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Physics Astrophysics

astro-ph Watch Astrophysics

Friday 17 October

High Energy Astrophysical Phenomena

Rapidly rotating hot nuclear and hypernuclear compact stars: integral parameters and universal relations

Stefanos Tsiopelas, Armen Sedrakian and Micaela Oertel

In this work, we investigate hot, isentropic compact stars in the limiting cases of static and maximally rotating configurations, focusing on how variations in the symmetry energy of the equation of state derived from covariant density functional theory affect stellar properties. We consider both nucleonic and hyperonic matter with systematically varied symmetry energy slopes, fixed entropies per baryon $s / k_B=1$ and 3, and electron fractions $Y_e=0.1$ and $Y_e=0.4$, representative of conditions in binary neutron star mergers and proto-neutron stars. We compute and analyze mass–radius and moment–of–inertia–mass relations, as well as the dependence of the Keplerian (mass-shedding) frequency on mass, angular momentum, and the ratio of kinetic to gravitational energy. Furthermore, we show that several universal relations between global properties remain valid across both nucleonic and hyperonic equations of state with varying symmetry energy, both in the static and Keplerian limit, and for various combinations of the fixed entropy and electron fraction.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Observational constraints on the modified cosmology inspired by string T-duality

G.G.Luciano, A. Paliathanasis and A. Sheykhi

We explore the cosmological consequences of a modified cosmology inspired by string T-duality. We incorporate the zero-point length correction, $l_0$, into the gravitational potential and derive the modified Friedmann equations via thermodynamic approach at the apparent horizon of a Friedmann-Robertson-Walker (FRW) universe. The resulting framework introduces a dimensionless coupling parameter $\beta\sim l_0^2H_0^2$ quantifying deviations from the standard $\Lambda$CDM model. Using Bayesian inference with Cobaya and MCMC sampling, we constrain the model parameter against late-time observations, including PantheonPlus and Union3 Type~Ia supernovae, cosmic chronometers, DESI~DR2 BAO measurements, and Amati-calibrated GRBs. The joint analysis yields an upper bound $\beta \lesssim \mathcal{O}(10^{-3})$ (68% C.L.), implying that departures from $\Lambda$CDM are extremely small within current precision. Model comparison through the Akaike Information Criterion shows that the $\Lambda$CDM and T-duality models provide statistically equivalent fits to the data, exhibiting only a marginal preference for $\Lambda$CDM. These results provide the first quantitative observational constraints on string T-duality inspired modified cosmology and underscore the potential of future high-precision surveys to test quantum-gravity induced corrections in a late-time universe.
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Astrophysics of Galaxies

The factors that influence protostellar multiplicity II. Gas temperature and mass in Perseus with APEX

N. M. Murillo, C. M. Fuchs, D. Harsono et al. (+5 more)

Protostellar multiplicity is a common outcome of the star formation process. To fully understand the formation and evolution of these systems, the physical parameters of the molecular gas together with the dust must be systematically characterized. Using observations of molecular gas tracers, we characterize the physical properties of cloud cores in the Perseus molecular cloud (average distance of 295 pc) at envelope scales (5000-8000 AU). We used Atacama Pathfinder EXperiment (APEX) and Nobeyama 45m Radio Observatory (NRO) observations of DCO$^+$, H$_2$CO and c-C$_3$H$_2$ in several transitions to derive the physical parameters of the gas toward 31 protostellar systems in Perseus. Gas kinetic temperature was obtained from DCO$^+$, H$_2$CO and c-C$_3$H$_2$ line ratios. Column densities and gas masses were then calculated for each species and transition. Gas kinetic temperature and gas masses were compared with bolometric luminosity, envelope dust mass, and multiplicity to search for statistically significant correlations. Gas kinetic temperature derived from DCO$^+$, H$_2$CO and c-C$_3$H$_2$ line ratios have average values of 14 K, 26 and 16 K, respectively, with a range of 10-26 K for DCO$^+$ and c-C$_3$H$_2$. The gas kinetic temperature obtained from H$_2$CO line ratios have a range of 13-82 K. Column densities of all three molecular species are on the order of 10$^{11}$ to 10$^{14}$ cm$^{-2}$, resulting in gas masses of 10$^{-11}$ to 10$^{-9}$ M$_{\odot}$. Statistical analysis of the physical parameters finds: i) similar envelope gas and dust masses for single and binary protostellar systems; ii) multiple (>2 components) protostellar systems tend to have slightly higher gas and dust masses than binaries and single protostars; iii) a continuous distribution of gas and dust masses is observed regardless of separation between components in protostellar systems.
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All authors N. M. Murillo, C. M. Fuchs, D. Harsono, T.-H. Hsieh, D. Johnstone, R. Mignon-Risse, M. V. Persson, N. Sakai

Solar and Stellar Astrophysics

One century data of {\tau} CMa: a (2+1)+1 system with a short-period overcontact binary and an eccentric intermediate orbit with probably no apsidal motion

Sophie Rosu, Jesús Maíz Apellániz, Luca Sciarini et al. (+4 more)

{\tau} Canis Majoris (CMa) is an intriguing system that has captured astronomers' attention for more than a century. The two main components Aa and Ab are two evolved O stars on a 350 years orbit. Aa is itself a SB1 with a 155-days period and a 0.3 eccentricity. Since Hipparcos, we know that a 1.28-days period eclipsing binary (EB) is hidden somewhere in Aa or Ab, but nowhere else. Our recent analysis finally disentangles the system. We calculated the visual Aa-Ab orbit from AstraLux imaging. We detected the SB2 nature of Aa based on STIS spectra, the companion of the O star (Aa1) being a B+B binary (Aa2 = Aa2a + Aa2b). Multiple lines of evidence point towards Aa2 being the EB: time delays in the eclipsing orbit detected by TESS, high mass for Aa2 from SB1 from constraints from the orbit of Aa1, and a lack of radial-velocity motion of Ab synchronised with the eclipsing orbit. This remains as a tentative conclusion pending further analysis. We detect secular changes in the SB1 orbit of Aa1 on a baseline longer than a century. At this stage, the effect is most likely caused by the change in velocity of the Aa center of mass due to the Aa-Ab visual orbit. Apsidal motion is most probably not the culprit.
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All authors Sophie Rosu, Jesús Maíz Apellániz, Luca Sciarini, Roberto C. Gamen, Juan A. Molina-Calzada, Gonzalo Holgado, Rodolfo H. Barbá

Solar and Stellar Astrophysics

Modelling the Future of Gaia Neutron Star-Main Sequence Binaries: From Eccentric Orbits to Millisecond Pulsar-White Dwarfs

Debatri Chattopadhyay, Kyle A. Rocha, Seth Gossage et al. (+3 more)

