor A. Hutchison, Rebecca L. Larson, Pablo Arrabal Haro et al. (+46 more)

To date, many spectroscopic confirmations of z>7 galaxies have been obtained using JWST/NIRSpec prism observations, with most of their physical properties inferred from these observations and corresponding imaging. What is needed are higher-resolution spectra at deeper depths to study these sources in detail. We present The High-(Redshift+Ionization) Line Search (THRILS) program: deep (>8 hr) observations in two pointings of JWST/NIRSpec G395M spectroscopy to 1) probe high ionization spectral features in z>8 galaxies that are indicative of top-heavy initial mass functions or growing massive black holes, 2) search for accreting supermassive black holes in typical galaxies at z~4-9 through broad Balmer line emission, and 3) probe the stellar-mass growth histories of massive galaxies. We include spectroscopic redshift measurements for 89 sources from the THRILS data, as well as a detection threshold for the full and half depth integration times of the program.
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All authors Taylor A. Hutchison, Rebecca L. Larson, Pablo Arrabal Haro, Erini Lambrides, Katherine Chworowsky, Gourav Khullar, Kelcey Davis, Steven L. Finkelstein, Jane R. Rigby, Guillermo Barro, Nikko J. Cleri, Dale Kocevski, Jacqueline Antwi-Danso, Mic Bagley, Danielle A. Berg, Volker Bromm, Oscar Chavez Ortiz, John Chisholm, Sadie C. Coffin, M. C. Cooper, Olivia Cooper, Isa G. Cox, Mark Dickinson, Harry Ferguson, Maximilien Franco, Jonathan P. Gardner, Ananya Ganapathy, Norman Grogin, Michaela Hirschmann, Marc Huertas-Company, Intae Jung, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Ray A. Lucas, Elizabeth McGrath, Alexa M. Morales, Grace M. Olivier, Casey Papovich, Pablo G. Perez-Gonzalez, Nor Pirzkal, Rachel S. Somerville, Anthony J. Taylor, Jonathan R. Trump, Brittany Vanderhoof, Benjamin Weiner, Brian Welch, L. Y. Aaron Yung, Jorge A. Zavala, the THRILS collaboration

Instrumentation and Methods for Astrophysics

Mega-MUSE Nearby Galaxy Serendipity Survey

Martin M. Roth

The Mega-MUSE Nearby Galaxy Serendipity Survey is a White Paper in response to the ESO Expanding Horizons Call for Ideas. It is based on the notion that novel observing facilities for astronomy almost always make discoveries that were not anticipated at the time of planning and conceptual design. Unlike other surveys it is not targeting the solution of a single specific astrophysical problem, but enables the exploitation of data obtained from other surveys in Local Group and Local Volume galaxies to clarify open science questions that cannot be solved otherwise. The most prominent objectives are to obtain solid statistics of rare stars and gaseous nebulae as well as their properties that are not accessible in the Milky Way disk due to extinction and selection effects, and the discovery of as yet unknown phenomena.
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Saturday 13 December

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

An Enhanced "Flux-Corrected Transport"-Based Plasmasphere Refilling Model

Jaden Fitzpatrick, Kausik Chatterjee and Naomi Maruyama

A previously developed multi-ion, two-stream Flux-Corrected Transport (FCT) hydrodynamic model for plasmasphere refilling has been extended to incorporate self-consistent electron temperature evolution. The past assumption of a constant temperature along the modeled flux tube has been replaced by solving the electron energy equation, permitting spatially and temporally varying temperature. This improvement provides a more physically complete representation of the pressure and ambipolar electric-field gradients that influence ion transport. The extended model allows us to investigate two-stage refilling behavior established by prior observations and simulations. The model continues to reproduce the expected dominance of H+, enhanced early-time O+ contributions, and the coupling between H+ and He+ through the ambipolar electric field during the transition between stages. Sensitivity experiments with modified initial ion concentrations, including cases representing seasonal effects, highlight the distinct roles of each ion species in shaping the refilling trajectory. Comparisons across L-shells 3 and 4 further confirm the robustness of the model framework for future extension to three-dimensional geometries. Overall, by incorporating more realistic temperature variations, this enhanced model strengthens the physical understanding for interpreting complex multi-ion transport processes during plasmasphere recovery following geomagnetic storms.
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Cosmology and Nongalactic Astrophysics

