Cosmology and Nongalactic Astrophysics
Primordial power spectrum reconstructions from BOSS + eBOSS
Guillermo Martínez-Somonte, Héctor Gil-Marín, Airam Marcos-Caballero et al. (+1 more)
We reconstruct the
primordial power spectrum $P_{\mathcal{R}}(k)$ from the BOSS DR 12 LRG and eBOSS DR 16 QSO catalogs with a non-parametric
Bayesian method. The $P_{\mathcal{R}}(k)$ is reconstructed by linearly interpolating $N$
knots in the $\{ \log k, \log P_{\mathcal{R}}(k) \}$ plane. We use a parametric model to describe the
galaxy power spectra of the BOSS+eBOSS catalogs, assuming any power-law deviations and BAO contributions separately from the matter power spectrum template, composed of seven parameters $\Theta_{\text{model}}$. This parametric model enables us to reconstruct $P_{\mathcal{R}}(k)$ at non-linear scales, reaching $k = 0.3 \text{ h} \text{ Mpc}^{-1}$. The method is validated by applying it to different
Primordial Features (PF) templates and by recovering the input power law of MD-Patchy and EZmock mock catalogs, representative of the BOSS and eBOSS data. These mocks provide additional information on $\Theta_{\text{model}}$, enabling us to impose Gaussian correlated priors on $\Theta_{\text{model}}$. This prior set allows us to reconstruct $P_{\mathcal{R}}(k)$ more precisely and to alleviate the degeneracies between the model and
knot parameters. The results for both individual and combined $z$-bins and
galactic caps of the BOSS and eBOSS catalogs are consistent, showing no evidence of the presence of PF in $P_{\mathcal{R}}(k)$ and pointing to a quasi-scale-invariant power law as the preferred model for $P_{\mathcal{R}}(k)$, as predicted by most slow-roll inflationary models. With a different prior set that relaxes the Gaussian constraints on $\Theta_{\text{model}}$ and imposes Planck-based priors on the extreme
knots, the results also favor the power law. From the
knot reconstructions, we robustly constrain the spectral index $n_s = 0.976 \pm 0.021$, compatible with the Planck value.
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All authors Guillermo Martínez-Somonte, Héctor Gil-Marín, Airam Marcos-Caballero, Enrique Martínez-González
Instrumentation and Methods for Astrophysics
Towards the Deployment of the First NectarCAM, a Medium-Sized-Telescope Camera for the Cherenkov Telescope Array Observatory
Pablo Correa and CTAO NectarCAM Collaboration
NectarCAM is a Cherenkov camera designed to detect gamma rays with energies between 80 GeV and 50 TeV. It will equip nine medium-sized
telescopes (MSTs) of the Cherenkov
Telescope Array Observatory. NectarCAM consists of 1855 pixels distributed over 265 modules. Each pixel consists of a photomultiplier tube that is connected to a NECTAr3 chip. This NECTAr3 chip contains a 12-bit digitizer with a GHz sampling rate, and has a typical readout deadtime of ${\sim}0.7$ $\mu$s. In these proceedings, we highlight the performance of the NectarCAM in terms of time resolution and charge resolution. We also present the latest calibration techniques that were recently implemented for the camera. Finally, we briefly present the current status and plans of the NectarCAM production; the first production-line NectarCAM will be ready for shipment by Summer 2026, and it is planned to equip one of the MST pathfinders of CTAO.
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High Energy Astrophysical Phenomena · Cross-listed from nucl-th
Astrophysics equation of state inference with Bayesian chiral effective field theory uncertainties
Melissa Mendes, Hannah Göttling, Anna Hensel et al. (+5 more)
We investigate
Bayesian chiral effective field theory ($\chi$EFT) uncertainties, which assign a statistical interpretation to equation of state (EOS) distributions near
nuclear saturation density, n$_0$, as well as constraints from perturbative
quantum chromodynamics (pQCD) to
Bayesian EOS inference from LIGO/Virgo, NICER and
pulsar mass observations. The tails of the $\chi$EFT uncertainties allow for broader pressure ranges in our priors, but large parts of these are excluded by the astrophysical observations, so that the EOS and the resulting mass-radius posteriors are still very consistent with our earlier work. Within our broad prior ranges, we observe a clear stiffening of the EOS at $n \gtrsim 3 n_0$. Moreover, the impact of the pQCD constraints on the posterior EOS and mass-radius range is negligible due to the astrophysics constraints. Exploiting the strong correlation between pure
neutron matter and matter in beta
equilibrium, we infer the
symmetry energy slope parameter $L$ from astrophysics. For the $68\%$ credible interval, we obtain $L=42.6-52$ MeV and $L=44.2-56.7$ MeV using piecewise-polytrope and speed-of-sound high-density extensions, respectively. The $L$ posterior is mainly driven by the combination of GW170817 LIGO/Virgo and PSR J0740+6620, PSR J0437-4715, and PSR J0614-3329 NICER observations.
