High Energy Astrophysical Phenomena
The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study
Eftychia Madika, Bia Boccardi, Luca Ricci et al. (+5 more)
We present a comprehensive multifrequency VLBI analysis of the FRII, high-excitation
radio galaxy 3C 452, aiming to resolve and analyze for the first time its twin-jet structure on sub-parsec scales. Our data set comprises High Sensitivity Array (HSA) observations at 4.9, 8.4, 15.4, 23.6, and 43.2 GHz. Through fitting methods performed in both the visibility and the image plane, we trace the jet expansion from scales of a few thousand to nearly $10^5$ Schwarzschild radii ($R_S$) on both the approaching and receding jets. Additionally, we derive the core brightness temperatures and Doppler factors to constrain the jet's orientation and intrinsic speed. Our study provides the first detailed description of the twin-jet system in 3C 452 on VLBI scales, confirming it as a rare FRII source with jets detected down to millimeter wavelengths. We resolve both jet and counter-jet down to scales of a few thousand $R_S$, revealing a symmetric, parabolically expanding structure with power-law indices $k \approx 0.66$ (jet) and $k \approx 0.47$ (counter-jet). The brightness temperature analysis yields low Doppler factors ($\delta \sim 0.03$-$0.83$), indicative of Doppler de-boosting due to the large viewing angle ($\theta \approx 70^\circ$) and/or a magnetically dominated jet base. A spectral index analysis reveals a strongly inverted core spectrum ($\alpha > 2$) with additional absorption at the highest frequencies, followed by a sharp steepening ($\alpha \sim -2.5$) to optically thin values in the innermost jet. Finally, a comparison between broad- and narrow-line high-excitation
radio galaxies shows that jets in narrow-line sources such as 3C 452 and Cygnus A complete collimation at $\leq 10^5 R_S$, whereas broad-line sources exhibit shape transitions at $10^6$-$10^7 R_S$, suggesting that orientation plays an important role in the observed collimation scales.
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All authors Eftychia Madika, Bia Boccardi, Luca Ricci, Paola Grandi, Eleonora Torresi, Gabriele Giovannini, Matthias Kadler, J. Anton Zensus
Solar and Stellar Astrophysics
The Cold Debris Disk Surveys I. Host Star Properties
Scott J. Kenyon, Benjamin C. Bromley and Joan R. Najita
We describe the dynamical, photometric, and spectroscopic data available for
stars targeted by Spitzer and Herschel to search for cold circumstellar
dust emission from debris disks, a collection that we name the Cold Debris Disk Surveys (CDDS). These data include Hipparcos and
Gaia parallaxes, 0.4-1250 micron photometry, spectral types, effective temperatures, gravities, bolometric luminosities, visual extinctions,
metallicities, lithium abundances, rotational periods, projected rotational velocities, the Ca~II HK and IR triplet activity indicators, and X-ray luminosities for 3675
stars. Within this sample, we investigate the frequency of
stellar and planetary companions (including potential new proper motion companions); use the data to assign CDDS
stars to the field or one of many moving groups, open clusters, or
stellar associations; and investigate correlations between
stellar activity indicators. In future papers, we plan to explore the magnitude and frequency of infrared excess emission as a function of host
star properties; to search for new companions with
Gaia; and to examine the
evolution of infrared excesses with the ages of
stars in clusters and the field.
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Solar and Stellar Astrophysics
Identifying highly magnetized white dwarfs: A dimensionality reduction framework for estimating magnetic fields
Surajit Kalita, Akhil Uniyal, Tomasz Bulik et al. (+1 more)
Magnetic fields play a crucial role in compact object physics, particularly in white
dwarfs (WDs), where high densities can sustain strong
magnetic fields. Observations have revealed magnetized WDs (MWDs) with surface fields reaching approximately $10^9\rm\,G$, although high-field MWDs are fewer in number in current catalogs owing to their intrinsic faintness and limitations in conventional electromagnetic surveys. In this study, we apply unsupervised machine learning (ML) techniques to systematically analyze a sample of hydrogen-atmosphere (DA) WDs. Using Uniform
Manifold Approximation and Projection (UMAP) for dimensionality reduction and Density-Based Spatial Clustering of Applications with Noise (DBSCAN) for cluster identification, we classify distinct subpopulations within the DA WD sample. Each cluster exhibits unique intrinsic properties such as mass, surface
gravity, temperature, and age. Our analysis further reveals that these subgroups effectively differentiate MWDs from non-
magnetic or weakly
magnetic counterparts. Moreover, utilizing a set of previously confirmed MWDs, we estimate the field strengths of all other MWDs lacking
magnetic field measurements. This study underscores the effectiveness of ML-based approaches in astrophysical discovery, particularly detecting magnetized compact objects when direct measurements are unavailable.
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All authors Surajit Kalita, Akhil Uniyal, Tomasz Bulik, Yosuke Mizuno
High Energy Astrophysical Phenomena · Cross-listed from hep-ph
B.O. Kerbikov
The
neutron to mirror
neutron transitions in
neutron stars would possibly result in significant effects. In this work we show that collisional decoherence entails exponential relaxation in lieu of oscillations. Decoherence is a great many orders of magnitude faster than the expected oscillations. The admixture of mirror
neutrons at all times remains very small with respect to ordinary
neutrons component.
