High Energy Astrophysical Phenomena
The pair-instability origin of supernova 2023vbw
Daichi Hiramatsu, Edo Berger, Daichi Tsuna et al. (+18 more)
Stars in the initial and carbon-oxygen core mass ranges of $\sim140-260$ and $50-130$ M$_\odot$, respectively, with low
metallicity are predicted to experience copious electron-positron pair production in their cores, leading to a runaway thermonuclear explosion that obliterates the entire
star in a luminous and long-duration pair-instability
supernova explosion. Some previous
supernovae have been interpreted in this context but lack the full range of predicted properties. Here, we report detailed observations and modeling of the hydrogen-rich
supernova 2023vbw, which exploded in a low-
metallicity ($\sim0.1$ Z$_\odot$) environment in a
dwarf star-forming
galaxy at a
redshift of $0.088$. Its light curve exhibits a luminous ($1.6\times10^{43}$ erg s$^{-1}$) and long-duration ($190$ days) main peak, resulting in a total radiated energy of $3\times10^{50}$ erg, more than an order of magnitude greater than canonical core-collapse
supernovae. Semi-analytical light-curve modeling yields a blue supergiant-like progenitor with an ejecta mass of $170-350$ M$_\odot$, radioactive nickel mass of $1.2-1.6$ M$_\odot$, and explosion energy of $(6-13)\times10^{52}$ erg, well matched by pair-instability models. The early and late-phase light curve and spectra also show evidence for interaction of the
supernova ejecta with an aspherical circumstellar medium. Discoveries of numerous such events with the upcoming Rubin Observatory and Roman Space
Telescope will shed light on the deaths of the most massive
stars in the Universe.
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All authors Daichi Hiramatsu, Edo Berger, Daichi Tsuna, Sebastian Gomez, Harsh Kumar, Peter K. Blanchard, Walter W. Golay, Anya E. Nugent, Takashi J. Moriya, D. Andrew Howell, Alexei V. Filippenko, Thomas G. Brink, WeiKang Zheng, Yi Yang, Moira Andrews, K. Azalee Bostroem, Joseph Farah, Curtis McCully, Megan Newsome, Estefania Padilla Gonzalez, Giacomo Terreran
Instrumentation and Methods for Astrophysics · Cross-listed from stat.ML
StAD: Stein Amortized Divergence for Fast Likelihoods with Diffusion and Flow
Gurjeet Jagwani, Stephen Thorp, Sinan Deger et al. (+1 more)
Diffusion and flow-based models are ubiquitously used for generative modelling and density estimation. They admit a deterministic probability flow ordinary differential equation (PF-ODE), analogous to continuous normalizing flows (CNFs), which describes the transport of the probability mass. Obtaining the likelihood from these models is of interest to many workflows, especially
Bayesian analysis, and requires solving the trace of the Jacobian to compute the divergence of the learned PF-ODE, which is either $\mathcal{O}(D^2)$ to compute exactly or $\mathcal{O}(D)$ with a noisy estimate. We introduce StAD, a new
distillation method to predict and learn the divergence of the PF-ODE using the Langevin-Stein operator without ever computing the Jacobian. We show that our method is competitive with the Hutchinson and Hutch++ on CIFAR-10, ImageNet and other density estimation tasks, consistently improving the variance and speed of the likelihood predictions compared to the Hutchinson. We additionally show our method will generalize to a varied class of generative models, and show that under some regularity conditions these learned vector fields can be made to satisfy the Stein class.
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All authors Gurjeet Jagwani, Stephen Thorp, Sinan Deger, Hiranya Peiris
Astrophysics of Galaxies
Halo-driven Origin and Evolution of Overmassive Black Holes and Little Red Dots
Ritik Sharma and Mahavir Sharma
We present a theoretical model in which the recently detected overmassive black holes (OBHs), and potentially Little Red Dots (LRDs), arise during a
halo-driven transient phase preceding the established coevolution of supermassive black holes (SMBHs) and their host
galaxies. In this model,
halo gravity drives an early phase of rapid black hole growth, leading to systems in high-
redshift haloes that lie above the local scaling relations. As the
halo evolves, a transition in
halo thermodynamics leads to the onset of a hot, pressure-supported medium that suppresses
accretion, reducing the black hole growth rate and driving the system toward the local black hole mass$-$
stellar mass relation. LRDs may represent an observational manifestation of the rapid,
halo-driven growth phase, while OBHs trace its direct mass signature. Our model thus provides a unified framework in which these systems form and evolve toward the regulated coevolution observed in the local Universe.
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Astrophysics of Galaxies
The limits of feedback from active galactic nuclei
Andrew Pontzen, Hiranya V. Peiris, Joop Schaye et al. (+1 more)
We use FLAMINGO to investigate why feedback from active
galactic nuclei (AGN) significantly depletes gas in
galaxy groups but is ineffective in clusters. We delineate three radial zones: an inner zone where AGN feedback heats
halo gas via shocks; an intermediate buoyancy zone where the heated
halo gas rises; and an outer zone where the
outflow may stall in a termination shock. Heating in the inner zone self-limits because, once the gas is sufficiently hot, shocks become too weak to deposit further
entropy. Consequently,
outflows have a ceiling
entropy value ($360\, {\rm keV\, cm^2}$) that is nearly independent of
halo mass. These values (and trends with
redshift and feedback variants) are explained using an argument based on the Rankine-Hugoniot relations.
Outflows rise at fixed
entropy through the buoyancy zone, escaping the
halo if the ceiling value is sufficiently elevated over that of the inflowing gas. This condition is satisfied only for
halo masses $M_{\rm 200m}<10^{13.7}\,{\rm M_\odot}$, because inflow
entropy tracks the virial relation. Variants with stronger (or weaker) feedback have a higher (or lower)
entropy ceiling and a correspondingly modified critical mass of $M_{\rm 200m}=10^{14.0}\,{\rm M_\odot}$ (or $10^{13.5}\,{\rm M_\odot}$). In clusters above the critical mass, the increased inflow
entropy causes the
outflow to stall and potentially shock at the 'splashback' radius. We derive an expression for the time
evolution of the virial gas fraction, which shows how lingering gas is reincorporated as the
halo virial radius expands. This effect dominates over
outflows unless they rejoin the Hubble flow; as a result, virial gas fractions rise as a function of mass starting at $M_{\rm 200m} = 10^{13.0}\,{\rm M_\odot}$. These effects explain why groups have depleted gas, while clusters have close to the
cosmic baryon fraction.