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All authors Melissa Mendes, Hannah Göttling, Anna Hensel, Isak Svensson, Kai Hebeler, Achim Schwenk, Nathan Rutherford, Anna Watts
High Energy Astrophysical Phenomena
A 4200-hour HyperFlash and ÉCLAT campaign on the hyperactive FRB 20240114A: constraining energetics with the most brilliant bursts
O.S. Ould-Boukattine, A.J. Cooper, A.M. Cook et al. (+14 more)
Hyperactive repeaters provide a unique window into the evolving environments and energy budgets of fast
radio burst (FRB) sources, though they may not be representative of the FRB population in general. High-cadence observations are key to capturing the rarest and most energetic bursts, which occur only once per hundreds to thousands of hours. Here we present an unprecedented $4{,}200$-hour observing campaign targeting FRB 20240114A as part of the HyperFlash and ÉCLAT FRB monitoring programs. Over $806$ days, we detected $178$ high-energy ($\sim$$10^{40-42}$ erg) bursts with HyperFlash, which together amount to $4.4 \times 10^{42}$ erg of released
radio energy (assuming isotropic emission and 1-GHz emission bandwidth). The cumulative energy of the HyperFlash bursts is about twice that of $\sim$$11{,}000$ lower-energy bursts detected with FAST, emphasising the significant role that the highest-energy bursts play in depleting the central engine's stored energy. In fact, the single most brilliant burst from our sample, which we term the STROOP, contributes roughly $1/3$ of all the energy we measure, and is at the maximum energy seen in studies of both repeating and apparently one-off FRBs alike. We also find a break in the burst energy distribution at $\sim$$2\times10^{40}$ erg and a linear dispersion measure (DM) increase of $+0.96 \pm 0.06$ pc cm$^{-3}$ over a period of $318$ days. We discuss these findings in the context of a magnetar source model and highlight comparisons with the energetics of intermediate and
giant X-ray/$\gamma$-ray
flares from
Galactic sources.
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All authors O.S. Ould-Boukattine, A.J. Cooper, A.M. Cook, J.W.T. Hessels, D.M. Hewitt, J. Huang, I. Cognard, T.J. Dijkema, M.P. Gawroński, W. Herrmann, F. Kirsten, A. Moroianu, Z. Pleunis, W. Puchalska, S. Ranguin, M.P. Snelders, T. Telkamp
Instrumentation and Methods for Astrophysics
Datacubes of H$\alpha$, CaII K, CaII H and H$\epsilon$ line profiles of the full solar disk recorded daily at Meudon observatory since 2017 and some typical profiles of solar features
Jean-Marie Malherbe
Systematic observations of the Sun are performed at Meudon observatory since 1908 under the form of monochromatic images. However, a major technical improvement occurred in 2017; since this date, spectroscopic datacubes are obtained daily with the spectroheliograph and a fast CCD camera. Line profiles of H$\alpha$ (6562.8 A), CaII K (3933.7 A), CaII H (3968.5 A) and H$\epsilon$ (3970.1 A in the wing of CaII H) are recorded over the full
solar disk under the form of 3D FITS files (x, y, $\lambda$). The optical spectral resolution is 0.15 A for Calcium and H$\epsilon$ (0.093 A/pixel), and 0.25 A for H$\alpha$ (0.155 A/pixel); the spatial sampling is about 1 arc sec (the usual seeing is 2 arcsec). Datacubes are freely available since July 2017 in raw TIF (level 0) or processed FITS (level 1) format. Access to observations and typical line profiles associated to
solar features are presented.
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Cosmology and Nongalactic Astrophysics
(An)Isotropy in Pantheon+ and Type Ia supernova samples: intrinsic limits of directional tests
Antonio Quintana-Estellés and Pilar Ruiz-Lapuente
The use of methods that investigate the value of the Hubble constant H$_0$ in different patches (60 $ or 90$^ size) across the sky to probe the statistical isotropy of the Universe using large SNe Ia databases has led to contradictory claims of either anisotropy or isotropy. The anisotropy directions vary amongst research works. The objective of this paper is to clarify the abovementioned claims and study the lack of basis for depicting directions of anisotropy with the present SNe Ia samples. We explain the type of limitation embedded in the SN Ia lightcurve method to determine the isotropy of H_0 and the corresponding consequences. The widely used analysis through the Region Fitting and the Hemisphere Comparison methods is done here using the Pantheon+ database, simulating 2000 distinct directions in the sky within a
Bayesian Markov Chain Monte Carlo approach. We also study a smaller SNe Ia database, the Carnegie
Supernova Project sample, leading to a similar kind of result as that from the Pantheon+ sample. We investigate the validity of the directions found for anisotropy within these analyses. We have found that within the tests used here, the Region Fitting method and the Hemisphere Comparison method, one can not determine with robustness the direction of an anisotropy of H$_0$ using the present SNe Ia large data samples. This is intrinsic to the way H$_0$ is obtained with the SN Ia lightcurve method. Achieving robust constraints will require a quite uniform sky coverage from larger SNe Ia samples with improved systematics.
