Cosmology and Nongalactic Astrophysics
Redshift Dependence of $H_0$ Dipole in Pantheon+ Supernovae
M.H. Jalali-Kanafi, E. Ó Colgáin, S. Pourojaghi et al. (+1 more)
We examine the dipole structure in the local
cosmic expansion rate $H_0$ using the Pantheon+ compilation of Type Ia
supernovae. We reconstruct directional maps of $H_0$ across the sky within the low-
redshift regime, $z_{\rm min} < z < 0.2$, evaluated in the CMB rest frame. To perform a tomographic analysis, we choose 10 different values for $z_{\rm min}\in [0.015 , 0.045]$ range. We find a dipole amplitude $A_{\rm dip}=1.16\pm0.28\ {\rm km/s/Mpc}$ for the lowest bin, monotonically decreasing to $A_{\rm dip} =0.35\pm0.52\ {\rm km/s/Mpc}$, as we increase $z_{\rm min}$. Since $A_{\rm dip}$ is positive-definite, by construction $A_{\rm dip} > 0$ even in an isotropic universe, thus to assess the statistical significance, we compare to $A_{\rm dip}$ generated from 1000 Monte Carlo simulations based on the $\Lambda$CDM model and the same
redshift ranges. Our results reveal a $2-3\sigma$ dipole pattern (depending on how the significance level is computed) for $z_{\rm min}\lesssim 0.032$. For
redshift thresholds where the signal remains statistically significant ($>2\sigma$), the inferred dipole direction points close to Shapley supercluster and CMB dipole directions. As $z_{\rm min}$ increases, the dipole amplitude diminishes in line with expectations of a higher
redshift isotropic Universe, but the difference in maximal antipodal $H_0$ determinations, $\Delta\text{H}_0^{\rm max}$, retain a signal of an anisotropy that disappears in the dipole ansatz. The decreasing statistical significance of all
estimators with increasing $z_{\rm min}$ suggests that any $H_0$ dipole is a low-
redshift feature.
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All authors M.H. Jalali-Kanafi, E. Ó Colgáin, S. Pourojaghi, M.M. Sheikh-Jabbari
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
New Universal Relations for Magnetized Neutron Stars
Byon N. Jayawiguna, Siddarth Ajith, Takuya Katagiri et al. (+2 more)
Unlike the
neutron-
star mass–radius relation, which depends sensitively on the internal
stellar structure through the equation of state of dense
nuclear matter, certain
neutron-
star properties obey approximately universal relations that exhibit only weak dependence on the equation of state. In this paper, we explore new universal relations for magnetized
neutron stars. We focus on a purely poloidal dipolar
magnetic-field configuration and treat the effects of the
magnetic field perturbatively to second order in the field strength. After appropriately normalizing the relevant quantities by the
stellar mass and the
magnetic-field strength at the pole, we first identify an exact universal relation between the
magnetic dipole moment and
stellar compactness. We then find approximate universal relations among the
magnetic dipole moment, magnetically-induced
stellar quadrupole moment, and ellipticity, with fractional variations due to the equation of state at the level of O(10%). We support these numerical findings with analytic estimates for Newtonian polytropes, which reproduce the observed behavior. Among the newly discovered relations, the one connecting the
magnetic dipole moment and
stellar quadrupole moment may be particularly useful for the analysis of gravitational-wave signals from binaries containing magnetized
neutron stars.
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All authors Byon N. Jayawiguna, Siddarth Ajith, Takuya Katagiri, Kent Yagi, Piyabut Burikham
Solar and Stellar Astrophysics
Instability and Angular Momentum Transfer in Thick Circumbinary Disks
Allen R. Murray and Paul C. Duffell
Utilizing the DISCO code we solve the 2D hydrodynamic equations for an equal mass circular restricted binary with an isothermal circumbinary disk (CBD). This work explores the eccentric instability in the CBD and angular momentum transfer, and their dependence on disk thickness and viscosity. The focus is specifically on thicker CBDs, with Mach number, ($\mathcal{M}\leq 14$). The growth rate of the disk cavity's eccentricity and the angular momentum transport from
accretion is calculated as diagnostics for binary-disk morphology
evolution. Calculations of CBD torques must run long enough to allow saturation of the disk's eccentric instability. The torque on the binary rapidly flips sign once the instability saturates. Thus we connect eccentricity saturation to a reversal in migration behavior for the binary. The binary experiences inward migration while the disk is in an evolving state, and outward migration once the disk eccentricity has saturated. The saturation time was found to be non-linearly dependent on Mach number and thicker CBDs require longer
evolution time for a quasi-steady state to appear. The eccentric instability is not present for $\mathcal{M} \lesssim 5$, and therefore migration reverses direction, leading to robustly inward migration for very thick disks $h/r \gtrsim 0.2$. The results of this work were found to be consistent with community results for Mach numbers $\mathcal{M} \ge 10$.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Enlightening dark moments of neutrino with superradiance
Indra Kumar Banerjee, Ujjal Kumar Dey and Anna John
Neutrinos can acquire electromagnetic moments either within the Standard Model through higher order radiative corrections or within the domain of new physics. In this study we focus on probing these beyond the standard model
neutrino moments through quenched superradiance of black holes where fermionic pairs can be produced from the superradiant bosonic cloud. We consider the production of dark
photons from black hole superradiance and quenching occurs through the production of
neutrino-antineutrino pairs from the dark
photons. The efficiency of the pair production depends on the effective coupling between the dark
photons and
neutrinos, i.e., the dark electromagnetic moments. We also discuss bounds on
primordial black hole abundance from
neutrino background arising from this quenched superradiance mechanism.