We model the evolution of 21 Gaia neutron star (NS)-main-sequence binaries (orbital period $P_{\mathrm{orb}}\sim200$–$1000$ days, eccentricity $e\gtrsim0.2$) using binary evolution with MESA. We examine eccentric mass transfer and models assuming prior circularization. All systems end as NS-white dwarf (WD) binaries, but transfer modes yield distinct outcomes. Under eccentric transfer, binaries are driven to higher $e$, forming orbits with $e\gtrsim0.6$ and $P_{\rm orb}\sim1000$-$4000$ days. Periastron bursts are brief ($\lesssim10^6$ yr), transfer only a few $\times10^{-2}$ M$_\odot$, and produce mildly recycled pulsars ($P_{\mathrm{spin}}\gtrsim50$ ms) with low-mass He WDs. Artificially circularized transfer gives $P_{\rm orb}\sim200$-$2000$ days, lasts $\sim10^7$ yr, and allows NSs to accrete $\sim0.1$ M$_\odot$, forming fully recycled MSPs ($P_{\mathrm{spin}}\sim$ few-30 ms) with CO WDs. Allowing super-Eddington accretion up to $100\times$ the canonical rate makes even eccentric systems efficient MSP producers, though torque coupling remains uncertain. Using an adaptive, field-dependent magnetic-field decay timescale, we find MSPs stay radio-active over Gyr spans. Gaia systems undergoing stable mass transfer remain wide and fail to match the Galactic MSP-WD population, where most, nearly circular systems have $P_{\rm orb}\lesssim100$ days. Binaries with different mass ratios and initial configurations – likely leading to unstable mass transfer – are needed to reproduce the observed MSP-WD distribution.
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All authors Debatri Chattopadhyay, Kyle A. Rocha, Seth Gossage, Vicky Kalogera, Kareem El-Badry, Alexander Tchekhovskoy

High Energy Astrophysical Phenomena

Charged pion chain decay and the cosmic ray positron flux

Paolo Lipari

The study of the cosmic ray positron flux has attracted intense attention in recent years, especially because the observations suggest that it could receive contributions from sources such as pulsars or the self–annihilation or decay of dark matter particles. The main known source of relativistic positrons, that form the background to possible additional contributions, is the chain decay of $\pi^+$ produced in the inelastic interactions of cosmic rays with interstellar and circumstellar gas. The shape of the energy spectrum of positrons produced in these pion decays can be calculated exactly and is well known. However, surprisingly, some estimates of the contribution of the standard mechanism to the positron flux have adopted an incorrect spectral shape of the positron produced in these decays, following an error present in a 1997 paper of Moskalenko and Strong. In this work we report this error, discuss its origin, and estimate its impact on the numerical results.
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Instrumentation and Methods for Astrophysics

Why every solar eclipse viewing event needs a disco ball

Valerie A. Rapson, Alex Pietrow, Robert J. Cumming et al. (+12 more)

Solar eclipses offer unparalleled opportunities for public engagement in astronomy. Large groups of people often gather to view eclipses, and these events require affordable and easy to use tools to safely observe the Sun. One unique way to observe a solar eclipse is by using a disco ball. Here, we present an analysis of the experiences of educators who used a disco ball as a solar projector during various public outreach events. Through a survey conducted shortly after the April 2024 total solar eclipse and the March 2025 partial solar eclipse, we collected data on the use, engagement, and perceived educational value of a disco ball projector from 31 individual events. The results suggest that disco balls were not only affordable and safe, but also popular and educational.
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All authors Valerie A. Rapson, Alex Pietrow, Robert J. Cumming, Jill Burns, Ali Cotton, Yashashree Jadhav, Maria D. Kazachenko, Casper A.L. Pietrow, Livia M.A. Pietrow, Dennis Schatz, Elliot Severn, Sundar Srinivasan, William Thornburgh, Andrea Warkentin, Hayley Yasui

High Energy Astrophysical Phenomena

Searching for Exotrojans in Pulsar Systems

Jackson D. Taylor, Emmanuel Fonseca, Lankeswar Dey et al. (+50 more)

Trojan asteroids are found in the equilateral triangle Lagrange points of the Sun-Jupiter system in great number, though they also exist less prolifically in other parts of the Solar System. Despite up to planetary mass Trojans being predicted in extrasolar systems (i.e. exotrojans), they remain unconfirmed, although strong candidate evidence has emerged recently. For the first time, we extend the search for exotrojans to radio pulsars with low-mass ($\sim0.01\,\rm{M}_\odot$) companions using accurately measured pulse times of arrival. With techniques developed for detecting the reflex motion of a star due to a librating Trojan, we place $\sim 1\,\rm{M}_\oplus$ upper mass constraints on potential exotrojans around eight pulsars observed in the NANOGrav 15-year data set. We find weak evidence consistent with $\sim2$–4$\,\rm{M}_{\rm J}$ exotrojans in the PSR~J0023+0923 and PSR~J1705$-$1903 binary systems, though the signals likely have a different, unknown source. We also place a libration-independent upper mass constraint of $\sim8$\,M$_{\rm J}$ on exotrojans in the PSR~J1641+8049 system by looking for an inconsistency between the times of superior conjunction as measured by optical light curves and those predicted by radio timing. These results offer initial observational constraints on the existence of exotrojans around pulsars, while their possible formation mechanisms remain unexplored.
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All authors Jackson D. Taylor, Emmanuel Fonseca, Lankeswar Dey, Sergey Zharikov, Aida Kirichenko, Joseph Glaser, Gabriella Agazie, Akash Anumarlapudi, Anne M. Archibald, Zaven Arzoumanian, Paul T. Baker, Paul R. Brook, H. Thankful Cromartie, Kathryn Crowter, Megan E. DeCesar, Paul B. Demorest, Timothy Dolch, Elizabeth C. Ferrara, William Fiore, Gabriel E. Freedman, Nate Garver-Daniels, Peter A. Gentile, Deborah C. Good, Jeffrey S. Hazboun, Ross J. Jennings, Megan L. Jones, David L. Kaplan, Matthew Kerr, Michael T. Lam, T. Joseph W. Lazio, Duncan R. Lorimer, Jing Luo, Ryan S. Lynch, Alexander McEwen, Maura A. McLaughlin, Natasha McMann, Bradley W. Meyers, Cherry Ng, David J. Nice, Timothy T. Pennucci, Benetge B. P. Perera, Nihan S. Pol, Henri A. Radovan, Scott M. Ransom, Paul S. Ray, Ann Schmiedekamp, Carl Schmiedekamp, Brent J. Shapiro-Albert, Ingrid H. Stairs, Kevin Stovall, Abhimanyu Susobhanan, Joseph K. Swiggum, Haley M. Wahl