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Astrophysics of Galaxies
Velocity-resolved [O I] 63,145 um, [C II] 158 um, and OH mapping along the Orion BN/KL explosive outflow and irradiated shocks
J. R. Goicoechea, R. Güsten, B. Godard et al. (+6 more)
Stellar mergers produce explosive
outflows that serve as transient sources of IR line luminosity and inject mechanical energy early into the natal molecular cloud. We present the first velocity-resolved maps of the [O I] 63 and 145 um fine-structure line emission from the wide-angle
outflow in Orion BN/KL, the nearest explosive
outflow. The data were obtained with SOFIA and include sensitive [C II] 158 um and OH maps. They allowed us to disentangle the quiescent cloud gas from the
outflow, traced by a broader [O I] component with a line FWHM of about 20-30 km/s and exhibiting a spatial distribution similar to that of the shock-excited H2 emission seen with JWST. The OH 119 um line shows a prominent P-Cygni profile covering 160 km/s, similar to the very broad CO lines. The total [O I] 63 and 145 line luminosity is remarkably high, 86.5 L_sun, comparable to the H2 and CO line luminosities, implying an
outflow mass-loss rate of (9.1+/-2.6)x10^-3 M_sun/yr and a mass of 3.3-5.9 M_sun. The [O I] 63 / 145 and [O I] 63 / [C II] 158 intensity ratios reach very high values in the line wings (20-30 and 40-60, respectively), exceeding those found in PDRs. These ratios are consistent with the presence of dense (10^5 to 10^6 cm^-3 ) and warm (~500 K) postshock gas. We analyzed the fine-structure line-wing intensities using magnetized shock models that include UV irradiation, to which the [C II] 158 line intensity is particularly sensitive. We find that the [O I] and [C II] intensities are consistent with emission from dissociative J-type shocks with velocities of 30-40 km/s and preshock gas densities of a few 10^4 cm^-3, illuminated by external UV radiation generated by surrounding fast shocks and possibly by massive (proto)
stars in the region. We also report a broad [O I] emission feature around the BN
star, which we attribute to an unresolved
outflow or wind bow shock.
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All authors J. R. Goicoechea, R. Güsten, B. Godard, H. Wiesemeyer, R. Higgins, A. Gusdorf, M. G. Santa-Maria, M. Zannese, K. M. Menten
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Probing (sub-)solar-mass black holes and superspinars with current and next-generation gravitational-wave observatories
K. S. Sruthy, N. V. Krishnendu, Chandrachur Chakraborty et al. (+1 more)
Gravitational-wave observations provide a powerful probe of compact objects and strong-field
gravity. In this work, we investigate the detectability of binaries containing (sub-)
solar-mass black holes and superspinars with current and next-generation gravitational-wave observatories. Such objects may arise from
primordial formation channels or from more exotic high-energy scenarios, and their detection would provide important insights into the population of low-mass compact objects and the physics of extreme gravitational fields. We model the gravitational-wave signals using the frequency-domain post-Newtonian inspiral waveform model TaylorF2, and truncate the signal at the innermost stable circular orbit (ISCO) to avoid contamination from the post-inspiral regime. We assess the observability of these systems using the sensitivities of current detectors such as Advanced LIGO and upcoming third-generation observatories including the Einstein
Telescope and
Cosmic Explorer. Our results show that while current detectors have limited reach for very low-mass binaries, third-generation observatories can enhance both detection capability and parameter-estimation precision. Their improved strain sensitivity and extended low-frequency coverage allow these observatories to track the inspiral phase over a substantially larger number of gravitational-wave cycles. As a result, they achieve considerably higher signal-to-noise ratios and provide dramatically improved constraints on binary parameters. In particular, it is possible to measure the primary
spin parameter with precision $\Delta \chi_{1z}~\sim~10^{-4}-10^{-3}$, potentially allowing clear observational discrimination between near-extremal black holes and superspinars in the mass range $0.1~M_\odot-2~M_\odot$ and with signal-to-noise ratio of $\sim 100-350$.