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Earth and Planetary Astrophysics
Pyrat Bay 2.0: an Upgraded Framework for Exoplanet Atmosphere Modeling in the JWST Era
Patricio E. Cubillos, Jasmina Blecic, Denis Shulyak et al. (+1 more)
This article presents a major update to the open-source Pyrat Bay modeling framework (version 2.0), tailored for the characterization of
exoplanet atmospheres with the observational capabilities of current facilities, such as the James Webb Space
Telescope (JWST), and future missions. The upgraded framework introduces a standalone chemistry package, chemcat, enabling the modeling of
exoplanet atmospheres via thermochemical-
equilibrium calculations with custom compositions, self-consistent radiative-
equilibrium thermal profiles, and
equilibrium-chemistry
retrievals. Other key implementations include nested sampling via MultiNest, isotopic-ratio fitting, transit light source modeling, and free-chemistry parameterization allowing vertical variations in volume mixing ratios. We validated the thermochemical- and radiative-
equilibrium modules via comparisons with the GGchem, FastChem, and HELIOS codes. We present an application of the new modeling framework by conducting an atmospheric characterization study using simulated transmission spectra of high signal-to-noise observations for a gas-
giant exoplanet. We found that vertical abundance variations in planetary atmospheres can be accurately recovered using data of JWST quality. In such cases,
retrievals that neglect these variations may result in biased abundance constraints, e.g., when employing the commonly used free-chemistry approach assuming constant abundance profiles. The results highlight the potential of JWST to study the multi-dimensional nature of exoplanetary atmospheres and their underlying physical processes.
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All authors Patricio E. Cubillos, Jasmina Blecic, Denis Shulyak, Luca Fossati
Astrophysics of Galaxies
Parameter-free prediction of the asymptotic acceleration scale confirmed by weak lensing
Maurice H.P.M. van Putten
The de Sitter transition scale $a_{dS}=cH$ to anomalous
galaxy dynamics was previously derived from first principles based on the background Hubble expansion $H$ and the velocity of light $c$. It introduces a $C^0$-transition across $a_{dS}$, which is in tension with predictions of $\Lambda$CDM
galaxy models at a significance of $6\sigma$. Tracing late-time cosmology, it predicted the deep-asymptotic acceleration scale $a_0^{th}\simeq 1.66\times 10^{-8}{\rm cm\,s^{-2}}$ without adjustable parameters based on $H$ and the deceleration parameter $q$. A recent weak-
lensing determination of $a_0^{WL} = 1.63_{-0.20}^{+0.23}\times 10^{-8}{\rm cm\,s}^{-2}$ now provides an independent empirical test of this prediction. The agreement with the updated theoretical value $a_0^{th} = 1.63_{-0.14}^{+0.13}\times 10^{-8}{\rm cm\,s^{-2}}$ closes the logical chain between cosmology,
galaxy dynamics, and asymptotic scaling. Unlike phenomenological fits to rotation curves, this result does not rely on tuning or baryonic modeling but follows directly from background cosmology. This parameter-free model suggests some further tests by
galaxy surveys extending over a finite range of
redshifts.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
Full analytic expressions of overlap reduction functions for anisotropies of the stochastic gravitational-wave background with pulsar timing arrays
Kun Zhou and Jin Li
Pulsar timing arrays (PTAs) have detected a
stochastic gravitational-wave background (SGWB) in the nanohertz band, enabling tests of
gravity and cosmology, as well as studies of supermassive black holes and early Universe physics. PTA data analysis relies on cross-correlating timing residuals, where overlap reduction functions (ORFs) critically determine sensitivity. Conventional ORF calculations using the short-wavelength approximation break down for the scalar longitudinal mode and cannot handle frequency dependence or anisotropies. This work rigorously derives the full response functions within an analytical cross-correlation framework. We reveal, for the first time, intrinsic
symmetries and relations among anisotropic ORF integrals for all polarizations. By variable substitutions and coordinate rotations, we transform complex integrals into tractable forms, resolving divergences in vector and scalar longitudinal modes. Building on this, we establish a universal framework yielding fully analytical expressions for anisotropic ORFs to arbitrary order for all six modes. As a direct application, we give complete expressions up to l <= 5. This framework is free of approximations; its (0,0) component recovers the isotropic Hellings–Downs curve. Compared with numerical integration, our results offer broader applicability, faster computation, and higher precision, providing a valuable foundation for anisotropic sky mapping, polarization-mode separation, and new-physics searches with PTA data.
