High Energy Astrophysical Phenomena
Testing the QGP Star Hypothesis: The oMEGACat BH-2 System as a Candidate Color-Superconducting Quark-Gluon Plasma Star
Herman J. Mosquera Cuesta
The recent discovery of a long-period binary system in the globular cluster $\omega$ Centauri, oMEGACat BH-2 \citep{Whitaker2026}, provides an unprecedented opportunity to probe the true nature of compact dark objects. The system's massive, dark companion has been inferred to have a mass of $4.46_{-1.01}^{+1.22} M_\odot$, which places it in the “mass gap” between
neutron stars and the canonical
stellar-mass black holes. In this Letter, we explore the hypothesis that the oMEGACat BH-2 companion is not a classical black hole, but a stable, self-bound
Quark-
Gluon Plasma (QGP)
star, as described by recent general relativistic models that incorporate Nonlinear Electrodynamics (NLED) and the asymptotic freedom of
Quantum Chromodynamics (QCD) \citep{Mosquera2025}. We compare the inferred mass of the companion with the novel Mass-Radius ($M$-$R$) relation predicted by the QGP
star model. We find that the inferred mass of the $\omega$ Centauri object lies squarely within the wide mass spectrum predicted for hypermassive QGP
stars ($0$ to $>7$ $M_\odot$). Although this consistency is not enough for claiming evidence, it suggests that oMEGACat BH-2 may be the first observed candidate for a QGP
star, representing a stable, non-singular end-state of
stellar collapse. We argue that future astrometric monitoring with JWST and
radio-
telescopes like FAST and SKA can further constrain the oMEGACat BH-2 orbital parameters. Meanwhile, gravitational-wave follow-ups for the $f$ and $g$ modes by observatories like LIGO, VIRGO, KAGRA, LISA, ET and CE can be crucial for distinguishing a classical black hole from the “gravitational eternally collapsing `kompact' object” (\GECKO) state of our QGP
star model.
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Cosmology and Nongalactic Astrophysics
Constraining the Tilted Dipole Cosmology with Primordial Nucleosynthesis and Baryogenesis
Mohsen Khodadi
The tilted dipole cosmology extends the standard model by incorporating a preferred spatial direction and separate bulk velocities for matter and radiation, offering a potential explanation for observed large-scale bulk flows and the CMB dipole. We conduct a comprehensive analysis of this anisotropic framework using Big Bang Nucleosynthesis (BBN) and Gravitational Baryogenesis (GB) to impose stringent constraints on the radiation tilt parameter $\beta_r$, which quantifies the magnitude of the radiation bulk flow. By deriving the modified expansion rate $H(T)$ and its impact on light element abundances, we find that the combined $2\sigma$ BBN limits from
primordial helium-4 and deuterium are $|\beta_r| \lesssim 0.03$ for maximal shear (tightest bound) and $|\beta_r| \lesssim 0.2$ for minimal shear (weakest bound). These bounds are consistent with, but tighter than, those inferred from the effective
neutrino species count $|\Delta N_{\mathrm{eff}}| \lesssim 0.4$. Importantly, both bounds are upper limits; $\beta_r = 0$ is always allowed by the BBN constraints. The lithium-7 problem persists, as the $\beta_r$ values required to resolve it are excluded by helium-4 data. Furthermore, GB – operating at decoupling temperatures $T_D \gtrsim 10^{12}$ GeV under the scaling $\beta_r \propto T$ – yields far more severe constraints, limiting $|\beta_r^{\mathrm{BBN}}| \lesssim 10^{-8}$ for such high-scale baryogenesis scenarios.
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Solar and Stellar Astrophysics
Evolutionary Period-Change Modeling of Delta Cephei with MESA Tracks
Zuhoor Elahi, Christopher Sirola and Wafa Gull
Cepheid period changes provide a direct evolutionary diagnostic because the pulsation period responds to changes in
stellar radius as a
star crosses the instability strip. We present a controlled MESA evolutionary-track analysis for Delta Cephei using nonrotating, no-wind models. The period is estimated from the period-mean-density relation, P = Q [(R/Rsun)^3/(M/Msun)]^(1/2), with Q = 0.033 d. The adopted comparison values are P = 5.366531 d and dP/dt = -0.1006 s yr^-1. A
solar-
metallicity mass and overshoot grid identifies clean blueward solutions with negative dP/dt near the observed period, but the best such case has dP/dt = -0.6813 s yr^-1, a factor of 6.77 too large in magnitude. A
metallicity pilot grid improves the result substantially, with a best clean-blueward model at M = 5.90 Msun, Z = 0.012, and fov,core = 0.010, giving P = 5.370677 d and dP/dt = -0.2460 s yr^-1. A local refinement around this solution does not improve beyond the same model. Thus,
metallicity refinement reduces the period-change mismatch by a factor of about 2.77 relative to the
solar-
metallicity baseline, but the final nonrotating, no-wind, fixed-Q model still overpredicts the observed magnitude of dP/dt by a factor of about 2.45. The result supports a blueward evolutionary interpretation while identifying rotation, mass loss, binary-related effects, and structure-dependent pulsation periods as the natural scope of a follow-up study.