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All authors K. S. Sruthy, N. V. Krishnendu, Chandrachur Chakraborty, Nami Uchikata
High Energy Astrophysical Phenomena
Iron line diagnostics of the stellar wind in X1908+075
J. Planelles-Villalva, J.M. Torrejón, J.J. Rodes-Roca et al. (+1 more)
Aims. X1908+075 is a supergiant X-ray binary (SgXB) composed of an evolved OB
star and a
neutron star (NS) in a 4.4-day orbit. We aim to characterize the
stellar wind, constrain the system geometry, and investigate the origin and variability of the Fe K$\alpha$ fluorescence line. Methods. We analyzed three Chandra HETGS observations at different orbital phases. The continuum was modeled with a bulk motion Comptonization (Bmc) model including partial-covering absorption. We performed a blind line search with Monte Carlo simulations accounting for the look-elsewhere effect. The orbital modulation of $N_{\rm H}$ was modeled through particle swarm optimization to constrain the inclination and donor mass-loss rate. Results. Fe K$\alpha$ emission is detected in all observations and remains significant after correcting for multiple trials (global $p < 0.005$). Tentative features include a Compton shoulder and Fe K$\beta$ emission, indicating dense reprocessing material. No highly ionized Fe XXV or Fe XXVI lines are confirmed. The Fe K$\alpha$ flux correlates with the continuum flux, while its equivalent width anticorrelates with both $N_{\rm H}$ and orbital phase, opposite to the canonical curve of growth seen in many HMXBs. Line broadening reaches velocities of $\sim3000$ km s$^{-1}$. Modeling of the $N_{\rm H}$ variability yields an inclination $i = 46 \pm 3^\circ$ and a donor mass-loss rate $\dot{M}_{\rm W} = (9.1 \pm 1.6) \times 10^{-7}\,M_\odot\,{\rm yr}^{-1}$. Conclusions. X1908+075 is a classical wind-fed SgXB in the direct
accretion regime. The observed continuum and line variability reflect the complex density and ionization structure of the
stellar wind.
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All authors J. Planelles-Villalva, J.M. Torrejón, J.J. Rodes-Roca, G. Sanjurjo-Ferrín
Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Cosmological perturbations of TDiff fields
Antonio L. Maroto, Prado Martín-Moruno and Diego Tessainer
We study scalar field theories that break diffeomorphism invariance down to the subgroup of transverse diffeomorphisms through the matter sector in cosmological backgrounds. We focus on single- and multi-field models and develop the corresponding cosmological perturbation theory. We analyze the different contributions to the pressure perturbation, discussing the adiabaticity and the effects in the perturbation coefficients of the interactions that arise in the multi-field case as a consequence of the
symmetry breaking. We also consider the stability of the perturbations in terms of the effective speed of sound and present particular models that could be of phenomenological interest.
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Solar and Stellar Astrophysics
7DT Insight: Variability in Young Stellar Objects
Mi-Ryang Kim, Jeong-Eun Lee, Myungshin Im et al. (+11 more)
Photometric variability in young
stellar objects (YSOs) provides critical insight into the mechanisms of mass
accretion, disk
evolution, and circumstellar extinction in early
stellar evolution. We present an analysis of day-timescale optical variability in the Orion A central region using two-night 7-Dimensional
Telescope (7DT) medium-band photometry obtained on March 23 and 24, 2024. The 7DT observations provide optical spectral sampling with 16 medium-band filters spanning 400–825 nm, enabling direct two-epoch comparisons. To remove satellite-trail contamination, we used an SSIM-based ResNet classifier (accuracy 0.97; F1 = 0.93) to exclude affected exposures. Subsequent photometry and two-epoch variability measurements yielded a working sample of 769 YSO candidates, among which we identified 110 variables ($\sim$14%), including seven extreme cases with $|\Delta m_\lambda|>0.5$ mag. To describe the wavelength dependence of the variability, we compared five simple templates: extinction-like changes ($R_V =$ 3.1 and 5.5), a gray (wavelength-independent) change, and two spot-like toy models (hot and cold) implemented as two-temperature surface mixtures. The best-fit results are dominated by spot-like templates (37 cold-spot and 22 hot-spot objects), with 37 sources best matched by extinction-like templates and 14 by the gray template. The m650 excess fraction is higher in the hot-spot and gray templates than in the others. This could be compatible with more frequent line/veiling-related contributions in those groups, although the m650 excess is not a direct
accretion diagnostic.