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All authors Andrew Pontzen, Hiranya V. Peiris, Joop Schaye, Matthieu Schaller
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Freeze-in Warm Dark Matter via Dimension-6 Operators in 3-3-1 Models
Fuwei Liu and Fujun Liu
We propose a natural resolution to the fine-tuning problem inherent in the freeze-in dark matter paradigm by
embedding a sterile singlet within a 3-3-1 electroweak extension. By imposing an exact $Z_{13}$ discrete gauge
symmetry, we formally suppress all low-dimensional portals to ensure that the dark sector communicates with the Standard Model (SM) exclusively through a dimension-six operator. This theoretical structure allows the extraordinarily small coupling required for dark matter production to emerge naturally from the profound hierarchy between the electroweak scale and the ultra-high Peccei-Quinn
symmetry breaking scale. Detailed numerical integration of the Boltzmann equations demonstrates that the sterile singlet can be produced via the infrared freeze-in mechanism to match the observed relic abundance of $\Omega_S h^2 = 0.12$. The resulting keV-scale warm dark matter candidate remains consistent with stringent Lyman-alpha forest constraints while offering a viable solution to
galactic-scale discrepancies such as the cusp-core and missing satellites problems. Ultimately, this framework provides a self-consistent unification of dark matter genesis and the strong CP solution that is completely independent of ad hoc parameter adjustments.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Particle Dynamics, Shadow and Hawking Sparsity of a Kalb-Ramond Black Hole Coupled to Nonlinear Electrodynamics
Faizuddin Ahmed, Ahmad Al-Badawi and İzzet Sakall{\i}
We study the timelike and null geodesic structure of a static, spherically symmetric black hole sourced by a Kalb–Ramond (KR) field coupled to nonlinear electrodynamics (NED). The geometry is characterized by the mass $M$, the
magnetic monopole charge $q$, and the Lorentz-violating parameters $(\gamma,\lambda)$. Closed-form expressions are derived for the effective potential, as well as the specific energy and angular momentum of massive particles on circular orbits. We further analyze the
photon sphere, black hole shadow, and the Lyapunov exponent associated with unstable null circular geodesics. The latter determines the eikonal quasinormal-mode frequencies through $\omega_{\rm eik}=(\ell+1/2)\,\Omega_c-i(n+1/2)\,|\lambda_L|$. The shadow radius is compared with the Event Horizon
Telescope (EHT) observations of M87$^\ast$ and Sgr~A$^\ast$, allowing us to identify the viable region in the $(q,\gamma)$ parameter space. Finally, we compute the Hawking temperature, horizon area, and the Gray–Visser sparsity parameter. We demonstrate that the combined effects of the KR field and
magnetic monopole charge increase the sparsity parameter from the Schwarzschild value $16\pi^3 \simeq 496$ to nearly $1.7\times10^3$. This indicates a significantly sparser Hawking cascade compared to the Schwarzschild case, while the
photon ring remains consistent with the EHT $1\sigma$ observational bounds across most of the physically allowed parameter range.
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High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph
Stern–Gerlach Spin Sorting and Dynamical Feedback in Relativistic Pair-Plasma Reconnection
K. Nykyri
We derive a Stern–Gerlach control parameter, $\Xi$, comparing
spin-driven cross-sheet displacement with the relativistic Larmor radius. It places heliospheric
plasmas and most astrophysical jets in the negligible regime, some
stellar-mass black-hole coronae in a transitional regime, and magnetar sheets in a strong, near-QED regime. Relativistic pair-
plasma simulations at fixed $\gamma_{\rm tr}=2$ show that increasing $\Xi$ produces, within the coupled SG+$\mathbf J_M$ model,
magnetic-moment sorting and magnetization-current feedback that enhance the normalized flux-growth rate through an additional
spin-kinetic pathway beyond classical pressure- and geometry-controlled pair-
plasma reconnection.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Irreducible Graviton Floor from Reheating
James M. Cline and Yong Xu
Inflaton decay inevitably emits gravitons through bremsstrahlung during reheating. We show that the soft part of this emission amplitude, fixed by Weinberg's soft-graviton
theorem, becomes an irreducible
stochastic gravitational-wave (GW) background after accounting for cosmological
evolution. The
theorem fixes the infrared branch of the spectrum, $\Omega_{\rm GW}\propto f$, independently of the microscopic operator responsible for inflaton decay, while the normalization is controlled by the hard inflaton decay rate and by a phase-space factor. We carry this out for inflaton $n$-body decays, including the phase-space integrals, finding that the maximum of the spectrum scales as $2/n$ relative to the $n=2$ case. The signal can reach $\Omega_{\rm GW}h^2\sim \mathcal O(10^{-17})$ at frequencies above the GHz scale. This predicts a
stochastic graviton floor from perturbative reheating: a larger signal would require either other processes beyond perturbative bremsstrahlung or inflationary scenarios beyond conventional single-field slow roll.