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Solar and Stellar Astrophysics
Fourier Morphology Diagnostics of Delta Cephei MESA-RSP Light Curves
Zuhoor Elahi, Christopher Sirola and Wafa Gull
Fourier morphology provides a compact test of whether a nonlinear Cepheid model reproduces the observed harmonic content and asymmetric light-curve shape, not only the pulsation period. We apply
Fourier and time-domain morphology diagnostics to observed and synthetic Delta Cephei light curves. The observed Johnson V-band light curve is represented by a
Fourier template and compared with MIST bolometric-correction V-band light curves generated from the calibrated MESA-RSP model sequence. This study is framed as a target-specific morphology assessment rather than as a new
Fourier-decomposition method. The observed template has Delta V = 0.8390 mag, R21 = 0.3401, R31 = 0.1943, rise fraction = 0.2775, and asymmetry index = 0.4450. The final accepted model, with RSP_alfam = 0.400 and RSP_alfat = 0.095, gives Delta V_syn = 0.0367 mag, R21 = 0.0185, R31 = 0.0028, rise fraction = 0.4860, and asymmetry index = 0.0280. The accepted calibration sequence therefore improves the synthetic amplitude only modestly and leaves the synthetic curve much too sinusoidal. The result shows that the accepted sequence is period-calibrated and morphology-tested, but it is not yet an observed-amplitude solution for Delta Cephei.
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Solar and Stellar Astrophysics
Modified electron dispersion relations in degenerate white dwarfs
Edson Otoniel and Iarley P. Lobo
We investigate how modified electron dispersion relations affect the structure of cold white
dwarfs (WDs). The deformation is introduced only in the degenerate electron equation of state, through the energy of a single particle and the group velocity, while the
stellar mass density remains dominated by ions through the relation $\rho\simeq \mu_e m_u n_e$ imposed by charge neutrality. The resulting equations of state are coupled to the standard TOV equations, with no modification of the gravitational field equations. For a baseline composed of carbon and oxygen with $\mu_e=2$, the undeformed limit recovers a maximum mass in the Chandrasekhar scale, validating the normalization of the calculation before the modified cases are considered. The first model of modified dispersion produces only a modest stiffening over the parameter range studied, whereas the logarithmic model depends strongly on the sign of the deformation parameter: negative values increase the pressure and the maximum mass, while positive values soften the sequence. These results show that WDs can isolate the impact of modified electron kinematics on compact
star structure, but the logarithmic branch in particular requires further restrictions from its physical domain, stability conditions, and observational constraints on mass and radius.
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Cosmology and Nongalactic Astrophysics
A Multimodal Approach to Star–Galaxy Separation using SPHEREx Spectrophotometry and DESI Legacy Survey Imaging
Kendrick Nguyen, Richard M. Feder, Sean Bruton et al. (+1 more)
Stellar contamination is a critical systematic for increasingly precise large-scale structure analyses from ongoing and next-generation surveys. Experiments targeting constraints on local
primordial non-Gaussianity with $\sigma(f_{\rm NL}^{\rm loc}) \sim \mathcal{O}(1)$ demand sub-percent
stellar contamination rates to avoid misidentifying spurious large-scale power induced by
Galactic structure as true cosmological signal. In this work, we explore the use of
multimodal models for
star–
galaxy separation, harnessing the information from both optical broad-band imaging data and SPHEREx near-infrared low-resolution spectrophotometry. The two modalities are integrated using contrastive learning, which projects image- and spectrum-based
embeddings into a shared latent space. We find that classifiers trained on these transformed representations outperform those trained on the original
embeddings and show less performance degradation when simpler classifiers are used. These results suggest that
multimodal alignment organizes the
embedding space along dimensions that are better suited to source classification. The improvement is particularly strong for image-based classification, which we connect to increased predictability of highly-discriminative infrared spectral features from the transformed image
embeddings. Applying
redshift error-based selections and extrapolating to the full SPHEREx footprint, we demonstrate that
stellar contamination can be controlled at the sub-percent level across most of the extragalactic sky, with completeness tradeoffs largely confined to low
redshift. Our work highlights the utility of
multimodal methods for modern
galaxy surveys such as SPHEREx and $\textit{Rubin}$ LSST.