Cosmology and Nongalactic Astrophysics

Updated Cosmological Constraints from 2D BAO Measurements: A New Compilation and Comparison with DESI DR2

Miguel A. Sabogal, Rafael C. Nunes, Felipe Avila et al. (+1 more)

We investigate and update observational constraints on cosmological parameters within the $\Lambda$CDM and dynamical dark energy frameworks, using a new compilation of the transverse (or 2D) BAO data, measurements that provide a relatively model-independent estimate of the BAO angular scale at a given redshift. Firstly, we assess the consistency of this compilation with CMB-Planck data and recent BAO results from the DESI collaboration. After confirming minimal tension with CMB data, we perform a series of joint analyses combining CMB data with the 2D~BAO compilation, as well as with several recent Type Ia supernova (SNIa) samples. In all cases, we compare the constraining power of the 2D~BAO data with that of DESI~DR2 data (3D~BAO). Our results indicate that combining 2D~BAO with CMB and SNIa data provides observational constraints that are in agreement with those obtained using DESI~DR2 data. Although the precision of DESI~DR2 results remains higher, as expected due to the more accurate 3D measurements, the 2D~BAO compilation combined with other probes yields strong constraints. For example, in the $\Lambda$CDM context, we find (CMB + 2D~BAO): $H_0 = 68.16^{+0.41}_{-0.37} \,\,\, \text{km s}^{-1}\,\text{Mpc}^{-1}$ and $\Sigma m_{\nu} < 0.081~\mathrm{eV}$ (95%~CL). These results are consistent with analogous analyses using DESI~DR2. Several other cases are investigated and discussed in the main text. Our results show that this new 2D~BAO compilation is robust and delivers meaningful cosmological constraints. Parameters from 2D~BAO alone agree with CMB-only results, with no significant tension. Overall, 2D and 3D BAO provide consistent and complementary information when combined with other probes.
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All authors Miguel A. Sabogal, Rafael C. Nunes, Felipe Avila, Armando Bernui

Earth and Planetary Astrophysics

Order-by-order Modeling of Exoplanet Radial Velocity Data

Zachary Langford, Cullen Blake, Samuel Halverson et al. (+12 more)

Precise radial velocity (RV) measurements are a crucial tool for exoplanet discovery and characterization. Today, the majority of these measurements are derived from Echelle spectra in the optical wavelength region using cross-correlation techniques. Although for certain stars these approaches can produce RVs with sub-1 m~s$^{-1}$ measurement errors, for many others, we are now in a regime where instrumental precision is fundamentally below the intrinsic RV variations of the star that result from astrophysical processes that can be correlated in both time and wavelength. We explore new methods for measuring exoplanet orbital parameters that take advantage of the fact that RV data sets are fundamentally multi-wavelength. By analyzing NEID extremely precise radial velocity (EPRV) data of three known exoplanet systems, we show that fitting a single Keplerian model to multi-wavelength RVs can produce a factor of 1.5 – 6.8 better $M_p \sin i$ uncertainties compared to fitting RVs that are derived from a weighted average across wavelength.
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All authors Zachary Langford, Cullen Blake, Samuel Halverson, Eric B. Ford, Suvrath Mahadevan, Mark R. Giovinazzi, Arvind F. Gupta, Paul Robertson, Jaime A. Alvarado-Montes, Chad F. Bender, Daniel M. Krolikowski, Arpita Roy, Christian Schwab, Ryan C. Terrien, Jason T. Wright

Solar and Stellar Astrophysics

Neutron Star-Main Sequence Collisions Robustly Form Dynamically Stable Thorne-Żytkow Objects

Lauryn E. Williams, Philip Chang, Emily M. Levesque et al. (+1 more)

Thorne-Żytkow Objects (TŻOs) are hypothetical hybrid stars with a neutron star at the core of a large, diffuse envelope. (TŻOs) may be formed when a newly formed neutron star that is kicked by its supernova collides with its main-sequence companion. Using a moving-mesh hydrodynamics solver integrated into the parallel-code Charm N-body GrAvity solver, we demonstrate that these “impact scenario” formation processes robustly form (TŻOs) for periastron distances less than one stellar radius. These (TŻOs) are dynamically stable and they can serve as initial models for further evolutionary studies.
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All authors Lauryn E. Williams, Philip Chang, Emily M. Levesque, Thomas R. Quinn

Astrophysics of Galaxies

Effective cosmic ray diffusion in multiphase galactic environments

Timon Thomas, Christoph Pfrommer, Rüdiger Pakmor et al. (+2 more)

Cosmic-ray (CR) feedback is widely recognized as a key regulator of galaxy formation. After being accelerated at supernova remnant shocks, CRs propagate through the interstellar medium (ISM), establishing smooth large-scale distributions and driving galactic outflows. The efficiency of this feedback is controlled by the effective transport speed of the CR population, which in turn depends on the competition between CR-driven plasma instabilities and wave damping processes that vary strongly with ISM phase. In cold, dense gas, ion-neutral damping dominates, whereas in warm, diffuse environments, weaker non-linear Landau damping prevails, leading to enhanced CR scattering and slower transport. To investigate these effects, we employ the moving-mesh code Arepo and model CR transport using a two-moment description within the multiphase ISM framework Crisp, which self-consistently computes CR diffusion coefficients and transport velocities from coarse-grained plasma physics. The intrinsic CR diffusion coefficient depends inversely on the scattering rate of CRs and Alfvén waves, covering 15 orders of magnitude. In contrast, we show that the effective CR diffusion coefficient, which quantifies the propagation speed of CRs through the ISM, converges toward the canonical range of $10^{28}$-$10^{29}$ cm$^2$ s$^{-1}$. Simulations with only non-linear Landau damping yield transport rates up to an order of magnitude slower than those including both Landau and ion-neutral damping. Overall, CR transport speeds increase systematically with gas density, for which we provide a density-dependent fit of the effective CR diffusion coefficient. We demonstrate that, despite strong ion-neutral damping in the cold and warm phases of the galactic disk, CRs are transported at speeds only a few times the local Alfvén speed as they traverse alternating ISM phases on their way out of the galaxy.
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All authors Timon Thomas, Christoph Pfrommer, Rüdiger Pakmor, Rouven Lemmerz, Mohamad Shalaby

High Energy Astrophysical Phenomena

Tracking optical variability and outflows across the accretion states of the black hole transient MAXI J1820+070