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Astrophysics of Galaxies
Formation of globular cluster-rich ultra-diffuse galaxies through mergers
Truman Tapia, Hidenori Matsui, Kenji Bekki et al. (+6 more)
We use high-resolution, idealized hydrodynamic simulations of gas-rich
dwarf-
galaxy mergers to test whether such encounters can form ultra-diffuse
galaxies (UDGs) with globular cluster (GC) systems. We simulate 1:1 and 1:2 mergers alongside an isolated control model and identify
stellar overdensities as GC candidates (GCCs). The remnants evolve into dispersion-supported, UDG-like systems with three-dimensional
stellar half-mass radii $r^{3D} \sim 1.9-2.6$ kpc, while the isolated
dwarf remains rotationally supported and forms no GCCs.
Tidal heating and
stellar feedback expel a large fraction of the gas beyond the dark matter (DM)
halo, leaving
stellar-dominated remnants whose DM
haloes remain cuspy. Merger-driven
star formation is highly clustered: the fraction of newly formed
stellar mass bound in massive clusters exceeds 0.5 after the first pericentric passage and remains elevated thereafter. By the final snapshot, the remnants host GC populations numbering 20 (1:1) and 39 (1:2), more centrally concentrated than the field
stars and consistent with the observed GC number-
halo mass relation. The GCCs match observed
star clusters in the planes of mass versus size, velocity dispersion, and density. More massive clusters exhibit stronger internal rotation and broader
metallicity spreads. In one case, the merger produces a nucleated UDG via cluster inspiral followed by sustained in-situ
star formation. These results demonstrate that gas-rich
dwarf mergers are a viable pathway to GC-rich (and sometimes nucleated) UDGs, and predict correlated cluster mass, rotation, and
metallicity-dispersion trends testable with observations.
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All authors Truman Tapia, Hidenori Matsui, Kenji Bekki, O. Ivy Wong, Connor Bottrell, Brent Groves, Duncan A. Forbes, Jean P. Brodie, Warrick J. Couch
High Energy Astrophysical Phenomena
Constraining Circum-burst Environments of GRBs with Jet Break Features in X-ray Afterglows
Si-Ji Xin, Yu-Qi Zhou, Sheng-Jin Sun et al. (+6 more)
The nature of the circum-burst medium serves as a key diagnostic for probing the progenitor systems and the physics of relativistic jet propagation in gamma-ray bursts (GRBs). In this work, we systematically infer the density profile index $k$ (where $n \propto r^{-k}$) from the change in the temporal decay index at the jet break ($\Delta\alpha$). Within the framework of the uniform jet model, the two quantities are linked by the relation $\Delta\alpha = (3 - k)/(4 - k)$. We apply this diagnostic to a substantial and uniformly selected sample of 170 GRBs with clear jet breaks, identified from over 1,400 Swift/XRT X-ray afterglows observed from 2004 to 2024. By fitting the light curves with a broken power-law model, we obtain $\Delta\alpha$ for each burst and subsequently derive the corresponding $k$ value. We then use the derived $k$ values to classify the circum-burst environment of each GRB. Our results reveal a near-even split: 82 bursts ($\sim48\%$) are consistent with a constant-density interstellar medium (ISM, $k \approx 0$), while 88 bursts ($\sim52\%$) favor a wind environment ($k \approx 2$). For the 35 bursts with optical data, our X-ray-based classifications are generally consistent with independent multi-band analyses. Additionally, we derive jet opening angles and true beaming-corrected energies for bursts with known
redshifts.