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All authors Kendrick Nguyen, Richard M. Feder, Sean Bruton, Uroš Seljak
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Numerical Backreaction and Finite-Time Energy Transfer in Post-Recombination Magnetogenesis from Ultralight Dark Matter
Aviv David and Robert Brandenberger
We study gauge-field production in a post-recombination magnetogenesis scenario driven by an ultralight pseudoscalar dark matter field $\phi$ coupled to electromagnetism. Previous analytical work has shown that a homogeneous oscillating $\phi$ background can excite gauge field modes through tachyonic and narrow-band resonance channels. Here we follow the coupled
evolution of the homogeneous $\phi$ mode and gauge field modes of both helicities in an expanding background. Our numerical work confirms the results for the spectra of produced gauge particles obtained in the previous approximate analytical treatments. In addition, our study allows us to determine when back-reaction shuts off the resonance. We find that for parameter values for which the tachyonic instability band is open, back-reaction does not shut off the resonance until a fraction ${\cal{F}}$ of order one of the initial dark matter energy density has been transferred to
photons, and this happens on a time scale which is short compared to the Hubble time. In the parameter range in which only the narrow resonance band is open, ${\cal{F}}$ eventually reaches order one, but the time when this happens increases as the effective coupling constant decreases, and therefore ${\cal{F}}$ may remain negligible until the present time.
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Instrumentation and Methods for Astrophysics
The High-Energy Light Isotope eXperiment (HELIX) Instrument
P. S. Allison, M. Baiocchi, J. J. Beatty et al. (+27 more)
The HELIX detector is a balloon-borne instrument designed to measure the flux of light (Z < 11)
cosmic-ray isotopes. In this paper we describe the initial configuration of HELIX, optimized for measurements in the energy range from approximately 0.2 GeV per nucleon to beyond 3 GeV per nucleon. In addition to a spectrometer, which is based on a one-tesla superconducting magnet and a drift-chamber tracker, HELIX employs scintillator-based time-of-flight counters and a ring-imaging Cherenkov detector, allowing event-by-event reconstruction of primary particle mass, charge, and
magnetic rigidity.
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All authors P. S. Allison, M. Baiocchi, J. J. Beatty, L. Beaufore, D. H. Calderon, Y. Chen, S. Coutu, E. Ellingwood, D. Fuehne, N. Green, D. Hanna, H. B. Jeon, B. Kunkler, M. Lang, R. Mbarek, K. McBride, C. E. McGrath, I. Mognet, J. Musser, S. Nutter, S. O'Brien, N. Park, T. Rosin, K. Sakai, M. Tabata, G. Tarle, G. Visser, S. P. Wakely, I. G. Wisher, M. Yu
Solar and Stellar Astrophysics
The Effects of Magnetic Accretion on the Spatial Extent of White Dwarf Pollution
Dang Pham, Aster G. Taylor and Tim Cunningham
Many white
dwarfs are polluted by metals, which are generally understood to be the accreted remnants of a planetary system. Modeling these systems typically assumes that the metal concentration is homogeneous throughout the white
dwarf's atmosphere. However, the
magnetic fields of a white
dwarf may affect the
accretion geometry of the white
dwarf via magnetospheric
accretion. Convection in the white
dwarf's photosphere will then transport the metals across the surface, with a structure set by the relative sinking versus spreading timescales. In this work, we construct models for the
accretion geometry, subsequent spreading, and observed pollution of
magnetic white
dwarfs. We show that the
magnetic fields will initially concentrate the pollution into a narrow region of the white
dwarf's surface. The relative spreading and sinking timescales determine whether the metals become uniformly distributed or remain confined to localized patches. If the
magnetic field and
spin poles are misaligned, then patchy white
dwarfs exhibit periodically variable pollution signatures, which enable constraints on the patch area. We explore this model as a possible explanation for the recent detection of periodically variable pollution signatures in
magnetic white
dwarfs. Finally, we also demonstrate that the concentration of material due to the
magnetic field may lead to systematic underestimates of the mass
accretion rate onto these objects.
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Solar and Stellar Astrophysics
An extensive grid of DARWIN models for M-type AGB stars II. Effects of pulsation periods on wind properties
Emelie Siderud, Kjell Eriksson, Susanne Höfner et al. (+1 more)
Mass loss from asymptotic
giant branch (AGB)
stars is the result of a complex interplay between pulsation, atmospheric dynamics,
dust formation, and radiative acceleration. Pulsation periods are a key input in dynamical atmosphere and wind models, and different prescriptions for assigning periods based on
stellar parameters may lead to systematic differences in the predicted wind properties. To better constrain this critical parameter, we investigated how the choice of pulsation period affects the wind properties of dynamical atmosphere and wind models of M-type AGB
stars by comparing models based on an empirical period-luminosity (P-L) relation with corresponding ones that adopt a period-mean density relation derived from 3D pulsation models. We analysed two grids of DARWIN models that cover a range of current
stellar masses, luminosities, and effective temperatures. For each grid, pulsation periods were assigned using either the P-L relation or the period-mean density relation, allowing for a direct comparison of the resulting dynamical structures and wind properties for pairs of models differing by period only. Independent of the adopted period prescription, the time-averaged wind properties correlate strongly with $L_\star/M_\star$. The pulsation period affects the atmospheric dynamics through changes in the relative timing of shock propagation and
dust formation, which affect both wind formation and the resulting wind properties. Shorter periods favour the onset of a wind, and models differing only in pulsation period can exhibit significantly different wind properties. The period-mean density relation provides a physically motivated alternative to the empirical P-L relation by accounting for
stellar parameters beyond luminosity, and enables a more direct comparison between DARWIN models and observed Mira variables.