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Astrophysics of Galaxies
No Stream Left Unscathed: The imprint of a host galaxy
Arpit Arora, Peter S. Ferguson, Jacob Nibauer et al. (+11 more)
Stellar streams from disrupted globular clusters are excellent probes of dark matter (DM) subhalos. Observed Milky Way streams display a remarkable diversity of features: spurs, gaps, kinks, cocoons, and density variations, many attributed to subhalo encounters. But how much of this diversity arises from the host itself? We simulate $\sim$15,000 globular cluster streams across four Milky Way-mass
halos from the FIRE-2 cosmological simulations, evolved in basis function expansion potentials capturing the evolving disk,
halo, and large-scale structure while excluding small-scale perturbers such as DM subhalos and
giant molecular clouds. We find that roughly three quarters of streams develop complex features from the host potential, such as spurs, kinks, and cocoon-like envelopes. Even the smoothest streams exhibit 10–25% width variation along their track and host overdensities and gaps at scales of ${\sim}2^\circ$, squarely in the $1^\circ$–$5^\circ$ range predicted for subhalo-induced gaps. Pericentric distance is the primary predictor of stream morphology, with ${\sim}15$ kpc separating smooth from disturbed streams and circular orbits beyond $\sim$20 kpc producing the smoothest streams. Only $\sim$70 out of $\sim$15,000 streams are free of detectable wiggles in the track at any scale. Analogs to observed features, such as the GD-1 spur and the ATLAS–Aliqa Uma kink, emerge even without the presence of subhalos. As next-generation surveys (LSST, Euclid, and Roman) resolve stream structure across hundreds of streams, the baseline established here, streams evolved without small-scale perturbers, becomes essential for extracting DM substructure constraints.
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All authors Arpit Arora, Peter S. Ferguson, Jacob Nibauer, Nora Shipp, Videep Reddy, Eugene Vasiliev, Jack Kohm, Laurella C. Marin, Adrian M. Price-Whelan, Denis Erkal, Sarah Pearson, Andrew Wetzel, Jeremy Bailin, Robert Feldmann
Astrophysics of Galaxies
Simulations of gas inflow in the Milky Way I. Stellar-Feedback-Regulated Transport from the Central Molecular Zone to the Circumnuclear disk
Zi-Xuan Feng, Mattia C. Sormani, Robin G. Tress et al. (+18 more)
We perform hydrodynamical simulations with radially varying resolution to study the effects of
stellar feedback on the radial inflow of gas from the Central Molecular Zone (CMZ, $R\sim200$ pc) to the Circumnuclear Disk (CND, $R\sim5$ pc) of the Milky Way. The simulations include a realistic Milky Way barred gravitational potential, a cooling function coupled to a non-
equilibrium chemical network, gas self-
gravity,
star formation,
supernova feedback, and radiation feedback from massive
stars computed via on-the-fly radiative transfer. Our main findings are as follows: 1)
Stellar feedback drives a radial inflow that decreases monotonically with decreasing Galactocentric radius. The time-averaged inflow rate in our fiducial SNRad simulation, which includes both
supernova and radiation feedback, declines from $\langle \dot{M} \rangle\sim5\times10^{-3}$ Msun/yr at $R\sim100$ pc, to $\langle\dot{M}\rangle\sim10^{-4}$ Msun/yr at $R\sim10$ pc, to $\langle\dot{M}\rangle\sim10^{-6}$ Msun/yr at $R\sim1$ pc. 2) The total inflow rate can be broken down into two components driven by two distinct mechanisms. First, feedback-driven
turbulence redistributes the angular momentum of gas clouds, producing a smooth (secular) transport of mass inward, similar to a Shakura-Sunyaev viscous
accretion disk. This component contributes inflow rates that vary from $\dot{M}\sim5\times10^{-4}$ Msun/yr at $R\sim100$ pc to $\dot{M}\sim10^{-7}$ Msun/yr at $R\sim1$ pc. Second, episodic inflow events can transiently increase the inflow rate by several orders of magnitude, reaching $\dot{M}\sim10^{-3}$ Msun/yr over timescales of $\Delta t\sim3$-$5$ Myr at $R=10$ pc. 3) The
stellar feedback model significantly affects the episodic inflow but has little impact on the smooth component. Simulations including radiation feedback produce substantially more episodic events than those with
supernova feedback alone.
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All authors Zi-Xuan Feng, Mattia C. Sormani, Robin G. Tress, Simon C. O. Glover, Ralf S. Klessen, Jonathan Petersson, Michaela Hirschmann, Ashley T. Barnes, Cara Battersby, Marco Donati, Karl Fiteni, Jonathan D. Henshaw, Adam Ginsburg, Savannah Gramze, Xingchen Li, Dani R. Lipman, Steven N. Longmore, Elisabeth Mills, Maya A. Petkova, Yoshiaki Sofue, Arianna Vasini
Cosmology and Nongalactic Astrophysics
Analytical method for computing the covariance matrix of cosmic shear two-point correlation function
Kosuke Nagura, Ryo Terasawa, Taisei Terawaki et al. (+1 more)
Accurate estimation of the covariance
matrix of
cosmic shear statistics is essential for cosmological analyses using current and upcoming wide-area weak
lensing surveys. In this work, we investigate analytical methods for computing the Gaussian covariance
matrix of the
cosmic shear two-point correlation function (2PCF), taking into account the effects of finite survey geometry. We compute the covariance of 2PCF based on the improved Narrow Kernel Approximation (iNKA), with a projection using the Legendre transformation. We also consider other analytical covariance
estimators, the $f_{\mathrm{sky}}$ approximation and the weighted quartic-counts method. We evaluate the accuracy of those analytical methods using the convergence fields with the HSC Year 3 survey mask as a test case. We find that the covariance of the 2PCF obtained by using the iNKA does not reproduce the covariance measured directly from Gaussian simulations. Although the iNKA accurately models the diagonal structure of the harmonic-space covariance, residual inaccuracies in the off-diagonal components propagate through the Legendre transformation and significantly affect the real-space covariance. In contrast, the weighted quartic-counts method shows better agreement with the simulations. Our results demonstrate that accurate modeling of the off-diagonal structure of the harmonic-space covariance is crucial for obtaining reliable covariance estimates of real-space weak
lensing statistics in the presence of survey window effects.
