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Physics Astrophysics

astro-ph Watch Astrophysics

Saturday 30 May

Solar and Stellar Astrophysics

Ca-bearing cyanopolyynes in IRC+10216

T. J. Millar

In recent years, a number of metal-containing, carbon-chain species have been detected in the external circumstellar envelope of the carbon-rich AGB star IRC+10216. The most common metal detected in such species is Mg, for which molecules as large as MgC$_5$N, MgC$_5$N$^+$, MgC$_6$H and MgC$_6$H$^+$ have been observed. In this paper, we calculate the likely abundances of the Ca-bearing cyanopolyynes, CaC$_{2n+1}$N for n = 1-4, drawing the conclusion that the observed abundance of CaNC must be made from much larger Ca-terminated cyanopolyyne ions, which requires considerable rearrangement in their dissociative recombination. We pay particular attention to the detectability of CaC$_3$N whose rotational spectrum has recently been measured.
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Earth and Planetary Astrophysics

The Mass, Orbit and Location of Planet Nine Derived from Classical Astrophysics

Mattia Galiazzo and Robert Finch

Our models are based on the planet's presumed influence on the orbits of 12 TNOs: the original 6 analyzed by the CalTech astronomers and an additional 6 selected by the authors using similar although broader criteria. An additional goal of our approach is to make the rationale behind each claim regarding Planet X mathematically visible and verifiable to readers, who can use the models for further analysis with their own TNO data sets should they choose, which is rarely an option with simulations. Using the math models described herein we found that each orbit of the 12 asteroids (12As) lies close to the same plane, presumably the planet's orbital plane. This geometric near-symmetry was found to uniquely determine the planet's orbital elements longitude of the ascending node at +107.7 degree and inclination of +19.5 degree. The planet's argument of perihelion is 307.5 degree. The planet's off axis Lagrange points L4 and L5 were found to play a key role in producing this cluster. At least 4 and possibly 5 of the 12As appear to be in mean motion resonance with the planet, which led to an estimate of the orbit's semi major axis length of a 510 AU and a period of 11512 years. The eccentricity for the planet is 0.39 implying a perihelion of 311 AU. The estimation for the planet's location looks like this: the first two locations estimated to be in the lower region of Taurus, the latter in the upper region of Libra. A wide orbital ellipse that completely surrounds half the asteroids' orbits nearest to their perihelia. The estimated planet's mass is 7.1 Earth masses.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Searching for a waveform-agnostic gravitational wave signal in pulsar timing arrays

Heling Deng, Bence Becsy, Yuri Levin et al. (+2 more)

Pulsar timing arrays have recently provided compelling evidence for a nanohertz stochastic gravitational wave background, motivating searches for gravitational waves from localized sources. Most existing searches assume specific waveform templates, which can be computationally demanding and potentially insensitive to unexpected signals. We introduce a waveform-agnostic framework that models signal-induced timing residuals via a Fourier expansion. A Lorentzian hyperprior is imposed on the variances of the Fourier coefficients, providing a flexible spectral envelope that captures the signal's dominant frequency and bandwidth while remaining agnostic to its exact shape. Analytical marginalization over the Fourier coefficients then yields a Bayesian hierarchical framework that concurrently infers the source sky location, its frequency content, and the stochastic background. To mitigate contamination from unmodeled pulsar noise, we further allow for additional flat-spectrum features for each pulsar. Tests on simulated datasets show that the method is robust and provides a flexible tool for future PTA searches, with sensitivity to both expected and unexpected gravitational wave phenomena.
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All authors Heling Deng, Bence Becsy, Yuri Levin, Neil J. Cornish, Xavier Siemens

Instrumentation and Methods for Astrophysics

Prospects for Astrobiology and Technosignature Searches with the Vera C. Rubin Observatory Legacy Survey of Space and Time

Andjelka B Kovacevic, Nigel J. Mason, Aleksandra Ciprijanovic et al. (+4 more)

he Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will repeatedly measure the brightness of billions of astrophysical sources using six colour filters. We test a simple idea: real physical signals tend to appear as coordinated patterns across several measurements, whereas noise does not; we call this behaviour “coherence”. We illustrate the approach with three LSST-like simulations: (i) five Kuiper Belt Object (KBO) surface/activity states, (ii) 648 exoplanet spectra with an injected vegetation-red-edge-like spectral feature, and (iii) 500 multiband light curves. In idealised simulations, a weakly active KBO is strongly separated from its inactive counterpart when all five colours are combined. The injected red-edge-like feature becomes detectable at effective visible-disc coverage of about 13%, and stacking about 150 detections could reach 5$\sigma$. Coherent, natural, and noise-like variability occupy distinct regions. A small Gaia DR3 stellar sample illustrates how colour and stability may help prioritise Galactic search regions.
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All authors Andjelka B Kovacevic, Nigel J. Mason, Aleksandra Ciprijanovic, Becky Long, Dominika Korczakowska, Maia Moore, Juulia Vuolukka

Astrophysics of Galaxies

Non-detection of HC(S)SH: Estimating Upper Limits and Constraining Chemistry

Dipen Sahu, Ankan Das, Prasanta Gorai et al. (+4 more)

The search for dithioformic acid (t-HC(S)SH) in star-forming regions is crucial for understanding interstellar sulfur chemistry and addressing the 'missing sulfur' problem. Motivated by a recent claim of t-HC(S)SH detection in NGC 1333 IRAS 4A2, we independently reanalyzed the same ALMA dataset using comprehensive spectral and chemical modeling. We find no credible evidence for t-HC(S)SH: all reported transitions are fully accounted for by known, abundant molecules, with no unblended features unique to t-HC(S)SH. We critically reassess the reported detection, deriving stringent upper limits on the column density (N_t-HC(S)SH <= 4 x 10^14 cm^-2) and the fractional abundance (<= 1 x 10^-10 relative to H2). Our astrochemical models place these limits in context, showing the claimed detection likely results from spectral blending and inconsistent modeling assumptions. The non-detection aligns with chemical expectations given the rarity of complex and doubly sulfur-substituted molecules in hot corino environments. Furthermore, our analysis establishes a rigorous framework to guide future searches for sulfur-bearing species and highlights the critical importance of thorough line identification and modeling practices in astrochemistry.
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All authors Dipen Sahu, Ankan Das, Prasanta Gorai, Victor M. Rivilla, Sheng-Yuan Liu, Bhalamurugan Sivaraman, Paola Caselli