Structural Limitations on Constraining the Time Evolution of Dark Energy

Seokcheon Lee

Cosmological constraints on a time-varying dark energy equation of state are fundamentally limited by the integral structure through which the equation of state enters cosmological observables. We rigorously derive the linear response kernel that maps perturbations in the equation of state \omega(z) to comoving distance fluctuations \delta D(z). By adopting a Fourier mode expansion \delta \omega(z) = \sin(kz), we obtain the exact analytic form of the distance response in terms of Sine and Cosine integrals. We show that this mapping involves a double integration over redshift, which acts as an intrinsic low-pass filter with a characteristic \sim k^{-2} scaling in redshift space. This structural limitation is visualized in a schematic diagram and confirmed by observational verification using the full covariance matrix of the Pantheon+ supernova dataset. Our analysis reveals a steep hierarchy in Fisher eigenvalues where the information content drops by an order of magnitude already at the second eigenmode. Consequently, distance-based probes effectively constrain only a single dominant mode of \omega(z). This implies that the difficulty in constraining dynamical parameters such as w_a is not due to data precision, but is a necessary consequence of the observable's integral nature, which renders it structurally blind to the instantaneous rate of change d\omega/da.
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High Energy Astrophysical Phenomena

Magnetohydrodynamic Simulations of Transonic Accretion Flows

Raj Kishor Joshi, Antonios Tsokaros, Sanjit Debnath et al. (+2 more)

Theoretical studies of transonic accretion onto black holes reveal a wide range of possible solutions, broadly classified into smooth flows and flows featuring shocks. Accretion solutions that involve the formation of shocks are particularly intriguing, as they are expected to naturally produce observable variability features. However, despite their theoretical significance, time-dependent studies exploring the stability and evolution of such shocked solutions remain relatively scarce. To address this gap, we perform simulations of transonic accretion flows around a black hole in an ideal magnetohydrodynamic framework. Our simulations are initialized using boundary conditions derived from semi-analytical hydrodynamical models, allowing us to explore the stability of these flows under varying magnetic field strengths. Our results indicate that mildly magnetized flows in a uniform vertical magnetic field alter the accretion dynamics through magnetic pressure, with the resulting force imbalance driving oscillations in the shock front. Variations in the emitted luminosity arising from shock oscillations appear as quasi-periodic oscillations (QPOs), a characteristic feature commonly observed in accreting black holes. We find that the QPO frequency is determined by the radial position of the shock front: oscillations occurring closer to the black hole produce frequencies of tens of hertz, whereas shocks located farther out yield sub-hertz frequencies.
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All authors Raj Kishor Joshi, Antonios Tsokaros, Sanjit Debnath, Indranil Chattopadhyay, Ramiz Aktar

High Energy Astrophysical Phenomena

Readdressing the contribution of photonuclear reactions to the muon content of extensive air showers: a heuristic approach

Nickolay S. Martynenko

The indirect ground-based observations of cosmic rays through extensive air showers in modern experiments typically involve the use of Monte Carlo simulations to determine the characteristics of the primary particles. These simulations necessitate assumptions about particle interactions at energies that have not yet been experimentally probed, which introduces systematic uncertainties in key observables, particularly the number of muons. Current research on this uncertainty primarily focuses on hadronic interaction models, the dominant source of muon production. This study presents an approach that takes into account another significant mechanism for muon generation: photonuclear reactions. A robust heuristic technique has been developed to estimate the contribution of these interactions to the total number of muons over a wide range of extensive air shower parameters (including primary particle type, energy, and slant atmospheric depth) and photonuclear interaction models, with an absolute percentage error on the order of $10\%$ in the estimated number of muons. Furthermore, several potential applications of the suggested method in relation to modern challenges in extensive air shower physics are discussed.
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High Energy Astrophysical Phenomena

eROSITA-RU Tidal Disruption Events with Keck-I/LRIS: Sample Selection, Optical Properties, and Host Galaxy Demographics