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All authors Kosuke Nagura, Ryo Terasawa, Taisei Terawaki, Masahiro Takada
Instrumentation and Methods for Astrophysics
Rapid data quality investigations of gravitational-wave events with the Data Quality Report Builder toolkit
Derek Davis, Zach Yarbrough, Joseph Areeda et al. (+30 more)
We present the Data Quality Report Builder toolkit, DQRbuild, a suite of data quality tools that have been developed to vet gravitational-wave events in preparation for the fourth LIGO-Virgo-KAGRA observing run. We explain the main functionality and the many scientific tests that we support. To validate the performance of the tools included in the toolkit, we run a series of tests on all significant candidates shared as public alerts in the third observing run to compare against what was manually reported using human intervention. We find that these automated tools can now identify 96% of the problems identified by humans during this previous observing run, with a 24% false alarm rate. We conclude with a commentary on the prospects and potential challenges for fully automating the process of vetting the data quality for gravitational-wave events identified in future observing runs.
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All authors Derek Davis, Zach Yarbrough, Joseph Areeda, Ronaldas Macas, Nicolas Arnaud, Adrian Helmling-Cornell, Paolina Doliva, Olivia Godwin, Hirotaka Yuzurihara, Benjamin Mannix, Sofia Alvarez-Lopez, Max Trevor, Rachael Huxford, Philippe Nguyen, Beverly Berger, Chayan Chatterjee, Francesco Di Renzo, Christiano Palomba, Viola Sordini, Dimitrios Pesios, Marissa Walker, Airene Ahuja, Man Leong Chan, Julian Ding, Raymond Frey, Franz Herbst, Yannick Lecoeuche, Annudesh Liyanage, Jess McIver, Raymond Ng, Sophie Perry, Caitlin Rawcliffe, Robert Schofield
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Observational signatures of negative mass wormholes through their shadows
Shin'ichi Nojiri, Sergei D. Odintsov and Diego Sáez-Chillón Gómez
Systems containing objects with negative mass (NMOs) are considered. Such a system consists of one object with positive mass and one NMO, where a bound state exists even though the force exerted by the NMO on the object with positive-mass is repulsive. Unlike standard binaries composed of positive-mass objects, the emitted gravitational waves exhibit decreasing frequency and amplitude over time. We propose a model that removes the ghost appearing in the construction of the Ellis-Bronnikov wormhole, a candidate for a NMO. Furthermore, we perform numerical simulations to obtain the optical appearance of such NMOs. The observed luminosity is also compared with the Schwarzschild black hole and with the Simpson-Visser wormhole, revealing clear differences in the
photon ring substructure around the central object.
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Cosmology and Nongalactic Astrophysics
Study of dark interactions through strong gravitational lenses
F. Villalobos, J. Magana and T. Verdugo
The possible interaction between the dark components of the Universe (dark matter and dark energy) stands as an attractive alternative to the standard $\Lambda$CDM cosmological model. In this work, we present a novel analysis of three sign-changeable interaction models whose coupling term $Q$ depends explicitly on the deceleration parameter $q$ and is proportional to different energy densities: dark matter, dark energy, and total energy density. To constrain these models, we combine strong gravitational
lensing data on two complementary scales: a sample of early-type
galaxies acting as lenses and the
galaxy cluster Abell 1689. Our results show that the interaction strength $\beta$ depends on the choice of the coupling term $Q$, with all models yielding negative values of $\beta$, indicative of a dark interaction scenario. The $\beta$ values obtained in this work are significantly larger in magnitude than those previously reported using Type Ia
supernovae, CMB, and BAO. The strong-
lensing constraints indicate a transition to
cosmic acceleration at earlier
redshifts ($z_t \sim 1.8-2.1$) than that predicted by the $\Lambda$CDM model, while remaining consistent with
cosmic chronometer measurements within the reconstructed confidence regions. Therefore, our study shows that strong gravitational
lensing data provide an independent and competitive cosmological probe capable of testing interacting dark energy scenarios. The sensitivity of
lensing observables to the expansion history enables access to complementary information about dark-sector dynamics beyond standard cosmological probes.
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High Energy Astrophysical Phenomena
Nucleosynthesis in the fast ejecta of a neutron star merger
Lukas Schnabel, Stephan Rosswog, Friedrich-Karl Thielemann et al. (+1 more)
Neutron star mergers are today considered a major production site for rapid
neutron capture elements. While the bulk of the matter escapes at fast, but non-relativistic velocities (${\sim} 0.2\,c$), a small amount of the dynamically ejected mass reaches mildly relativistic velocities (${\gtrsim}0.6\,c$). It has been suggested earlier, that in such ejecta parts
neutrons may avoid being captured and that their decay could power an early blue precursor to the main kilonova event. Here we study in detail the nucleosynthesis in such fast ejecta with
nuclear network calculations along both parametrized and numerical relativity trajectories. We find that the nucleosynthesis can be divided into three channels, in one of which a substantial amount of free
neutrons survives when the main r-process has frozen out. We provide a (semi-)analytical model for surviving free
neutrons which agrees very well with the network calculations. If the mass fraction of the free
neutrons exceeds ${\sim} 0.05$, their $\beta^-$-decay dominates the
nuclear heating rate between ${\sim} 100$ and ${\sim} 10^4$ seconds. This dominance leads to a pronounced kilonova precursor that should for plausible ejecta parameters be visible for ULTRASAT out to ${\sim}200\,\rm Mpc$. Since at low electron fractions free
neutrons can survive even for moderate velocities, mergers with large
tidal ejecta, such as asymmetric
neutron star mergers or favorable
neutron star black hole mergers, may produce particularly bright blue precursors to their subsequent kilonovae.