M. C. Baglio, K. Alabarta, D. M. Russell et al. (+10 more)

We present a study of the minute-timescale optical variability and spectroscopic outflow signatures in the black hole X-ray binary MAXI J1820+070 during its 2018 outburst and re-brightenings. Minute-cadence, multi-filter optical light curves were obtained with the Las Cumbres Observatory network and the Al Sadeem Observatory (UAE) over 2018-2020, complemented by X-ray data from Swift/BAT, XRT, and MAXI. We also acquired contemporaneous low-resolution optical spectra with the 2.1 m OAN San Pedro Martir and OAGH Cananea telescopes (Mexico) and the 1.5 m G.D. Cassini telescope at Loiano (Italy). The optical fractional rms peaks in the hard state and is dominated by short-timescale flickering that strengthens toward longer wavelengths, suggesting that the variability is jet-driven. In this scenario, inflow fluctuations inject velocity perturbations at the jet base (internal shock model). The variability is quenched in the soft state, with any residual signal likely linked to accretion-flow fluctuations. This behaviour supports the accretion-ejection coupling in black hole binaries and shows that the jet's variability signature extends to optical wavelengths in all hard states. In the faintest hard states, residual optical variability may instead trace cyclo-synchrotron emission from the hot flow. The spectra show double-peaked emission lines and tentative cold-wind signatures during the hard state. Such winds were reported during the main 2018 outburst; here we find evidence of their presence also in later re-brightenings. Their absence in the soft state likely reflects stronger disc ionisation from the higher X-ray flux, suppressing low-ionisation optical features.
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All authors M. C. Baglio, K. Alabarta, D. M. Russell, N. Masetti, M. M. Messa, T. Muñoz Darias, F. M. Vincentelli, S. K. Rout, P. Saikia, A. Gabuya, V. Chavushyan, T. Al Qaissieh, A. Palado

High Energy Astrophysical Phenomena

GRB 230307A Formed No Dust or Was Not a Binary Neutron Star Merger

Prasiddha Arunachalam, Phillip Macias and Ryan. J. Foley

We present a new analysis of the JWST infrared spectra of GRB 230307A (AT 2023vfi), a long gamma-ray burst (GRB) with an infrared excess and spectral lines suggestive of significant heavy $r$-process production. The spectra, taken 29 and 61~days after the GRB trigger, have blackbody-like continua with $T_{\rm eff} \approx 550$ K and an emission line near $2.1$ $\mu$m, previously attributed to [Te III]. This line identification has been used as evidence for an $r$-process-powered kilonova (KN), despite no KN model producing a blackbody-like spectrum at late times. Such an infrared continuum could be emitted by newly formed dust, and we model the thermal emission to infer dust properties, including composition and mass. We find that the emission requires at least 3–$6 \times 10^{-3}$~M$_{\odot}$ of carbon or silicate dust, which is inconsistent with $r$-process yields expected from a neutron star merger. Alternatively, the continuum could be from $2\times 10^{-3}$~M$_{\odot}$ of metallic iron dust, which is mildly consistent (at 3$\sigma$) with KN models, but such dust is unlikely to form in the expanding ejecta. GRB 230307A's low late-time luminosity also constrains the amount of radioactive $^{56}$Ni produced to $<2.6 \times 10^{-3}$~M$_{\odot}$ (3$\sigma$). No KN model can simultaneously form the necessary dust for the infrared continuum and heavy elements for the [Te III] line. We conclude that the blackbody continuum is not due to dust emission, or GRB 230307A did not originate from a binary compact-object merger.
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Cosmology and Nongalactic Astrophysics

Spectral Chebyshev Approximation of Cosmic Expansion in $f(R)$ Gravity

Akshay Rana

We present a numerical framework to study the cosmological background evolution in $f(R)$ gravity by employing a spectral Chebyshev collocation approach. Unlike standard integration methods such as Runge–Kutta that often encounter stiffness and accuracy issues, this formulation expands the normalized Hubble function $E(z) = H(z)/H_0$ as a finite Chebyshev series. The modified Friedmann equation is then enforced at selected Chebyshev–Gauss–Lobatto points, converting the original nonlinear differential equation into a system of algebraic relations for the series coefficients. This transformation yields exponentially convergent and numerically stable solutions over the entire redshift domain, $0<z<z_{max}$, eliminating the need for adaptive step-size control. We apply the method to two widely studied $f(R)$ models, Hu–Sawicki and Starobinsky, and perform a combined analysis using cosmic chronometer $H(z)$ data and the Union~3.0 supernova compilation. The reconstructed expansion histories match observations to within $2\sigma$ over $0 < z < 2$, producing best-fit parameters of approximately $(\Omega_{m0}, H_0, \Lambda_{\mathrm{eff}}) \simeq (0.29, 68, 1.2\text{--}2.5\,H_0^2)$. These results indicates that both models reproduce the observed late-time acceleration while permitting small geometric corrections to $\Lambda$CDM. Overall, the spectral Chebyshev method provides a precise and computationally efficient framework for probing modified-gravity cosmologies in the precision-data era.
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Astrophysics of Galaxies

The ALPINE-CRISTAL-JWST Survey: NIRSpec IFU Data Processing and Spatially-resolved Views of Chemical Enrichment in Normal Galaxies at z=4-6

Seiji Fujimoto, Andreas L. Faisst, Akiyoshi Tsujita et al. (+37 more)

We present a statistical study of spatially resolved chemical enrichment in 18 main-sequence galaxies at $z=4$–6, observed with \jwst/NIRSpec IFU as part of the ALPINE-CRISTAL-\jwst\ survey. Performing an optimized reduction and calibration procedure, including local background subtraction, light-leakage masking, stripe removal, and astrometry refinement, we achieve robust emission-line mapping on kiloparsec scales. Although line-ratio distributions vary across galaxies in our sample, we generally find mild central enhancements in [O\,iii]/H$\beta$, [O\,ii]/[O\,iii], [S\,ii]$_{6732}$/[S\,ii]$_{6718}$, H$\alpha$/H$\beta$, and $L_{\rm H\alpha}/L_{\rm UV}$, consistent with elevated electron density, dust obscuration, and bursty star formation accompanied by reduced metallicity and ionization parameter. These features point to inside-out growth fueled by recent inflows of pristine gas. Nevertheless, the median metallicity gradient is nearly flat over a few kpc scale, $\Delta \log({\rm O/H}) = 0.02 \pm 0.01$ dex kpc$^{-1}$, implying efficient chemical mixing through inflows, outflows, and mergers. From pixel-by-pixel stellar and emission-line characterizations, we further investigate the resolved Fundamental Metallicity Relation (rFMR). Metallicity is described by a fundamental plane with stellar mass and SFR surface densities, but with a stronger dependence on $\Sigma_{\rm SFR}$ than seen in local galaxies. Our results indicate that the regulatory processes linking star formation, gas flows, and metal enrichment were already vigorous $\sim$1 Gyr after the Big Bang, producing the nearly flat metallicity gradient and a stronger coupling between star formation and metallicity than observed in evolved systems in the local universe.
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All authors Seiji Fujimoto, Andreas L. Faisst, Akiyoshi Tsujita, Mahsa Kohandel, Lilian L. Lee, Hannah Übler, Federica Loiacono, Negin Nezhad, Andrea Pallottini, Manuel Aravena, Roberto J. Assef, Andrew J. Battisti, Matthieu Béthermin, Médéric Boquien, Elisabete da Cunha, Andrea Ferrara, Maximilien Franco, Michele Ginolfi, Ali Hadi, Aryana Haghjoo, Rodrigo Herrera-Camus, Hanae Inami, Anton M. Koekemoer, Brian C. Lemaux, Yuan Li, Lun-Jun Liu, Juan Molina, Ambra Nanni, Francesca Pozzi, Monica Relano, Michael Romano, David B. Sanders, Natascha M. Förster Schreiber, John Silverman, Justin Spilker, Kseniia Telikova, Vicente Villanueva, Livia Vallini, Wuji Wang, Giovanni Zamorani