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All authors Mi-Ryang Kim, Jeong-Eun Lee, Myungshin Im, Jinho Lee, Ji Hoon Kim, Seo-Won Chang, Gregory S. H. Paek, Hyeonho Choi, Donggeun Tak, Donghwan Hyun, Won-Hyeong Lee, Hyeyoon Lee, ShinGeon Kim, S. Thomas Megeath
Earth and Planetary Astrophysics
A fast tree algorithm for multi-component coagulation equation
Taichi K. Watanabe and Akimasa Kataoka
Dust properties, such as mass and porosity, impact
planet formation directly. Understanding the time
evolution of
dust distribution across multiple properties requires numerical computation. However, available ways to calculate the multi-component coagulation-fragmentation are highly time-consuming. This study aims to develop a fast and accurate
algorithm for multi-component coagulation. We assumed that two pairs of colliding aggregates reproduce a similar outcome if the
dust properties are similar, and that the ratio of
dust properties in logarithmic space gives the similarity as a "distance". These assumptions enable us to apply the tree
algorithm, which groups distant bins and calculates interactions together, to coagulation. The
algorithm reduces the computational complexity from $O (N^{2d})$ to $O (d N^d \log N)$, considering $N$ bins per $d$ components. We tested the
algorithm by comparing it with the conventional direct method for cases where analytic solutions are known. We measured the dependencies of the wall-clock time, $L_2$ error in the distribution, and relative error of the total mass, on the $d, N$, opening angle $\theta_c$, and maximum
dust distribution width after coagulation $k_c$. The
algorithms are found to calculate coagulation consistently. For $d=1$, the tree method is faster than the direct method for a specific range of parameters. For $d=2$, however, the tree method is faster for all parameter regions surveyed, speeding it up by tens of times. Increasing $N$ and decreasing $\theta_c$ or $k_c$ made it slower and more accurate. Additionally, using a small $k_c$ performs worse than when using a large $k_c$, suggesting that limiting $k_c$ is unnecessary. We present a fast tree
algorithm for the multi-component coagulation equation. It will enable us to evolve the multi-component
dust distribution, such as in mass-porosity space, in protoplanetary disks.
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High Energy Astrophysical Phenomena
A Three-Dimensional Exploration of Magnetic Fields, Rotation, and Shock Revival in a $39 M_\odot$ Core-Collapse Supernova Progenitor
Liubov Kovalenko, Evan O'Connor, Haakon Andresen et al. (+1 more)
We present three-dimensional hydrodynamic and magnetohydrodynamic core-collapse
supernova simulations of a rapidly rotating, high-compactness $39 M_\odot$ progenitor to investigate the roles of rotation and
magnetic fields in shock revival and
outflow morphology. This study is designed to separate
neutrino-driven expansion, rotation-induced deformation, and magnetically aided polar
outflow within the same progenitor. We evolve three models: a non-rotating hydrodynamic baseline, a rotating hydrodynamic model, and a rotating magnetized model. All three models reach runaway shock expansion within the simulated interval, but with markedly different morphologies and timescales. The magnetized model revives first and develops the clearest bipolar
outflow. The rotating non-magnetized model undergoes the latest shock revival and remains comparatively compact at the end of the simulation. The non-rotating model also undergoes shock revival, but subsequently collapses to a black hole about one second after core bounce. In the magnetized model, Maxwell stresses redistribute angular momentum and extract energy from the differential rotation of the protoneutron
star, reducing the inner-core
spin and helping channel rotational free energy into the emerging polar
outflow.
Neutrino emission provides an additional, though smaller, angular-momentum sink in both rotating models. We find that rapid rotation and strong
magnetic fields can launch an early magnetically aided polar
outflow in 3D, while the resulting dynamics remain intrinsically non-axisymmetric. In this extreme progenitor, rotation also provides significant support against
prompt black-hole formation, although the longer-term remnant stability remains uncertain beyond the simulated interval.
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All authors Liubov Kovalenko, Evan O'Connor, Haakon Andresen, Sean M. Couch
Cosmology and Nongalactic Astrophysics
Probing Reionization up to the Mark: The Marked Power Spectrum to unveil the HI 21-cm signal from the EoR
Vishrut Pandya, Leon Noble, Suman Majumdar et al. (+3 more)
The redshifted 21-cm signal from neutral hydrogen (HI) in the intergalactic medium (IGM) is a powerful probe of the Epoch of Reionization (EoR). Owing to the complex growth and morphology of ionized regions, the 21-cm brightness-temperature field becomes strongly non-Gaussian during the EoR, limiting the information captured by the standard power spectrum alone. While higher-order statistics such as the bispectrum can recover part of this information, they are computationally expensive and often less straightforward to interpret. In this work, we investigate marked statistics as an alternative framework for characterizing the EoR 21-cm signal. Using semi-numerical 21-cm simulations, we introduce a set of EoR-tailored mark functions, inspired by and extending existing marked-statistics ideas, and study both the power spectrum of the mark and that of the marked field. We show that suitably chosen marks can selectively enhance contributions from different IGM environments, capture additional non-Gaussian information beyond the standard power spectrum, and improve the statistical constraining power on EoR model parameters in a Fisher-
matrix analysis. These results demonstrate that
Fourier-space marked statistics, including both the power spectrum of the mark and that of the marked field, provide a computationally simple and flexible extension of standard two-point statistics for extracting astrophysical information from the EoR 21-cm signal.