Cosmology and Nongalactic Astrophysics

Calibrating Self-Weighted Shear Estimation via Field Distortion

Zhenjie Liu, Jiarui Sun, Jun Zhang et al. (+1 more)

Accurate cosmic shear measurement is the key to fully realize the scientific potential of large scale galaxy surveys. The Fourier\_Quad shear measurement method has been significantly developed to avoid biases caused by various factors, including the point spread function (PSF), photon noise, pixelation effect, selection effects, etc.. Shear statistics has also been optimized by using the PDF-SYM method (symmetrization of the Probability Distribution Function of the shear estimators) to achieve the minimal statistical error, without introducing systematic error. Nevertheless, the iterative nature of the PDF-SYM method makes it computationally expensive for massive datasets. To substantially improve efficiency, we introduce the Self-Weighted Shear Estimation (SWSE) method, which employs a specific self-weighting scheme to naturally suppress the shape noise as well as to balance the contributions from the bright and faint galaxies. Because SWSE is intrinsically biased, the primary focus of this work is to robustly evaluate whether the field-distortion test (FD test, a way to calibrate the shear bias on real data) can accurately recover and correct these inherent multiplicative and additive biases. Using both simulated and real survey data, we demonstrate that the FD test can successfully calibrate SWSE on galaxy-galaxy lensing measurements, enabling it to achieve statistical precision comparable to PDF-SYM. Our results establish the combination of FD test and SWSE as a robust shear estimation approach for forthcoming large-scale weak lensing surveys.
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All authors Zhenjie Liu, Jiarui Sun, Jun Zhang, Cong Liu

Cosmology and Nongalactic Astrophysics

Testing the cosmological principle with quasars

Shuangnan Chen, Xiaofeng Yang, Yunliang Ren et al. (+5 more)

The inferred velocity is consistent at the 1.56 {\sigma} level with the value of 370 km/s from a purely kinematic interpretation of the CMB dipole. Based on the motion direction component analysis, we have not found any significant deviation from cosmological principle in current released quasars data. The cosmological principle posits that the universe is homogeneous and isotropic on the large scales. In history, the cosmological principle was confirmed by various cosmological observations from CMB to large scale structure. However, several new challenges to the cosmological principle were reported in recent years, particularly in radio observations from overdispersed radio source counts to quasars. Here, we firstly present studies on the peculiar velocity of large-scale anisotropy by measuring the dipole signal from the DESI DR1 catalogue with a sample of 1,176,570 quasars (0.8 < z < 3.0). Our analysis reveals the peculiar velocity of $|v| = 443.8 \pm 204.1$ km/s towards $(l, b) = (107.4^\circ \pm 86.8^\circ, 28.4^\circ \pm 45.2^\circ)$ in Galactic coordinates.The motion direction deviates from the CMB dipole (264.02$^\circ$, 48.253$^\circ$). The inferred velocity is consistent at the 1.56 $\sigma$ level with the value of 370 km/s from a purely kinematic interpretation of the CMB dipole. Based on the motion direction component analysis, we have not found any significant deviation from cosmological principle in current released quasars data.
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All authors Shuangnan Chen, Xiaofeng Yang, Yunliang Ren, Xuwei Zhang, Yangjun Shi, Cheng Cheng, Xiaolong He, Sufen Guo

Cosmology and Nongalactic Astrophysics

Interacting dark energy constraints from Fermi GRBs and Pantheon+ SNe Ia with full GRB covariance

Jianfeng Meng, Xiaofeng Yang, Yunliang Ren et al. (+4 more)

The standard $\Lambda$CDM model faces long-standing theoretical and observational problems, such as the Hubble tension, which motivate extensions beyond $\Lambda$CDM, including interacting dark energy (IDE). Type Ia supernovae (SNe Ia) are precise probes of the late-time expansion history, while gamma-ray bursts (GRBs) can extend the Hubble diagram to higher redshifts. However, GRB cosmology depends on careful calibration and uncertainty modeling. Using an Amati relation calibrated with the corresponding low-redshift GRBs, we construct distance moduli for the high-redshift subsets of the 15-year Fermi/GBM GOLD and FULL samples and combine them with Pantheon+ SNe Ia to compare flat $\Lambda$CDM, $w$CDM, IDE-$\rho_{\rm de}$, and IDE-$\rho_{\rm c}$ models. The covariance of the calibrated Amati intercept and slope is propagated into a full, non-diagonal GRB distance-modulus covariance, and an effective residual scatter, $\sigma_{\rm res,\mu}$, is fitted jointly with the cosmological parameters. The GOLD and FULL samples yield very similar constraints on the main cosmological parameters. With either the full or diagonal GRB covariance, the IDE models do not improve likelihood sufficiently to compensate for their additional parameters, and the BIC favors $\Lambda$CDM. A SNe-only comparison shows that the additional constraining power of the GRBs is generally modest. Current GRB and Pantheon+ distance measurements provide no significant evidence for either of the two IDE interactions considered here.
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All authors Jianfeng Meng, Xiaofeng Yang, Yunliang Ren, Yangjun Shi, Bohao Wang, Jingze Li, Xiongwei Liu