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All authors Emelie Siderud, Kjell Eriksson, Susanne Höfner, Arief Ahmad
Astrophysics of Galaxies
Hierarchical Bayesian inference with compositional score modeling for stellar streams
Giuseppe Viterbo, Jonas Arruda and Tobias Buck
Context:
Stellar streams trace the gravitational potential of the Milky Way over a wide range of Galactocentric radii. Since different streams sample different regions of the
Galaxy, combining several of them can constrain the global mass distribution more tightly than modeling any single stream in isolation. Most of the existing multi-stream analyses rely on likelihood-based methods that require a new inference run whenever additional streams or kinematic measurements become available. Aims: We aim to infer the Milky Way potential from multiple
stellar streams combined with an additional constraint through the
Galactic circular velocity curve within a single hierarchical framework. Methods: We model the problem hierarchically, separating parameters that are common to all streams from parameters that are specific to each progenitor. We train score-based neural posterior
estimators on a library of simulated streams and combine information from different streams through compositional score modeling as a post-training step. Results: Tests on independent simulations show that the inferred posteriors are well calibrated and accurate. Combining several streams reduces the uncertainties on the global potential parameters relative to single-stream analyses. Applied to
Gaia data, the model favours a mildly oblate dark matter
halo with axis ratio $q_{NFW} = 0.77$, scale radius $a_{NFW} = 9.3$ kpc, a disk mass of $4.3 \times 10^{10} M_\odot$, and a local dark matter density $\rho_{NFW,\odot} = 0.01153 M_\odot pc^{-3}$ consistent with recent stream-based studies. Conclusions: This hierarchical framework provides a practical way to combine information from multiple
stellar streams without repeating the full inference procedure for each new
dataset. The method is adaptable to new
datasets, like future
Gaia DR4, or new spectroscopic surveys, with minimal computational cost.
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Cosmology and Nongalactic Astrophysics
Probing the baryonic–dark matter connection in galaxy clusters using X-rays with gated recurrent unit neural networks
Asif Iqbal, Subhabrata Majumdar, Weiguang Cui et al. (+3 more)
Accurate cluster mass measurements are crucial for cosmology, yet conventional hydrostatic
equilibrium (HSE) methods can suffer from systematic biases, particularly in dynamically disturbed systems. We present a gated recurrent unit (GRU) based deep learning framework for predicting three-dimensional mass profiles of
galaxy clusters from spherically averaged intra-cluster medium (ICM) radial profiles. By treating ICM profiles as sequential data, the GRU captures radial dependencies and naturally handles profiles with different radial samplings. We train and validate the model using high-resolution hydrodynamical simulations from The Three Hundred Project, achieving unbiased mass predictions with a typical 1$\sigma$ scatter of $\sim$5% over most of the cluster region, significantly improving upon HSE estimates. The model provides radius-dependent uncertainty estimates and remains robust against variations in data quality and cluster morphology. When trained jointly on independent simulation suites (GIZMO-SIMBA and GADGET-X), it successfully generalises across both simulations. Feature importance analysis shows that enclosed gas mass is the dominant predictor, with pressure and temperature providing additional information on the radial mass distribution. We further apply the GRU model to X-ray observations of the REXCESS and X-COP cluster samples from XMM-Newton and compare the inferred mass profiles with HSE estimates. The HSE masses are systematically lower than the GRU predictions for the higher-mass X-COP sample, while the REXCESS sample shows mass differences that are close to zero on average. This work provides a data-driven framework for cluster mass inference that bridges simulations and observations and can be extended to multi-wavelength
datasets, including Sunyaev-Zel'dovich and optical observations.
