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Physics Astrophysics

astro-ph Watch Astrophysics

Saturday 4 July

Astrophysics of Galaxies

Excitation of warm HD and H$_2$ and implications for the interstellar deuterium abundance

David A. Neufeld

Rotational emission from HD provides a direct probe of deuterium in warm molecular gas, but the conversion of HD line fluxes to total column densities is sensitive to non-LTE excitation. I present calculations of the non-LTE rotational excitation of HD and H$_2$ and a method for fitting their rotational diagrams simultaneously. Because HD remains subthermally excited to higher densities than H$_2$, the combined analysis constrains the thermal pressure as well as the HD/H$_2$ abundance ratio. I apply this method to JWST/MIRI observations of 26 positions in 10 protostellar outflows from the JOYS survey, published previously by Francis et al. (2025), including 17 positions with secure detections of at least three HD lines. Two-temperature, isobaric models generally provide good fits to the measured H$_2$ and HD level populations. For 11 positions with HD detections, the inferred pressures span $p/k_B=8.4\times10^7$ - $5.1\times10^9\ {\rm K\,cm^{-3}}$; for the remaining six, the HD populations approach LTE and provide only lower limits on the gas pressure. Excluding two positions toward IRAS 4B with comparatively poor fits, the median and mean inferred HD/H$_2$ abundance ratios are $1.42\times10^{-5}$ and $1.47\times10^{-5}$, respectively, only 28$\%$ and 29$\%$ of the value expected if all primordial deuterium were present as HD. These results confirm that HD/H$_2$ is generally low in shocked molecular gas while demonstrating that pressure-dependent non-LTE excitation must be modeled explicitly. Translating the measured HD/H$_2$ ratios into elemental D/H requires additional modeling of shock chemistry.
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Astrophysics of Galaxies

A Homogeneous Determination of the Interstellar Extinction Law and Metallicity for 105 Galactic Open Clusters

Tahereh Ramezani

This work presents an investigation of the interstellar extinction law toward 105 Galactic open clusters using ultraviolet-optical photometry. The photometric dataset and reddening determinations were previously introduced in Ramezani et al., (2025); hereafter Paper I, where homogeneous ultraviolet photometry and colour-colour diagrams were used to derive reddening values for the cluster sample. In the present study, we extend that analysis by determining extinction law and metallicity parameters for the same clusters. Ground-based U-band observations obtained with the 2.15-m CASLEO telescope and the 1.54-m Danish telescope are combined with Gaia Data Release 3 photometry to construct ultraviolet-optical colour indices. Using extinction-ratio combinations and the Fitzpatrick, (1999) extinction model, we derive mean total-to-selective extinction law values (RV) for all clusters. The resulting RV values span approximately 2.5-4.5, with a mean value of RV = 3.53, indicating significant spatial variations of dust properties across the Galactic disk and deviations from the canonical diffuse interstellar medium value RV = 3.1. Afterward, by fitting the isochrone to the Colour-Magnitude Diagrams, we estimated the metallicity values of 105 observed clusters. This work provides the first homogeneous catalogue of extinction law measurements based on combined ground-based ultraviolet and Gaia photometry for a large open-cluster sample, establishing an important reference for studies of Galactic dust structure and stellar parameter corrections.
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Cosmology and Nongalactic Astrophysics

Baryonic assembly bias in X-ray-selected galaxy groups and clusters: insights from the Magneticum simulation

Ilaria Marini, Tiago Castro, Paola Popesso et al. (+5 more)

Galaxy groups and clusters trace the large-scale matter distribution, with their clustering usually interpreted mainly as a function of halo mass. Yet, at fixed mass, their baryonic properties retain information about halo growth, gas accretion, and feedback. The intrinsic scatter in X-ray luminosity and gas fraction suggests that X-ray-selected systems may not be a random subset of the halo population. If these observables correlate with halo assembly, they may trace secondary variations in halo bias. We test this using the Magneticum hydrodynamical simulation, measuring the clustering of systems selected by X-ray luminosity and gas fraction at fixed halo mass. We construct mass-matched subsamples by ranking halos in percentiles of X-ray luminosity and derive the linear halo-matter bias from the halo-matter cross-power spectrum. X-ray-bright halos are more strongly clustered than X-ray-faint halos at fixed mass. For the 84th-16th percentile split, we find $\Delta b_{\rm lin}=0.17\pm0.03$, corresponding to a $\sim17\%$ enhancement relative to the X-ray-faint sample. A 67th-33rd split gives a consistent signal, with $\Delta b_{\rm lin}=0.12\pm0.02$ and a $\sim12\%$ enhancement. The effect is strongest at group scales and negligible for cluster-size halos. Gas fraction shows an even stronger clustering dependence, with relative enhancements of $\sim39\%$ and $\sim26\%$ for the two percentile splits. This signal is present from $z\simeq2$, whereas X-ray luminosity becomes significant only at $z\simeq0.3$, once the gas thermodynamic state is more closely coupled to baryon retention. Matching halos by both mass and formation time reduces the large-scale bias difference to below $2\sigma$, indicating that formation time captures much of the signal. These results show that, in Magneticum, X-ray luminosity traces a baryonic manifestation of halo assembly bias beyond mass.
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All authors Ilaria Marini, Tiago Castro, Paola Popesso, Klaus Dolag, Veronica Biffi, Natanael de Isídio, Daudi Mazengo, Victoria Toptun

Instrumentation and Methods for Astrophysics

Source Finding and Characterisation for SKAO Science

Sabyasachi Pal, Souvik Manik, M Carmen Toribio et al. (+8 more)