Zirui Zhang, Yuhan Yao, Marat Gilfanov et al. (+10 more)

We select seventy tidal disruption event (TDE) candidates among X-ray transients discovered during the eROSITA all-sky surveys in the Eastern Galactic hemisphere between December 2020 and February 2022 (eRASS1–5). We cross-match each X-ray source to a host galaxy in archival optical surveys using Bayesian likelihood-ratio techniques and obtain Keck/LRIS spectroscopy for all 70 host galaxies. Host properties are inferred through SED fitting with Prospector and emission line analysis with pPXF. We develop a robust classification scheme using X-ray and broad line luminosities, narrow-line ionization diagnostics, and optical variability to identify high-confidence TDEs, for which we analyze optical spectral features, light curve properties, and host galaxy demographics. Our final sample contains 52 TDEs with redshifts of $0.018 \leq z\leq0.714$, comprising 41 gold (high-confidence) and 11 silver (lower-confidence) events. The vast majority (93%) of gold TDEs are intrinsically brighter in the X-ray band, with $L_{\rm X,peak} > L_{\rm opt,peak}$. Among 23 events with detected optical flares, delayed X-ray peak is commonly observed. We identify transient spectral features in eight events, including six with prominent broad \ion{He}{II} $\lambda$4686 and/or H$\alpha$ emission and two coronal-line TDEs. Host galaxy demographics reveal modest over-representation in green valley ($\times1.8$) and quiescent Balmer-strong ($\times5.3$) galaxies, significantly weaker than previous TDE samples, demonstrating greater diversity in star formation histories than previously recognized. Most TDE hosts exhibit suppressed star formation relative to the main sequence, consistent with X-ray selection biases against dusty star-forming galaxies.
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All authors Zirui Zhang, Yuhan Yao, Marat Gilfanov, Sergey Sazonov, Pavel Medvedev, Georgii Khorunzhev, Rashid Sunyaev, Vikram Ravi, S. R. Kulkarni, Jean Somalwar, Ryan Chornock, Ilfan Bikmaev, Mark A. Gorbachev

Astrophysics of Galaxies

Exploiting light coherence in astrophysics

Vitalii Sliusar, Domenico Della Volpe, Benjamin Garcia et al. (+7 more)

The Hanbury Brown-Twiss (HBT) effect, discovered in the 1950s and further developed in the 1960s, was originally used to estimate stellar angular diameters through intensity correlations measured by spatially separated detectors. Further developments started from HBT experiments to exploit quantum bunching of photons in incoherent light sources played foundational role in the development of quantum optics. When the two detectors in an HBT experiment are co-located, typically implemented using a beam splitter, a zero-baseline intensity correlation is obtained, which after deconvolution of the detector response function, yields the temporal component of the second-order coherence function. Unlike spatial correlations, this function is independent of the source brightness distribution, or its size, giving direct insight into the properties of the source's emission process itself - photon statistics. Along with photometric and spectral information, the second order coherence function can be used to constrain the emission mechanisms and discriminate between thermal, synchrotron, bremsstrahlung and stimulated emission processes. Evolution of the emission processes would likewise drive changes in the second order coherence. Light coherence information along with multi-wavelength observations, can become a complementary "messenger", carrying internal information about the astronomical source.
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All authors Vitalii Sliusar, Domenico Della Volpe, Benjamin Garcia, Gilles Koziol, Etienne Lyard, Nicolas Produit, Aramis Raiola, Prasenjit Saha, Lucijana Stanic, Roland Walter

Instrumentation and Methods for Astrophysics

Nyx-RT: Adaptive Ray Tracing in the Nyx Hydrodynamical Code

Nathan X. Marshak, Kathlynn Simotas, Zarija Lukić et al. (+3 more)