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All authors Asif Iqbal, Subhabrata Majumdar, Weiguang Cui, Elena Rasia, Gabriel W. Pratt, Daniel de Andres
Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Evolving Dark Energy from the Back-Reaction of Cosmological Perturbations
Vincent Comeau
We argue that the back-reaction of inhomogeneities, when combined with a pure cosmological constant, produces an effective dark energy whose equation of state can evolve with time. Our analytical computation agrees with recent numerical simulations, which have shown that the back-reaction of inhomogeneities can impact dark energy around the present time. Here, its equation of state is derived from an average version of the Friedmann equations, for which the average is taken over a fixed volume of matter particles (not necessarily global). When treating the inhomogeneities as small perturbations around a homogeneous background, dark energy acquires a time dependence which reduces to a boundary term, at least at quadratic order in the scalar modes of the perturbations. Thus, it becomes negligible for a global average. On the other hand, when treating instead the spatial derivatives of the inhomogeneities perturbatively in a gradient expansion, dark energy gets the same time dependence, which no longer reduces to a boundary term. This time dependence can be significant even for a global average or when the constant spatial
curvature induced by the inhomogeneities is negligible. It also keeps the same sign around the present time, thus causing dark energy to remain either always phantom or never phantom, not allowing for any crossing from one type of behaviour to the other.
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Instrumentation and Methods for Astrophysics
RAMSES-GPU: Cell-by-Cell Adaptive Mesh Refinement with Magneto-Hydrodynamics and Self-Gravity on Graphics Processing Units
Romain Teyssier, Eric Moseley, Robert V. Caddy et al. (+9 more)
We present the implementation and optimization of the cosmological simulation code RAMSES on Graphics Processing Units (GPUs) using CUDA Fortran. This accelerated version ports the main computational routines, including hydrodynamics, particle dynamics, and self-
gravity, to multi-GPU architectures. We detail our strategy for managing
cell-by-
cell Adaptive Mesh Refinement (AMR) on the GPU, utilizing bucket sort with prefix sums for AMR level sorting, radix sort via the CUB library for
Hilbert key ordering, and an fnv64 hash table with linear probing for fast spatial indexing. Portability across diverse hardware architectures is achieved via a dispatcher and C-Fortran wrappers, calling CUDA, HIP, and Metal kernels directly translated from the CUDA Fortran framework. Hydrodynamics updates are executed via a Godunov MUSCL-Hancock HLLC Riemann solver managed through a three-tier shared-
memory kernel architecture (named rock, paper, and scissor). Particle mass deposition uses Cloud-in-
Cell (CIC) interpolation optimized with atomic additions or prefix sums, combined with a kick-drift-kick time integration pusher. Self-
gravity is handled via a Multigrid (MG) Poisson solver performing hierarchical V-cycles on individual levels. Performance
benchmarks conducted on NVIDIA A100 and H200 GPUs demonstrate substantial accelerations compared to multi-core CPUs, yielding 10x up to a 100x speedup for standard test problems such as the Sedov blast wave, molecular core collapse, and cosmological simulations. Finally, we briefly discuss additional accelerated physics modules, including
equilibrium cooling, polytropic equations of state, ideal and non-ideal magneto-hydrodynamics (MHD), and
stellar feedback.
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All authors Romain Teyssier, Eric Moseley, Robert V. Caddy, Troels Haugbolle, James Sunseri, Jun-Young Lee, Jonathan Palafoutas, Yue Pan, William Groger, Tom Abel, Burlen Loring, Guillaume Barnier
High Energy Astrophysical Phenomena
Reaction-constrained composition \(g\)-modes in neutron stars with antikaon condensates, hyperons, and \(\Delta(1232)\) resonances
Prashant Thakur and Ishfaq Ahmad Rather
We study core composition \(g_1\) modes of cold, nonrotating
neutron stars containing antikaon condensates, hyperons, and \(\Delta(1232)\)
baryons and present, to our knowledge, the first calculation in full general relativity of the continuous-composition \(g_1\)-mode frequency and gravitational-wave damping time for
stars with a \(K^-\) condensate. Using the BigApple relativistic mean-field equation of state, we compute frequencies, damping times, and frozen-composition
tidal overlaps, and identify the buoyancy channels with a species-resolved Ledoux decomposition validated by mode-frequency sensitivities. We compare fully frozen matter with a fast-\(K\) limit for \(n\leftrightarrow p+K^-\) and a strong-
equilibrium limit for the \(\Delta\) quartet. Fast-\(K\) equilibration retains \(36\%\)–\(44\%\) of the peak local kaon buoyancy and \(65.7\%\)–\(73.4\%\) of the frozen terminal-configuration frequencies, while increasing the damping times by factors of \(14.4\)–\(31.8\); the mode remains above the nucleonic band. Strong \(\Delta\) equilibration removes most of the direct \(\Delta\)-induced enhancement, returning the \(N\Delta\) mode toward the nucleonic band, whereas the high-frequency \(NY\Delta\) branch survives through the frozen \(\Lambda\) gradient. Eigenfunction tracking confirms a continuous \(g_1\) branch, and representative DD-ME2 calculations reproduce this hierarchy. The direct full-GR frozen-composition phase shifts satisfy \(|\Delta\Phi_{g_1}|\leq1.410\times10^{-3}\) rad, a factor of 21 below the \(0.03\)-rad favorable-event scale for the Einstein