The advancements in highly sensitive and powerful radio telescopes, including the Square Kilometre Array Observatory (SKAO) and its precursors, MeerKAT, ASKAP, MWA, and HERA, will enable us to create the deepest radio images of the sky. However, due to the sheer scale of the datasets, manually classifying and analyzing this data is computationally expensive, time-consuming, and laborious. Therefore, the development of automated algorithms to detect and classify complex morphological radio sources from large astronomical surveys is the need of the time. In this chapter, we examine the recent advancements and challenges in source-finding techniques triggered by the analysis of SKAO precursor data and the SKA Data Challenges, both in spectral and continuum modes, along with the growing demand for computational resources and automated source detection methods using machine learning (ML) algorithms for the identification and characterisation of new populations of sources, addressing their complex and diffuse morphologies, as well as transient nature. Additionally, we discuss the critical factors affecting the quality and limitations of automated source-finding techniques, including artefacts from residual continuum emission, sidelobes, radio frequency interference (RFI), technical failures, calibration issues, flagging methods and false positives. With the availability of the full operational phase of the SKAO, continued advancements in source detection algorithms and computational infrastructure will be essential to fully exploit its scientific potential.
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All authors Sabyasachi Pal, Souvik Manik, M Carmen Toribio, Geferson Lucatelli, Omkar Bait, Simone Riggio, Antxon Alberdi, Philippa Hartley, Javier Moldon, Mamta Pandey-Pommier, Rob Beswick

Astrophysics of Galaxies

Tracing the Origin of Circular-Symmetry Diffuse Radio Sources in the Next-Generation SKAO Era

Sabyasachi Pal, Shobha Kumari, Souvik Manik et al. (+2 more)

Circularly symmetric diffuse radio sources represent a recently recognized and rare class of extragalactic synchrotron-emitting structures whose physical origin remains uncertain. The currently known population is small, and the limited frequency coverage, sensitivity, and polarimetric information available from existing observations hinder robust discrimination among the various proposed formation scenarios. This chapter presents an overview of four representative circular diffuse sources featuring diverse morphologies, such as a horseshoe-shaped inner ring, circular diffuse emission surrounded by inner S/Z-shaped structures and a double-ring diffuse source. The angular sizes of these sources are in the range of 60-100 arcsec, similar to odd radio circles (ORCs). These systems illustrate the morphological diversity of circular diffuse radio emission and provide valuable clues to the physical processes responsible for their formation. The Square Kilometre Array Observatory (SKAO), with its unprecedented sensitivity, wide instantaneous bandwidth, sub-arcsecond imaging capability, and high-fidelity polarimetric performance, will significantly advance the study of these objects. SKAO surveys are expected to detect a substantially larger population of faint circular diffuse radio sources, enabling the first statistically meaningful studies of their occurrence, environments, and evolution. An expanded sample will allow investigations of their cosmological evolution and help determine whether their origin is linked to AGN feedback processes, episodic jet activity, galaxy interactions, or large-scale structure formation. This chapter summarizes the current observational knowledge of circular diffuse radio sources and describes how next-generation radio facilities, particularly the SKAO, will enable progress in understanding the origin and physical nature of these enigmatic radio structures.
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All authors Sabyasachi Pal, Shobha Kumari, Souvik Manik, Marek Jamrozy, Ananda Hota

Astrophysics of Galaxies

Exploring the Influence of the Jet-environment Interactions on the Observed Asymmetry of Extragalactic Radio Sources with SKAO

Sabyasachi Pal, Souvik Manik, Shobha Kumari et al. (+1 more)

Several observational and theoretical studies have suggested that the observed arm-length asymmetries in extragalactic radio sources are primarily driven by interactions between radio jets and an inhomogeneous ambient medium, although orientation effects may also contribute to the observed asymmetry. However, the observational evidence supporting these interpretations comes from only a small sample of FR II radio sources, in which the brighter hotspots are typically found on the side of the shorter jet arm. We aim to investigate the interactions between powerful jets and their surrounding environments in radio sources, with a particular focus on how these interactions shape the morphology and asymmetry of the radio lobes. The unprecedented sensitivity and angular resolution of the Square Kilometre Array Observatory (SKAO) will facilitate the discovery and detailed characterization of asymmetric radio sources across a wide range of physical scales and redshifts. Using a combination of 3D magnetohydrodynamic simulations and observational data from the SKAO, one can explore the influence of clumpy interstellar and intergalactic media on jet propagation and the resulting asymmetries in radio sources at various redshifts. The study will analyze how environmental factors, such as density and turbulence, decelerate jets, leading to observable asymmetries in smaller, higher-redshift sources. In this chapter, we review existing simulation and observational results on jet-environment interactions in radio galaxies and discuss how SKAO capabilities will further advance our understanding of AGN feedback and its role in shaping large-scale cosmic structure.
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All authors Sabyasachi Pal, Souvik Manik, Shobha Kumari, Chiranjib Konar

Earth and Planetary Astrophysics

Three body Simulations of a Primordial Black Hole Encounter with the TOI 2796 System

S.V. Vereshchagin and M. I. Kamentsev

We present numerical three body simulations of a primordial black hole (PBH) encounter with the TOI 2796 system, which hosts a hot Jupiter. Over 100 days, we identify three outcomes: (i) ejection of the planet; (ii) a bound triple system (a approx. 1 au, P approx. 5 days); and (iii) capture of the planet by the PBH into a binary (0.0194 au, 15 days) that escapes the star. These results highlight the rich dynamical diversity of PBH encounters and their potential role in shaping planetary systems.
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High Energy Astrophysical Phenomena

A NICER view of the millisecond pulsar PSR J2124$-$3358: evidence for a helium atmosphere

Denis González-Caniulef, Sebastien Guillot, Pierre Stammler et al. (+12 more)