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All authors Si-Ji Xin, Yu-Qi Zhou, Sheng-Jin Sun, Cheng-Jie Sun, Shuang-Xi Yi, Yuan-Chuan Zou, Yu-Peng Yang, Yan-Kun Qu, Fa-Yin Wang
Solar and Stellar Astrophysics
Euclid Early Release Observations of the Barnard 30 dark cloud I. Brown dwarfs and planetary mass candidate members at the core of the association
D. Barrado, H. Bouy, E. L. Martin et al. (+8 more)
This study aims to identify very low-mass members within the Barnard 30 dark cloud, with a particular focus on detecting objects in the substellar domain, including those within the planetary-mass regime. We employed deep photometric observations using data from the Euclid mission, incorporating advanced data processing techniques, in particular the DeNeb tool for optimized source detection. We also analyze multi-wavelength ancillary observations and perform a Spectral Energy Distribution analysis for each candidate member. In addition, low-resolution near-infrared spectroscopy was obtained for one candidate to further assess its nature and properties. Our initial photometric analysis yielded a initial sample of nearly one hundred candidate members in the substellar mass range. A subsample of 23 probable members, located close to the 3 Myr isochrone, has been identified. Thus, we have substantially expanded the known population of faint, cool sources associated with the region. Low resolution near-IR spectroscopic analysis of one candidate reveals an L2 spectral type with low
gravity, consistent with a young ultra-cool
dwarf. If its membership is confirmed, its estimated mass lies in the range 15-20 Mjup. These findings validate the reliability of our multi-wavelength photometric selection methodology. These results offer valuable insights into the low-mass end of the initial mass function (IMF) and demonstrate the effectiveness of Euclid in identifying brown
dwarf and planetary-mass candidates in nearby, densely packed
star-forming regions.
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All authors D. Barrado, H. Bouy, E. L. Martin, J.-C. Cuillandre, N. Huélamo, M. Žerjal, C. Dominguez-Tagle, S. Muñoz Torres, J.-Y. Zhang, E. Bertin, J. Olivares
Cosmology and Nongalactic Astrophysics
Revisiting 2D and 3D Dainotti Correlations for GRBs Using Bayesian Neural Networks
Nilanjana Bagchi Aurpa, Abha Dev Habib and Nisha Rani
Gamma-ray bursts (GRBs) are promising cosmological probes, but their use as standard candles is limited by the circularity problem, necessitating model-independent calibration of GRB luminosity correlations. We revisit the two-dimensional (2D) and three-dimensional (3D) Dainotti correlations using
Bayesian Neural Networks (BNNs) trained on the updated Observational Hubble Data (OHD) and Pantheon+ Type Ia
Supernova sample. The reconstructed luminosity distances are used to calibrate the Platinum and Narendra et al. GRB samples. We constrain the parameters of the 2D Dainotti relation and the 3D fundamental plane, and examine the impact of calibration
datasets and GRB sample selection. Calibration achieved using Pantheon+ yields tighter constraints than OHD, while the 3D correlation exhibits lower intrinsic scatter than the 2D relation. Our results demonstrate that BNNs provide a robust framework for model independent calibration of GRB luminosity correlations with reliable uncertainty propagation. Further, the underlying distance probe is a key factor in model-independent calibration, determining both the size of the GRB samples and the precision of the resulting constraints.
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Cosmology and Nongalactic Astrophysics
Cosmography for a General Spacetime Centred at Arbitrary Redshift
Jonas Broe Bendtsen, Asta Heinesen and Sofie Marie Koksbang
With upcoming surveys providing large volumes of highly precise observational data across a wide range of
redshifts, it is increasingly important to have tools that can translate observational data into
geometric and dynamical information without imposing a predetermined cosmological model. General cosmographic expansions centred at arbitrary
redshift provide exactly such a tool. We here present the formalism for general cosmographic expansions centred at an arbitrary
redshift, valid for 4-dimensional Lorentzian spacetimes. We then apply the expansion formalism to test its ability to reproduce the
redshift-distance relation in two examples of large-scale
cosmic structures (an underdensity and an overdensity) modelled by the Lemaître-Tolman-Bondi metric, where we examine the effect of choosing different
redshift intervals for the cosmographic series expansions. This quantifies the extent to which cosmographic coefficients inferred from
redshift-distance observations retain their interpretation as local
geometric and dynamical quantities, as is expected in standard FLRW cosmology. Similarly to earlier results, we here find that in more general spacetimes the coefficients instead become effective parameters reflecting the finite observational range probed. Lastly, we discuss possible strategies for using the expansions to constrain dynamical and
geometric quantities.