Cosmology and Nongalactic Astrophysics

Decaying Dark Matter as a Possible Solution for Cosmological Tensions

Yaman Acharya and Ryan E. Johnson

Large-scale structure measurements have revealed persistent tensions between early- and late-time cosmological probes, most notably the long-standing discrepancy in the structure-growth parameter $S_8$. In this work, we explore how a model including decaying dark matter (DDM) can alleviate this tension by suppressing the growth of matter fluctuations at late times. Specifically, we consider a neutrinophilic decay channel in which a heavy dark matter particle $\chi$ slowly decays into a Standard Model neutrino and a light invisible fermion, $\chi \rightarrow \nu + \phi$, modifying both the background evolution and the clustering of structure. Using the DES Year~1 redshift distributions, we construct a baseline matter power spectrum and compute the galaxy-galaxy, shear-shear, and galaxy-shear angular power spectra under both $\Lambda$CDM and DDM-inspired scenarios. We find that slow dark matter decay produces a scale-dependent suppression of clustering that remains consistent with DES measurements while naturally shifting the predicted structure amplitude toward the lower values favored by weak lensing surveys. Our results suggest that decaying dark matter is a compelling and physically motivated framework for addressing the $S_8$ tension.
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High Energy Astrophysical Phenomena

Suppression of boosted relic neutrinos by photon backgrounds during ultra-high-energy cosmic ray propagation

Gabriel Azeredo and Vitor de Souza

Constraining the cosmic neutrino background (C$\nu$B) represents a major experimental challenge in cosmology. Recent studies have suggested that relic neutrinos boosted by ultra-high-energy cosmic rays (UHECRs) may generate observable diffuse neutrino fluxes. Previous estimates have not effectively propagated the primary cosmic rays, often neglecting crucial energy losses and the unavoidable, competing interactions with diffuse photon backgrounds. Here we revisit these expectations using a realistic Monte Carlo propagation framework. This approach allows us to consistently incorporate cosmic ray energy losses, nuclear photodisintegration, and production of secondary neutrinos. We show that interactions with diffuse photon backgrounds strongly suppress the boosted relic neutrino flux predicted in simplified propagation scenarios. Furthermore, we demonstrate that to produce any observable suppression on the UHECR energy spectrum at Earth, or for the boosted C$\nu$B component to become comparable to the cosmogenic neutrino flux, the C$\nu$B density must be enhanced by a factor, the so-called overdensity, of extreme magnitude ($\eta \gtrsim 10^{8}$).
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Solar and Stellar Astrophysics

Non-linear Dynamical Stability of Magnetic Polytropes

Bryan M Johnson

This work analyzes the non-linear dynamical stability of ideal-gas polytropes under homologous flow. A non-constant density profile requires the inclusion of magnetic fields, which is done by introducing a mean-field model that treats the spherically-averaged radial Lorentz force self-consistently and has the following properties: 1) The only essential simplifications are the Cowling approximation and a dominant radial flow. 2) The average radial Lorentz force due to an isotropic field is $-\frac13 dP_B/dr$, not $-dP_B/dr$ as is typically assumed. 3) A central peak in the magnetic field requires isotropy there; all other configurations are zero at the origin due to magnetic tension. 4) Solutions with negligible surface fields require $\gtrsim1/2$ of the magnetic energy to be in the radial component. 5) Solutions that resemble Lane-Emden solutions are restricted to $\gamma = 4/3$, where $\gamma$ is the material adiabatic index, and exhibit either collapse or escape. 6) Solutions for general $\gamma$ have a harmonic enthalpy profile and allow for non-linear radial pulsations. 7) A harmonic-enthalpy homologous flow becomes unbound when an overpressure satisfies $\delta = \Delta P_0/P_{\rm eq} > \frac{3\gamma - 4}{1 + 3(\gamma-1)\alpha_0}$, where $P$ is the total pressure, $P_{\rm eq}$ is its equilibrium value, $\alpha$ is the ratio of radiation to material pressure, and a zero subscript denotes minimum volume. This indicates that radiation pressure can unbind a linearly-stable polytrope in the presence of small but finite radial perturbations. The condition to unbind a fully-ionized $n = 3$ polytrope with $2/3$ of its magnetic energy in the radial component is $\delta \gtrsim 0.15\mu^{-1}\left(300M_\odot/M\right)^{1/2}$, where $\mu$ is the mean molecular weight. This non-linear dynamical instability threshold may have some relevance for mass loss in and dispersal of evolved high-mass stars.
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Instrumentation and Methods for Astrophysics

Some Thoughts on the Future of Technosignature Searches: Constraining the Fermi Paradox

Ian A. Crawford

This paper examines how future technosignature searches may constrain competing resolutions of the Fermi Paradox, with particular attention to the possibility that technologically capable entities (TCEs) are either intrinsically rare or deliberately concealed. I propose a multi-pronged observational strategy comprising expanded radio and optical SETI, spectroscopic searches for biosignatures and technosignatures in exoplanet atmospheres, astronomical searches for large-scale extraterrestrial engineering, and Solar System searches for extraterrestrial artefacts (SETA). The latter is identified as having a distinctive temporal advantage because it can probe evidence accumulated over Solar System (and perhaps even Galactic) history, rather than requiring temporal overlap with TCEs. In this context, I argue that searches for micron-scale interstellar technological debris (Arkhipov particles) in lunar and planetary regoliths may provide an additional constraint on past Galactic technological activity. Additionally, I briefly argue that this programme of scientific exploration should be considered alongside its geopolitical and governance implications.
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Earth and Planetary Astrophysics

Jets and Superrotation in Deep and Shallow Planetary Atmospheres

Geoffrey K Vallis, Loren I. Matilsky and Quentin Nicolas

Zonal flows in planetary atmospheres are ubiquitous, and nearly all the planets in the Solar System have flows that are zonally rather than meridionally aligned. Jets, which are essentially concentrated streams of fluid that are distinct from a more quiescent background, are less common but can also be found in both deep and shallow atmospheres. Superrotation, which in most circumstances simply means prograde motion (relative to the planetary rotation) at the equator, is less common but can be found in both deep and shallow planetary atmospheres, and in both quickly and slowly-rotating atmospheres: Jupiter, Saturn, Venus and Titan all have superrotating atmospheres. Jets, especially superrotating jets, imply some form of `antidiffusion' of momentum, meaning that momentum (or angular momentum) must be transported upgradient. This article discusses some of the mechanisms that give rise to jets and superrotation in both deep and shallow planetary atmospheres, on both slow and fast rotators, contrasting and comparing the processes involved. Topics discussed include the roles of convection in deep atmospheres, geometric and topographic $\beta$-effects, potential-vorticity homogenization, wave–mean-flow interaction and tidal locking in exoplanets.
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Solar and Stellar Astrophysics