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All authors Vishrut Pandya, Leon Noble, Suman Majumdar, Debanjan Sarkar, Mohd Kamran, Abhirup Datta
Solar and Stellar Astrophysics
Discovery of variable polarization in H$\alpha$ profile of symbiotic star Y Gem: A case for orbital-phase dependent variation of Raman-scattered Ly$\beta$ emission
Arijit Maiti, Mudit K. Srivastava and Vipin Kumar
The geometry and morphology of symbiotic
stars are conducive to exhibit a variety of scattering phenomena. The prominent among them is the Raman scattering of O VI doublet $\lambda \lambda$ 1032,1038 angstrom, which often show strongly polarized features in the visible spectrum. Similar Raman scattering of Ly$\beta$
photons has also been predicted to occur in symbiotic
stars, though with fewer detections and with weak polarization amplitudes. Here, we present the discovery of strong variable polarization in the H$\alpha$ profile of a recently established symbiotic system Y Gem, over a period of nearly 22 months. This is, most likely, a very rare detection of the strongly polarized Raman scattered Ly$\beta$
photons, falling at the H$\alpha$ emission. Monte-Carlo simulations have been conducted to confirm the underlying Raman scattering process causing the polarized line profile, and a simple orbital model is constructed with typical parameters available in the recent literature along with a complementary low-resolution spectroscopic data. These simulations and models are then used to validate the observed polarization variation of H$\alpha$ at different orbital phases corresponding to the epochs of observations. The possibility of such strong variable H$\alpha$ polarization, being caused by Raman scattering of Ly$\beta$, would thus open up avenues of exploring such effects in various other astrophysical situations having similar morphology.
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Cosmology and Nongalactic Astrophysics
Primary gravitational waves at high frequencies II: Emergence of the exponential cut-off in the power spectrum
Alipriyo Hoory, Jerome Martin, Arnab Paul et al. (+1 more)
[Abridged] In slow roll inflation, the power spectrum (PS) of primary gravitational waves (PGWs) generated from the
quantum vacuum rises as $k^2$ over wave numbers $k$ which never leave the Hubble radius. In fact, over such small scales, the PS exhibits a similar behavior at any time after inflation. In a recent work, we had argued that the PS of PGWs has to be regularized to truncate the unphysical quadratic rise at large wave numbers. Assuming instantaneous transitions from inflation to the epochs of radiation and matter domination, we had shown that the regularized PS oscillates with a constant amplitude about a vanishing mean over small scales during these epochs. We had also smoothed the transition (actually, the `effective potential' governing the equation of motion of GWs) from inflation to radiation domination using a linear function and evaluated the regularized PS of PGWs post inflation. In such a case, we had shown that, over small scales, while the regularized PS continues to oscillate about zero, its amplitude decreases as $k^{-1}$. In this work, using the Born approximation, we examine the behavior of the regularized PS of PGWs over small scales when they are evolved through smoother and smoother transitions from inflation to the epochs of radiation and matter domination. We illustrate that, at small scales or high frequencies, the suppression in the regularized PS of PGWs occurs more and more sharply as the transition is smoothed further and further. With the help of examples, we also show that, in the case of transitions described by infinitely differentiable `effective potentials', the regularized PS of PGWs exhibits an exponential suppression on small scales. We argue that the observation of the exponential drop in the PS of PGWs can help us determine the energy scale and the time of the end of inflation. We clarify related issues and discuss the wider implications.
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All authors Alipriyo Hoory, Jerome Martin, Arnab Paul, L. Sriramkumar
Astrophysics of Galaxies
Constraining the magnetic field strength of a flaring radio core in the compact steep spectrum source 3C 138
Shan Li, Sang-Sung Lee, Whee Yeon Cheong et al. (+3 more)
Compact steep spectrum (CSS) sources generally show weak Doppler boosting, yet some exceptions show multi-year-scale
radio flux variability and high-energy activity. Since 2022, the CSS
quasar 3C 138 has been in a
radio high state accompanied by multiple gamma-ray outbursts, offering unique opportunities to study changes in jet physical conditions. We estimated the synchrotron self-absorption (SSA)
magnetic field ($B_{\rm SSA}$) in the SSA core of 3C 138 during its high state and compared it with the equipartition
magnetic field ($B_{\rm eq}$) to assess the core field environment. Using extended Korean Very long-baseline interferometry Network (KVN) data at 22, 43, 86, and 129 GHz (2024-2025), we calibrated the visibilities and modeled resolved components with circular Gaussians. A single-zone SSA model fitted to the core spectrum provided the turnover frequency and peak flux density, from which we estimated the $B_{\rm SSA}$ and $B_{\rm eq}$. We used Very Large Array and Atacama Large Millimeter/submillimeter Array data to constrain the broadband spectra with the same model. The KVN SSA core shows a turnover at about 33 GHz and a peak flux of about 1.45 Jy. The inferred $B_{\rm SSA}$ is far below equipartition, with $B_{\rm SSA}/B_{\rm eq}\approx0.05$. The flux variability of 3C 138 is driven by a compact, particle-dominated core. Shock-driven particle injection in the inner jet could account for the core brightening and the production of X-ray/gamma-ray emissions through an inverse-Compton process without requiring extreme relativistic beaming effects.