Numerical methods for radiative transfer play a key role in modern-day astrophysics and cosmology, including study of the inhomogeneous reionization process. In this context, ray tracing methods are well-regarded for accuracy but notorious for high computational cost. In this work, we extend the capabilities of the Nyx N-body / hydrodynamics code, coupling radiation to gravitational and gas dynamics. We formulate adaptive ray tracing as a novel series of filters and transformations that can be used with AMReX particle abstractions, simplifying implementation and enabling portability across Exascale GPU architectures. To address computational cost, we present a new algorithm for merging sources, which significantly accelerates computation once reionization is well underway. Furthermore, we develop a novel prescription for geometric overlap correction with low-density neighbor cells. We perform verification and validation against standard analytic and numerical test problems. Finally, we demonstrate scaling to up to 1024 nodes and 4096 GPUs running multiphysics cosmological simulations, with 4096^3 Eulerian gas cells, 4096^3 dark matter particles, and ray tracing on a 1024^3 coarse grid. For these full cosmological simulations, we demonstrate convergence in terms of reionization history and post-ionization Lyman-alpha forest flux.
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All authors Nathan X. Marshak, Kathlynn Simotas, Zarija Lukić, Hyunbae Park, James Ahrens, Chris R. Johnson

Instrumentation and Methods for Astrophysics

Transients as Determinants of Habitability

Fatemeh Zahra Majidi, Katia Biazzo, Maria Tsantaki et al. (+7 more)

Stellar magnetic activity, manifested through spots (faculae and flares), fundamentally shapes the exoplanets' environments. For low-mass stars in particular, where most habitable-zone planets reside, the variable magnetic phenomena can dominate atmospheric chemistry, surface radiation levels, long-term atmospheric escape, and ultimately habitability. However, physical characteristics of these transients (e.g. energy and temperature) and their spectra remain ill-constrained due to limitations in cadence and magnitude access of current spectroscopic facilities. A next-generation 12-m class ground-based observatory equipped with integral-field spectroscopy (IFS) and multi-object spectroscopy (MOS) at R$\sim$4,000 and $\sim$40,000 offers a transformational opportunity to characterize stellar activity in the time domain across large samples of exoplanet host stars. Such a facility would enable simultaneous monitoring of continuum variability, chromospheric and coronal line diagnostics, and particle-accelerated flare signatures, resolving the physics driving space weather and quantifying its impact on planetary atmospheres.
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All authors Fatemeh Zahra Majidi, Katia Biazzo, Maria Tsantaki, Amelia Bayo, Gražina Tautvaišienė, Valentin D. Ivanov, Germano Sacco, Richard I. Anderson, Avraham Binnenfeld, David Montes

Instrumentation and Methods for Astrophysics

Toward the time-domain spectroscopic study of the dynamic life of stars: from accretion to magnetic activity

Fatemeh Zahra Majidi, Amelia Bayo, Marc Audard et al. (+11 more)

Stars and planets can be seen as the second fundamental building blocks of baryons in the universe (only second to the dust and gas in molecular clouds). Their formation involves dust grain growth of many orders of magnitude and a myriad of processes operating at time scales from a few tens to millions of years. Thus, investigating the formation and evolution of young stellar objects (YSOs) is of great importance in modern astronomy. Addressing this goal requires overcoming long-standing challenges in characterizing multifaceted phenomena that span a broad range of astrophysical processes (from protoplanetary disk evolution and planet formation to accretion dynamics and transient stellar events). Also, YSOs are complex systems that consist of several components: a central forming object, surrounded by a medium or disk from which the accretion process is at work, supersonic ejection of plasma in the form of collimated bipolar jets (which interact with the ambient medium through which they propagate) and all these components emit in a wide range of wavelengths. A facility capable of simultaneously tackling these diverse questions must deliver long-term, high-cadence spectroscopic monitoring of YSOs over time spans of at least a decade; especially because accretion/ejection processes in YSOs are characterized by a wide range of temporal variability: from short-term (hours-days) to long-term (months-years) variability due to rotation, accretion, magnetic activity, etc. Such a mission demands a spectroscopic platform considering a solid time-domain astronomy framework, providing repeated observations over wide fields and supporting multiple cadence strategies tailored to distinct scientific objectives.
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All authors Fatemeh Zahra Majidi, Amelia Bayo, Marc Audard, Francisco José Galindo-Guil, Rosaria Bonito, Katia Biazzo, Loredana Prisinzano, Mario Giuseppe Guarcello, Eleonora Fiorellino, Innocenza Busa, Germano Sacco, Richard I. Anderson, Avraham Binnenfeld, David Montes