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

Hybrid waveform model for asymmetric spinning binaries: Self-force meets post-Newtonian theory

Loïc Honet, Adam Pound and Geoffrey Compère

We develop and implement a new hybrid waveform model for quasicircular inspirals with a spinning primary and nonspinning secondary, excluding the merger and ringdown. This model, which is a core component of the more extensive WaSABI-C model, consistently assembles all available first-order self-force and post-Newtonian results through a hybridization procedure without any tuning to numerical relativity, making it particularly suited for intermediate to extreme mass ratios. For almost all masses and primary spins, the resulting hybrid model significantly improves the faithfulness of both post-Newtonian and adiabatic self-force waveforms considered separately. We provide detailed comparisons with 50 simulations from the SXS catalog with mass ratios ranging from 1 to 15 and primary spins ranging from -0.8 to 0.8. The hybrid model improves the median mismatch against numerical relativity waveforms by a factor of 2000 with respect to adiabatic waveforms and 40 with respect to post-Newtonian waveforms. The mismatches are comparable to those obtained from the SEOBNRv5EHM model in the quasicircular limit over most of the parameter space covered by NR simulations.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Post-adiabatic self-force waveforms: slowly spinning primary and precessing secondary

Josh Mathews, Barry Wardell, Adam Pound et al. (+1 more)

Recent progress in gravitational self-force theory has led to the development of a first post-adiabatic (1PA) waveform model for nonspinning, quasicircular compact binaries [Phys. Rev. Lett. 130, 241402 (2023)]. In this paper, we extend that model to allow for a slowly spinning primary black hole and a generic, precessing spin on the secondary object, restricting to the case of small misalignment between the primary spin and the orbital angular momentum. We demonstrate excellent agreement between our waveforms and fully nonlinear numerical relativity simulations for mass ratios $q\gtrsim 5$ and primary spins $|\chi_1|\lesssim 0.1$ and arbitrary secondary spin $\chi_2 \lesssim 1$. In particular we present the re-summed 1PAT1R waveform model, which significantly improves the accuracy of the original 1PAT1 waveforms for comparable masses and increasing primary spin. Our models are publicly available in the WaSABI package.
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All authors Josh Mathews, Barry Wardell, Adam Pound, Niels Warburton

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

Spin-aligned inspiral waveforms from self-force and post-Newtonian theory

Loïc Honet, Josh Mathews, Geoffrey Compère et al. (+5 more)

We present the state-of-the-art waveform model WaSABI-C for quasicircular inspirals of spinning black hole binaries with aligned or anti-aligned spins. Our model synthesizes the most up-to-date first- and second-order gravitational self-force results with high-order post-Newtonian expansions through a systematic hybridization procedure. This approach captures both strong-field and weak-field dynamics with high fidelity, enabling accurate modeling of spin-(anti)aligned inspirals across a wide parameter space. The resulting waveforms mark a significant advance in the precision of self-force-based templates, providing critical input for the detection and interpretation of gravitational waves from compact binaries with future observatories such as LISA and ET. We accompany this work with the release of WaSABI (Waveform Simulations of Asymmetric Binary Inspirals), a public package implementing our model for community use and further development.
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All authors Loïc Honet, Josh Mathews, Geoffrey Compère, Adam Pound, Barry Wardell, Gabriel Andres Piovano, Maarten van de Meent, Niels Warburton

Astrophysics of Galaxies

The ALPINE-CRISTAL-JWST Survey: JWST/IFU Optical Observations for 18 Main-Sequence Galaxies at z=4-6

A. L. Faisst, S. Fujimoto, A. Tsujita et al. (+80 more)

To fully characterize the formation and evolution of galaxies, we need to observe their stars, gas, and dust on resolved spatial scales. We present the ALPINE-CRISTAL-JWST survey, which combines kpc-resolved imaging and spectroscopy from HST, JWST, and ALMA for 18 representative main-sequence galaxies at z=4-6 and log(M/$M_\odot$) > 9.5 to study their star formation, chemical properties, and extended gas reservoirs. The co-spatial measurements resolving the ionized gas, molecular gas, stars, and dust on 1-2 kpc scales make this a unique benchmark sample for the study of galaxy formation and evolution at $z\sim5$, connecting the Epoch of Reionization with the cosmic noon. In this paper, we outline the survey goals and sample selection, and present a summary of the available data for the 18 galaxies. In addition, we measure spatially integrated quantities (such as global gas metallicity), test different star formation rate indicators, and quantify the presence of H$\alpha$ halos. Our targeted galaxies are relatively metal rich (10-70% solar), complementary to JWST samples at lower stellar mass, and there is broad agreement between different star formation indicators. One galaxy has the signature of an active galactic nuclei (AGN) based on its emission line ratios. Six show broad H$\alpha$~emission suggesting type 1 AGN candidates. We conclude with an outlook on the exciting science that will be pursued with this unique sample in forthcoming papers.
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All authors A. L. Faisst, S. Fujimoto, A. Tsujita, W. Wang, N. Nezhad, F. Loiacono, H. Übler, M. Béthermin, P. Cassata, M. Dessauges-Zavadsky, R. Herrera-Camus, D. Schaerer, J. Silverman, L. Yan, M. Aravena, I. De Looze, N. M. Förster Schreiber, J. González-López, J. Spilker, K. Tadaki, C. M. Casey, M. Franco, S. Harish, H. J. McCracken, J. S. Kartaltepe, A. M. Koekemoer, A. A. Khostovan, D. Liu, J. Rhodes, B. E. Robertson, R. Amorin, R. J. Assef, A. J. Battisti, J. E. Birkin, M. Boquien, E. Da Cunha, P. Dam, R. L. Davies, D. A. Gómez-Espinoza, A. Ferrara, Y. Fudamoto, S. Gillman, M. Ginolfi, G. Gozaliasl, C. Gruppioni, A. Hadi, N. Hathi, E. Ibar, R. Ikeda, H. Inami, G. C. Jones, M. Kohandel, Y. Li, Y-H. Lin, Z. Liu, L-J. Liu, A. S. Long, G. Magdis, C. Maraston, C. L. Martin, I. Mitsuhashi, B. Mobasher, J. Molina, A. Nanni, M. Palla, A. Pallottini, F. Pozzi, M. Relano, W. Ren, D. A. Riechers, M. Romano, D. B. Sanders, P. Sawant, M. Shuntov, R. Smit, L. Sommovigo, M. Talia, L. Vallini, E. Veraldi, D. Vergani, A. P. Vijayan, V. Villanueva, G. Zamorani