Pulse profile modeling has proven to be a powerful technique for determining the mass and radius of neutron stars. To date, this method has been applied to a handful of millisecond pulsars observed by the Neutron Star Interior Composition Explorer (NICER). However, analyses of more millisecond pulsars are necessary to determine tight constraints on the equation of state of superdense matter. In this study, we present an analysis of the isolated, rotation-powered millisecond pulsar PSR J2124$-$3358 using the X-ray Pulse Simulation and Inference (X-PSI) package, a publicly available state-of-the-art code for neutron-star relativistic ray tracing and Bayesian parameter inference. We use NICER and Chandra observations of this pulsar, exploring different neutron star atmospheric compositions and different configurations of the hot polar caps responsible for the pulsed X-ray emission. Our analyses favor a helium atmospheric composition, plausibly originating from accretion and subsequent evaporation of a former hydrogen-depleted binary companion. For this composition, and given the faint nature of the source and the low signal-to-noise of the data sets, we obtain broad posterior distributions yielding a mass $M = 1.8\pm0.5\,M_\odot$ and an equatorial radius $R_{\mathrm{eq}} = 11.7^{+2.6}_{-3.0}$ km (medians and $68\%$ credible intervals), and infer a configuration consisting of two slightly non-antipodal hot spots. By contrast, when using a hydrogen atmosphere model, the mass and radius decrease by $\sim 0.5\,M_\odot$ and $\sim 1$ km, respectively. Future multiwavelength studies, particularly those incorporating radio and gamma-ray pulse-emission, may provide tighter constraints on the geometry and physical properties of this source.
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All authors Denis González-Caniulef, Sebastien Guillot, Pierre Stammler, Lucien Mauviard, Christine Kazantsev, Anna L. Watts, Devarshi Choudhury, Bas Dorsman, Mariska Hoogkamer, Daniela Huppenkothen, Yves Kini, Tuomo Salmi, Lucas Guillemot, Ismaël Cognard, Gilles Theureau

Solar and Stellar Astrophysics

SADCat: A Catalog of Supra-Arcade Downflow Events in Solar Flares

Trestan F. Simon and Ryan J. French

Supra-arcade downflows (SADs) are sunward-traveling features routinely observed in hot fan structures above magnetic loop arcades during eruptive solar flares. We manually compiled a catalog (SADCat) of 178 SAD-productive flares imaged by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during solar cycle 24 as a resource for the wider solar flare community. We conducted a preliminary analysis of the SADCat, comparing the flare X-ray and CME properties between eruptive solar flares with and without SADs. We found that peak GOES X-ray flux, flare duration, CME speed, and CME mass have a strong influence on whether a flare produces visible SADs, whereas flare impulsivity and CME acceleration have little effect.
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High Energy Astrophysical Phenomena

Different Jet Dissipation Mechanisms Underlying the Variability in Blazars

Hong-bin Tan and Ruo-yu Liu

Blazars are among the most extreme classes of active galactic nuclei. They are powered by relativistic jets, but the way in which the jet energy is dissipated is still unclear. The flat radio spectrum and the core-shift effect trace the distributions of magnetic fields and relativistic particles along the jet, while variability carries information about time-dependent dissipation. However, a unified framework connecting these observables to the underlying jet physics has been lacking. Here we present a multi-frequency analysis of the prototypical blazar Mrk~501. We model its core-shift measurements, spectral energy distributions (SEDs), and power spectral densities (PSDs) with a conical jet model that conserves magnetic power. The core-shift data localize the radio emitting regions and constrain the electron-density and dissipation-rate profiles along the jet. With a single radial distribution of jet parameters, the model reproduces the core-shift relation and SED, but it underpredicts the observed variability at high radio frequencies and in the optical to $\gamma$-ray bands. We therefore introduce different blob distributions for the inner ($\lesssim$~0.1\,pc) and outer ($\gtrsim$~0.1\,pc) jet regions. With this extended model, the simulated PSDs are consistent with the multiwavelength observations of Mrk~501 during its 2017–2019 low state. This result points to different dissipation behavior in the inner and outer jet. Our study demonstrates that spectro–timing–astrometric jet modeling, which combines SEDs, multiwavelength PSDs, and radio core-shift measurements, can constrain jet stratification and scale-dependent dissipation in blazars.
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Astrophysics of Galaxies

A population of LIGO-Virgo-KAGRA mergers happening inside active galactic nuclei

Dhruv Kumar and Alejandro Torres-Orjuela

Multimessenger observations of compact binary mergers in active galactic nuclei (AGN) offer unique probes of black hole formation channels and cosmology. We analyse 18 gravitational-wave events from LVK observing runs O3–O4b paired with 28 candidate AGN counterparts, incorporating supermassive black hole (SMBH)-induced environmental redshift corrections and electromagnetic sky-localisation constraints. Of the 28 candidate pairs, 21 are positive-to-strong-favoured (the most compelling being GW190412\_053044 $\leftrightarrow$ J143041.67$+$355703.8 at $\ln\mathcal{B} = +17.35$), three are inconclusive, and four are negative-to-strong-disfavoured. From the 21 positive-to-strong-favoured binary black hole (BBH) AGN pairs, 13 unique associations can be identified as the preferred ones. The cumulative Bayes factor across all 13 BBH-AGN confirmed associations yields $\ln\mathcal{B}_{\rm comb} \approx +81$, establishing strong collective support for AGN-hosted merger associations. Sky localisation is the dominant driver of model selection; at current detector sensitivity, the environmental redshift corrections are neither decisively confirmed nor ruled out, motivating future high-precision multimessenger follow-up.
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Cosmology and Nongalactic Astrophysics

Hubble constant measurement with 13 bright standard sirens from binary black hole mergers inside active galactic nuclei

Dhruv Kumar and Alejandro Torres-Orjuela

We measure the Hubble constant $H_0$ using 13 gravitational-wave binary black hole mergers associated with active galactic nucleus hosts. We find $H_0=70.50^{+3.37}_{-2.89}\,({\rm stat})\pm1.56\,({\rm cal})$\,km\,s$^{-1}$\,Mpc$^{-1}$ ($4.4\%$ precision), consistent with both Planck\,2018 ($0.98\sigma$) and SH0ES\,2024 ($0.76\sigma$), with no significant preference between the two. Combining with the bright siren GW170817 sharpens the constraint to $H_0=70.31^{+3.00}_{-2.85}\,({\rm stat})\pm1.55\,({\rm cal})$\,km\,s$^{-1}$\,Mpc$^{-1}$ ($4.2\%$ precision), and further combining with an independent dark-and-bright-siren sample tightens it to $H_0=69.71^{+2.55}_{-2.40}\,({\rm stat})\pm1.54\,({\rm cal})$\,km\,s$^{-1}$\,Mpc$^{-1}$ ($3.5\%$ precision). Assuming a luminosity-distance prior centered around the value related to a fixed cosmology in turn, recovers $H_0=67.62\pm0.72$ (Planck-anchored) and $H_0=72.91\pm0.72$\,km\,s$^{-1}$\,Mpc$^{-1}$ (SH0ES-anchored). We show that under such an assumption, a rejection of $\gtrsim4\sigma$ to the opposing anchor is obtained.
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Earth and Planetary Astrophysics