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Instrumentation and Methods for Astrophysics
AS-Bridge: A Bidirectional Generative Framework Bridging Next-Generation Astronomical Surveys
Dichang Zhang, Yixuan Shao, Simon Birrer et al. (+1 more)
The upcoming decade of observational cosmology will be shaped by large sky surveys, such as the ground-based LSST at the Vera C. Rubin Observatory and the space-based Euclid mission. While they promise an unprecedented view of the Universe across depth, resolution, and wavelength, their differences in observational modality, sky coverage, point-spread function, and scanning cadence make joint analysis beneficial, but also challenging. To facilitate joint analysis, we introduce A(stronomical)S(urvey)-Bridge, a bidirectional generative model that translates between ground- and space-based observations. AS-Bridge learns a
diffusion model that employs a
stochastic Brownian Bridge process between the LSST and Euclid observations. The two surveys have overlapping sky regions, where we can explicitly model the conditional probabilistic distribution between them. We show that this formulation enables new scientific capabilities beyond single-survey analysis, including faithful probabilistic predictions of missing survey observations and inter-survey detection of rare events. These results establish the feasibility of inter-survey generative modeling. AS-Bridge is therefore well-positioned to serve as a complementary component of future LSST-Euclid joint data pipelines, enhancing the scientific return once data from both surveys become available. Data and code are available at https://github.com/ZHANG7DC/AS-Bridge.
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All authors Dichang Zhang, Yixuan Shao, Simon Birrer, Dimitris Samaras
High Energy Astrophysical Phenomena
Unveiling the biconical geometry of the outflow in the ultraluminous X-ray source NGC 5204 X-1
S. Caserta, C. Pinto, T. Di Salvo et al. (+9 more)
Ultraluminous X-ray sources (ULXs) are non-
nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 Msun black hole. The majority of these sources are thought to be
stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3c and emission lines typically found at their laboratory wavelengths. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3c. The aim of this study is to examine the geometry and physical properties of the
accretion disc and the relativistic
outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the RGS instrument which allowed us to resolve both emission and absorption lines. The
outflows features were characterised using physical models of
plasma in collisional-ionisation and photoionisation
equilibrium. We identify collisionally-ionised blueshifted and redshifted components at about 0.3c. These findings have high statistical significance and suggest a biconical structure for the
outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting
plasma, e.g. electron density (ne $\sim 10^{10}$ cm$^{-3}$) and temperature (Te $ \geq 1.5 \times 10^5$ K). A hybrid
plasma whose ionisation balance is affected by both collisions and radiation is favoured.
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All authors S. Caserta, C. Pinto, T. Di Salvo, F. Pintore, P. Kosec, F. Barra, D. J. Walton, A. D'Aì, M. Del Santo, A. Gúrpide, A. Fabian, A. Wolter
Astrophysics of Galaxies
\HI 21-cm Line Properties of the Nearby LIRG IRAS 04296+2923
Guixiang Feng, Zhongzu Wu, Chuanpeng Zhang et al. (+1 more)
We present an analysis of archival Very Large Array (VLA) and Five-hundred-meter Aperture Spherical
radio Telescope (FAST) \HI\ 21 cm data, together with archival multi-band
radio continuum observations, of the nearby luminous infrared
galaxy IRAS~04296+2923. The system, located behind the Taurus dark cloud at a distance of $\sim$29 Mpc, forms a small
galaxy group consisting of five members as revealed by the \HI\ imaging. IRAS~04296+2923 has a close companion, HI~0432+2926, with a projected separation of $\sim$40 kpc, a small line-of-sight velocity difference of $\Delta$ v = 26 km s$^{-1}$, and comparable total \HI\ masses of order $10^{9}$~$M_{\odot}$. Both
galaxies exhibit regular \HI\ velocity fields and characteristic double-horn profiles in the VLA and FAST data, accompanied by only subtle asymmetries and extended \HI\ structures, indicating rotation-dominated kinematics with early signs of weak
tidal interaction.
Radio continuum emission is detected only from IRAS~04296+2923 and is confined to its
nuclear region, consistent with previous studies. Modeling of its multi-band
radio spectrum reveals a significant contribution from free–free emission at high frequencies ($>$30 GHz) and a high FIR-to-
radio flux ratio ($q_{8.4}\simeq3.2$), implying a young,
dust-obscured
nuclear starburst. Taken together, the regular \HI\ kinematics, the small velocity offset, and the group-scale environment favor an interpretation in which IRAS~04296+2923 and HI~0432+2926 form a gravitationally bound, orbiting
galaxy pair embedded in a small group, rather than an advanced merger. In this context, the luminous infrared
galaxy (LIRG) nature of IRAS~04296+2923 is more plausibly driven by internal processes, such as bar-induced gas inflow, possibly modulated by long-timescale, low-level
tidal interactions with nearby group companions.
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All authors Guixiang Feng, Zhongzu Wu, Chuanpeng Zhang, Ming Zhu
Cosmology and Nongalactic Astrophysics
Cosmological gravity on all scales V: MCMC forecasts combining large scale structure and CMB lensing for binned phenomenological modified gravity
Sankarshana Srinivasan, Shreya Prabhu, Kai Lehman et al. (+2 more)
As cosmology rapidly approaches the data-dominated phase of stage IV large scale structure surveys, the modelling of nonlinear scales has become a serious challenge that faces the community, particularly when analysing models beyond $w$CDM. In this work, we emulate the matter power spectrum in a phenomenological parameterisation of modified
gravity in which a time-varying effective gravitational constant $\mu$ and a gravitational slip $\eta$ are binned in
redshift. We are able to achieve accuracy $<1\%$ in the modified
gravity boost relative to COLA (COmoving Lagrangian Acceleration) simulations. We forecast the constraining power for each bin using a simulated $3\times 2$pt LSST Y10-like data vector and a $6\times 2$pt LSST Y10 x Simons Observatory
cosmic microwave background (CMB)
lensing data vector. We recover the characteristic degeneracy between $\mu$ and $\eta$ previously identified in Fisher forecasts and demonstrate that the best-constrained direction corresponds to the combination $\Sigma=\mu(1+\eta)/2$ which governs the
lensing potential. We show that while large scale structure is sensitive to growth of structure at low
redshift, CMB
lensing extends the sensitivity to a higher
redshift range. These results demonstrate that fast emulation of nonlinear modified-
gravity effects enables full
Bayesian analyses of model-agnostic
gravity parameterisations with realistic survey data vectors and astrophysical systematics.