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Astrophysics of Galaxies
Smoothed particle magnetohydrodynamics for simulations of galaxy and cosmic structure formation
Orestis A. Karapiperis, Matthieu Schaller, Nikyta Shchutskyi et al. (+2 more)
We introduce a novel formulation of cosmological smoothed particle magnetohydrodynamics (SPMHD), designed to model
magnetic field physics in a vast array of nonlinear astrophysical systems, and which we have implemented in the highly-parallel, entirely modular, and open-source simulation code SWIFT. Our numerical scheme is designed to offer optimal performance at a minimal computational cost, keep a low
memory footprint, and most notably couple robustly to effective sub-resolution recipes of
galaxy formation. This is achieved through expressing our
evolution equations in a density-energy conservative form, and augmenting them with discontinuity-capturing terms tailored to high dynamic range simulations, which are further modulated by adaptive switches that drastically improve coupling to sub-grid models and limit spurious dissipation. We moreover present novel suggestions for the two major regularisation techniques used in modern SPMHD, namely a tensile instability correction and mixed
hyperbolic/
parabolic divergence-cleaning scheme, to ensure code stability in highly dynamical scenarios. We evaluate the performance of our method on a series of problems of increasing complexity, culminating in three astrophysical applications which have historically proven challenging for mesh-less methods: we study jet launching from a forming proto-
stellar core, dynamo amplification in a massive
galaxy cluster and
magnetic field
evolution in a Milky Way-like disk
galaxy; the latter constitutes the first reported coupling of the EAGLE
galaxy formation model to a magnetohydrodynamics solver. Keeping model hyperparameters fixed across our test suite to provide a transparent picture of our method's capabilities in production, we demonstrate sound performance and convergence with resolution on standard `laboratory' numerical experiments, as well as competitive capabilities in realistic applications.
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All authors Orestis A. Karapiperis, Matthieu Schaller, Nikyta Shchutskyi, Federico A. Stasyszyn, Maarten Elion
Solar and Stellar Astrophysics
Discovery of the $\lambda$ Bootis phenomenon in an early B-type star
J. Olbermann, P. Aschenbrenner and N. Przybilla
Chemically peculiar A-type
stars of the $\lambda$ Boo class are characterised by depletions of refractory elements and normal volatile species. Their formation channel(s) have not yet been fully established, but they provide insight into
star-disc-
planet interactions during
star formation,
star-
planet interactions in later system
evolution, or interactions between
stars and the interstellar medium. We report the first detection of the $\lambda$ Boo phenomenon in a massive early B-type
star, HD 37356, a member of Ori OB1c. We analysed a high-quality spectrum using a hybrid non-LTE model-atmosphere technique. We determined atmospheric parameters and elemental abundances using neural networks to emulate synthetic spectra, coupled with a Markov chain Monte Carlo
algorithm. We also derived the fundamental parameters of the
star using
Gaia data and
stellar evolution models. HD 37356 closely resembles the Morgan-Keenan standard $\gamma$ Peg, closely agreeing in spectral signatures except for the lines of refractory elements, which are systematically weaker. The quantitative analysis yields similar
stellar parameters. The abundances of volatile elements are compatible with
cosmic standard abundances, while refractory species are depleted by 0.25 to 0.45dex. In particular, the lower iron abundance explains the different pulsational behaviour of HD 37356 compared to $\gamma$ Peg. Of the different formation channels for $\lambda$ Boo
stars, only the
accretion of gas depleted in refractory elements from the circumstellar disc during
star formation appears to be feasible in this case. This scenario requires HD 37356 to be a true slow rotator in order to prevent the abundance pattern from being erased by internal mixing processes such as meridional circulation.
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High Energy Astrophysical Phenomena
Parameter Estimation for Eccentric Supermassive Black Hole Binaries with Pulsar Timing Arrays
Sara Manzini and Stanislav Babak
Pulsar timing array (PTA) experiments are searching for gravitational waves (GWs) in the nanohertz band. The primary GW sources targeted by PTAs include populations of inspiralling supermassive black hole binaries (SMBHBs) in the local Universe, some of which may emit detectable continuous gravitational-wave (CGW) signals. In this paper, we focus on the detection strategy for individual binaries on eccentric orbits. Using simulated
datasets based on the EPTA DR2new configuration, we perform injection-recovery studies across the parameter space. We demonstrate that individual component masses can be measured when an eccentric SMBHB is detected at high GW frequencies. We further show a correlation between the CGW signal and the
stochastic gravitational-wave background (SGWB) at low frequencies, which makes CGW identification challenging. Finally, the developed software is GPU-compatible, enabling efficient
Bayesian inference. This work lays the groundwork for future applications to real PTA data.