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All authors Lukas Schnabel, Stephan Rosswog, Friedrich-Karl Thielemann, Moritz Reichert
Solar and Stellar Astrophysics
Energy Evolution from the Chromosphere to the Heliosphere in the 2021 October 28 Solar Eruption
Katharine K. Reeves, Daniel B. Seaton, Cynthia Cattell et al. (+20 more)
We perform a detailed study of the energetics for a well-observed
solar eruption and
flare that occurred on 28 October 2021. This event included a GOES class X1.0
flare, a global EUV wave, and a coronal mass ejection that reached speeds of >2000 km/s. The event was observed from a variety of spacecraft in NASA's Heliophysics System Observatory, including multiple missions near Earth, STEREO-A off the Sun-Earth line, and
Solar Orbiter, near the Sun-Earth line at about 0.8 au. Using remote sensing, in situ observations, and in some cases scaling laws based on previous observations, we characterize the following quantities: free
magnetic energy, energy in non-thermal electrons, energy in non-thermal ions, bolometric energy, energy deposited in the chromosphere, thermal energy radiated in the
flare loops, energy dissipated by the EUV wave, CME kinetic and gravitational potential energy, CME energy flux in the heliosphere, and the energy partition in the CME shock. We find that the total energy released during the event is consistent with estimates of the pre-event stored
magnetic energy, and the CME kinetic + potential energy dominates the energy partition.
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All authors Katharine K. Reeves, Daniel B. Seaton, Cynthia Cattell, Bin Chen, Liam David, Federico Fraschetti, Joe Giacalone, Phillip Hess, Andryi Koval, Dana W. Longcope, Surajit Mondal, Christopher S. Moore, Sophie Musset, Tatiana Niembro, Daniel Pacheco, Yeimy J. Rivera, Soumya Roy, Xudong Sun, Durgesh Tripathi, Domenico Trotta, Matthew J. West, Sijie Yu, Chunming Zhu
Astrophysics of Galaxies
Kinematic hints of a nuclear bar in the Milky Way
Karl Fiteni, Mattia C. Sormani, Victor P. Debattista et al. (+8 more)
The Milky Way hosts a flattened
nuclear stellar disc (NSD) that dominates the gravitational potential in the inner few hundred parsecs. Whether the NSD is purely axisymmetric or contains a
nuclear bar remains an open question. We test for the presence of a
nuclear bar using kinematic diagnostics by combining line-of-sight velocities from the KMOS NSD survey with proper motions from VIRAC2 to construct the $ (v_\ell, v_\mathrm{los}) $ velocity ellipse. After applying strict quality cuts to minimise contamination from large-scale bar
stars, we measure the vertex deviation $ l_v $ and anisotropy $ \beta $ for several subsamples. For our primary sample ($ |\ell| < 0.9^\circ $, $ -0.4^\circ < b < 0.25^\circ $, $ \mathrm{[Fe/H]} > -0.3 $), we find a significant negative vertex deviation $ l_v = -54.8^{+13.1}_{-14.8}\,^\circ $ with moderate anisotropy $ \beta = 0.16^{+0.08}_{-0.05} $. A subsample restricted to the innermost four fields yields an even stronger signal with $ l_v = -64.3^{+12.1}_{-12.2}\,^\circ $ and $ \beta = 0.38^{+0.12}_{-0.07} $. The direction of maximum velocity dispersion is oriented along
Galactic longitude, opposite to that observed in large-scale bar-dominated samples. These signatures are robust against extinction-driven incompleteness, primary-bar contamination, and the choice of
metallicity threshold. They are inconsistent with an axisymmetric NSD or one oriented orthogonally to the primary bar, but match expectations for a
nuclear bar oriented at $ \alpha \approx 60^\circ $-$75^\circ$ to the Sun-
Galactic-Centre line with its near side pointing toward positive
Galactic longitude. While definitive confirmation awaits larger and more precise samples from upcoming surveys, our results provide the first kinematic indication of a possible
nuclear bar in the Milky Way.
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All authors Karl Fiteni, Mattia C. Sormani, Victor P. Debattista, Francisco Nogueras-Lara, Rainer Schödel, Jason L. Sanders, Mathias Schultheis, Xingchen Li, Arianna Vasini, Zi-Xuan Feng, Marco Donati
Cosmology and Nongalactic Astrophysics
A model-independent test of the cosmic distance-duality relation using galaxy clusters and Type Ia supernovae matched pairs
Jian Hu, Yi Liu, Jian-Ping Hu et al. (+1 more)
The
cosmic distance-duality relation (CDDR), expressed as $ D_L/D_A(1+z)^{-2}=1 $, is a fundamental relation in cosmology connecting luminosity distance ($ D_L $) and angular diameter distance ($ D_A $). Any departure from this relation would indicate new physics such as
photon non-conservation,
cosmic opacity, or non-metric
gravity. We perform a stringent, model-independent test of the CDDR using a matched sample of 38
galaxy clusters from the Bonamente et al. compilation and Type Ia
supernovae from the Pantheon+ sample. Employing the matched-pair technique, we simultaneously constrain the CDDR-violation parameter $ \eta $ and a possible
redshift evolution of the SNe Ia absolute magnitude, parameterized as $ M_B(z)=M_0 + \varepsilon z $. We assess the robustness against matching tolerance and further supplement the analysis with DESI 2024 BAO measurements. Our results yield $ \eta = 0.050^{+0.348}_{-0.307} $ and $ \varepsilon = -0.184^{+0.724}_{-0.574} $ (68% CL), showing no statistically significant evidence for CDDR violation or SNe Ia
evolution. The conclusions remain unchanged with stricter matching criteria and the inclusion of DESI BAO data. We also derive a cosmology-independent calibration of $ M_0 = -19.460^{+0.126}_{-0.124} $ mag. The standard cosmological model remains robust under this model-independent scrutiny.