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All authors Sankarshana Srinivasan, Shreya Prabhu, Kai Lehman, Ajiv Krishnan V., Jochen Weller
Instrumentation and Methods for Astrophysics · Cross-listed from physics.optics
Phase Retrieval using Nonlinear Curvature Sensing within Convergent Beams
Justin R. Crepp, Caleb G. Abbott, James Smous et al. (+2 more)
Path-length diversity methods may be used for adaptive optics (AO) systems to retrieve phase and amplitude information by measuring intensity across multiple planes. Observations that rely on free-space propagation, such as the nonlinear
curvature wavefront sensor (WFS), have been shown to offer excellent sensitivity and robustness to scintillation. However, the default design results in a large opto-mechanical footprint due to unavoidable
geometric-optics and wave-optics effects. Measurements recorded in a convergent beam would improve instrument compactness, while concentrating light into smaller detector regions of interest, improving signal-to-noise ratio and possibly wavefront reconstruction speed. In this paper, we study path-length diversity wavefront sensing using four planes of contemporaneous intensity measurements made in a convergent beam. We develop a physical optics propagation model and validate the model by performing wavefront reconstructions in both simulations and lab experiments. The manuscripts core contribution is a practical, intensity-domain,
Fourier-transform-based recipe to use a conventional multi-plane Gerchberg-Saxton (or comparable) reconstruction pipeline with convergent-beam measurements, enabling a compact optical layout. We find that this approach offers practical benefits over an equivalent free-space wavefront sensor, in particular reducing size, weight, complexity and cost.
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All authors Justin R. Crepp, Caleb G. Abbott, James Smous, Matthew Engstrom, Brian Sands
Astrophysics of Galaxies
Energy dependence of the X-ray power spectrum in NGC4051 and NGC4395
V. A. Diamantopoulos, I. E. Papadakis, A. Akylas et al. (+3 more)
Active
galactic nuclei (AGNs) exhibit strong variability across the electromagnetic spectrum on a wide range of timescales, particularly in X-rays where fluctuations are both rapid and high amplitude. Power spectral density (PSD) analysis is commonly used to characterise this variability. Although AGN PSDs are typically well described by a bending power-law model, the dependence of the model parameters on
photon energy has not been systematically investigated. We examine whether PSD parameters depend on energy using two highly variable, low-mass Seyfert
galaxies, NGC4051 and NGC4395, as case studies. Using archival observations from XMM-Newton, Suzaku, and NuSTAR, we computed power spectra in six energy bands spanning 0.3-20 keV and fitted them with a bending power-law model to study the energy dependence of the PSD parameters. Power spectra derived from light curves obtained with different satellites and at different epochs are consistent within uncertainties, indicating that the X-ray variability process in both sources is stationary. For both AGNs we find that (i) the PSD bending frequency is consistent with being independent of energy, (ii) the high-frequency slope becomes flatter at higher energies, and (iii) the PSD amplitude decreases with increasing energy. These results place important constraints on models of AGN X-ray variability, such as the fluctuating
accretion-rate model. Similar analyses of larger AGN samples are required to fully characterise the energy dependence of AGN power spectra.
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All authors V. A. Diamantopoulos, I. E. Papadakis, A. Akylas, A. Zoghbi, E. Kammoun, B. Rani
Cosmology and Nongalactic Astrophysics
The impact of baryons on weak lensing statistics as a function of halo mass and radius
Max E. Lee, Zoltan Haiman and Shy Genel
Upcoming weak
lensing (WL) surveys such as those by Euclid, LSST, and Roman require percent-level control over systematic effects. A common approach to mitigating baryonic effects uses semi-analytic
baryon correction models (BCMs) that modify
halo profiles in dark matter-only (DMO) simulations, calibrated to statistics from hydrodynamic simulations. We investigate the limits of this approach by progressively replacing larger regions around
halos of decreasing mass in DMO simulations with their hydrodynamical counterparts. We compare multiple statistics – the matter ($P(k)$) and weak-
lensing ($C_\ell$) power spectra, peak counts, minima, one-point PDFs, and Minkowski functionals – from "Replace" fields against hydrodynamical and DMO simulations. We find that replacing all
halos with $M\geq10^{12}\,h^{-1}\,{\rm M}_\odot$ out to $r\leq5R_{200}$ recovers $\sim 90\%$ of the baryonic suppression in $P(k)$ and $C_\ell$ with the remaining $\sim 10\%$ originating from lower-mass
halos or material farther outside of DM
halos. Each statistic has distinct sensitivities to
baryons: $P(k)$ and $C_\ell$ are sensitive to a broad range of masses and radii, whereas WL peaks are primarily affected by the cores of massive
halos. We show that BCMs applied to massive
halos and calibrated to match hydrodynamical $P(k)$ make two cancelling "mistakes": they underpredict core masses and compensate by overpredicting baryonic impacts at larger radii, thereby explaining previously reported failures of peak statistics in these models. We provide a framework for diagnosing critical mass/radius regions in baryonic modeling for a range of statistics for next-generation BCMs.
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Astrophysics of Galaxies
Chemo-dynamical reconstruction of Milky Way globular cluster progenitors: Age-metallicity relations and the universality of multiple stellar populations
Carmela Lardo, David Valcin and Raul Jimenez
Globular clusters encode the hierarchical assembly history of the Milky Way and the physics of multiple
stellar populations. Using homogeneous
stellar parameters for 69
Galactic globular clusters derived while modelling multiple populations, we reconstruct progenitor-specific age–
metallicity relations (AMRs) and test whether helium-related multiple-population (MP) properties depend on progenitor origin once cluster mass and
metallicity are controlled for. Ages, helium spreads ($\delta Y$), mean helium abundances ($\bar{Y}$), and first-population fractions ($f_{\rm P1}$) are drawn from hierarchical
Bayesian CMD modelling. Progenitor families are identified via chemo-dynamical clustering, AMRs reconstructed within a hierarchical
Bayesian framework, and MP indicators tested for environmental dependence. Enrichment timescales are consistent with $\tau \lesssim 2$\,Gyr, though individual progenitors prefer shorter values when fitted independently. The primary distinction is the extent of chemical
evolution: most systems reach $\Delta[\mathrm{Fe/H}] \sim 1.1$–$1.3$\,dex while Sagittarius achieves ${\sim}1.6$\,dex and higher terminal
metallicities.