Spectral Mixture Modeling with Laboratory Near-Infrared Data II: Effects of Grain Size and Implications for Europa

A. Emran

Spectral analysis using linear mixture (LM) and radiative transfer-based (RT) intimate mixture modeling based on Hapke theory at near-infrared wavelengths are applied to estimate the abundance of surface materials on Europa. Previously, Emran (2026) compared these approaches against the laboratory spectra of H$_2$O ice and H$_2$SO$_4$$\cdot$8H$_2$O mixtures with $\sim$100 $\mu$m grains. Here, the effect of particle size on spectral modeling accuracy was assessed using laboratory spectra of H$_2$O ice mixtures with small ($\sim$70 $\mu$m spherical) and coarse ($\sim$1 mm irregular) grains, measured over the $\sim$1.2-2.5 $\mu$m wavelength range at 100 K and 120 K (Stephan et al., 2021). Modeled abundance estimates at both temperatures show consistent trends across all mixing ratios, with only minor temperature-dependent variations. The discrepancy in abundance estimates from both LM and RT models remains within $\pm$10% across all mixtures, with the error reduced to $\pm$5% when fine grains dominate. Across all mixtures, the average difference between RT- and LM-derived abundance estimates remains within $\pm$2% for mixtures containing both small and large grains. In contrast, mixtures composed solely of smaller grains render larger deviations between the models, with RT producing more accurate estimates (Emran, 2026) – indicating that the presence of coarse H$_2$O ice grains minimizes abundance differences between LM and RT modeling. Thus, I posit that Hapke-based RT modeling is the preferred spectral modeling approach – regardless of grain size or compositional mixture – for constraining Europa's surface composition. Nonetheless, LM modeling remains a reliable approach for compositional analysis of terrains containing H$_2$O ice with $\sim$mm-sized grains.
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Friday 3 July

High Energy Astrophysical Phenomena

Understanding Pulsar Magnetospheres with the SKAO

L. S. Oswald, A. Basu, M. Chakraborty et al. (+21 more)

The SKA telescopes will bring unparalleled sensitivity across a broad radio band, a wide field of view across the Southern sky, and the capacity for sub-arraying, all of which make them the ideal instruments for studying the pulsar magnetosphere. This chapter describes the advances that have been made in pulsar magnetosphere physics over the last decade, and details how these have been made possible through the advances of modern radio telescopes, particularly SKA precursors and pathfinders. It explains how the SKA telescopes would transform the field of pulsar magnetosphere physics through a combination of large-scale monitoring surveys and in-depth follow-up observations of unique sources and new discoveries. Finally, it describes how the specific observing opportunities available with the AA* and AA4 configurations will achieve the advances necessary to solve the problem of pulsar radio emission physics in the coming years.
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All authors L. S. Oswald, A. Basu, M. Chakraborty, B. C. Joshi, N. Lewandowska, K. Liu, M. E. Lower, A. Philippov, X. Song, P. Tarafdar, J. van Leeuwen, A. L. Watts, P. Weltevrede, G. Wright, J. Benáček, A. Beri, S. Cao, P. Esposito, F. Jankowski, J. C. Jiang, A. Karastergiou, K. J. Lee, N. Rea, D. Vohl

2607.05432 · 3 Jul · Published in Advancing Astrophysics with the SKA II (AASKAII), 2026 (arXiv:2606.20366). Report-no:AASKAII/Oswald01. Advancing Astrophysics with the SKA II (AASKAII) outlines the transformative scientific advances that will be enabled by the SKA telescopes. N.B. a version of this article was previously published as part of a Special Issue, linked at arxiv:2512.16157

Abstract PDF $PLSR ×5$RDIO ×3$TSCP ×3

Earth and Planetary Astrophysics

Comparing Monte Carlo Models of Impact Alteration of Planetary Atmospheres

MIkayla R. Huffman, David A. Brain, Kelsi N. Singer et al. (+2 more)

One process that affects atmospheric surface pressure is impact bombardment. The evolution of a planet's atmosphere under impact bombardment is an open question. We use a Monte Carlo method to evolve a range (0.006 to 92.5 bar) of initial atmospheres at Mars, Earth, and Venus under bombardment of 5x10^6 impactors using seven individual models. Since these seven models are best suited for specific impactor size regimes, we also combine these models into a composite model and compare it to other existing composites. Alterations to the existing models are required to apply to broad initial conditions. If we use each component model for every impactor, starting from present-day atmospheric pressure, we find about two or three orders of magnitude spread in the final atmospheric pressure. Given these differences, we suggest that the use of any one model to determine atmospheric change due to impact bombardment is risky. Most models and starting parameters result in net growth between +0.01 and +100 bar. Our composite model shows that the atmospheres of Venus, Earth, and Mars tend to grow under bombardment, with Earth's atmosphere growing most quickly. For an early Martian (P_0=1 bar) and an early terrestrial (with an initial pressure of P_0=0.25 bar) atmosphere, both tend to grow under bombardment. The results suggested here, where the models are universally applied, suggest that impact bombardment was likely a significant source of volatiles in the early Solar System. Additional work and careful consideration of how impact events affect the evolution of planetary atmospheres is needed.
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All authors MIkayla R. Huffman, David A. Brain, Kelsi N. Singer, Julia C. Johnston, Caitlin A. Caldwell