Instrumentation and Methods for Astrophysics

Linear Image Regridding and Coaddition with Oversampled Point Spread Functions: Lessons from 1D

Kaili Cao

Image regridding and coaddition have a wide range of applications in astronomical observations. Imcom, an algorithm that provides control over point spread function (PSF) and noise in coadded images, has been found to meet the stringent requirements of weak gravitational lensing cosmology with the forthcoming Nancy Grace Roman Space Telescope. In this work, I introduce a new algorithm, Fast Imcom, which outperforms traditional Imcom in terms of both efficiency and quality. After explaining the underlying philosophy and mathematical formalism, I conduct systematic comparisons between Imcom and Fast Imcom in terms of PSF reconstruction in 1D. While a 2D implementation is beyond the scope of this paper, I demonstrate how to generalize Fast Imcom to 2D and discuss practical issues involved. This new algorithm has the potential of reducing both the computational costs and storage requirements (current estimates are $\sim 100\,{\rm M}$ core-hours and $\sim 1.5 \,{\rm PB}$, respectively) of the Roman High Latitude Imaging Survey (HLIS) by an order of magnitude. Meanwhile, it provides implications for the dithering patterns of Roman surveys (extrapolated from 1D to 2D). I also address potential applications of Fast Imcom beyond the Roman HLIS, with focus on other weak lensing programs and Roman time domain surveys; the actual range of use cases is likely beyond what is discussed here.
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High Energy Astrophysical Phenomena

Detection of Compton scattering in the jet of 3C 84

Ioannis Liodakis, Sudip Chakraborty, Frédéric Marin et al. (+77 more)

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $\gamma$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $\gamma$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rm\Pi_X=4.2\pm1.3\%$ ($\sim3.2\sigma$ confidence) at an X-ray electric vector polarization angle of $\rm \psi_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons – specifically Synchrotron Self-Compton – that takes places downstream, away from the supermassive black hole.
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All authors Ioannis Liodakis, Sudip Chakraborty, Frédéric Marin, Steven R. Ehlert, Thibault Barnouin, Pouya M. Kouch, Kari Nilsson, Elina Lindfors, Tapio Pursimo, Georgios F. Paraschos, Riccardo Middei, Anna Trindade Falcão, Svetlana Jorstad, Iván Agudo, Yuri Y. Kovalev, Jacob J. Casey, Laura Di Gesu, Philip Kaaret, Dawoon E. Kim, Fabian Kislat, Ajay Ratheesh, M. Lynne Saade, Francesco Tombesi, Alan Marscher, Francisco José Aceituno, Giacomo Bonnoli, Víctor Casanova, Gabriel Emery, Juan Escudero Pedrosa, Daniel Morcuende, Jorge Otero-Santos, Alfredo Sota, Vilppu Piirola, Rumen Bachev, Anton Strigachev, George A. Borman, Tatiana S. Grishina, Vladimir A. Hagen-Thorn, Evgenia N. Kopatskaya, Elena G. Larionova, Daria A. Morozova, Sergey S. Savchenko, Ekaterina V. Shishkina, Ivan S. Troitskiy, Yulia V. Troitskaya, Andrey A. Vasilyev, Alexey V. Zhovtan, Ioannis Myserlis, Mark Gurwell, Garrett Keating, Ramprasad Rao, Sincheol Kang, Sang-Sung Lee, Sanghyun Kim, Whee Yeon Cheong, Hyeon-Woo Jeong, Chanwoo Song, Shan Li, Myeong-Seok Nam, Diego Álvarez-Ortega, Carolina Casadio, Emmanouil Angelakis, Alexander Kraus, Jenni Jormanainen, Vandad Fallah Ramazani, Chien-Ting Chen, Enrico Costa, Eugene Churazov, Riccardo Ferrazzoli, Giorgio Galanti, Ildar Khabibulin, Stephen L. O'Dell, Luigi Pacciani, Marco Roncadelli, Oliver J. Roberts, Paolo Soffitta, Douglas A. Swartz, Fabrizio Tavecchio, Martin C. Weisskopf, Irina Zhuravleva

High Energy Astrophysical Phenomena

Early X-ray emission of short Gamma-Ray Bursts: insights into physics and multi-messenger prospects

Annarita Ierardi, Gor Oganesyan, Stefano Ascenzi et al. (+3 more)

Early X-ray emission of Gamma-Ray Bursts (GRBs) traces the transition between the prompt emission and the afterglow radiation, and its rapid flux decline is often interpreted as the tail of the prompt emission. As such, it can offer insights into the emission mechanisms active during the prompt emission and the physics of GRB jets. In this work, we focus on merger-driven GRBs, which are sources of gravitational waves (GWs) detectable by ground-based interferometers, such as LIGO, Virgo, and KAGRA. We present a systematic analysis of the early X-ray emission ($t < 10^3 \ \mathrm{s}$) of a sample of 16 merger-driven GRB candidates detected by the Neil Gehrels Swift Observatory (hereafter, Swift). We performed a time-resolved spectral analysis of soft and hard X-ray data (0.3-150 keV) by fitting two curved spectral models to the spectra: a physical synchrotron model and an empirical smoothly broken power law model. We characterized the evolution of the peak energy and bolometric flux, and derived the intrinsic properties of the 10 bursts with measured redshift. We discovered a tight correlation between the rest-frame peak energy of the spectra and the isotropic-equivalent luminosity. Specifically, we obtained $\nu_{c,z} \propto L_{\rm iso}^{(0.64 \pm 0.03)}$ when adopting the synchrotron model, and $E_{p,z} \propto L_{\rm iso}^{(0.58 \pm 0.04)}$ when adopting the smoothly broken power law. Both relations were extrapolated to the typical prompt emission energies and well describe the properties of short GRBs detected in the MeV gamma-rays. These results suggest a common origin for the prompt and steep-decay emissions in merger-driven GRBs, and rule out high-latitude emission as the dominant process shaping the early X-ray tails. Finally, we assessed the detectability of these sources with the Wide-field X-ray Telescope onboard the Einstein Probe mission.
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All authors Annarita Ierardi, Gor Oganesyan, Stefano Ascenzi, Marica Branchesi, Biswajit Banerjee, Samuele Ronchini