Narrowing the solar surface flux transport parameter space through nonlinear feedbacks

M. Alhosani, M. H. Talafha and M. A. Al-Wardat

The surface flux transport (SFT) model describes the evolution of the Sun's large-scale photospheric magnetic field. While linear transport processes are relatively well constrained, nonlinear feedbacks such as tilt and latitude quenching remain less explored, despite their potential role in regulating the solar cycle. We aim to determine how nonlinear quenching mechanisms and flux decay influence the admissible parameter space of the SFT model, and to identify parameter combinations that reproduce the observed characteristics of the solar polar magnetic field. We extend the parameter-space optimisation by introducing analytic prescriptions for tilt quenching (TQ) and latitude quenching (LQ) in the source term, together with a tunable flux-decay term. The model is solved numerically over a grid of meridional flow speeds ($u_{0}$), surface diffusivities ($\eta$), and decay timescales ($\tau$). Admissible solutions are defined by agreement with {observationally motivated constraints on the {polar-field minimum-to-extremum amplitude ratio}, reversal timing, and the latitude of the polar cap boundary. Both TQ and LQ reduce the admissible parameter domain, with LQ exerting the stronger influence. Their combined action produces a pronounced saturation ("ceiling") that limits axial dipole amplification. The inclusion of a finite flux-decay timescale ($\tau \simeq 8$–$10$~yr) further narrows the admissible domains within the present parameterised source formulation, whereas the non-decaying case may lead to unrealistically persistent dipole fields in this modelling framework. Correlations between $u_{0}$, $\eta$, and $\tau$ reveal a coupled balance between advection, diffusion, and phenomenological flux loss.
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Friday 29 May

High Energy Astrophysical Phenomena

A synchro-curvature treatment of gamma-ray luminosity trends in pulsars

A. Pathania, K. K. Singh and K. K. Yadav

In recent years, the Large Area Telescope (LAT) onboard the Fermi satellite has detected more than 300 pulsars in the high energy range. The population studies of high energy pulsars show that the gamma ray luminosity of a pulsar ($L_\gamma$) can be expressed in terms of the spin down luminosity ($\dot{E}$) as $L_\gamma \propto \dot {E}^s$ having exponent $s\sim 0.68$. This high energy emission, assumed to originate far from the stellar surface and near the light cylinder, is usually studied in either purely curvature or purely synchrotron regime. In this work, we adopt a synchro-curvature radiation framework to understand the origin of gamma ray emission from the pulsar and its implications at the population-level. By comparing the observed cutoff energies of the differential gamma-ray spectra with the theoretical synchro-curvature predictions and enforcing radiation reaction approximation, we determine the equilibrium Lorentz factor and pitch angle of the emitting charged particles. This approach allows to quantify the relative roles of curvature and synchrotron radiation to the radiative losses, thereby providing a physically grounded interpretation of the luminosity trend across the pulsar population.
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Astrophysics of Galaxies

Vision-Language Model Ensembles Achieve Human-Expert Accuracy for Galaxy Merger Classification

Marco Chiaberge, Elias Stengel-Eskin, Massimo Stiavelli et al. (+1 more)

Models (VLMs) combined using a Bayesian statistical framework can classify galaxy merger morphologies with accuracy comparable to trained human experts. We deploy 15 VLM classifier configurations, spanning four model architectures (Gemma-4 E2B, Gemma-4 E4B, Qwen2.5-VL, and Qwen3-VL) tested with up to four prompt engineering strategies each. We evaluate their performance against a truth-known sample of 41 VELA+SUNRISE mock galaxy images from Lambrides et al. (2021a). As a proof-of-concept, all validation is performed on these mock images; application to real observational samples will require additional calibration and observational validation. The VLM ensemble achieves 83.3% accuracy on confident classifications (merger probability pM >= 0.8 or pM <= 0.2) and 58.3% completeness, with 5 misclassified galaxies, compared to 85% for both human accuracy and completeness. The ensemble recovers the population merger fraction to within 0.66 sigma of the truth (fM = 0.52 +/- 0.09 vs. true value of 0.585). Bayesian weighting improves overall accuracy by 17.1 percentage points over simple majority voting, with sensitivity improving by 29.2 percentage points. The ensemble produces 5 misclassifications (2 false positives, 3 false negatives), comparable to the 6 misclassifications (5 false positives, 1 false negative) reported for human classifiers by L21. The error-profile differences are not statistically significant for this sample. VLMs also produce more moderate per-galaxy merger probability distributions (27% uncertain) than the more polarized human distributions (15% uncertain), though this difference is also consistent with statistical fluctuation. These results establish VLMs as scalable, reproducible alternatives to human classifiers within a Bayesian probabilistic merger-fraction framework, for large-survey applications.
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All authors Marco Chiaberge, Elias Stengel-Eskin, Massimo Stiavelli, Colin Norman

2606.00415 · 29 May · This work used LLMs as research assistants for code development, statistical analysis, and drafting under close human supervision; the workflow is described in Appendix A. Submitted to the Astronomical Journal. Revised version includes responses to the referee's comments and the requested additional tests, minor changes to Fig 2 and Table 8. Appendices C1 and C2 titles corrected (swapped)

Abstract PDF $GLXY ×5$BAYS ×3$LANG ×1$PRMT ×1 +1

Cosmology and Nongalactic Astrophysics

The sound of dynamical dark energy and modified gravity

João Rebouças, Guilherme Brando, Felipe T. Falciano et al. (+1 more)