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All authors Jian Hu, Yi Liu, Jian-Ping Hu, Zhongmu Li
Astrophysics of Galaxies
The Low-$\alpha$ Splash Population in the Milky Way
Lais Borbolato, João A. S. Amarante, Hélio D. Perottoni et al. (+5 more)
The Milky Way in-situ
halo, also known as the Splash, consists of old (age $>$ 10 Gyr), metal-rich ([Fe/H] $> -0.7$), high-$\alpha$
stars, i.e., thick disk-like chemistry, on
halo-like orbits (eccentricity $>$ 0.6). Its origin is linked to
stars formed in the disk and dynamically heated by either internal or external
agents. In this work, we investigate its low-$\alpha$ counterpart, the low-$\alpha$ Splash, motivated by recent findings of an old thin disk population. We
conjecture that any mechanism capable of heating disk
stars should affect both of present-day high- and low-$\alpha$ old populations. Using data from the APOGEE DR17 spectroscopic catalog, we identify metal-rich low-$\alpha$
stars with
halo-like kinematics similar to those of the classical high-$\alpha$ Splash. We investigate their possible heating mechanisms using the GASTRO suite of simulations, which allows us to explore the effects of
star-forming clumps as well as a major merger in the proto-disk of a Milky Way analog
galaxy. Our main results show that only clumpy Milky Way models are able to produce Splash populations through scattering by clumps in the early
Galaxy, including the low-$\alpha$ counterpart, whereas the model including only the merger and without an early clumpy phase fails to produce these populations. In the models, the low-$\alpha$ Splash corresponds to a subset of the old thin disk that was dynamically heated by the same mechanism responsible for the formation of the high-$\alpha$ Splash.
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All authors Lais Borbolato, João A. S. Amarante, Hélio D. Perottoni, Silvia Rossi, Victor P. Debattista, Zhao-Yu Li, Nathan Deg, Tigran Khachaturyants
High Energy Astrophysical Phenomena
Temporal evolution of the periodic GeV signal from 4FGL J1913.2+0512 and analysis of the SS 433 / W50 lobes
Ömer Faruk Çoban, Diego F. Torres, Jian Li et al. (+3 more)
SS 433 is a microquasar whose relativistic jets precess every ~162 days, providing a laboratory for jet-interstellar medium interactions. We present a comprehensive analysis of 16 years of
Fermi Large Area
Telescope data (August 2008-September 2024) of the SS 433/W50 field, using events in the 0.3-300 GeV range and employing
pulsar gating to mitigate contamination from the bright nearby
pulsar PSR J1907+0602. We detect the GeV source 4FGL J1913.2+0512 (TS = 45, where TS denotes the likelihood-ratio Test Statistic) with a power-law spectrum (
photon index 2.61 +- 0.08) and confirm a GeV excess at the western lobe (TS = 17). The eastern lobe of SS 433 is hinted at with lower significance. One additional GeV excess,
Fermi J1909.6+0552 (TS = 20; TS = 28 over 0.1-300 GeV), located outside the SS 433 / W50 system, is revealed after gating. Exposure-corrected Lomb-Scargle periodograms and precessional phase-folded light curves show a ~162-day modulation in 4FGL J1913.2+0512. This periodicity is prominent during the first 10 years of the mission (2008-2018) but disappears thereafter, with the phase-folded flux concentrated in precessional phases 0.0-0.5. Over the full 16-year
dataset, the modulation remains detectable but with reduced significance, consistent with dilution by the later non-modulated epoch. These results indicate that the efficiency and/or geometry of gamma-ray production in the SS 433 environment evolves on multi-year timescales.
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All authors Ömer Faruk Çoban, Diego F. Torres, Jian Li, Daniela Hadasch, Agnibha De Sarkar, Matthew Kerr
High Energy Astrophysical Phenomena
X-Ray Polarization from the Gamma-Ray Binary LS I +61 303
Philip Kaaret, Sudip Chakraborty, Daniel Golonka et al. (+5 more)
The gamma-ray emitting binary
stellar system LS I +61 303 was observed with the Imaging X-ray Polarimetry Explorer (IXPE) on two successive orbits over orbital phases of 0.74 to 1.05. Polarization is detected at a significance of 4.2$\sigma$ with an average polarization degree of $13.1\% \pm 3.0\%$ in the 2-8~keV band after background subtraction. This is the second detection of polarization of the X-ray synchrotron emission from a gamma-ray binary and, again, suggests that the
magnetic field in the particle acceleration region has a significant ordered component. The orbital motion on the sky of LS I +61 303 is not well determined, which leads to ambiguity in interpretation of the X-ray electric vector polarization angle (EVPA) measurement. Use of orbital elements determined via radial velocity measurements combined with
radio imaging of variable nebular emission, suggests an offset between the X-ray EVPA and the compact object-massive
star axis on the order of ~30$^{\circ}$. Such an offset could be produced by Coriolis forces due to binary motion. Use of two different sets orbital elements determined via optical polarimetry suggest either no offset or a perpendicular orientation, but require an unexpectedly high inclination. Use of orbital elements derived from modeling of the keV/TeV light curves suggest good
alignment between the X-ray EVPA and the compact object-massive
star axis. Such
alignment was found for the gamma-ray binary PSR B1259-63. If the same physical situation holds for LS I +61 303, that would favor the orbital elements derived from the keV/TeV light curves.
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All authors Philip Kaaret, Sudip Chakraborty, Daniel Golonka, Oliver J. Roberts, Ioannis Liodakis, Andrea Gnarini, Steven R. Ehlert, Joel B. Coley
Cosmology and Nongalactic Astrophysics
Dipoles for everyone: the pseudo-$C_\ell$ approach to directional stacking
Lea Harscouet, Amy Wayland and David Alonso
Stacking (i.e. averaging) the value of a given astrophysical field around sources allows us to detect new cosmological signatures, such as the kinematic Sunyaev-Zel'dovich, and gain insight on the astrophysical properties of
galaxies and their environment. Further information may be gained by orienting these stacks along preferred axes defined by a local directed field, such as the transverse
galaxy velocities,
galaxy shapes, or the local
tidal forces. Examples of this are searches for the moving lens effect, the detection of dipole signatures, or the study of
cosmic filaments. Here we show that all directional stacking signals may be reconstructed, without loss of information, in terms of the cross-power spectrum between the quantity of interest and the $E$ and $B$ modes of the
spin field used to define the preferred axes weighted by the local
galaxy density. The power spectrum approach has several practical advantages, in terms of speed, finite-resolution effects, data visualisation, and combination with other cosmological probes. We also argue that, in some cases, such as stacking using velocities or
tidal forces reconstructed from the density field, the recovered signal may be dominated by information that is already present in the cross-spectrum between the target field and the
galaxy overdensity itself.