Gaia–Sausage–Enceladus and low-energy/Kraken are the dominant
accretion events. Neither $\delta Y$ nor $\bar{Y}$ depends on progenitor origin; the mass–MP scaling is indistinguishable across in-situ and accreted systems. Sequoia clusters alone show higher $f_{\rm P1}$ at fixed mass and
metallicity. AMRs carry fossil signatures of progenitor chemical
evolution and mass hierarchy. Helium enrichment amplitude is regulated by cluster mass and blind to environment, pointing to universal cluster-scale formation physics, with the sole exception of a residual dependence in $f_{\rm P1}$, suggesting the enriched-
star fraction retains a secondary environmental imprint.
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Astrophysics of Galaxies
Binary disruption during the early phase of open clusters
Zepeng Zheng, Long Wang and Holger Baumgardt
The binary fraction in young open clusters exceeds that of field
stars, making the study of binary dynamical
evolution in clusters essential for understanding the origins and
evolution of field binaries. Using N-body simulations based on
Gaia DR3 open cluster observations and assuming a 100%
primordial binary fraction, we investigated the early
evolution of binary survival fractions in open clusters. We find that binary disruption has two stages, an initial rapid decline followed by a slower decrease, well described by two piecewise linear functions. The early disruption rate, $k_1$, follows a power-law relation with the cluster's initial density ($ \rho_\mathrm{0} $), with an index of approximately 0.56, driven by the disruption of wide binaries via close encounters. The transition time between the two phases, $t_\mathrm{b}$, also exhibits a power-law dependence on $\rho_\mathrm{0}$ with an index of about -0.46. The disruption rate also depends on binary parameters: high-$q$ and wide binaries are disrupted faster, while the dependence on eccentricity $e$ is less clear, likely due to its strong
evolution. We developed and publicly released a Python tool to predict binary survival fraction
evolution based on $\rho_0$, $P$ and $q$. Additionally, we also investigate how open cluster binaries contribute to the field population, and find that the escaped
stars have a systematically lower binary fraction, likely due to mass segregation. Both populations show similar distributions of $ P $ and $e$, but lower-$q$ systems preferentially remain bound within clusters, the origin of which remains uncertain.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Analytic Singular Slow-roll Inflation
V.K. Oikonomou
We study a class of minimally coupled scalar field theories which leads to analytic solutions for the Hubble rate and the scalar field, where the scalar field obeys a generalized tracking law $\dot{\phi}^2\sim H^{-m}$. The inflationary phenomenology for this class of models can be studied fully analytically. The resulting phenomenology is compatible with the ACT data and for limiting cases, the spectral index is bluer than the ACT constraints and tends to the value $n_{\mathcal{S}}=0.98$, while in the limiting case, the tensor-to-scalar ratio takes very small values, nearly zero. In addition, we prove analytically that the phenomenology is a one-parameter model, and the inflationary observables encode the scaling exponent $m$ of the generalized kinetic attractor $\dot{\phi}^2\sim H^{-m}$. Furthermore, the tensor-to-scalar ratio and the spectral index have a simple linear and $m$-dependent relation. More importantly, the resulting cosmology describes a Universe that has a finite scale factor at $t=0$, thus non-singular, evolves and expands realizing a slow-roll inflationary era and after that it reaches classically a pressure singularity. Classically, the Universe can pass through this singularity, and a turnaround cosmology is realized with the Universe contracting after the turnaround point. However, before the singularity is realized classically, the
quantum phenomena dominate the
evolution, avoiding the singularity. Specifically we consider the Nojiri-Odintsov conformal anomaly mechanism and we qualitatively show that the conformal anomaly erases the classical singular
evolution and at the same time it enhances particle creation, which eventually reheats the Universe. Thus in this model the scalar field oscillations and the numerous couplings of the inflaton to the Standard Model particles are not required for reheating.
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Astrophysics of Galaxies
Continuum Reverberation in Bright Quasars Using NASA/ATLAS
Zachary Steyn, Christian Wolf, Christopher Onken et al. (+5 more)
Variable continuum emission from AGN can be used to probe the structure of their
accretion disks via reverberation mapping. Assuming a variable, hot inner light source irradiates the surrounding
accretion disk, time delays between different continuum band light curves reveal light-travel times between their respective emission regions. Inter-band delays measured in several low-luminosity AGN are ubiquitously $\sim 3$ times longer than expected from standard disk theory, with evidence this size discrepancy may decrease in more luminous AGN. We have analysed high-cadence light curves of 9,498 of the brightest
quasars between
redshift 0.3-2.5 in the largest continuum reverberation study to date. Given the large sample size, we construct bins and fit delays jointly to combine inference across the parameter space and improve lag detections. We find that the size discrepancy persists in our high-luminosity sample, and that the previously seen anti-correlation with luminosity is likely driven by wavelength effects. The complex, non-monotonic wavelength dependence of delay amplitudes strongly suggests that contamination of inter-band delays by variable diffuse emission is widespread in the AGN population. We test delay behaviour against a variety of
quasar properties finding longer lags in
quasars with: higher Eddington ratios, redder colours, stronger optical FeII equivalent widths, higher iron ratios (both UV FeII/MgII and optical FeII/H$\beta$), CIV broad absorption troughs, and lower CIV blueshift.