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Instrumentation and Methods for Astrophysics
The Kepler and TESS Asteroseismic Consortia - A Historical Overview
Mikkel N. Lund and Tiago L. Campante
This overview is, in short, about people. About an idea that grew into a consortium, then a community, and finally a way of doing asteroseismology in the space era. The Kepler Asteroseismic Science Consortium (KASC) and later the TESS Asteroseismic Science Consortium (TASC) are not just organisational structures to access data. They became collaborative ecosystems in which students, postdocs, and senior scientists learned to work together across continents, time zones, and occasionally differing opinions about filters. The idea for this booklet did not emerge from a formal meeting or a funding call. It emerged-appropriately enough for an international collaboration-over a beer. More precisely, over a beer (truth be told, more than one) at Beer Lab University in Honolulu, during the TASC7/KASC14 Workshop in 2023. Google Maps now informs us that this particular Beer Lab location is "permanently closed", which seemed at first like an ominous metaphor for fading inspiration. Fortunately, we have since learned that Beer Lab simply moved across the street, proving that good ideas, like good consortia, tend to reinvent themselves rather than disappear entirely. Somewhere between discussions that night and many others over the years, we found ourselves revisiting stories we had long heard from our former supervisors and more senior colleagues; stories of how KASC was negotiated, how the first working groups were formed, how data were filtered (a saga in its own right!), and how trust was built. This book is therefore an attempt to capture some of that collective
memory, while we still have access to the primary sources. It is timed to celebrate the return of the KASC/TASC Workshop Series to Aarhus in 2026, where the consortia's original framework was set in motion. There is something fitting about reflecting on the journey of KASC and TASC as the community gathers again where it all began.
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Cosmology and Nongalactic Astrophysics
Joint Geometric and Dynamical Constraints on Cosmology from Anisotropies in Galaxy Intrinsic-Alignment Correlations
Teppei Okumura
We present the first joint cosmological analysis to extract both
geometric and dynamical information from
galaxy intrinsic
alignments (IAs). Using BOSS spectroscopy cross-matched with
galaxy shape measurements from the DESI Legacy Imaging Surveys, we measure anisotropic
galaxy density–intrinsic ellipticity (GI) and intrinsic ellipticity (II) correlations over $0.43\leq z\leq0.7$ and decompose their
spin-dependent angular structure into associated Legendre multipoles. The measured GI correlation exhibits the expected
baryon acoustic oscillation (BAO) structure, while its anisotropy provides
geometric information complementary to
galaxy clustering. By jointly modeling
redshift-space and Alcock-Paczynski distortions, we constrain the growth rate parameter $f\sigma_8$, the angular-diameter distance $D_A$, and the Hubble expansion rate $H$. Relative to
galaxy clustering alone, adding IA reduces their fractional uncertainties by 32%, 18%, and 29%, respectively. By mapping these constraints onto a flat $w_0$CDM model, IA also tightens the constraints on $w_0$, $\Omega_m$, and $H_0$; however, the $w_0$ constraint is sensitive to the minimum scale included in the analysis. Our results establish anisotropic
galaxy shapes as an additional source of
geometric and dynamical information for spectroscopic cosmology.
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Cosmology and Nongalactic Astrophysics
Optical Signatures of Sgr A* and M87* with Dark Matter Halos
Noraiz Tahir, Muhammad Ali Paracha and Mubasher Jamil
The event horizon
telescope (EHT) has opened a new window onto the strong-field regime by imaging the shadows of the supermassive black holes (SMBHs) Sgr A* and M87*. These observations provide a unique laboratory for probing the dark matter (DM) distribution around black holes. In this work we systematically investigate the imprints of two distinct DM
halo models, the cold DM (CDM), and the cored scalar field DM (SFDM) profiles on the gravitational
lensing signatures of Sgr A* and M87*. We compute the
photon spheres, shadows, weak and strong
lensing observables, and caustic structures for both models. We then compared the obtained values of the shadow diameters with the EHT data, using $\chi^2$ statistics. We found that all the models are within $1.2\sigma$ of the measured shadow diameters with the small $\chi^2$ differences, $\Delta\chi^2 \lesssim 1.2$. The caustic analysis reveals distinct topological regimes, Sgr A* retains both tangential and radial critical curves, while M87* may show only tangential critical curves due to its larger mass. This topological difference provides a clear observational signature for future observations.
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Earth and Planetary Astrophysics
An Observational Study of Systematics in Differential Transmission Spectroscopy Using HFOSC on the Himalayan Chandra Telescope
Manjunath Bestha, Athira Unni, T Sivarani et al. (+2 more)
Ground-based low-resolution transmission spectroscopy requires photometric precision of a few hundred parts per million, making it sensitive to instrumental and atmospheric systematics. This work studies the systematic effects affecting differential transmission spectroscopy using the Hanle Faint Object Spectrograph Camera (HFOSC) on the 2-m Himalayan Chandra
Telescope (HCT). The study was motivated by an additional flux drop observed in the white-light curve of HAT-P-1 b. HAT-P-1 b is an ideal target for differential spectrophotometry because it has a visual binary companion with similar brightness at a suitable separation, allowing the companion
star to be used as a reference. To investigate the origin of this feature, we analyzed several observational parameters, including FWHM variations, spectral trace motion, centroid drift, and spectral shifts. We also observed WASP-33 b in slitless mode to test whether differential slit losses could explain the observed systematic. In addition, observations of WASP-12 b were used to derive a broadband optical transmission spectrum using common-mode correction. The additional flux drop is unlikely to be caused only by differential slit losses, since similar differential centroid and spectral shifts are present in both slit and slitless observations. The results suggest that the observed systematic may be related to field-dependent distortions and pointing-dependent instrumental flexure, although its exact cause is still unknown. Overall, this work highlights the importance of understanding and reducing observational systematics in ground-based
exoplanet transmission spectroscopy, especially for measurements that require photometric precision of a few hundred parts per million.