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High Energy Astrophysical Phenomena
Bridging X-ray Polarization with Timing & Spectroscopic Parameters of a galactic black hole: Swift J1727.8-1613
Arka Chatterjee, Sujoy K. Nath, Kaushik Chatterjee et al. (+5 more)
We report the discovery of a correlated energy-dependent time lag and degree of polarization for Swift J1727.8-1613 during its 2023 outburst. The energy-dependent time lag is measured around the type-C quasi-periodic oscillations (QPO) observed by IXPE on 2023-09-07, while the degree of polarization is obtained from energy-resolved polarimetric measurements. The Spearman correlation coefficient was found to be 0.8, with a null hypothesis probability of 4.2%. Furthermore, the correlation value drops as the quality factor, or Q value, of the observed QPO frequencies decreases. The spectral properties of Swift J1727.8-1613 are analyzed using simultaneous Insight/HXMT data. Thereafter, we present model-independent theoretical arguments to show that processes other than inverse Comptonization also contributes to both the observed polarization and time lags. This correlation may therefore point to additional mechanisms contributing to the connection between the spectral, temporal, and polarimetric properties of black hole binaries in their hard state.
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All authors Arka Chatterjee, Sujoy K. Nath, Kaushik Chatterjee, Samar Safi-Harb, Broja G. Dutta, Indranil Chattopadhyay, Sudip K. Garain, Hsiang-Kuang Chang
High Energy Astrophysical Phenomena
Imaging the radio-wave emission from extensive air showers
Juan Ammerman-Yebra, Harm Schoorlemmer, Anne Timmermans et al. (+1 more)
We propose a new way to observe
cosmic-ray-induced air showers by imaging the
radio emission. With simulations we demonstrate key features for imaging the
radio-wave emission from air showers, which show similarities to the well-established atmospheric imaging Cherenkov technique in gamma-ray astronomy. In addition, we find that imaging the emission with a camera, consisting of multiple
antennas, resolves emission that is not accessible to a single
antenna. Pursuing this technique, with a camera operating in the GHz frequency domain, might be beneficial ultra-high-energy gamma-ray astronomy and other studies that include detailed observations of air showers.
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All authors Juan Ammerman-Yebra, Harm Schoorlemmer, Anne Timmermans, Sebastian Achim Mueller
Solar and Stellar Astrophysics
Mechanisms for magnetic braking boost and disruption: the role of irradiation-driven winds and convective turnover time spike in cataclysmic variables
Vladislav Dodon, Xiang-Dong Li, Xiao-jie Xu et al. (+2 more)
The saturated, boosted, and disrupted
magnetic braking (SBD MB) model is an empirical prescription that has recently gained support from close-binary observations. Different boosting ($K$) and disruption ($\eta$) parameters appear necessary for different systems, but their physical origins remain uncertain. We aim to identify the mechanisms that boost
magnetic braking (MB) and cause its disruption at the fully convective boundary in cataclysmic variables (CVs). We modelled CV
evolution with MESA and compared the results with observed CV properties. We computed the convective turnover time ($\tau_c$) directly from the donor's structure rather than adopting empirical relations. We also included irradiation from the accreting white
dwarf, which heats the donor's outer layers and can drive additional winds that enhance MB. The structure-based $\tau_c$ calculation reveals a pronounced spike as the donor approaches full convection, which drives the disruption parameter $\eta$ and initiates the period gap in CVs. The outcome of irradiation is sensitive to the
accretion, irradiation, and wind efficiencies, all of which are poorly constrained from observations. Despite these uncertainties, plausible parameter choices allow irradiation-driven winds to provide the required boost $K$ during accreting phases. We refer to the combined prescription as the i$\tau$SBD MB model and find that it yields evolutionary tracks broadly consistent with the main CV properties. Our i$\tau$SBD MB framework offers a physically motivated interpretation of the empirical boost and disruption factors in SBD MB for CV
evolution. We suggest that the convective turnover time spike at the fully convective boundary may drive MB disruption for fast-rotating
stars in the saturated regime, while irradiation-driven winds may be the dominant mechanism boosting MB in accreting binaries and other strongly irradiated close systems.
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All authors Vladislav Dodon, Xiang-Dong Li, Xiao-jie Xu, Ilkham Galiullin, Askar Sibgatullin
Cosmology and Nongalactic Astrophysics
Probing the faint end of simulated galaxy counts at z>3
Flaminia Fortuni, Emiliano Merlin, Marco Castellano et al. (+2 more)
Simulations and observations now probe comparable
redshift regimes with unprecedented accuracy, enabling direct consistency tests through forward modeling. In a previous work, we identified a faint-end discrepancy between observed and simulated near-infrared
galaxy counts in CANDELS GOODS-South. Here we investigate whether this tension originates from the forward-modeling procedure or from limitations of the underlying simulations, and we characterize the
galaxy populations responsible for the tension. Using the FORECAST forward-modeling code, we generated ten independent light-cone realizations and mock CANDELS images from the TNG100 and EAGLE simulations. We compared both the intrinsic light-cone catalogs and the mock-image detections with observations, testing dependencies on field and
redshift, and validating the pipeline through
stellar mass and multi-band analyses. The faint-end deficit is present in all CANDELS fields and appears at z>3 in both simulations. GOODS-South counts corrected for completeness exceed intrinsic simulation counts already at the 50% completeness limit, indicating that the missing population is not simply hidden below the detection threshold. Increasing the depth of mock images recovers the counts near the peak but overpredicts the faintest sources, showing that depth alone cannot resolve the discrepancy. Structural analyses reveal that compact
galaxies with bright central cores observed in GOODS-South are underproduced in simulations, which instead favor diffuse low-surface-brightness systems. We conclude that the discrepancy arises both from detection losses of diffuse
galaxies and, more fundamentally, from the inability of current hydrodynamical simulations to produce enough faint compact
galaxies at z>3. This tension points to the need for improved modeling of early
star formation, feedback, and
dust treatment.