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All authors Zachary Steyn, Christian Wolf, Christopher Onken, Ken Smith, Ji-Jia Tang, Andjelka B. Kovacevic, John Tonry, Alejandro Clocchiatti
Earth and Planetary Astrophysics
Equilibrium figure of Haumea and possible detection by stellar occultation
C. Staelen, N. Rambaux, F. Chambat et al. (+1 more)
The
equilibrium figure of
dwarf planet Haumea is studied to determine if the observed shape is compatible with a differentiated hydrostatic body. Three groups of interior models of Haumea are assumed, all with a rocky core and a volatile-rich outer shell that may contain some porosity. A third layer located between the core and the outer shell has a density suggesting partial differentiation or the presence of a large fraction of organic matter. Using the code BALEINES, which solves for the
equilibrium figures of the boundaries between layers, we show that the hydrostatic models closest to the shape derived by
stellar occultation approach a state of critical rotation, which translates into a pinched shape with large deviations from an ellipsoid (up to 110 km). The previous
stellar occultation and light curves cannot distinguish between the ellipsoid and the pinched shape, but we predict this figure could be observable on the next
stellar occultation of Haumea on May 4, 2026, if some chords are obtained in the northern or southern limbs of the shadow.
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All authors C. Staelen, N. Rambaux, F. Chambat, J. C. Castillo-Rogez
Astrophysics of Galaxies
Origin of open clusters revealed by the evolution of the m_max$-$M_ecl relation
J. W. Zhou, Sami Dib and Pavel Kroupa
Using the
Gaia DR3 open cluster catalog, we identified the most massive
star in each observed cluster. Examining the m_max$-$M_cluster relations across different age ranges, we find that as clusters age, the relation gradually deviates from the initial m_max$-$M_ecl relation and eventually exhibits clear age stratification. We conducted N$-$body simulations for both individual cluster
evolution and subcluster coalescence. Four gas expulsion modes were tested for individual clusters, and two scenarios were modeled for cluster coalescence. Under all four gas expulsion modes, the
evolution of the m_max$-$M_cluster relation follows a similar trajectory, differing mainly in evolutionary speed. The coalescence simulations show comparable behavior but align better with the observations, as both exhibit systematically lower m_max$-$M_cluster relations than individual cluster simulations. This systematically lower observed m_max$-$M_cluster relation suggests slower cluster mass loss and smaller masses for the most massive
stars$-$both conditions reproduced in the coalescence simulations. Observations also show that clusters older than 5 Myr have most massive
stars significantly deviating from the initial m_max$-$M_ecl relation. From this perspective, the coalescence simulations also provide a better match to the observations. In conclusion, the
evolution of the m_max$-$M_ecl relation supports subcluster coalescence as a dominant pathway for open cluster formation, consistent with our previous work.
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Solar and Stellar Astrophysics
Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae since 2003
N.P. Ikonnikova, V.I. Shenavrin, G.V. Komissarova et al. (+2 more)
We present photometric and spectral monitoring of the symbiotic system HM Sge, which consists of a Mira variable with a
dust shell and a hot white
dwarf ionizing the surrounding gaseous nebula. The system underwent a nova-like outburst in 1975 and experienced a high-activity episode during 2018-2021. UBV photometry from 2003 to 2025 shows monotonic fading at about 0.05 mag/yr until 2018, followed by a 0.3 mag brightening peaking in 2021 and a decline by 2025 to the faintest level in five decades of monitoring. Near-infrared observations (JHKLM, 2009-2025), combined with archival data, reveal Mira pulsations with a period of 532 d and long-term variability driven by changes in the optical depth of the
dust shell. Spectral monitoring (2016-2025) reveals a substantial
evolution in the emission spectrum of the gaseous envelope. The 2018-2021 high-activity episode was accompanied by enhanced fluxes in recombination lines (H I, He I, He II) and forbidden transitions ([O I], [Ar V], [Fe VI], [Ca VII]), together with a 17-fold brightening of the Raman-scattered O VI $\lambda$6725 line. We report the first detection of the [Fe X] $\lambda$6374 line in HM Sge. Monitoring this line from 2007 to 2025 shows its equivalent width growing through 2017 - indicating gradual coronal heating - then declining by 2021, likely reflecting altered
accretion conditions and/or hot-component properties during the high-activity episode. We propose that both the 1975 outburst and the 2018-2021 high-activity episode may be linked to periastron passage of the binary components; if so, the about 46-year interval would constrain the system's orbital period.
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All authors N.P. Ikonnikova, V.I. Shenavrin, G.V. Komissarova, M.A. Burlak, A.V. Dodin
Earth and Planetary Astrophysics
Self-consistent-field method for triaxial differentiated bodies in hydrostatic equilibrium
C. Staelen and J.-M. Huré
Recent observations and models of Haumea and Quaoar suggest that both bodies are triaxial, but their shapes are inconsistent with Jacobi ellipsoids. To determine whether these objects can be at hydrostatic
equilibrium, we propose a new numerical code, BALEINES, to study the hydrostatic shape of triaxial differentiated bodies. The fluid mass is assumed to be made of several homogeneous layers, which allowed us to rewrite the gravitational potential as a sum of proper surface integrals. In contrast to the classical self-consistent field method, we did not solve for the mass density, but for the shape of the boundary of all layers, meaning that only one point per layer is needed in the radial direction. The solution is still searched for iteratively. The code was benchmarked against analytical and numerical solutions. As a quick application, we studied the position of the axisymmetric-triaxial bifurcation point of two-layer systems. We show that the deviation from the Meyer bifurcation point in the single-layer case is below $10~\%$ in realistic cases. Based on this result, we conclude that the shape of Quaoar, as obtained in a recent work using a thermophysical model of the surface, is not compatible with a hydrostatic figure of
equilibrium.