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All authors Manjunath Bestha, Athira Unni, T Sivarani, Parvathy M, Devika Divakar
High Energy Astrophysical Phenomena
Probing Variations in Earth's Ionosphere Using Pulsars
Yanqing Cai, Ziwei Wu, Weiwei Zhu et al. (+24 more)
We present high cadence 10-minute rotation measure (RM) monitoring of PSR~J0814+7429 using the LOw-Frequency ARray, aiming to probe ionospheric variability along the
pulsar line of sight (LoS). By separating the ionospheric contribution from the observed RM, we quantitatively reconstruct the diurnal variation of the ionospheric electron density along the
pulsar LoS based on the World
Magnetic Model. The derived variations exhibit clear
solar-driven modulation, including the ionospheric noontime bite-out phenomenon, and show good agreement with the LoS total electron content reconstructed from independent global vertical total electron content maps. These results demonstrate the feasibility of using
pulsars as probes of temporal variations in the electron density of the Earth's ionosphere.
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All authors Yanqing Cai, Ziwei Wu, Weiwei Zhu, Joris P. W. Verbiest, Ningbo Wang, Moochickal Ambalappat Krishnakumar, Lars Künkel, Jörn Künsemöller, Yulan Liu, Nataliya Porayko, Golam M. Shaifullah, Caterina Tiburzi, Marcus Brüggen, Ralf-Jürgen Dettmar, Ziyao Fang, Qiuyang Fu, Jiawei Jin, Caisong Liu, Lingqi Meng, Xueli Miao, Jiarui Niu, Dominik J. Schwarz, Christian Vocks, Mengyao Xue, Mao Yuan, Youling Yue, Zhen Zhang
High Energy Astrophysical Phenomena
Evolution of low-mass He stars and implications for electron-capture supernova formation in close binaries
Jun-Qian Li, Ying Qin, Zi-Yuan Wang et al. (+3 more)
The
evolution of low-mass helium (He)
stars ($\sim2.5$–$5\,M_\odot$) with
neutron-
star (NS) companions in close binaries has been extensively studied, but the combined effects of rotation and
tidal interaction remain poorly understood. We investigate how rotation, mass transfer, and
tidal interactions affect the
evolution of low-mass He
stars, the formation of electron-capture
supernovae (ECSNe), and the properties of the resulting NSs. Using detailed
stellar and binary
evolution calculations that include mass loss, differential rotation, and
tidal interactions, we systematically explore the initial binary parameter space leading to ECSNe. We find that rotation has only a modest effect on the
evolution of low-mass He
stars. ECSNe occur within a narrow initial He-
star mass range of $2.42$–$2.67\,M_\odot$ at
solar metallicity ($Z_\odot$) and $2.37$–$2.62\,M_\odot$ at $0.01\,Z_\odot$. The resulting NSs have
spin periods of $7.7$–$83.8\,\mathrm{ms}$,
magnetic fields of order $10^{12}\,\mathrm{G}$, and rotational energies of $2.6\times10^{48}$–$2.5\times10^{50}\,\mathrm{erg}$, although these values would be substantially reduced if efficient angular-momentum transport mechanisms, such as the Spruit–Tayler dynamo, were included. We further show that the evolutionary outcome is highly sensitive to the initial orbital period, with shorter-period systems undergoing Roche-lobe overflow at earlier evolutionary stages and experiencing stronger binary interactions. Finally, comparison with
Galactic double NS systems indicates that most observed binaries can be reproduced in the eccentricity–orbital-period plane by adopting relatively large natal kick velocities.