Astrophysics of Galaxies

The ALPINE-CRISTAL-JWST Survey: The Fast Metal Enrichment of Massive Galaxies at z~5

Andreas L. Faisst, Lun-Jun Liu, Yohan Dubois et al. (+54 more)

We present the stellar mass-metallicity relation (MZR) and mass-metallicity-star formation relation ("fundamental metallicity relation"; FMR) of 18 massive (log(M/Msun) = 9.5 - 11) main-sequence galaxies at z~5 from the ALPINE-CRISTAL-JWST sample. This sample complements recent studies by JWST at up to two orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines, and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z~5 and cosmic noon (z~2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z=5 exhibits a 5x larger scatter (preferentially to lower metallicities) compared the local FMR relation. This scatter can be explained by a bursty star formation and the direct build-up of metals in early galaxies as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over three orders of stellar mass is generally in good agreement with results from cosmological simulation, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE-CRISTAL-JWST galaxies likely joined the current FMR at z~10 and will evolve into massive (log(M/Msun)~11.4) galaxies with super-solar metallicities by z=0.
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All authors Andreas L. Faisst, Lun-Jun Liu, Yohan Dubois, Yurina Nakazato, Omima Osman, Andrea Pallottini, Livia Vallini, Seiji Fujimoto, Bahram Mobasher, Wuji Wang, Yu-Heng Lin, Ricardo O. Amorín, Manuel Aravena, R. J. Assef, Andrew J. Battisti, Matthieu Béthermin, Médéric Boquien, Paolo Cassata, Elisabete da Cunha, Poulomi Dam, Gabriella de Lucia, Ilse De Looze, Miroslava Dessauges-Zavadsky, Andrea Ferrara, Kyle Finner, Fabio Fontanot, Michele Ginolfi, Diego A. Gómez-Espinoza, Carlotta Gruppioni, Nicol Gutiérrez-Vera, Ali Hadi, Rodrigo Herrera-Camus, Michaela Hirschmann, Eduardo Ibar, Hanae Inami, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Mahsa Kohandel, Lilian L. Lee, Yuan Li, Juan Molina, Ambra Nanni, Desika Narayanan, Francesca Pozzi, Monica Relano, Michael Romano, David B. Sanders, John D. Silverman, Laura Sommovigo, Justin Spilker, Akiyoshi Tsujita, Hannah Übler, Keerthi Vasan G.C., Enrico Veraldi, Vincente Villanueva, Lizhi Xie, Gianni Zamorani

Earth and Planetary Astrophysics

Surprise non-detection of Upsilon Andromedae b with MIRC-X and MYSTIC at the CHARA Array

Tyler Gardner, John D. Monnier, Stefan Kraus et al. (+11 more)

Ground-based long baseline interferometry is a powerful tool for characterizing exoplanets which are too close to their host star to be imaged with single-dish telescopes. The CHARA Array can resolve companions down to 0.5 milli-arcseconds, allowing us in principle to directly measure the near-infrared spectra of non-transiting "Hot Jupiter" exoplanets. We present data taken with the MIRC-X and MYSTIC instruments at the CHARA Array on the Hot Jupiter Upsilon Andromedae b. By resolving the star-planet system, we attempt to directly detect the flux from the planet. We describe our self-calibration methods for modeling systematics in the closure phase data, which allows us to reach sub-degree precision. Through combining multiple nights of data across two MIRC-X runs in 2019 and 2021, we achieved a very tentative detection of Ups And b in the H-band at a planet/star contrast of 2-3 x 10^-4. Unfortunately, we cannot confirm this detection with 2021 MYSTIC data in the K-band, or in a 2023 joint MIRC-X and MYSTIC dataset. We run updated global circulation models and create post-processed spectra for this planet, and report the resulting model spectra in H- and K-bands as a function of orbital phase. We then run planetary injection tests to explore H/K-band contrast limits, and find that we can confidently recover planets down to a planet/star contrast of 1-2 x 10^-4. We show that we are probing contrasts fainter than predicted by the model, making our non-detection surprising. We discuss prospects for the future in using this method to characterize companions with interferometry.
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All authors Tyler Gardner, John D. Monnier, Stefan Kraus, Emily Rauscher, Isaac Malsky, Jean-Baptiste Le Bouquin, Narsireddy Anugu, Sorabh Chhabra, Isabelle Codron, Claire L. Davies, Noura Ibrahim, Cyprien Lanthermann, Gail Schaefer, Benjamin R. Setterholm

Astrophysics of Galaxies

The SDSS-V Black Hole Mapper Reverberation Mapping Project: Light Echoes of the Coronal Line Region in a Luminous Quasar

Theodore B. Smith, Logan B. Fries, Jonathan R. Trump et al. (+19 more)

We present a reverberation mapping analysis of the coronal line [Ne V]$\lambda$3427 emitting region of the quasar COS168 (SDSS J095910.30+020732.2). [Ne V]$\lambda$3427 is known as one of the "coronal lines," which are a species of emission lines present in AGN spectra with high ionization potentials ($\geq$ 100 eV) that can serve as tracers for AGN activity. The spatial extent of the coronal line region has been studied with only spatial resolving techniques that are not sensitive to the innermost regions of AGN. Through our reverberation mapping analysis of [Ne V]$\lambda$3427, we measure a nominal `optimal emission radius' for [Ne V]$\lambda$3427 of $381.1^{+16}_{-22}$ light days (observed-frame). We place the coronal line region in context with other AGN regions by comparing it with the characteristic radius of H$\alpha$, the dust-sublimation radius, and the dusty torus. The coronal line region is located at a larger radius from the black hole than the characteristic radius of the dusty torus, measured using a torus-radius luminosity relationship. The virial product ($v^2 R/G$) of both H$\alpha$ and [Ne V]$\lambda$3427 is consistent within the uncertainties, implying that the coronal line region, as probed by the [Ne V]$\lambda$3427 line, may be in a virialized orbit that is dominated by the gravitational potential of the black hole. This plausibly suggests that coronal lines could be an effective method for estimating black hole masses.
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All authors Theodore B. Smith, Logan B. Fries, Jonathan R. Trump, Catherine J. Grier, Yue Shen, Scott F. Anderson, W. N. Brandt, Megan C. Davis, Tom Dwelly, P. B. Hall, Keith Horne, Y. Homayouni, J. McKaig, Sean Morrison, Hugh W. Sharp, Roberto J. Assef, Franz E. Bauer, Anton M. Koekemoer, Donald P. Schneider, Benny Trakhtenbrot, Hector Javier Ibarra-Medel, Castalia Alenka Negrete Peñaloza