Instrumentation and Methods for Astrophysics

SIMLA: The Spitzer Infrared Spectrograph Mapping Legacy Archive

Grant P. Donnelly, Cory M. Whitcomb, Lindsey Hands et al. (+9 more)

We present the Spitzer/IRS Mapping Legacy Archive (SIMLA); a complete set of mid-infrared spectral cubes built from low-resolution mapping-mode fixed-target observations from Spitzer/IRS (5.2-38 micron, R~60-130). Contained in this dataset are spectral maps for several hundred spatially-resolved and unresolved objects, including galaxies, molecular clouds, supernova remnants, HII regions, and more. Each cube has been carefully treated to remove astronomical foregrounds and backgrounds as well as detector effects using a novel pipeline. Cube assembly was facilitated by the CUBISM code, which included automatic detection and removal of bad pixels. We describe the SIMLA pipeline for reducing and validating the cubes, and we show that synthetic photometry derived from SIMLA spectra and corresponding WISE photometry typically agree within a few percent. SIMLA products and documentation related to their use will soon be available at the NASA/IPAC Infrared Science Archive (DOI:10.26131/IRSA655).
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All authors Grant P. Donnelly, Cory M. Whitcomb, Lindsey Hands, Sara E. Duval, J.-D. T. Smith, Karin Sandstrom, David Carroll, McKenna Dowd, Brandon S. Hensley, Leslie K. Hunt, Edward Walsh, Julie Watson

Solar and Stellar Astrophysics

Thread Separation and Expansion Observed in Multi-Stranded Solar Coronal Loops

David H. Brooks and Harry P. Warren

The theoretical expectation that coronal loops should expand with height contrasts with observations that typically show constant cross-sections. We investigate the idea that this discrepancy results from loops being composed of fine threads whose expansion occurs below the resolution limits of instruments like SDO/AIA. In this paper, we present two significant findings: (1) several extended loops exhibit measurable expansion, suggesting length as a critical factor in detection capability, and (2) high-resolution Solar Orbiter/EUI observations have captured expanding loops in active regions. For both AIA and EUI data, we observe cases where thread separation is directly visible as the loops evolve. These findings complement our previous work indicating AIA loops may consist of relatively few threads. Collectively, these observations provide compeling evidence supporting the multi-thread model and offer a potential resolution to the long-standing loop expansion problem in solar coronal physics. However, the high densities and narrow temperature distributions of observed coronal loops remain unresolved.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Hybrid algorithm combining matched filtering and convolutional neural networks for searching gravitational waves from binary black hole mergers

Takahiro S. Yamamoto, Kipp Cannon, Hayato Motohashi et al. (+1 more)

Efficient searches for gravitational waves from compact binary coalescence are crucial for gravitational wave observations. We present a proof-of-concept for a method that utilizes a neural network taking an SNR map, a stack of SNR time series calculated by the matched filter, as input and predicting the presence or absence of gravitational waves in observational data. We demonstrate our algorithm by applying it to a dataset of gravitational-wave signals from stellar-mass black hole mergers injected into stationary Gaussian noise. Our algorithm exhibits comparable performance to the standard matched-filter pipeline and to the machine-learning algorithms that participated in the mock data challenge, MLGWSC-1. The demonstration also shows that our algorithm achieves reasonable sensitivity with practical computational resources.
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All authors Takahiro S. Yamamoto, Kipp Cannon, Hayato Motohashi, Hiroaki W. H. Tahara

Astrophysics of Galaxies

First mapping of prebiotic molecule CH2NH in a pre-stellar core

Yuxin Lin, Silvia Spezzano, Olli Sipilä et al. (+2 more)