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Solar and Stellar Astrophysics
Reconstruction of Cepheid Radial Velocity Curves from the shape of the V-band Light Curves
V. Hocdé, P. Moskalik, N. Nardetto et al. (+17 more)
This paper aims to develop the first method to reconstruct the shape of the RV curves of short-period fundamental-mode Cepheids, based exclusively on their pulsation period and the morphology of their $V$-band light curves (LCs). We compiled a
dataset of high-quality spectroscopic and photometric measurements from the literature for 81 short-period fundamental-mode
Galactic Cepheids up to a pulsation period of 8\,days, enabling precise determination of the
Fourier parameters and their uncertainties. We investigated correlations between LC and RV
Fourier parameters and used these relations to reconstruct the RV curves. We further assessed the accuracy of these reconstructions by examining potential
metallicity effects with an additional
dataset of 23 metal-poor Cepheids. For pulsation periods between 3.5 and 7.0\,days, we found tight correlations between different combinations of LC and RV
Fourier parameters up to order 7, in particular $R_{21}(RV)/R_{21}(LC)$ and $R_{31}(RV)/R_{31}(LC)$ are correlated with the pulsation period. These relationships enable the reconstruction of RV curves of Cepheids with their LC. The reconstructed curve has an uncertainty of about 0.60${\rm km\,s}^{-1}$ relative to the
Fourier fit of true spectroscopic RV measurements. For individual Cepheids, the reconstructed RV curves integrated along the pulsation cycle (i.e. the linear radius variations) are accurate to less than 1% and precise to within 4.16% in comparison to the integrated true spectroscopic RV curves. This approach provides a valuable tool for the reconstruction of RV curves for extragalactic Cepheids through photometric data alone. It opens the road to a purely photometric parallax-of-pulsation method in the context of photometric surveys, such as the Vera Rubin
Telescope.
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All authors V. Hocdé, P. Moskalik, N. Nardetto, P. Kervella, B. Pilecki, R. Smolec, G. Pietrzyński, W. Gieren, G. Hajdu, A. Gallenne, M. C. Bailleul, G. Bras, P. Wielgórski, L. Breuval, A. Mérand, R. S. Rathour, F. Espinoza-Arancibia, W. Kiviaho, B. Apostolova, K. Sivkova
Astrophysics of Galaxies
Merger-driven buildup of the $M_{\rm BH}$ - $M_*$ relation bridging high-$z$ overmassive black holes with the local relation
Takumi S. Tanaka, John D. Silverman, Kazuhiro Shimasaku et al. (+3 more)
The origin of the mass scaling relation between supermassive black holes (SMBHs, $M_{\rm BH}$) and
galaxies ($M_*$) remains a key open question. Rather than invoking AGN feedback, a non-causal mechanism has been proposed in which multiple mergers average out the $M_{\rm BH}/M_*$ ratio, thus decreasing its scatter ($\sigma$) and forming a tight local mass relation over
cosmic history. A larger scatter in the relation at higher
redshift suggested from a non-causal evolutionary scenario may be evident from recent JWST observations of overmassive SMBHs at high
redshift. Here, we carry out a Monte Carlo simulation of solely merger-induced
evolution of
galaxies and their SMBHs which incorporates recent high-
redshift observational constraints on $\sigma$ and the
galaxy merger rate. We find that the dispersion in the local mass relation can be reproduced, even when starting from a highly scattered population at $z\sim6$ with $\sigma=0.8\,{\rm dex}$ or $1.0\,{\rm dex}$, which are in agreement with recent JWST studies. The
redshift evolution of the scatter is highly sensitive to the mass ratio between merging pairs and the merger rate, and minor mergers with higher frequency than major mergers can also contribute to the scatter
evolution, highlighting the importance of accurately constraining these parameters at high
redshift through observations. Furthermore, statistical surveys aimed at determining the $M_*$-dependence of $\sigma$ and constraining $\sigma$ at $z\sim3-4$ will be effective in testing this scenario.
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All authors Takumi S. Tanaka, John D. Silverman, Kazuhiro Shimasaku, Knud Jahnke, Junyao Li, Makoto Ando
Solar and Stellar Astrophysics
The Inner and Outer Shock Layers of Bow Shocks in Cataclysmic Variables
Krystian Ilkiewicz, Christian Knigge, Simone Scaringi et al. (+3 more)
Bow shocks around cataclysmic variables (CVs) have traditionally been identified with a single bright optical arc. This feature has been interpreted as the bow shock formed by the interaction between a sustained
outflow and the interstellar medium (ISM). We show that this interpretation is incomplete. Generic wind-ISM interaction theory predicts a two-shock configuration consisting of an inner terminal wind (reverse) shock and an outer forward shock, separated by a hot, low-density shocked wind cavity. Using archival ultraviolet, optical, and infrared imaging of the nova-like systems BZ Cam and V341 Ara, and the polar 1RXS J052832.5+283824, we find that the nebulae around all three systems exhibit this layered structure. In each case, the previously identified bow shock bright in Halpha and [OIII] corresponds to a compact inner arc, while additional emission components reveal a more extended morphology. Specifically, each system shows an outer arc detected in mid-infrared images, and the region between the optical and infrared arcs is filled with faint Halpha emission and, where available, far ultraviolet emission. We identify this infrared arc, reported here for the first time in these systems, as the sweep-up boundary of the forward shock, while the bright inner optical arc corresponds to the terminal wind shock rather than the forward shock as previously assumed. These results reveal that the true extent and layered structure of bow shocks around CVs only become apparent when observations extend beyond the optical band.