Solar and Stellar Astrophysics

Ultracool dwarf Science with MachIne LEarning (USMILE). I. Scalable Tree-Based Models for Photometric Spectral Classification and New Discoveries from LSST Data Preview 1 and Euclid Quick Data Release 1

Zhoujian Zhang and Yanxia Li

We present the Ultracool dwarf Science with MachIne LEarning (USMILE), a program developing machine-learning tools for the discovery and characterization of ultracool dwarfs. We introduce USMILE Avocado, a spectral classification framework that uses broadband photometry from wide-field surveys – Rubin Observatory LSST Data Preview 1, VISTA Hemisphere Survey, and CatWISE – as input features. The framework has two gradient-boosted decision-tree models scalable to the massive data volumes of modern surveys: the classifier, which distinguishes ultracool dwarfs from stellar/extragalactic contaminants, and the regressor, which predicts spectral types. A key strength is its ability to natively handle missing photometric features, whereas earlier machine-learning approaches required complete multi-band detections or relied on imputation, thereby excluding genuine ultracool dwarfs or introducing bias. Trained on an augmented labeled dataset of >2 million sources built from known ultracool dwarfs, reddened early-type stars, and quasars, the models achieve strong performance: the classifier attains an ROC AUC of 0.976 and an F1 score of 0.92, while the regressor yields a mean-squared error of 0.88 subtypes. Applying these models, we carried out the first ultracool dwarf search with LSST DP1, cross-matched against VHS and CatWISE. Crucially, Euclid Quick Data Release 1 provided near-IR spectra for hundreds of candidates, enabling a rare, large-scale external spectroscopic validation. This confirmed 15 M6–L2 discoveries, verified USMILE performance, and clarified regimes where USMILE predictions are most reliable. Building on these insights, we identified 25 additional M6–L9 photometric candidates. These demonstrate the effectiveness of machine-learning methods in the data-rich era of wide-field surveys, highlighting the synergy between LSST and Euclid in expanding the ultracool dwarf census.
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High Energy Astrophysical Phenomena

A merger within a merger: Chandra pinpoints the short GRB 230906A in a peculiar environment

S. Dichiara, E. Troja, B. O'Connor et al. (+6 more)

We report the precise X-ray localization of GRB 230906A, a short duration ($T_{90}\sim$0.9 s) burst with no optical or radio counterpart. Deep imaging with the Hubble Space Telescope detects a faint galaxy (G$^\ast$; $F160W\simeq26$ AB mag) coincident with the sub-arcsecond X-ray position. Compared with standard GRB galaxies, its faintness, compact size and color would suggest a high redshift ($z\gtrsim$3) host. However, our observations also reveal the presence of a galaxy group at $z\!\sim$0.453, confirmed spectroscopically with VLT/MUSE, with clear signs of interactions and mergers among group members. The GRB and its putative host project onto an extended ($\approx$180 kpc) tidal tail emerging from the group's central galaxy. The probability of a chance alignment is small ($P_{cc}\!\lesssim\!4$%), we thus argue that the GRB and its galaxy G$^*$ reside within the group. Their peculiar location along the tidal debris suggests that an enhanced burst of star formation, induced by the galaxy merger, might have formed the progenitor compact binary $\lesssim$700 Myr ago. The compact binary later evolved in a neutron star merger which produced GRB 230906A and injected $r$-process material into the surrounding circumgalactic medium.
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All authors S. Dichiara, E. Troja, B. O'Connor, Y.-H. Yang, P. Beniamini, A. Galvan-Gamez, T. Sakamoto, Y. Kawakubo, J. C. Charlton

High Energy Astrophysical Phenomena

From Localization to Discovery: Bayesian Ranking of Electromagnetic Counterparts to Gravitational-Wave Events

Kendall Ackley

The robust association of electromagnetic candidates discovered during follow-up of gravitational-wave alerts is challenging, not only due to the large sky areas and broad distance uncertainties, but also due to the tens to hundreds of unrelated optical transients that are observed per event. We present a Bayesian ranking method to identify electromagnetic counterparts to GW events using only location information. The framework combines three-dimensional gravitational wave skymaps with host-galaxy information, a morphology-aware host association, empirical offset priors, and peculiar velocity corrections. We apply the method to GW170817 where it ranks AT2017gfo as the top candidate and correctly selects NGC\,4993 as the host. The approach is directly applicable to transient candidates with only location information and enables more efficient follow-up with prioritized candidates and leads to more reliable counterpart identification in current and future observing runs.
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Cosmology and Nongalactic Astrophysics

Constraints on neutrino mass and dark energy agnostic to the sound horizon

Ravi Kumar Sharma and Julien Lesgourgues

Recent BAO observations from DESI DR2 either hint at a possible dynamical dark energy component, which would worsen the Hubble tension, or at a 95% credible interval for the summed neutrino mass hardly compatible with neutrino oscillation experiments. In this context, it is interesting to investigate constraints on neutrino masses, dark energy and the Hubble parameter that are agnostic to some aspects of the cosmological model. Here we choose to be agnostic to the value of the sound horizon at recombination, while sticking to standard assumptions regarding the time of recombination and the growth of structures. To be consistent, we also disregard information on the full shape of the CMB temperature and polarization spectrum on sub-degree scale. With such agnostic and conservative assumptions, using data mainly on uncalibrated distances, the growth of structures, and laboratory bounds on tritium $\beta$-decay, we find that: (i) the dark energy evolution is well constrained by uncalibrated data on angular and luminosity distances, with a mild preference for dynamical dark energy, independently of the value of the sound horizon; (ii) large values of the Hubble rate are favored, $H_0=74.7^{+3.4}_{-4.4}$ km/s/Mpc (68%CL), together with low values of the sound horizon, $r_{\rm s}=131.1^{+6.8}_{-6.9}$ Mpc (68%CL); the SH0ES value of $H_0$ is thus marginally preferred over the low value returned by the standard inverse distance ladder analysis; (iii) the cosmological neutrino mass bound relaxes to $\sum m_\nu = 0.69^{+0.33}_{-0.47}$ eV (68%CL) and becomes well compatible with the normal and inverted neutrino mass schemes.
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