We present the first spatially resolved map of methanimine CH2NH in the prestellar core L1544 using the IRAM 30m telescope. The 2$_{0,2}$-1$_{0,1}$ line at 127 GHz was mapped with 20" resolution ($\sim$2800 au), revealing extended CH2NH emission across the core. The peak line intensity coincides with the well-known c-C3H2 peak, while the integrated intensity peaks between the HNCO and dust continuum peaks due to broader linewidths in the latter region. Column densities of CH2NH are $\sim$(0.5-1.4$\times$)10$^{12}$ cm$^{-2}$, corresponding to fractional abundances of $5\times10^{-11}$-$1\times10^{-10}$, with a trend decreasing from the southern, carbon-chain rich region to the dust and HNCO peak in the north. Comparison with complementary molecular maps and the gas-grain chemical model of Sipilä et al. suggests that neutral-neutral gas-phase reactions and dissociative recombination dominate in the outer carbon-chain shell. This study demonstrates that CH2NH, a simple nitrogen- and carbon-bearing molecule previously detected with pointed observations in other cold cores, is present and spatially extended in the evolved pre-stellar core L1544. This indicates that prebiotic nitrogen-carbon chemistry continues efficiently up to the onset of gravitational collapse, providing key constraints for astrochemical models and the early stages of chemical complexity leading to amino acids.
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All authors Yuxin Lin, Silvia Spezzano, Olli Sipilä, Jaime E. Pineda, Paola Caselli

Earth and Planetary Astrophysics

On the use of polarized thermal emission to constrain cloud grain size and temperature structure of sub-stellar objects

Fei Wang, Yuka Fujii, Ben Burningham et al. (+1 more)

Emission spectroscopy is an invaluable tool for probing the atmospheres of brown dwarfs and exoplanets, but interpretations based on flux spectra alone often suffer from degeneracies among temperature structure, chemical composition, and cloud properties. Thermal emission spectropolarimetry offers complementary sensitivity to these atmospheric characteristics. Previous studies have shown that linear polarization in fixed bandpasses depends on emission angle, temperature profile, and cloud scattering. In this study, we revisit these dependencies, emphasizing the wavelength-dependent effects that shape polarized spectra. We show that thermal polarization spectrum is primarily governed by: (1) a combination of temperature, temperature gradient, and wavelength; (2) cloud particle size; and (3) cloud optical thickness. Using the 3D Monte Carlo radiative transfer code ARTES, we simulate polarization spectra from a modeled 1D atmosphere. We find that, for a fixed cloud optical thickness, the polarization exhibits peaks at size parameters near 0.2 and 1. However, the dependence on cloud optical thickness is more pronounced and tends to dominate the broadband polarization. We further show that much narrower polarization features in molecular absorption band, can in principle trace the local temperature gradient at the photosphere of each wavelength. Future low-resolution (resolving power around 100) spectropolarimeter operating at 1-2 micron with sensitivities of 1e-5 would be able to capture these polarization features, and may provide a new diagnostic for breaking degeneracies that commonly affect flux-only retrievals. This work represents an incremental step toward the challenging goal of jointly interpreting atmosphere from both intensity and polarization spectra.
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All authors Fei Wang, Yuka Fujii, Ben Burningham, Jinping He

Instrumentation and Methods for Astrophysics

The Universal Bayesian Imaging Kit

Torsten Enßlin, Vincent Eberle, Matteo Guardiani et al. (+1 more)

Bayesian imaging of astrophysical measurement data shares universal properties across the electromagnetic spectrum: it requires probabilistic descriptions of possible images and spectra, and instrument responses. To unify Bayesian imaging, we present the Universal Bayesian Imaging Kit (UBIK). Currently, UBIK images data from Chandra, eROSITA, JWST, and ALMA. UBIK is based on information field theory (IFT), the mathematical theory of field inference, and on NIFTy, a package for numerical IFT. UBIK provides sky models that are instrument independent and instrument interfaces that share common parts of their response representations. It is open source, can provide spatio-spectral image cubes, jointly analyses data from several instruments, and separates diffuse emission, point sources, and extended emission regions.
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All authors Torsten Enßlin, Vincent Eberle, Matteo Guardiani, Margret Westerkamp