Solar and Stellar Astrophysics

Coronal Magnetography using Spectropolarimetry with SKA Telescopes

Deepan Patra, Puja Majee, Soham Dey et al. (+6 more)

The solar coronal magnetic field drives nearly every aspect of solar phenomena and activity – from flares, coronal mass ejections, and solar wind that governs space weather to the much weaker nanoflares. These magnetic fields are routinely measured at the visible surface of the Sun, the photosphere. However, detailed and direct measurements of the magnetic fields in the solar atmosphere, particularly in the coronal layer, have remained rather limited. Mostly, these are estimated from vector magnetic field measurements at photospheric heights through different extrapolation models. In the case of the corona, these extrapolations lack observational constraints from the corona, especially during periods of intense activity when magnetic structures evolve rapidly. Measurements of coronal magnetic fields from observations, therefore, remain one of the most crucial and unresolved challenges in solar and space-weather research. Radio observations of the Sun hold considerable potential in this regard. Observations of diverse emission mechanisms, ranging from plasma emissions at lower frequencies to thermal Bremsstrahlung and gyro-resonance at higher frequencies, provide multiple avenues to probe the coronal magnetic fields, unique at radio wavelengths. SKAO, with its broad frequency coverage (0.05 to 15 GHz), will allow us to probe wide range of coronal layers through unprecedented high-fidelity polarimetric imaging at high temporal, spectral, and spatial resolutions. This chapter details how the coronal magnetic field measurements can be achieved through spectro-polarimetric imaging of the Sun with the SKAO.
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All authors Deepan Patra, Puja Majee, Soham Dey, Devojyoti Kansabanik, Divya Oberoi, Anshu Kumari, Surajit Mondal, Ketaki Deshpande, Divya Paliwal

Solar and Stellar Astrophysics

Archival Diagnostics for Potential Background Contaminantsof Project Hephaistos Dyson Sphere Candidates

Tongtian Ren, Michael A. Garrett, Erik Zackrisson et al. (+4 more)

We report on the diagnostic investigation of nine Project Hephaistos Dyson Sphere candidate M-dwarfs based on archival data. By comparing the Gaia DR3 positions with epoch 2016.0, propagated to the AllWISE epoch, with the mid-infrared centroids measured from the AllWISE images, together with deep archival optical/near-infrared imaging, we identified significant background contamination in candidates B and C–candidate B coincident with a radio counterpart showing a power-law-like spectrum with a radio spectral index alpha = 0.63 +/- 0.11, while candidate C has a near-infrared companion at an offset of 3.75 arcsec. Candidate A provides suggestive evidence through a radio counterpart with spectral index alpha = 0.40 +/- 0.35, while candidates E, F, H and J show marginal evidence. These systems exhibit either significant astrometric offsets or visible interlopers, indicating that the mid-infrared excess likely arises from line-of-sight contamination by hot, dust-obscured galaxies. However, candidates D and I still lack obvious signs of contamination. Dedicated observations are therefore essential to characterise these potential interlopers, eliminate false positives, and ensure that technosignature searches focus on the most robust Dyson Sphere candidates.
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All authors Tongtian Ren, Michael A. Garrett, Erik Zackrisson, Andreas J. Korn, Andrew P. V. Siemion, Jason T. Wright, Alexis Brandeker

Instrumentation and Methods for Astrophysics

Machine Learning and the SKA for Cosmic Dawn and the Epoch of Reionization

Anshuman Acharya, Michele Bianco, Daniela Breitman et al. (+18 more)

When operational, the SKA will generate unprecedented amounts of data and provide exquisite sensitivity for 21 cm tomography of Cosmic Dawn (CD) and the Epoch of Reionization (EoR). With this comes opportunities for new data-driven algorithms that unlock new methods for instrument modelling, data analysis, theoretical simulation, and inference for understanding the high-redshift universe. In this chapter, we provide an overview of some machine learning algorithms that have been proposed for CD and EoR science with the SKA
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All authors Anshuman Acharya, Michele Bianco, Daniela Breitman, Huaxi Chen, Abhirup Datta, Kangning Diao, Sambit K. Giri, Caroline S. Heneka, Nicholas Kern, Adrian Liu, Yashrajsinh Mahida, Suman Majumdar, Samit Kumar Pal, Shulei Ni, Yannic Pietschke, Davide Piras, Abinash Kumar Shaw, Hayato Shimabukuro, Ce Sui, Anshuman Tripathi, Xiaosheng Zhao

Astrophysics of Galaxies

From Atomic Gas to Star Formation

Erik Rosolowsky, Eric Koch, Sambit Roychowdhury et al. (+9 more)

Since the 2015 SKA Chapters, the advent of panchromatic observations of nearby galaxies, capable of resolving the interstellar medium (ISM) into individual star-forming regions and molecular clouds, has revolutionized our understanding of the star formation cycle in galaxies. Despite these advances, our understanding of the gas cycle in galaxies remains limited because we know little about how the cold atomic phase of the ISM, which dominates its mass budget, participates in the cycle. Specifically, the processes regulating the condensation of atomic gas into molecular gas, the balance between warm and cold atomic gas, and their dependence on local environment and star formation activity have been explored only in the Milky Way and the Magellanic Clouds. The SKA presents a new opportunity to observe the atomic ISM within nearby galaxies at comparable resolution to other facilities and is the only facility that can make wide-area surveys of atomic gas across a statistical sample of galaxies at high (<100 pc) spatial resolution. Such observations will directly address the open questions about the evolution of the star-forming ISM. We present recent results from SKA-like observations of the Local Group made with SKA pathfinders and the Jansky VLA to illustrate the promise of replicating those observations for hundreds of nearby galaxies, reaching out to the Virgo cluster.
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All authors Erik Rosolowsky, Eric Koch, Sambit Roychowdhury, Luca Cortese, Filippo M. Maccagni, Barbara Catinella, Nushkia Chamba, Timothy A. Davis, Cosima Eibensteiner, Eric Murphy, Mamta Pandey-Pommier, Amidou Sorgho