Telescope. An exotic species therefore produces a distinct composition mode only if its composition gradient, or a coupled slowly equilibrating gradient, survives over the oscillation period.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Gravitational Wave Birefringence in generalized Palatini Chern Simons
Jose Perdiguero and Macarena Lagos
The cosmological propagation of gravitational waves (GWs) can exhibit amplitude and phase polarization distortions when parity
symmetry is broken, a phenomenon known as cosmological birefringence. In this paper, we investigate the phenomenology of GW birefringence in a gravitational model $f(R)$ coupled to a dynamical Chern-Simons (dCS) term, analyzed in the metric and Palatini formalisms. At the background level, this model can lead to dynamical dark energy, while for GW propagation we find that the Palatini formalism predicts both amplitude and velocity birefringence, whereas the metric formalism predicts amplitude birefringence only. We also find that the birefringence effects can either be suppressed or enhanced by the $f(R)$ interactions, depending on the specific form of $f(R)$. Considering three common $f(R)$ models (Hu-Sawicki, Exponential, and
Hyperbolic gravity) fit to recent cosmological data, we find that birefringence is enhanced relative to the $f(R) = R$ case, by $1-10\%$ in the Palatini formalism and up to a factor of 2 in the metric formalism. We also translate current GW birefringence constraints to bounds on the dCS coupling within our model. Finally, we show that in our model the birefringence effect grows polynomially with source
redshift, in contrast to the linear-distance scaling commonly assumed in phenomenological models of GW birefringence in the current literature.
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Solar and Stellar Astrophysics
Statistics on the population of distant substellar companions to nearby stars
Souleymane Ouedraogo, Étienne Artigau, Sié Zacharie Kam et al. (+5 more)
Context. Wide substellar companions provide important
benchmarks for studying the formation and
evolution of brown
dwarfs and
giant planets. Their association with
stellar primaries also provides age and
metallicity constraints that are difficult to obtain for isolated objects. Aims. We investigate the population of wide substellar companions to nearby main-sequence
stars using proper motions derived from multi-epoch WISE imaging. Our sample includes 2103
stars within 20 pc and is sensitive to companions down to $\sim300$ K at separations up to 20,000 AU. Methods. We use a
Bayesian Markov Chain Monte Carlo (MCMC) analysis to constrain the companion mass and semi-major axis distributions while accounting for observational selection effects, including the reduced detectability of faint companions. We assume power-law distributions, $d^2n \propto M^\alpha a^\beta dM da$. Results. We derive $\alpha=-0.21^{+0.31}_{-0.27}$, $\beta=-1.38\pm0.32$, and an occurrence rate of $1.07^{+0.31}_{-0.24}\%$ for companions with masses of 5–80 $M_J$ at separations of 1,000–20,000 AU. The occurrence rate increases towards smaller separations and for more massive host
stars. Conclusions. The inferred companion mass distribution is consistent with previous direct imaging surveys despite differences in target selection. The power-law distributions in both mass and separation also agree with those of
stellar binaries. Our sample is about an order of magnitude larger than those of previous imaging surveys, providing the most robust statistical characterisation of wide substellar companions to date. These results support the interpretation that wide substellar companions represent the low-mass extension of the
stellar binary population rather than a direct extension of close-in
giant planets.
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All authors Souleymane Ouedraogo, Étienne Artigau, Sié Zacharie Kam, Orlagh L. Creevey, Clémence Fontanive, Neil J. Cook, René Doyon, Fabrice Bado
Instrumentation and Methods for Astrophysics
A multi-band radio flux density catalog of ICRF3 sources using the Onsala Twin Telescopes
Alva Kinman, Rüdiger Haas, Karine Le Bail et al. (+2 more)
The VLBI Global Observing System (VGOS) is the next generation system for geodetic and astrometric Very Long Baseline Interferometry (VLBI). To optimize the observing time for each source in geodetic schedules, a flux density catalog is needed for the sources that are observed at the VGOS frequencies. The aim of this work is to monitor the flux densities of geodetic sources in the VGOS bands. The obtained flux density time series can be used for more effective scheduling of geodetic and astrometric VLBI experiments, as well as probing active
galactic nuclei (AGN) physics. The Onsala Twin
Telescopes have been used as a single baseline interferometer to measure flux densities of AGN that are part of the International Celestial Reference System (ICRF3). The
telescopes observed at 3.2, 5.5, 6.6 and 10.4 GHz simultaneously. Both locally planned flux monitoring sessions and international geodetic experiments were analyzed. The data were calibrated using the Common Astronomy Software Applications (CASA). The possibility of predicting geodetic signal-to-noise ratios (S/N) using the measured flux densities was also tested. Simultaneous light curves in up to four frequencies have been obtained for 361 sources. The majority of the sources vary significantly in flux density during the measurement period. Most sources have a flat or inverted spectrum, with only 6 % having a steep spectrum. Furthermore, the flux densities from this work were shown to more precisely predict geodetic signal-to-noise ratios compared to the standard VGOS flux density catalog, especially for the most variable sources. Flux density variation needs to be taken into account to obtain the most optimal VGOS schedules. The flux density catalog presented here is expected to be of use for both astronomy and geodesy. We plan to continue the monitoring program.