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All authors Krystian Ilkiewicz, Christian Knigge, Simone Scaringi, Noel Castro Segura, Santiago del Palacio, Martina Veresvarska
High Energy Astrophysical Phenomena
Revisiting early afterglows of gamma-ray bursts with finite-thickness ejecta: Implications from XRF 080330 and GRB 080710
Kaori Obayashi, Ryo Yamazaki, Yo Kusafuka et al. (+1 more)
We revisit the physical origin of the achromatic peaks and breaks observed several thousand seconds after the burst in the multi-wavelength afterglows of XRF 080330 and GRB 080710. Using a numerical afterglow model that consistently incorporates finite ejecta thickness and a generalized external density profile, we perform
Bayesian inference to estimate model parameters describing these events. Our analysis shows that the gradual rise and achromatic temporal features in both events are more naturally explained by jet dynamical
evolution with finite shell thickness rather than by off-axis viewing effects. The inferred initial radial width of the ejecta is of order $10^{13}$ cm for both bursts, implying a central engine activity timescale significantly longer than that suggested by the
prompt gamma-ray duration alone. Taken together, these results demonstrate that early afterglow light curves are strongly influenced by transition dynamics when finite ejecta thickness is properly taken into account, thereby providing a physical link between the
prompt and afterglow phases and highlighting limitations of simply applying the thin-shell approximation when interpreting early-time afterglows. Furthermore,
Bayesian model comparison favors a generalized circumburst density profile over the canonical uniform or steady-wind models, suggesting that fixing the external density structure to idealized profiles a priori may obscure crucial information about the progenitor's pre-burst activity.
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All authors Kaori Obayashi, Ryo Yamazaki, Yo Kusafuka, Katsuaki Asano
Cosmology and Nongalactic Astrophysics
A Unified Scalar-Field Mechanism for the $H_0$, $S_8$ and Evolving Dark-Energy Tensions
Gerasimos Kouniatalis
We propose a unified scalar-field framework that addresses, within standard general relativity, three current cosmological anomalies: the $H_0$ tension, the mild preference for reduced late-time clustering ($S_8$), and recent indications of evolving dark energy. The model contains a single minimally coupled canonical scalar field evolving in a smooth potential composed of a localized bump superimposed on an exponential tail. The bump generates a transient pre-recombination energy injection that increases the expansion rate before last scattering, reduces the sound horizon, and shifts the CMB-inferred value of $H_0$ upward. After the field is released, its energy density rapidly
redshifts through a kination-like phase, ensuring that the early modification does not persist as an unwanted late-time contribution. At low
redshift, the exponential tail drives quintessence-like
evolution, naturally yielding $w_0>-1$ and $w_a<0$ while suppressing linear structure growth and moving $S_8$ in the observationally preferred direction. The analysis shows explicitly how this smooth single-field potential can produce the required sequence of early enhancement, rapid dilution, and late-time thawing behavior.
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Solar and Stellar Astrophysics
pyKurucz: A Pure Python Reimplementation of Kurucz ATLAS12 and SYNTHE for Stellar Spectrum Synthesis
Elliot M. Kim and Yuan-Sen Ting
pyKurucz is a pure Python reimplementation of Kurucz's ATLAS12 and SYNTHE, the standard code tools for computing synthetic
stellar spectra. The original Fortran codes, written decades ago in a legacy dialect, are difficult to compile with modern toolchains without significant manual patching, and their long-term maintenance is uncertain following the passing of Robert L. Kurucz in 2025. pyKurucz delivers a complete, line-by-line Python translation of both engines, entirely removing the need for Fortran. Powered by NumPy, SciPy, and Numba, it combines the full ATLAS12 iteration loop (with direct opacity sampling and convection) with comprehensive physical treatments, from Saha-Boltzmann populations and molecular
equilibrium to radiative transfer and advanced line broadening. Validated against the original across 100 atmosphere models spanning 2500 K cool
giants to 44,000 K O
stars over 300-1800 nm at resolving power R = 300,000, it achieves sub-0.01% median agreement. The pure Python implementation enables direct integration with machine learning workflows and large-scale survey pipelines, while preserving an archival reference implementation of both ATLAS12 and SYNTHE in a modern, readable
language.
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Cosmology and Nongalactic Astrophysics
Constraints on Axion-Photon Mixing from Fast Radio Burst Dispersion Measures
Gunalan Muthusami and Gopal Kashyap
Fast
radio bursts (FRBs) offer a powerful probe of the ionized Universe through their dispersion measures (DM). While a significant fraction of the DM arises from the intergalactic medium (IGM), the contributions from the host
galaxy and the immediate environment of the source remain uncertain, and the physical origin of FRBs is still under active investigation. In this work, we investigated the possibility that FRBs originate from high-
magnetic-field
neutron stars (NS), whose magnetospheres can facilitate
axion-
photon mixing. Such mixing can modify
photon propagation and induce an effective contribution to the observed dispersion. Using a sample of localized FRBs with measured
redshifts, we perform a
Bayesian Markov Chain Monte Carlo (MCMC) analysis to constrain the
axion mass $m_a$ and
axion-
photon coupling $g_{a\gamma\gamma}$. Within a parametric cosmological framework, we obtain $m_a = 1.16^{+4.40}_{-1.08}\,\mu{\rm eV}$ and $g_{a\gamma\gamma} = (1.76^{+6.69}_{-1.64})\times10^{-16}\,{\rm GeV}^{-1}$, together with a physically consistent intergalactic
baryon fraction $f_{\rm IGM} = 0.837^{+0.053}_{-0.056}$. We further tested the robustness of our bounds against cosmological modeling assumptions by employing a non-parametric Gaussian Process reconstruction (GPR) of the DM-$z$ relation, which gives statistically consistent results.