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All authors Jun-Qian Li, Ying Qin, Zi-Yuan Wang, Qing-Wen Tang, Han-Feng Song, Georges Meynet
High Energy Astrophysical Phenomena
Radio Constraints on the Circumstellar Environment of the Type IIb Supernova SN 2024iss
Yuhei Iwata, Tomoki Matsuoka, Masanori Akimoto et al. (+6 more)
Type~IIb
supernovae exhibit diverse progenitor properties, and
radio observations offer a unique probe of their mass-loss histories shortly before the explosion. We present Japanese VLBI Network single-baseline monitoring of the nearby Type IIb SN 2024iss at 6.9 and 8.4 GHz, spanning approximately one year after its discovery. Our
radio observations have detected its emission at 10 and 23 days after the explosion, with subsequent epochs yielding non-detections. Based on the peak
radio luminosity and peak time, SN 2024iss exhibits
radio properties highly comparable to those of compact-envelope events. Using a synchrotron self-absorption (SSA) modeling, we estimate a progenitor mass-loss rate of $\dot{M} \approx 2.5 \times 10^{-6}\>M_{\odot}\>{\rm yr^{-1}}$ for a compact progenitor wind velocity of $100 \>{\rm km\>s^{-1}}$. Furthermore, our SSA analysis yields a mean expansion velocity of $V_{\rm sh} \approx 3.3 \times 10^4\>{\rm km\>s^{-1}}$, which exceeds the theoretical shock velocity derived from the self-similar solution by a factor of $\sim 2.4$. Even for the conservative upper-bound peak time, the SSA-derived velocity remains larger than the theoretical expectation by a factor of $\gtrsim 1.7$. To explain this velocity excess, we propose the presence of a confined, dense circumstellar matter (CSM) surrounding the progenitor. The shock emergence from this confined CSM may have accelerated the forward shock, pointing to a highly complex and non-steady mass-loss history shortly before the explosion.
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All authors Yuhei Iwata, Tomoki Matsuoka, Masanori Akimoto, Keiichi Maeda, Nozomu Tominaga, Yoshinori Yonekura, Takashi J. Moriya, Kotaro Niinuma, Kenta Fujisawa
Cosmology and Nongalactic Astrophysics
Say Hello, Wave Goodbye: Gravitational Waves from Hyperbolic PBH-SMBH Interactions
Laura Burn, Nelson Christensen and Richard Easther
Primordial black holes (PBHs) formed in the early Universe remain a viable dark matter candidate. Since dark matter is expected to be concentrated toward
galactic centres, a significant population of PBHs would reside near a supermassive black hole (SMBH), providing a promising environment for gravitational wave emission. We consider PBHs with masses from $10^{-15}$ to $10$
solar masses on
hyperbolic trajectories past an SMBH, that produce bursts of gravitational radiation during periastron passage. Sagittarius A* is an obvious location for these events, but we show that both M31* and M87* would generate similar signals, albeit at lower frequencies. We assess the detectability of both individual bursts and "popcorn" backgrounds, relative to LISA and the proposed $\mu$Ares mission. Comparing these results to plausible central
halo densities, we find that unbound PBHs are unlikely to yield detectable signals.
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Astrophysics of Galaxies
Detailed Abundance Determination of Metal-Poor Stars with X-Shooter II. - Chemically Disentangling the Halo, Disk and GSE
Benjamin D. C. Lowe, Luca Casagrande, Thomas Nordlander et al. (+2 more)
We present a detailed chemical analysis of seven extremely metal-poor (EMP)
star candidates observed with X-Shooter, combining them with 16 EMP candidates from Paper I. We measured abundances for 16 elements, showing excellent agreement with previously published results. The sample was further extended using high-resolution literature data for 315 metal-poor
stars. The full sample was then kinematically separated into prograde disk, retrograde disk,
Gaia-Sausage Enceladus (GSE) and
halo classifications. Combining dynamics with chemistry, we demonstrate that the prograde disk exhibits a distinct negative linear trend in [Sc/Mg] with increasing
metallicity, with a slope of -0.6 dex per dex. This contrasts with the
halo trend at -0.04 dex per dex, a difference significant at the 3.95$\sigma$ level. Within the
metallicity range -4.2 $\leq$ [Fe/H] $\leq$ -1.9, the prograde disk trend is driven by low [Mg/Fe] at lower
metallicities, along with low [Sc/Fe] at higher
metallicities. This could be due to reduced early Mg enrichment in the progenitor prograde disk, followed by subsequent Mg enrichment, possibly associated with a later gas
accretion event. However, the physical origin of the higher-
metallicity Sc depletion remains unexplained by current nucleosynthesis models. The result remains significant at the >3$\sigma$ level across kinematic classifications derived from a different
Galactic potential. Additionally, we also identified an r-I
star with enhanced Ti, moderately-enhanced Sc, and depleted in C (unrelated to its evolutionary state). These abundances suggest a massive jet-induced hypernova progenitor, though a measurement of Zn is needed to verify this.
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All authors Benjamin D. C. Lowe, Luca Casagrande, Thomas Nordlander, Gary S. Da Costa, Norbert Christlieb