Different candidate models are able to reproduce the dynamical dark energy signal preferred by combinations of recent distance measurements. These models may be distinguished by the behavior of their perturbations, which are controlled by the effective sound speed $c_s^2(k,a)$. To explore correlations between the dark energy sound speed and perturbative behavior, we test modified gravity (MG) scenarios in which the dark energy equation of state and sound speed determine modifications to the clustering of matter. We investigate the impact of varying the dark energy sound speed on several cosmological quantities in both General Relativity (GR) and MG. We constrain the dark energy and modified gravity parameters using measurements of the Cosmic Microwave Background (CMB) from Planck PR4, type Ia supernova luminosity distances (SN) from Pantheon+, Baryon Acoustic Oscillations (BAO) from DESI DR2, and cosmic shear from DES-Y3. Using the combination of CMB+BAO+SN, we find that, in the MG scenarios, the preference for dynamical dark energy is correlated with deviations from GR over redshifts $z < 2$ at over 95% confidence level. The significance of these deviations is not degraded when considering a dynamical or superluminal sound speed, but vanishes if we assume a cosmological constant. The inclusion of cosmic shear and CMB lensing data significantly shifts the constraints towards GR. Our framework enables the exploration of modified gravity models using the dark energy sound speed as a physically meaningful free parameter.
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All authors João Rebouças, Guilherme Brando, Felipe T. Falciano, Vivian Miranda

Astrophysics of Galaxies

Spectroscopic characterization of Young Stellar Populations and their Feedback in NGC 5253

Svea Hernandez, Linda J. Smith, Valentina Abril-Melgarejo et al. (+1 more)

We present the spectroscopic analysis of FUV observations taken with the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) targeting young massive clusters in the nearby, metal-poor, blue compact dwarf galaxy NGC 5253. We characterize the stellar populations observed across seven COS pointings and report on their inferred physical parameters, age, metallicity, mass, and reddening values. Comparison between our spectroscopic ages and those inferred using photometric methods show that the former are preferentially younger. We also investigate the impact of these young massive clusters on their surrounding ISM. Using Very Large Telescope/MUSE optical observations and matching the size of the COS aperture, we measured outflow velocities of the ionized gas along the line of sight of the COS pointing with values ranging from $\sim$125-300 km s$^{-1}$. We report on strong statistically-significant correlations between the outflow velocities and the stellar ages, masses, and total mechanical luminosity primarily driven by supernovae (SNe) as derived from our full-spectrum fitting analysis. Although theoretical models predict a delayed injection of mechanical energy and momentum in low-metallicity environments ($<$0.4 Z$_{\odot}$), our study shows that in this particular system (Z$\sim$ 0.3 Z$_{\odot}$), feedback from SNe appears evident at the nominal $>$ 5 Myr ages, with no apparent delay. One possible explanation is that the decrease and suppression of mechanical feedback due to SNe explosions might be dominant at even lower metallicities than those observed in NGC 5253.
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All authors Svea Hernandez, Linda J. Smith, Valentina Abril-Melgarejo, Bethan James

High Energy Astrophysical Phenomena

Hard X-ray Sources in Fermi UFOs

Alessandro Paggi, Ioannis Liodakis, John Antoniadis et al. (+7 more)

Identification and/or association of unidentified $\gamma$-ray sources with lower-energy counterparts represents a key challenge in modern astronomy, due to the relatively large positional uncertainty provided by $\gamma$-ray detectors. We selected Unidentified Fermi Objects (UFOs) positionally compatible with hard X-ray sources in the latest Palermo Swift-BAT hard X-ray Catalog and in the SRG/ART-XC all-sky X-ray survey Catalog, to identify lower-energy sources and possibly associate them to the UFOs. We found 17 UFOs with overlapping hard X-ray sources. We then collected soft X-ray data from Swift-XRT, Chandra-ACIS, XMM-Newton-EPIC, and eROSITA, identified 16 soft X-ray counterparts to the hard X-ray sources, and associate 15 with known astronomical objects, classified as: blazars/blazars candidates (2 sources), Seyfert galaxies (5 sources), X-ray binaries (2 sources), generic X-ray sources (1 source), cataclysmic variables (2 sources), and variable stars (3 sources). Blazars and Seyfert galaxies are likely lower-energy counterparts to the UFOs, since their mid-IR colors and broad-band spectral energy distributions suggest significant jetted, non-thermal emission. X-ray binaries can be potential lower-energy counterparts to the UFOs, since this class of sources has been already observed to emit $\gamma$-rays. The generic X-ray source has been proposed as a pulsar candidate, and we therefore suggest that it can be the lower-energy counterpart to the UFO. Cataclysmic variables have been suggested as potential $\gamma$-ray emitters so, if confirmed, the 2 sources classified as cataclysmic variables would represent the first $\gamma$-ray emitting sources of this kind. Finally, we consider the association of the 3 variable stars with the UFOs unlikely.
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All authors Alessandro Paggi, Ioannis Liodakis, John Antoniadis, Chien-Ting Chen, Steven R. Ehlert, Daniel Gruen, Philip Kaaret, Ignacio De La Calle Perez, Elena Jimenez Bailon, Mykhailo Ilin

Instrumentation and Methods for Astrophysics

Building a Global Astrotourism Community of Practice Through Astronomy for Development

Joyful E. Mdhluli

Astrotourism is an emerging interdisciplinary field situated at the intersection of astronomy, tourism, cultural heritage, and sustainable development. Despite growing global interest, the field remains loosely defined and under-theorized, with limited empirical studies documenting practitioner perspectives and community-led initiatives. This paper presents findings from the first Astrotourism Community Exchange hosted by the International Astronomical Union Office of Astronomy for Development (OAD), which brought together over 150 participants from Africa, Asia, Europe, the Middle East, and Latin America - including practitioners, researchers, educators, tourism processionals, and community developers. Using qualitative thematic analysis of a recorded online community session, we explore how practitioners conceptualize astrotourism, the models being implemented across diverse contexts, and the key challenges shaping the field's development. Five major themes emerged: (1) astrotourism as a tool for community development, (2) cultural heritage and knowledge systems, (3) environmental sustainability and dark sky protection, (4) accessibility and inclusive design, and (5) conceptual and methodological gaps in the field. Across all themes, astrotourism was consistently framed not merely as a form of niche tourism, but as a hybrid socio-cultural and environmental practice with significant implications for sustainable development and science engagement. We argue that astrotourism is evolving into a global community of practice rather than a fixed tourism category, and we highlight the need for further interdisciplinary research, data collection frameworks, and inclusive policy development to support its growth.
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Solar and Stellar Astrophysics