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All authors Flaminia Fortuni, Emiliano Merlin, Marco Castellano, Adriano Fontana, Paola Santini
High Energy Astrophysical Phenomena
Spectral-Regime Overlap and Transition-like Behavior in the Blazar Population from Multi-Instrument X-ray and TeV Observations
Javaid Tantry and Naseer Iqbal
Blazars are conventionally classified into BL Lac objects and flat-spectrum
radio quasars (FSRQs), commonly associated with radiatively inefficient and efficient
accretion onto supermassive black holes, respectively. Increasing evidence, however, suggests that this division is not strictly discrete, with several sources occupying intermediate or transition-like spectral states. Since the
photon index traces the
evolution of the non-thermal emission spectrum, it provides a useful probe of jet energetics, particle acceleration, and radiative processes across blazar populations. Using multi-mission observations from Swift-XRT, Swift-BAT, NuSTAR, ROSAT, Chandra, XMM-Newton, NICER, AstroSat, TeVcat, and archival VizieR compilations, we investigate the distribution and long-term
evolution of X-ray and TeV
photon indices across multiple blazar subclasses. We identify broad overlap regions in
photon-index space linking EHBL, HBL, IBL, LBL, and FSRQ-like populations, suggesting a continuous rather than sharply separated spectral distribution. Across X-ray instruments, the intermediate regime is concentrated near $\Gamma_{\mathrm{X}} \approx 2$, typically spanning $\Gamma_{\mathrm{X}} \sim 1.5$–$2.2$. Multi-epoch observations reveal substantial intra-source spectral
evolution, including
stochastic variability in Mrk~421 and state-dependent transitions in OJ~287. Several sources exhibit spectral-index changes of $\Delta \Gamma_{\mathrm{X}} > 0.5$, while occupancy of the intermediate regime reaches $\sim22$–$43%$ depending on instrument. Multiple objects repeatedly traverse spectral regions connecting traditionally distinct subclasses, supporting their interpretation as candidate transition-like blazars. Overall, the results favor a framework in which blazar subclasses represent overlapping and evolving spectral populations driven by long-term changes in jet emission and radiative processes.
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High Energy Astrophysical Phenomena
Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT
Rwitika Chatterjee, Vivek Kumar Agrawal, V. P. Shyam Prakash et al. (+2 more)
XSPECT instrument on-board XPoSat mission is a soft X-ray spectrometer sensitive in the energy band 0.8$-$15 keV. XSPECT has observed several bright
neutron star low mass X-ray binaries since launch. Two well known sources, Aql X-1 and 4U 1608-52 which are soft X-ray transients, were observed by XPoSat during the decay phase of their recent outbursts in September 2024 and February 2025 respectively. During XSPECT observations, 4U 1608-52 exhibited a superburst which is a long duration thermonuclear burst, believed to be triggered by carbon burning. We carry out a detailed spectro-temporal analysis of the superburst, tracing its onset, rise, and decay over the next several hours. Using time-resolved spectroscopy, we probe the spectral
evolution of the source and find that the persistent emission is suppressed during the superburst and the emission can be described by a gradually cooling blackbody component. The superburst was preceded by a precursor burst which is a normal type-I X-ray burst. We also observe a type-I burst $\sim 5$ days after the superburst, indicating resumption of burst activities which is typically quenched after a superburst. Aql X-1 also exhibited a type-I burst during XSPECT observations. The persistent emission of both the sources can be fitted using a combination of blackbody and disk blackbody emission or, alternatively, using a disk Comptonized by an optically thick
plasma. Using the latter model, we find a clear flux dependence of the Comptonization parameters, with both the sources exhibiting harder spectra at higher
accretion rates.
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All authors Rwitika Chatterjee, Vivek Kumar Agrawal, V. P. Shyam Prakash, Koushal Vadodariya, Radhakrishna V
Cosmology and Nongalactic Astrophysics
Accretion Effects on Primordial Black Hole Reheating Constraints
Chenhuan Wang
In this work, we study the effects of
accretion on the
primordial black hole (PBH) reheating scenario. PBHs could form from
primordial fluctuations. If they have the right mass and abundance, they could dominate the Universe and complete the reheating entirely through Hawking radiation. We find
accretion effects on the BH can not only increase the BH mass, but also prolong such early matter domination. The consequence of the
accretion is further investigated using isocurvature induced gravitational waves (GWs), which are generated right after the sudden evaporation of the BHs from the oscillation of the gravitational potential. Big Bang nucleosynthesis limits on the energy density of the GWs put important constraints on the PBH domination scenario. Inclusion of
accretion shifts such constraints significantly towards smaller formation mass and smaller initial abundance. Furthermore, the PBH could undergo mergers leading to extended mass functions. We find similar shifts in the allowed parameters with the inclusion of
accretion for the merger constraint. We find the constraints from isocurvature GWs typically stronger than the constraints from mergers.
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High Energy Astrophysical Phenomena
Average universal shower profile reconstruction using radio interferometry
J. Alvarez-Muñiz, W. R. Carvalho Jr., R. Conceição et al. (+1 more)
Radio detection of extensive air showers enables near-continuous observation and precise measurements of the shower geometry and the depth of shower maximum, $X_{\rm max}$. Beyond $X_{\rm max}$, the longitudinal shower development follows a Universal Shower Profile (USP), whose shape parameters contain information on primary mass composition and hadronic interaction models. While previous studies have focused on event-by-event reconstruction of profile parameters, in this work we investigate the reconstruction of the average USP using
radio interferometric techniques. Using Monte Carlo simulations, we reconstruct the average longitudinal profile directly from
radio data and extract the Gaisser-Hillas shape parameters $(R,L)$. We find that the
radio-derived average profiles provide enhanced separation between primary masses and hadronic interaction models compared to that obtained from fluorescence-equivalent longitudinal profiles. These results demonstrate that
radio interferometry can access higher-order information on the longitudinal shower development and that the use of the average USP significantly improves the sensitivity to composition and hadronic interaction studies at ultra-high energies.
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All authors J. Alvarez-Muñiz, W. R. Carvalho Jr., R. Conceição, D. Dias