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All authors Shan Li, Sang-Sung Lee, Whee Yeon Cheong, Tao An, Seiji Kameno, Ruediger Kneissl
Solar and Stellar Astrophysics
Sungrazer comets as analogs of star-planet magnetic interactions
L-S. Guité, A. Strugarek, A. J. Finley et al. (+5 more)
Star-
planet magnetic interactions (SPMIs) can transfer energy from an
exoplanet to its host
star via Alfvén waves when the
planet orbits within a sub-Alfvénic
stellar wind. Similar conditions were encountered by the sungrazing comet Lovejoy as it passed through the
solar corona in December 2011. The possibility that comets could trigger
solar activity via
magnetic interactions has never been investigated. The aim of this paper is to quantify the energetics of such interaction and to assess if enough energy could be deposited to either form a hotspot or trigger eruptions on the Sun. We used the magnetohydrodynamic WindPredict-AW model to reconstruct the coronal
magnetic field and
solar wind conditions along the comet's orbit, and determined the
magnetic footpoints that connected the comet to the
solar surface. By estimating the travel time of hypothetical Alfvén waves, we identified a brightening event observed by EUVI aboard STEREO-A that is spatially and temporally consistent with comet Lovejoy's passage. We then computed the SPMI power for all
magnetic field lines anchored within 5 degrees of the brightening using SPMI power scaling laws. The resulting power distribution spans approximately 10$^{14}$-10$^{16}$ W. In comparison, the estimated radiative power of the brightening event in the STEREO-A/EUVI 195$\r{A}$ channel is approximately 10$^{17}$ W. We find that comet Lovejoy does not generate sufficient SPMI power to be the energy source of the observed brightening intensity. However, it may still act as a perturbation to existing
magnetic structures, and lead to the triggering of
solar flares. Confirming this hypothesis would require additional observations that do not exist for this event, which make future passages of sungrazing comets valuable opportunities to study SPMI processes within our
solar system.
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All authors L-S. Guité, A. Strugarek, A. J. Finley, S. Parenti, V. Réville, A. Paul, A. S. Brun, J. de Carpentier
Astrophysics of Galaxies
Polarization Observations of a Sample of 6.7 GHz Methanol Masers
Paul Fallon and Derck P. Smits
Spectra of 6.7 GHz methanol masers from 21 pointings of known
star-forming regions are reported. The C-band observations, using the Green Bank
Telescope in full Stokes mode, have measured how polarization properties vary across the maser profiles in each spectrum and vary between different epochs of observation. Two-thirds of the sources are observed to have 6.7 GHz methanol masers, including one new detection (G240.316+0.071). Linear polarization is in the range 0 to 15% and circular polarization 0 to $\pm$9%, in line with previously reported values. The only instances where polarization is not observed is when these polarization ranges are below 3$\sigma$ detection limits. Zeeman splitting is observed in several sources, with splitting values derived from velocity separation between RCP and LCP components via Gaussian fitting. These values are seen to change with time and appear to correspond to changes in the linear and circular polarization. The polarization properties varying across the spectra and changing with time are most likely due to variations in the
magnetic fields.
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High Energy Astrophysical Phenomena
Two kinds of Galactic source populations could explain the cosmic-ray observation up to the "knee" region
Furong Li, Wei Liu, Yali Shao et al. (+1 more)
Observations of diffuse gamma rays above hundreds of TeV from the
Galactic disk provide strong evidence for the existence of PeV
cosmic-ray accelerators–so-called PeVatrons–in the
Galaxy. However, mounting observations have ruled out most
supernova remnants as likely PeVatron candidates, suggesting instead that multiple populations of
cosmic-ray sources exist in the
Galaxy. Recently, the LHAASO collaboration reported the detection of ultra-high-energy gamma rays from microquasars, establishing that the black holes in these systems, which accrete matter from companion
stars, are powerful PeV particle accelerators. In this work, we propose a two-component source model to explain the observed
cosmic-ray spectra and composition up to the PeV range. Below approximately 100 TeV,
supernova remnants serve as the dominant sources; above this energy, microquasars are considered the primary candidate population. Within this scenario, the assumption of a charge-dependent cutoff well accounts for the latest measurements, including the
proton and helium spectra up to the PeV range, the energy-dependent composition, and the all-particle spectrum. In contrast, the
nuclei-dependent cutoff hypothesis is ruled out by the data.