Halo stars harbour few wide ultracool companions

Jerry Jun-Yan Zhang, Nicolas Lodieu and Eduardo L. Martín

We conducted the first search for wide ultracool companions to metal-poor halo stars. A sample of nearby halo stars with spectroscopically determined metallicities and high proper motions was imaged in the $J$ band and examined for faint candidate companions. Follow-up imaging over baselines of two to four years enabled a search for common proper-motion sources. The survey reached average limiting magnitudes of $J_{\mathrm{lim}}=22.8$ and $23.0$ mag (Vega) in the first and second epochs, respectively, sufficient to detect extreme subdwarfs earlier than esdT0 out to 250 pc. No bona fide wide ultracool companion was identified over projected separations of a few hundred to a few thousand au. We therefore derive an upper limit of $4.0\%$ (at a $90\%$ confidence level) on the frequency of wide ultracool companions to metal-poor halo stars. Four wide stellar companions were recovered and confirmed with Gaia, yielding a wide stellar companion frequency of $6.1^{+7.2}_{-4.0}\%$ (at a $90\%$ confidence level). We conclude that wide ultracool companions are intrinsically rare around metal-poor halo stars and that their occurrence rate is, at most, comparable to that of wide stellar companions. Current observations provide no evidence for a metallicity dependence of the wide ultracool companion frequency around stars. Formation and retention processes in binary systems are likely to operate less efficiently for ultracool secondaries.
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Astrophysics of Galaxies

Constraining Extragalactic Proper Motion with Gaia Astrometry

Sean Maloney, Tianqing Zhang, Rupert Croft et al. (+1 more)

The Solar System's secular motion with respect to the cosmic microwave background (CMB) rest frame is inferred from the CMB dipole and should induce a tiny, coherent apparent drift in the positions of nearby galaxies, referred to as the extragalactic proper motion. We test the feasibility of a purely geometric measurement of this effect by combining Gaia DR2 and DR3 astrometry with low-redshift spectroscopic galaxy catalogs to build a large, full-sky sample of $67,173$ galaxies. Although we do not obtain a statistically significant detection of the expected dipole signal, we place the tightest constraint to date on the extragalactic proper motion $\bar{\pi}$. Using galaxies with comoving distance $D>5 {\, \rm Mpc}$, we also place the tightest constraints on cosmic extragalactic proper motion $\bar{\pi}_{\rm cosmic}$, with uncertainty $\sim 10\times$ the measured CMB dipole value. Our $1\sigma$ uncertainty on the near field extragalactic proper motion $\bar{\pi}_{\rm nf}$ is approximately $\sim 1.3\times$ the expected CMB measurement, demonstrating that Gaia astrometry is approaching the sensitivity required for a direct detection of near field Hubble constant in future releases.
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All authors Sean Maloney, Tianqing Zhang, Rupert Croft, Konstantin Malanchev

Astrophysics of Galaxies

OCEANS of Absorption: High-resolution NIRSpec Spectroscopy Reveals Diverse Balmer-line Absorption in Little Red Dots

Kelcey Davis, Madisyn Brooks, Raymond C. Simons et al. (+24 more)

The “Little Red Dots' (LRDs) that appeared in JWST deep field images have been the subject of significant study since their discovery. In this work, we present high-resolution follow-up spectroscopy from the OCEANS program of 10 LRDs with Ha coverage at 3<z<7 in the CEERS/EGS field. We find Balmer-line absorption in 4 of these LRDs, a detection rate higher than the fractions reported in lower-resolution NIRSpec surveys. All of the absorbers are presented in high-resolution for the first time here and two have Balmer-line absorption detected for the first time. Of the 10 LRDs, 7 are best fit by Ha profiles with exponential wings. We find that absorbers tend to be blue-shifted with a median velocity offset of (-49 km/s) and absorption equivalent width of 5.3 Angstroms. Trends are explored to compare LRD absorption properties along the sequence of LRDs. We confirm an LRD with statistically significant absorption velocity offsets between Ha and Hb. The diversity of absorption properties can be effectively explained by a model with a radial distribution of partial-covering absorbing gas that is often co-located near the broad-line emission regions, along with a radial gradient of close inflow and distant outflow velocities for the absorbing gas. We present other interesting LRDs, including an outflow-dominated LRD and an LRD with relatively blue UV-to-optical colors but clear Balmer-line absorption. This high occurrence of absorbing hydrogen in LRDs, evident by both the Balmer-line absorption features and Balmer break strengths, implies a near-ubiquitous presence of dense, excited n=2 hydrogen.
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All authors Kelcey Davis, Madisyn Brooks, Raymond C. Simons, Jonathan R. Trump, Guillermo Barro, Pablo Arrabal Haro, Bren E. Backhaus, Nikko J. Cleri, Alexander de la Vega, Steven L. Finkelstein, Mauro Giavalisco, Norman A. Grogin, Michaela Hirschmann, Taylor A. Hutchison, Dale Kocevski, Anton M. Koekemoer, Erini Lambrides, Mario Llerena, Ray A. Lucas, Madeline A. Marshall, Elizabeth J. McGrath, Casey Papovich, Aidan Starrs, Anthony J. Taylor, Phoebe R. Upton Sanderbeck, Xin Wang, Stijn Wuyts