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Instrumentation and Methods for Astrophysics
Exploring the Viability of Fisher Discriminants in Galaxy Morphology Classification
Sazatul Nadhilah Zakaria, Santtosh Muniyandy and John Y. H. Soo
One of the major challenges in astronomy involves accurately classifying
galaxies, particularly distinguishing between different
galaxy types. While many complex
algorithms have shown strong performance in classification tasks, their complexity often results in longer processing times and increased difficulty in understanding. This study addresses this issue by exploring the viability of Fisher discriminants, a much simpler
algorithm, in performing
galaxy morphology classification. We tested four machine learning
algorithms: the Fisher discriminant, Artificial Neural Networks (ANNs), Boosted Decision Trees (BDTs), and k-Nearest Neighbours (kNNs) to classify
galaxies by the shape of their central bulges. Using data from the Sloan Digital Sky Survey (SDSS), we utilised five pre-processing transformations: normalisation, decorrelation, principal component analysis (PCA), uniformisation, and Gaussianisation, and classified the shape of central bulge into either rounded or no-bulge, based on the
Galaxy Zoo Decision Tree. When compared to the
Galaxy Zoo 2 (GZ2) labels, the Fisher discriminant with uniformisation obtained the highest accuracy score of 0.9310, outperforming ANN, BDT, and kNN by 1.93%, 0.42%, and 3.08%, respectively.
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High Energy Astrophysical Phenomena
SN 2023axu: A Type IIP Supernova Interacted with a Low-Density Stellar Wind
Zeyi Wang, Jujia Zhang, Qian Zhai et al. (+8 more)
We present photometric and spectroscopic observations of Type IIP
supernova SN 2023axu, spanning $\sim$400 d after the explosion. Its light curve is typical of normal SNe IIP, with a V-band peak of $-17.25 \pm 0.06$ mag and no early-time excess indicative of strong circumstellar interaction. The early spectra exhibit a distinctive broad "ledge" near 4600 Å. Through spectral modeling and comparison, we attribute this feature to a blend of C, N, and He lines excited by weak interaction between the ejecta and a low-density
stellar wind. The late-time photometric
evolution shows no discernible contribution from interaction, arguing against strong late-time circumstellar material engagement and supporting the low-density wind scenario. From modeling, this SN synthesized $\sim 0.055\,M_\odot$ of $^{56}$Ni, and nebular spectrum analysis indicates a progenitor mass near $15\,M_\odot$. SN 2023axu thus exemplifies weak ejecta-wind interaction and highlights the diversity of mass-loss histories and circumstellar environments of SNe II progenitors.
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All authors Zeyi Wang, Jujia Zhang, Qian Zhai, Liping Li, G. Valerin, A. Reguitti, A. Pastorello, Zhenyu Wang, Zeyi Zhao, Tengfei Song, Yongzhi Cai
Cosmology and Nongalactic Astrophysics
Warm Inflation Beyond the Markovian Limit
Mayukh R. Gangopadhyay and Nilanjana Kumar
Warm inflation is commonly studied under the assumption that the
stochastic force sourcing inflaton fluctuations is Markovian. Realistic thermal systems, however, possess finite relaxation times and can therefore generate colored noise with non-zero correlation time. In this work, we investigate warm inflation beyond the Markovian limit and determine how finite correlation time modifies the
primordial scalar power spectrum. We show that
memory effects suppress the scalar spectrum relative to the standard white-noise result and derive a simple expression for this correction in terms of the background thermal dynamics. In particular, we relate the size of the non-Markovian effect directly to the thermal ratio between the bath temperature and the Hubble scale, thereby establishing a transparent link between warm-inflation background quantities and the validity of the Markovian approximation. We also derive the corresponding modification of the tensor-to-scalar ratio and the induced shifts in the scalar spectral index and the running of the scalar spectral index. Our results provide a simple and practical diagnostic for identifying when finite correlation-time effects become relevant in warm-inflation model building.
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Astrophysics of Galaxies
JWST's PEARLS: A clumpy ring galaxy at $z = 4.0148$
David Vizgan, Ming-Yang Zhuang, Ian Smail et al. (+22 more)
Ring
galaxies are an uncommon class of
galaxies whose morphology is closely related to dynamical processes that govern
galaxy evolution. Some ring
galaxies, known as "collisional ring
galaxies", are thought to form as a consequence of head-on collisions between
galaxies, and a number of high-
redshift collisional ring
galaxies have been discovered and/or studied in the era of the James Webb Space
Telescope (JWST). In this paper, we present HST/ACS, JWST/NIRCam, and JWST/NIRSpec observations of a candidate ring
galaxy at $z_{\rm spec} = 4.0148$, previously identified as a potential gravitational lens. The
galaxy exhibits a complex morphology, including three bright clumps along an apparent ring with radius $\approx 0.25$" $\simeq 1.8$ kpc. It has a total SFR $= 140^{+20}_{-30}$ ${\rm M}_{\rm \odot}$ yr$^{-1}$ and $\log(M_\ast/{\rm M}_\odot) = 10.41^{+0.11}_{-0.13}$, making it similar to other high-
redshift collisional ring
galaxies. Although we argue strongly in favor of the collisional ring explanation, we cannot entirely rule out a
galaxy-
galaxy strong
lensing explanation for the system's morphology, in which a foreground
galaxy at $z \simeq 1.7$ lenses a
galaxy at $z \simeq 4.0$ into an Einstein ring-like configuration; to confirm the nature of this source, we require kinematic information via high spectral resolution observations. We suggest that current and future gravitational lens surveys should consider high-
redshift ring
galaxies as possible but significant contaminants.
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All authors David Vizgan, Ming-Yang Zhuang, Ian Smail, Rogier Windhorst, Gibson Bowling, Cheng Cheng, Seth Cohen, Christopher Conselice, Jose Diego, Brenda Frye, Norman Grogin, Rolf Jansen, Patrick Kamieneski, Anton Koekemoer, Rafael Ortiz III, Massimo Ricotti, Bangzheng Sun, Hayley Williams, S.P. Willner, Haojing Yan, Aadya Agrawal, Manuel Solimano, Zachary Stone, Joaquin Vieira, Chentao Yang