Instrumentation and Methods for Astrophysics

SGN: A python framework for stream-processing pipelines

Yun-Jing Huang, Olivia Godwin, Chad Hanna et al. (+11 more)

We present the Stream Graph Navigator (SGN), a lightweight Python framework for building streaming data applications. In SGN, stream-processing pipelines are built by connecting computational components into directed acyclic graphs that run within an event loop. The time-series extension of the SGN library, SGN-TS, introduces signal processing methods to handle time series data. Together, SGN and SGN-TS provide the foundation for SGNL, a matched-filtering gravitational-wave search pipeline, and are being adopted by multiple projects across the low-latency gravitational-wave data analysis infrastructure as an extensible and maintainable framework for future gravitational-wave observations.
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All authors Yun-Jing Huang, Olivia Godwin, Chad Hanna, James Kennington, Jameson Rollins, Max Melching, Nathanael E Sovitzky, Aaron Viets, Madeline Wade, Zach Yarbrough, Yu-Kuang Chu, William Wyatt Phillips, Surabhi Sachdev, Rhiannon Udall

Cosmology and Nongalactic Astrophysics

Hubble tension in k-essence: Evidence for robust tension alleviation

Isaac Opio, Didam Duniya, Bishop Mongwane et al. (+1 more)

The Hubble tension has come to stay as a major problem in modern cosmology as it continues to plague the standard cosmological model ($\Lambda$CDM). As one of the viable, self-consistent dark energy theories, k-essence involves nontrivial self-interactions that can modify the background expansion beyond recombination; thereby impacting the sound horizon to last scattering, and hence, the inferred value of the Hubble constant. We examine this tension in two physically motivated k-essence models, dilaton and tachyon, using datasets from Planck and late-Universe probes including Pantheon+SH0ES, cosmic chronometer (CC), Supernova Cosmology Project Union compilation (Union3), Dark Energy Survey Year~5 (DESY5), and Dark Energy Spectroscopic Instrument (DESI) measurements. While $\Lambda$CDM exhibits inconsistent tension inferences, both k-essence models exhibit a substantial tension alleviation that is robust against the inclusion of the independent late-Universe cosmological datasets, giving consistent tension reduction irrespective of whether the observations are supernovae (Pantheon+SH0ES, Union3, DESY5) alone or in combination with cosmic chronometers (CC) and baryon acoustic oscillation measurements (DESI). The combined late-Universe dataset leads to only $0.14\sigma$ and $0.69\sigma$ offsets from the Planck prediction in the dilaton and tachyon models, respectively, compared to $5.89\sigma$ tension in $\Lambda$CDM. Both models demonstrate that the inferred tension alleviation is a stable, intrinsic consequence of the underlying k-essence dynamics rather than of model fine tuning: model parameters remain unchanged across datasets. The results establish that the apparent Hubble tension is not an unavoidable feature of late-Universe cosmology but depends critically on the description of dark energy.
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All authors Isaac Opio, Didam Duniya, Bishop Mongwane, Hassan Abdalla

Cosmology and Nongalactic Astrophysics

Inflation with nondynamic distortion to leading order in slow roll

Ali Hassan

We study inflation in metric-affine gravity. We write an action that contains all the second order algebraic distortion terms, and all first order distortion terms with a single covariant derivative, coupled to a scalar field. We include the Einstein–Hilbert term with nonminimal coupling, a scalar field potential, and impose projective invariance. The distortion equation of motion is algebraic by construction, and the distortion is integrated out analytically. This yields a kinetic term sourced entirely by distortion, with a kinetic coupling function determined by the 13 free coupling constants of the starting action. We compute inflationary observables for three model classes with a monomial distortion coupling. For a monomial potential, the spectral index and tensor-to-scalar ratio depend only on the ratio of the exponents, with the starting coupling constants dropping out entirely; however, this model lies outside the Planck + BK18 $2\sigma$ contours. For a potential of the $\alpha$-attractor form, the observables are governed by a single parameter and approach the Starobinsky predictions as a limit. Including a nonminimal coupling to the Ricci scalar with a monomial potential can also yield an asymptotically flat effective potential with the same modified Starobinsky observables.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Black Hole Memory Burden and its Signatures in Gravitational Waves from Mergers

Gia Dvali, Michael Zantedeschi and Sebastian Zell

Swift memory burden (MB) implies that the information stored in a black hole (BH) can modify its classical dynamics when the BH is perturbed. This influences the gravitational waves (GWs) emitted during BH mergers. In this paper, we investigate how the BH memory load is determined by the features of the collapsing source. We show that the memory load can vastly exceed the information content of its progenitor. An extreme example is a BH formed in a two-particle collision, which exhibits maximal MB. We then derive bounds for BHs formed through stellar collapse and examine the impact of swift MB on BH quasinormal modes, quantifying the MB-induced frequency shift of GWs. These findings imply that GW observations probe the fundamental mechanisms of BH information storage as well as their formation history.
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High Energy Astrophysical Phenomena

Neutrinos from super-Eddington Seyfert galaxies

Lucas M. Pasquevich, Gustavo E. Romero and Matías M. Reynoso

Multimessenger observations suggest that Seyfert galaxies are promising sources of high-energy neutrinos, but their dense inner environments can strongly suppress the emerging very-high-energy gamma-ray emission. Active galactic nuclei (AGN) undergoing intense accretion episodes can enter a super-Eddington state, in which the accretion flow becomes geometrically and optically thick within a critical radius and develops strong magnetic fields in its innermost region. At the same time, large amounts of matter are expelled from the disk surface in the form of powerful, radiation-driven winds. In this work, we explore a scenario in which the cores of super-Eddington AGN provide suitable conditions for the acceleration of relativistic particles, including hadrons, via magnetic reconnection in a magnetically confined region close to the supermassive black hole. The accelerated hadronic component interacts with the intense photon field of the disk, leading to a copious neutrino flux peaking at 10-100 TeV that may be detectable with current observatories such as IceCube and KM3NeT, while the surrounding outflow efficiently absorbs the accompanying gamma-ray emission from the inner core. We also apply the model to the nearby super-Eddington Seyfert 1 NGC 7469 as a representative case with two reported neutrino events. In this framework, super-Eddington AGN, in particular Seyfert galaxies undergoing transient intense accretion episodes, emerge as plausible hidden neutrino sources, offering a natural explanation for the coexistence of efficient neutrino production and a strongly attenuated gamma-ray counterpart.
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High Energy Astrophysical Phenomena