High Energy Astrophysical Phenomena

The Delta Resonance in the Neutrino Sky

Arifa Khatee Zathul, Ke Fang, Francis Halzen et al. (+1 more)

Recent measurements of the diffuse cosmic neutrino flux by IceCube show evidence for a spectral break at an energy near $E_\nu \sim 30$ TeV. In this letter, we suggest that this feature may be due to the $\Delta$-baryon resonance in $p\gamma$ interactions. We show that the measured spectrum, including the observed break, can be naturally accommodated by a flux of protons accelerated with a spectrum $dN_p /dE_p \propto E_p^{-3.1}$ interacting with X-rays of typical energy $E_{\gamma} \sim 0.3\,{\rm keV}$. We also point out that the presence of this spectral break significantly reduces the contribution of neutrino sources to the isotropic gamma-ray background, alleviating the longstanding tension between these measurements. In the $\Delta$-resonance scenario, the gamma rays accompanying neutrino production cascade down to MeV-GeV energies and contribute at the $\sim 10\%$ level to the isotropic gamma-ray background at $\sim 3$~GeV. If our proposal is realized, it may imply that we have identified the dominant sources that produce the extragalactic cosmic rays.
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All authors Arifa Khatee Zathul, Ke Fang, Francis Halzen, Dan Hooper

High Energy Astrophysical Phenomena

BBH-Genesis: Disentangling Binary Black Hole Formation Channels with GWTC-4

Shaunak Padhyegurjar and Suvodip Mukherjee

The detected population of binary black holes (BBHs) from the gravitational wave (GW) data has made it possible to decipher their formation and evolution history over cosmic time. The complexity of astrophysical modeling of binary mergers makes it challenging to predict key signatures for different formation channels. As a result, one of the major avenues to discover the presence of different channels from detected GW events is through a data-driven way which can isolate different scenarios. In this spirit, we developed a new inference pipeline BBH-Genesis and applied it on the fourth GW catalog (GWTC-4) to identify the presence of multiple underlying distinct populations. We find that the current population of all the binary events in GWTC-4 can be explained with the strongest evidence for only a two-channel scenario, hinting at the presence of a non-isolated binary formation channel. This sub-population can be further divided into a third channel with mild support towards formation in AGN exhibiting a slightly different effective spin and mass ratio correlation. In the future, with the detection of more events, it will be clearer whether it is necessary to consider at least three channels to explain the BBH events detected using GW observations.
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High Energy Astrophysical Phenomena

Comparative properties of X-Ray Flashes and Gamma-Ray Bursts from BeppoSAX observations of Fast X-ray Transients

L. Piro, G. Gianfagna, J.J.M. in't Zand et al. (+5 more)

We present the homogeneous and complete sample of 96 bona-fide Gamma Ray Bursts (GRBs) detected by the Wide Field Cameras and Gamma Ray Burst Monitor of BeppoSAX. We derive the spectral and temporal properties of the prompt emission, and assess the properties of the soft population of GRB, namely X-ray flashes (XRFs) in comparison with normal GRBs. We find that 36 events are XRFs, 40 X-ray rich events (XRR), and 20 normal GRBs. We analyze the distribution of the spectral parameters of the Band function, finding that the spectral indexes of the three classes are broadly similar. On the contrary the peak energy is the parameter driving the spectra shape, from 8.5 keV for XRF keV to 83 keV for GRBs. For the 67 events that are detected in both instruments we find that 9 events exhibit a soft X-ray precursor taking place from 14 to 105 s before the onset of the gamma-ray burst. About 90% of the events that were identified in real time exhibit an X-ray afterglow, with a similar fraction for the three classes. In the optical and radio the the corresponding fractions are 35% and 33%. All the similarities in the spectrum, duration and afterglow properties suggest common progenitors for the three classes, where the differences are likely a combination of the effect of different baryon loading, energy, structure and orientation of the jet with respect to the observer. A comparison with Einstein Probe shows that the latter, thanks to its sensitivity, reaches out to a population of fainter and more numerous events, whose presence was firstly hinted at by the unique very low luminosity BeppoSAX GRB980425.
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All authors L. Piro, G. Gianfagna, J.J.M. in't Zand, B. Gendre, C. Guidorzi, L. Amati, F. Frontera, E. Kuulkers

Astrophysics of Galaxies

It's Not Just Star Formation: A trend of low dark matter densities in the Andromeda dwarf galaxy system

Connor S. Pickett, Michelle L. M. Collins, Justin I. Read et al. (+8 more)

Dynamical mass modeling of Andromeda (M31) dwarf spheroidal (dSph) galaxies has revealed a growing trend of lower central dark matter (DM) densities than predicted by pure DM structure formation in Lambda Cold Dark Matter ($\Lambda$CDM) cosmology simulations and lower than most Milky Way (MW) satellites. So far, however, only four of the 35 confirmed M31 dSphs have been successfully mass modeled. In this second paper of a series, we aim to better understand growing Local Group (LG) dSph patterns by mass modeling seven more M31 dSphs: Andromeda I, III, V, VII, IX, XXXI, and XXXII. We update the kinematics of each dwarf and estimate their central dark matter densities at 150 pc using the dynamical Jeans modeling tool, GravSphere. We also update their DM halo mass, $M_{\rm{200}}$, via abundance matching. We find Andromeda III and V to have central DM densities in line with $\Lambda$CDM expectations, resembling dSphs around the Milky Way. The remaining five dwarfs have anomalously low central densities, continuing a growing trend seen for M31 satellites. We investigate each dwarf's star formation history and find that star formation-induced `DM heating' is disfavored as the sole explanation of these lower central densities. We consider the effect of tides and halo concentration scatter on these systems and predict that they should be on more plunging orbits than their denser counterparts. If this prediction is misaligned with the data, it could necessitate new physics beyond the Standard Cosmological Model.
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All authors Connor S. Pickett, Michelle L. M. Collins, Justin I. Read, R. Michael Rich, Emily J. E. Charles, Erik Tollerud, Nicolas Martin, Scott Chapman, Alan McConnachie, Alessandro Savino, Daniel R. Weisz