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All authors Alva Kinman, Rüdiger Haas, Karine Le Bail, Jun Yang, Eskil Varenius
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
A product template bank to search for compact binary coalescences microlensed by isolated point mass lenses
Sudhir Gholap, Sanjeev V. Dhurandhar and Shasvath J. Kapadia
Gravitational waves (GW) from
stellar mass compact binary coalescences (CBCs) that encounter a lens whose Schwarzschild radius is comparable to the GW wavelength, will be microlensed, leading to frequency-dependent modulations in the observed signal. Neglecting such wave-optics effects in standard templated searches for CBCs can result in signal-to-noise ratio (SNR) losses as large as 30%. Here, we present a prescription for constructing a template bank targeted at microlensed GWs. We consider CBCs microlensed by isolated point mass lenses, whose frequency-dependent amplification factor is known to have a closed-form analytical expression. However, to construct the template bank, it suffices to use the
geometric optics approximation, effectively modeling the microlensed signal as overlapping signals. Moreover, given that matched filtering is mostly sensitive to the phase of the GW signal, we use the phase of the amplification factor to compute the metric elements. The usual CBC parameter space is appended by two extra parameters: (i) the time-delay ($t_d$) between images and (ii) the relative-magnification ($\mu_r$). By identifying the region of the
lensing parameter space where the fitting factor (FF) falls below 97% due to wave-optics effects, we construct a template bank consisting of ~4000 templates covering the selected
lensing region, independent of the values of the unlensed CBCs' parameters. We show that the bank targeting microlensed CBCs is, to a very good approximation, a Cartesian product of the unlensed CBC bank and the bank spanning the
lensing parameters. We demonstrate the effectiveness of our method by evaluating fitting factors using both the unlensed CBC bank and the product template bank, finding a reduction of up to a factor of ~10 in the mismatch, as well as consistency with the prescribed minimal match criterion of 0.97 used in the construction of the banks.
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Cosmology and Nongalactic Astrophysics
When galaxies burst II. Implications of enhanced burstiness for the 21-cm Cosmic Dawn signal
Hovav Lazare, Sarah Libanore, Eleonora Vanzan et al. (+2 more)
Recent JWST observations suggest that
star formation in the early universe was substantially burstier than assumed in standard models. Such burstiness can be described as a
stochastic process characterized by the burst amplitude and the coherence time of
star formation epochs. In this paper, we investigate how bursty
star formation modifies the 21-cm power spectrum during
Cosmic Dawn through its impact on the non-local radiation fields that govern its
evolution, namely Lyman-$\alpha$ and X-ray backgrounds. To do so, we introduce an unequal-time correlation in the
star-formation-rate density sourcing the two fields and we compute its impact using the analytical framework implemented in the public code Zeus21. We find that the burstiness-induced time correlation produces a shot-noise-like contribution in the Lyman-$\alpha$ and X-ray fields, enhancing both their auto- and cross-power spectra while leaving the global 21-cm signal, $T_{21}(z)$, unchanged. As a result, the 21-cm power spectrum is strongly modified by a shot-noise-like contribution at the beginning of the
Cosmic Dawn, where the signal is dominated by lower-mass
halos ($M_h\lesssim10^{10}\,M_\odot$), and is boosted by a factor of a few near the Wouthuysen-Field absorption trough. Elsewhere at low
redshift, where the clustering signal dominates and larger
halos drive the signal, the burstiness component is negligible.
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All authors Hovav Lazare, Sarah Libanore, Eleonora Vanzan, Julian B. Muñoz, Ely D. Kovetz
Solar and Stellar Astrophysics
A Quiet Host in an Active Planet-Forming Disk: Optical Spectroscopy of WISPIT 2
C. Swastik, M. Bestha, L. S. Sonith et al. (+11 more)
WISPIT 2 is a young pre-main-sequence
star hosting a multi-ringed transition disk and two directly imaged protoplanets, including the accreting WISPIT 2b, making it the closest known analogue to PDS 70. We present the first optical spectrum of its central
star, obtained with HFOSC on the 2-m Himalayan Chandra
Telescope, and derive its atmospheric parameters, test its youth, and constrain its
accretion state. We analyse low-resolution spectra with iSpec and validate the pipeline at HFOSC resolution against
Gaia FGK
Benchmark Stars and K-type pre-main-sequence templates. We measure T_eff = 4551 +/- 150 K, log g = 4.32 +/- 0.18, and a low-resolution, model-dependent global
metallicity [M/H] = -0.17 +/- 0.16. The surface
gravity and Li I equivalent width support the pre-main-sequence nature of the host. H-alpha remains in net absorption but is partially filled by weak emission at only 1.5-2.0 sigma, approximately 1.1 dex below the expected chromospheric-noise level and therefore consistent with chromospheric activity rather than detectable
accretion. We place a 95% upper limit on the
stellar accretion rate of 3.6 x 10^-11
solar masses per year, below even the lowest monitored value for PDS 70 and implying a host-to-
planet accretion-rate ratio below approximately 18. Both known double-protoplanet hosts therefore show strongly suppressed or undetectable
stellar accretion. Larger spectroscopic samples are needed to determine whether this is common in multi-protoplanet transition disks.