Beyond the Impulsive Approximation: The Dynamics and Radio Emission of Tidal Disruption Jets

Fabio De Colle and Enrico Ramirez-Ruiz

Radio emission from relativistic tidal disruption events (TDEs) is commonly interpreted using the Blandford–McKee solution for an impulsive relativistic blast wave. Unlike gamma-ray bursts, however, TDE jets are powered by fallback accretion, injecting energy over weeks to months. We investigate how this sustained energy injection modifies the dynamics and radio emission of relativistic TDE jets using one- and two-dimensional relativistic hydrodynamic simulations coupled with synchrotron radiation calculations. For a fixed total energy, fallback-powered jets drive slower forward shocks, producing delayed and systematically fainter radio emission. We show that the impulsive approximation is valid only when the duration of energy injection is less than approximately ten percent of the deceleration time of an equivalent Blandford–McKee blast wave, a condition that is generally not satisfied for relativistic TDEs. As a result, continuously powered outflows are significantly less beamed than impulsive explosions, leading to a strong suppression of off-axis radio emission. These results demonstrate that the radio evolution of relativistic TDEs retains memory of the central engine and cannot, in general, be described by an impulsive solution. By substantially reducing the expected off-axis radio emission, sustained fallback-powered jets imply that relativistic jet production in tidal disruption events may be considerably more common than current radio observations suggest.
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Astrophysics of Galaxies

A MIGHTEE robust measurement of the star formation rate-radio correlation

Charlotte L. Jackson, Matt J. Jarvis, Imogen H. Whittam et al. (+9 more)

Determining the relationship between star-formation rate (SFR) and the radio luminosity ($L_{1.4}$) is critical if we are to trace the star-formation history of the Universe dust-agnostically using current and future radio facilities. However, until now, such work has relied on potentially biased binary classifications of sources to remove contaminating active galactic nuclei (AGN). We present a new, statistically-driven methodology for deriving the SFR – $L_{1.4}$ relation, removing the need for problematic cuts. We use a Bayesian hierarchical mixture model fit to the radio-detected sources in the deep MIGHTEE COSMOS DR1 catalogue, incorporating the full SFR posterior probability distributions generated by state-of-the-art spectral energy distribution fitting code GRAHSP. This allows us to probabilistically determine a mean SFR – $L_{1.4}$ relation for the SF dominated galaxies, whilst accounting for changing fractions of SF dominated sources across redshift, radio luminosity and stellar mass ranges. We find that the SFR – radio luminosity correlation exhibits a significant dependence on redshift, but a stellar mass dependence that is weaker than previous studies. Our resultant SFR-radio correlation is $\log_{10}(\text{SFR}/M_{\odot}\,\text{yr}^{-1}) = 0.790\times(\log _{10}(L_{1.4}/\text{W\,Hz}^{-1})-23) + 1.244 \times(1+z)^{0.122} -0.033 \times (\log_{10}(M_*/M_{\odot})-10)$, with an intrinsic scatter of 0.178 dex. We show that this redshift evolution could be explained by a moderate evolution in the radio spectral index of SF galaxies. We attribute the lack of observed strong dependence on stellar mass, compared to recent studies, to the novel statistical approach that does not rely on cuts to remove AGN.
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All authors Charlotte L. Jackson, Matt J. Jarvis, Imogen H. Whittam, James H. Matthews, Johannes Buchner, Catherine L. Hale, Joel Hamlett, S. Lyla Jung, Mara Salvato, Daniel J. B. Smith, Natalia Stylianou, Rohan G. Varadaraj

High Energy Astrophysical Phenomena

Multiwavelength properties of short gamma ray bursts with extended emission observed by Swift

M.M. Dinatolo, A. Mei, R. Brivio et al. (+11 more)

Short gamma-ray bursts with extended emission (SGRBEEs) are a particular class of long GRBs (LGRBs) which, despite their duration, share several observational features with short GRBs (SGRBs). They are composed by a short, hard initial pulse (IP) followed by a longer and softer extended emission (EE). We investigate whether SGRBEEs originate from the same progenitor as SGRBs despite their duration, representing a peculiar subclass of LGRBs, or if they constitute a distinct population. Given their respective duration, we tested if the IP and the EE share properties with short and long GRBs, respectively. We analysed SGRBEEs from the flux-limited, redshift-complete SBAT4 sample using prompt-emission data from Swift/BAT, Fermi/GBM and Konus-WIND, and X-ray afterglow observations from Swift/XRT. The temporal and spectral properties of the IP and EE components were compared with those of SGRBs from the extended SBAT4 sample and LGRBs from the BAT6 sample. Despite observing a clear spectral evolution during the prompt phase of each SGRBEE, IPs and EEs as well as short and long GRBs can not be distinguished by their hardness ratio only. All bursts analysed have a spectral lag consistent with zero. SGRBEEs XRT light curves are consistently more complex than those of SGRBs, requiring the addition of multiple breaks and showing the presence of steep decays and plateaus. Our results indicate that SGRBEEs are not an intermediate class. During prompt emission, despite common spectral features, IPs and EEs temporally differ from short and long GRBs, respectively. EEs are on average too faint to appear in the Amati plane, but IPs occupy the same parameter space of SGRBs. In the afterglow, SGRBEEs are more luminous than standard GRBs at early-time, suggesting a direct contribution from the EE; at later times, these events behave as standard SGRBs, and both remain systematically less luminous than LGRBs.
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All authors M.M. Dinatolo, A. Mei, R. Brivio, M. Ferro, M.G. Bernardini, P. D'Avanzo, S. Belova, S. Campana, S. Covino, D. Frederiks, B. Haskell, R. Salvaterra, B. Sbarufatti, A. Tsvetkova