Cosmology and Nongalactic Astrophysics

21cmEMUv3: a hybrid diffusion-LSTM emulator of 21cmFAST summary observables

Daniela Breitman, Andrei Mesinger, Steven G. Murray et al. (+2 more)

We are witnessing a surge in observations of the cosmic dawn (CD) and epoch of reionisation (EoR), driving an increasing demand for fast and robust theoretical interpretation frameworks. In response, machine learning (ML), and emulation in particular, has emerged as a powerful approach to accelerate and enhance inference pipelines. In this work, we present 21cmEMUv3, an emulator trained on 21cmFASTv3 simulations that model both atomically and molecularly cooling galaxies. 21cmEMUv3 is conditioned on $\sigma_8$ and ten astrophysical parameters to produce seven summary observables: (i) the cylindrical 21cm power spectrum (PS), emulated for the first time at such high resolution and accuracy across a wide redshift range of $z \sim$ 6–30; (ii) the spherically-averaged 21cm PS; (iii) the mean neutral fraction of the intergalactic medium (IGM); (iv) the mean 21cm spin temperature; (v) the global 21cm signal; (vi) the ultraviolet (UV) luminosity functions (LFs); and (vii) the Thomson scattering optical depth. Notably, the cylindrical 21cm PS is emulated via score-based diffusion, while the remaining six summaries are emulated via long-short term memory (LSTM) networks, all achieving sub-percent median accuracy. We use the emulator to reinterpret current 21cm PS upper limits from HERA, for the first time using state-of-the-art hydrodynamical simulations to inform priors on star formation inside molecularly cooling galaxies. We find that our inferred soft-band X-ray luminosity per unit star formation rate is consistent with extrapolations of high-mass X-ray binaries to the low-metallicity regimes expected in the first galaxies, excluding values below $10^{39.2}$ erg s$^{-1}M^{-1}_\odot \rm{yr}$ at $95\%$ confidence. Finally, we produce forecasts for the detection of the cosmic 21cm PS with the Square Kilometre Array for different array configurations. The 21cmEMU package is publicly available.
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All authors Daniela Breitman, Andrei Mesinger, Steven G. Murray, Ivan Nikolic, Roberto Trotta

Astrophysics of Galaxies

Prospects of resolving and localising individual supermassive black hole binaries with pulsar timing arrays: the host ranking challenge

Niccolò Veronesi, Maria Charisi, Polina Petrov et al. (+5 more)

Pulsar Timing Arrays (PTAs) are soon expected to detect individually resolved supermassive black hole (SMBH) binaries, opening the possibility for multi-messenger discoveries. The biggest challenge will be to pinpoint the host galaxy in a large localisation area. We simulate realistic binary populations consistent with the gravitational wave (GW) background, projecting the PTA sensitivity for the next 0-10 years. We inject the loudest binary on top of the background and use one of the standard detection pipelines to constrain its properties. We cross-match the localisation areas with comprehensive all-sky galaxy catalogues and estimate the number of candidate hosts in the localisation area assessing, for the first time, the number of missing galaxies due to incomplete coverage. We develop a ranking system that excludes galaxies with properties inconsistent with the GW posteriors, and prioritizes the remaining galaxies for follow-up observations. We find a $\approx$21, $\approx$38 and $\approx$51 percent probability of resolving a binary in the next 0, 5 and 10 years, respectively, reduced to 0.3, 3.8 and 14.1 percent if we require potentially well-constrained localisation areas. The localisation areas span hundreds of square degrees, but shrink significantly with the addition of more data. They contain on average $\approx$190,000 early type galaxies and $\approx$40,000 active galactic nuclei, with $\approx$25,000 missing candidate hosts. Our ranking method can exclude about half of the potential hosts and efficiently rank those remaining when the galaxy catalogue provides SMBH masses and redshifts, but becomes more inefficient when we rely on apparent magnitudes.
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All authors Niccolò Veronesi, Maria Charisi, Polina Petrov, Stephen R. Taylor, Jessie Runnoe, Daniel J. D'Orazio, Jacob Pilawa, Chung-Pei Ma

Instrumentation and Methods for Astrophysics

Hubble Science in the 2030s White Paper: High-Contrast Optical and UV Spectroscopy with HST/STIS

K. Ward-Duong, J. Debes, J. Aguilar et al. (+11 more)

The Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope currently stands as the sole space-based astronomical facility providing visible-light coronagraphic imaging – and the only facility anywhere that can perform both visible- and ultraviolet-light coronagraphic spectroscopy. In imaging, STIS offers unparalleled stability that rivals the performance of ground-based direct imaging in the optical, and a wide field of view that will complement the upcoming capabilities of Roman coronagraphy. STIS also has the capability for direct high-contrast visible and ultraviolet spectroscopy via two occulting bars in its 52”$\times$0.2” spectroscopic slit. By placing a bright astrophysical source behind an occulting bar, it is possible to use the STIS visible and NUV/FUV gratings to obtain spatially-resolved spectra of faint environments and companions, covering wavelengths from 1150-10,300Å at resolutions of $R\sim500-10{,}000$. In this white paper, we detail the use cases and performance of this under-utilized mode, with starlight subtraction enabling visible light spectral contrasts of $\sim10^{-4}-10^{-5}$. We describe the promise of STIS coronagraphic spectroscopy for a wide variety of astrophysical applications – planetary and brown dwarf companions, circumstellar disks, young stellar objects, evolved stars and binaries, and active galactic nuclei/galaxy host environments – throughout the 2030s. STIS high-contrast UV spectroscopy in particular could provide transformative science while pathfinding both techniques and observational studies for the Habitable Worlds Observatory.
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All authors K. Ward-Duong, J. Debes, J. Aguilar, T. Currie, J. Lomax, C. Xie, J. Hashimoto, J. Zhang, R. Michelson, E. Vrijmoet, C. Chen, E. Rickman, K. Hoch, K. Follette