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All authors C. Swastik, M. Bestha, L. S. Sonith, S. Facchini, T. Sivarani, R. K. Banyal, Z. Wahhaj, A. Choudhary, M. Gopinathan, P. Saraf, A. Mallick, M. P. Navaneeth, S. Biswas, K. Khushbu
Earth and Planetary Astrophysics
Evidence for a giant companion orbiting Tabby's star
Cristina Madurga-Favieres, P. A. Strøm, Thomas G. Wilson et al. (+2 more)
KIC 8462852, also known as Tabby's
star, shows deep, irregular flux variations up to 20 per cent in its Kepler light curve. These unusual events show no apparent periodicity. Multi-wavelength observations show the dips are consistent with transiting
dust, although its origin remains uncertain. The leading explanation is that
dust may originate from a family of exocomets or planetesimal fragments. TESS data reveal a unique, symmetric transit event consistent with a
planet or brown
dwarf. This would support the theory that incoming exocometary bodies may result from orbital perturbations by a
planet. The analysis of the TESS and, archival and newly obtained radial velocity (RV) data, suggests that a companion could orbit around Tabby's
star with a period of at least ~1030 d. When combined with the constraints imposed by Kepler and other photometric surveys, we find only five orbital period windows are possible. We conduct observations to follow-up the first two windows, significantly reducing the allowed ranges. Fitting the TESS transit using exocometary models produces a poor fit or the need of physically unrealistic parameters, favouring the
giant companion hypothesis. A 2.3 $\sigma$ detection of $M=9.4^{+4.9}_{-4.1}$ M$_{\mathrm{Jup}}$ (3$\sigma$ upper limit < 28.0 M$_{\mathrm{Jup}}$) at the transit orbital period provides tentative evidence of the companion, but more RV observations are required. Along with the radius $R=1.7\pm0.1$ R$_{\mathrm{Jup}}$, the companion would lie in the
giant planet-brown
dwarf regime. In the future
Gaia DR4 astrometry will reveal the nature of this object.
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All authors Cristina Madurga-Favieres, P. A. Strøm, Thomas G. Wilson, Neda Heidari, Flavien Kiefer
Instrumentation and Methods for Astrophysics
STORM: RDMA-based Monte Carlo Transport Scheme for Distributed-Memory Particle Simulations
Maor Mizrachi, Barak Raveh and Elad Steinberg
Monte Carlo particle transport enables high-fidelity astrophysical radiation and
neutrino simulations - from core-collapse
supernovae and
neutron-
star mergers to
accretion flows - by handling multidimensional geometries, frequency dependence, and moving media without angular discretization. However, inter-rank communication limits scalability on unstructured meshes: standard two-sided MPI requires receivers to post receives and poll completions, creating per-iteration progress overhead that grows with the number of communication partners. Such problems have not demonstrated high scaling efficiency at $O(10^4)$ cores. We present STORM (Scalable Transport via One-sided Remote
Memory), an open-source library for Monte Carlo transport on general meshes, physics, and boundary conditions. STORM provides a lock-free, mesh-independent communication layer that replaces MPI's matched-send/receive semantics with Remote Direct
Memory Access (RDMA) - one-sided operations that write directly into a remote rank's
memory without involving its CPU. Each rank pair shares a single-producer, single-consumer ring buffer; RDMA writes transfer particles while receivers remain passive. A two-sided MPI backend provides a portable fallback. In an
adversarial uniform-emission
benchmark, the RDMA backend sustains $>97\%$ weak-scaling and $>88\%$ strong-scaling efficiency up to 13,440~cores (112~cores per network adapter), with $1.14$-$1.27\times$ speedups over the two-sided alternative. In a Hohlraum IMC
benchmark at 4480 ranks, it is $1.41\times$ faster because MPI progress overhead is reduced by $6.1\times$. By decoupling communication from physics models and mesh representations, STORM removes a barrier to scaling Monte Carlo transport in astrophysical multiphysics codes, enabling coupled radiation-hydrodynamics with energy- and angle-resolved
photon or
neutrino transport on dynamically evolving meshes at scale.
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