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

Gravitational Wave Signatures of Cosmological Stasis: A Unified Spectral Template

Gabriela Barenboim and Anne-Katherine Burns

Proposed in 2022 by Dienes et al., stasis is a dynamical fixed point in the early universe in which the equation of state, $w_s$, is fixed at a constant value. In this study we show that the inflationary gravitational wave imprint of any stasis epoch is captured by a closed-form spectral template controlled by two physical inputs, the equation of state $w_s$ and the stasis duration $\Delta N_\mathrm{stasis}$, that applies uniformly across every microphysical realization. The template presented here yields two independently measurable observables, the spectral tilt of the spectra in the stasis band, $\alpha(w_s)$ and the amplitude step $C^2(w_s)$ at the beginning and end of the stasis band. Eliminating $w_s$ gives a one-parameter consistency curve $C^2 = C^2(\alpha)$ on which the data must lie if the underlying cosmology is any constant-$w$ era. This makes the spectrum falsifiable without knowing $w_s$ in advance: a measured $(\alpha, C^2)$ pair either lands on the curve or rules out the constant-$w$ class. We show that BBO and DECIGO can resolve the perpendicular displacement from the consistency curve to $\sigma_\perp \simeq 1.5\times10^{-5}$ at a tensor-to-scalar ratio, $r = 0.01$, four orders of magnitude below the curve's range in $C^2$ meaning that any off-curve deviation is detectable across a broad range of allowed $r$ values.
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Astrophysics of Galaxies

Pebbles to Gems: Intermediate-mass black holes in the first star clusters

Benedetta Mestichelli, Manuel Arca Sedda, Marta Volonteri et al. (+9 more)

The rapid assembly of supermassive black holes (SMBHs) observed at $z\gtrsim7$ requires efficient seeding mechanisms in the early Universe. Population III (Pop. III) star clusters have recently emerged as a promising pathway that may bridge the gap between traditional light- and heavy-seed scenarios by producing intermediate-mass black holes (IMBHs) with masses up to $\sim10^4\,\rm M_{\odot}$. We investigate the properties and number densities of IMBHs forming in Pop. III star clusters with masses $M_{\rm cl}\sim10^3-4\times10^5\,\rm M_{\odot}$, and hosted in isolated dark matter minihalos, using a suite of direct $N$-body simulations. We adopt cosmologically motivated initial conditions and explore different stellar evolution prescriptions, binary orbital parameter distributions, and cluster dynamical configurations. By $z\sim19$, the IMBH mass function consistently peaks at $m_{\rm IMBH}\sim200\,\rm M_{\odot}$, with number densities of $n_{\rm IMBH}\sim0.2-5\,\rm cMpc^{-3}$. In sufficiently dense and massive clusters, IMBHs with masses $>10^3\,\rm M_{\odot}$ can already form by $z\sim19$, reaching number densities of $n_{\rm IMBH}\sim10^{-4}-10^{-2}\,\rm cMpc^{-3}$. The most massive IMBHs in our models reach $\sim6200\,\rm M_{\odot}$ through the collapse of very massive stars assembled by repeated stellar collisions, a process enhanced in fractal clusters. Lower-mass IMBHs form instead predominantly through single and binary stellar evolution and binary stellar mergers. We find that models combining large stellar radii and tight binaries produce the highest IMBH abundances relative to isolated Pop. III evolution. Owing to the high retention fraction of IMBHs ($\gtrsim88\%$), massive dense Pop. III star clusters can act as efficient incubators of both light and heavy SMBH seeds, even if only a fraction of Pop. III stars formed in such environments.
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All authors Benedetta Mestichelli, Manuel Arca Sedda, Marta Volonteri, Michela Mapelli, Stefano Torniamenti, Alessandro Lupi, Marica Branchesi, Shingo Hirano, Tomoaki Ishiyama, Ralf S. Klessen, Veronika Lipatova, Boyuan Liu

Cosmology and Nongalactic Astrophysics

Pulsar Timing Sensitivity to Dark Matter Substructure in the Presence of a Stochastic Gravitational-Wave Background

Abhiram Cherukupalli, Vincent S. H. Lee, Kim Berghaus et al. (+1 more)

Pulsar timing arrays (PTAs) can detect dark matter (DM) substructure through the small shifts a transiting DM subhalo imprints on pulse arrival times. Recently found evidence for a stochastic gravitational-wave background (GWB) acts as red noise and competes with the substructure signal. Here we provide an analytic understanding of how this background degrades PTA sensitivity to DM substructure. We derive the full gauge-invariant proper-time observable induced by a transiting DM subhalo and develop a framework for the expected signal-to-noise ratio in the presence of red noise, accounting for the degeneracy with the pulsar timing model. From this we obtain simple scaling relations for the reach across the static, dynamic, and stochastic regimes, and numerically compute the reach for a Square Kilometre Array benchmark at three representative points of the NANOGrav 15-year posterior. The GWB background suppresses the sensitivity to DM substructure by one to three orders of magnitude compared to forecasts in the presence of only white noise, and the suppression depends on the amplitude and spectral index of the background within a factor of three. The dynamic Shapiro signal suffers the smallest suppression and gives the best sensitivity to DM substructure near $10^{-2}\, M_\odot$. Probing the regime where subhalos make up all or part of the DM remains a challenge even for surveys with more pulsars and longer observing time. Despite this, PTA measurements remain a competitive probe of DM substructure, and future surveys will increase in sensitivity by up to two orders of magnitude from existing NANOGrav limits.
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All authors Abhiram Cherukupalli, Vincent S. H. Lee, Kim Berghaus, Kathryn M. Zurek