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All authors Furong Li, Wei Liu, Yali Shao, Yi-Qing Guo
Earth and Planetary Astrophysics
FastChem 4: New chemical elements and improved convergence behaviour
Daniel Kitzmann, Joachim W. Stock and A. Beate C. Patzer
Chemical
equilibrium calculations are a key ingredient for modelling and interpreting spectroscopic observations of (exo)
planets, brown
dwarfs, cool
stars, and protoplanetary disks. As these applications increasingly probe non-
solar elemental abundances and previously underrepresented elements,
equilibrium chemistry solvers must be both numerically robust and capable of handling complex chemical systems. Here we present FastChem 4, a major update to the open-source FastChem
equilibrium chemistry code. We extend the gas-phase solver with a multidimensional Newton-method that mitigates the slow convergence previously encountered for strongly non-
solar elemental abundances. We further reformulate the gas-phase equations in logarithmic element densities, removing the dependence on quad-precision arithmetic and allowing FastChem to be applied at low temperatures on any platform supporting double precision. The condensate solver is upgraded with adaptive Levenberg-Marquardt regularisation, a perturbed-Hessian fallback, and a combined gas-condensate Newton solver. These changes lead to a strong increase in computational performance and stability. The thermochemical data is expanded using thermochemical data from the NIST-JANAF tables and the Barin compilation, and now comprises 800 gas-phase molecules and ions and 511 condensates spanning 44 elements. We apply the updated code to a wide pressure-temperature grid for both
solar and carbon-rich (C/O = 2) elemental compositions. The resulting grids reproduce the classical
solar-composition condensation sequence and reveal the marked shifts that occur under carbon-rich conditions. We also find that silicon monoxide is stable as a condensate over a limited pressure-temperature range, consistent with recent JWST observations of brown
dwarfs. FastChem 4 is released under the GPLv3 licence, together with a pre-compiled Python package.
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High Energy Astrophysical Phenomena
Sub-stellar Strange Quark Matter Objects: Predicting a New Class of Highly-Compact Candidates
Jonathan Joás Zapata Campos and Rodrigo Negreiros
We investigate the existence and stability of highly-compact sub-
stellar objects composed of strange
quark matter (SQM), focusing on finite-size strangelets with
baryon number $A \leq 100$. Motivated by the emergence of mass–radius outliers in the
Gaia DR3 era, we employ a
Bayesian exploration of the MIT bag-model parameter space, explicitly accounting for finite-size surface and
curvature contributions that become relevant at low
baryon number. Enforcing the bulk absolute-stability requirement for SQM ($E/A < 930~\mathrm{MeV}$), we find that self-gravitating
equilibrium sequences are confined to the sub-
stellar regime, with typical masses $M \simeq 10^{-2}$–$10^{-1}\,M_{\odot}$ and characteristic radii of order $10^{3}$–$10^{4}$ km. We further show that rapid rotation, treated through a self-consistent framework that incorporates relativistic thermodynamics, can substantially inflate the equatorial radius and extend the accessible mass–radius domain. While rotation does not eliminate the intrinsic high-density compactness of these configurations, it shifts the most extended models closer to the observational parameter space of massive
exoplanets. A comparison with objects from the NASA
Exoplanet Archive reveals a pronounced density gap separating standard atomic-matter
planets and brown
dwarfs from the strangelet-rich branch predicted here. We conclude that light strangelets cannot account for
solar-mass white
dwarfs, but they robustly predict a previously unexplored population of ultra-compact sub-
stellar objects, offering testable targets for future microlensing searches and high-cadence photometric surveys.
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Cosmology and Nongalactic Astrophysics
Phase-Space Analysis of Quadratic Dark Energy: Stable Attractors and Their Observational Signatures
Sahar Mohammadi, Ebrahim Yusofi and Kosar Asadi
We present a comprehensive phase-space analysis of a quadratic dark energy model where the pressure includes a nonlinear term proportional to the square of the energy density. This minimal extension beyond the $\Lambda$CDM framework introduces a dynamical parameter $\eta(z)$ that governs transitions between different cosmological regimes. Through dynamical systems theory, we identify critical points and their stability properties, revealing that negative $\eta$ values drive the system toward stable attractors (sinks) when $w_{\text{eff}} < 0$, which includes both quintessence and phantom regimes. Positive $\eta$ values correspond to unstable repellers (sources) when $w_{\text{eff}} > 0$. The model exhibits a distinctive asymptotic approach to the phantom divide ($w_{\rm eff}=-1$) from both quintessence and phantom sides without actual crossing, providing a non-crossing alternative to the phantom-crossing behavior preferred by recent DESI DR2 constraints. Our analysis shows that stable attractors produce enhanced Hubble expansion rates and more pronounced late-time acceleration, features that can be compared with recent DESI observations suggesting evolving dark energy.
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Solar and Stellar Astrophysics
Wolf-Rayet stellar evolution models with improved treatment of the atmosphere
Thomas Voje, Ana Palacios and Fabrice Martins
Evolutionary models of massive
stars are quasi-exclusively computed using an Eddington gray atmosphere. This approximation does not accurately describe the complex physical phenomena occurring in the atmosphere of massive
stars. We aim to include state-of-the-art atmosphere models in the
evolution computations of massive
stars and test how the Wolf-Rayet phase is impacted. We computed the
evolution of
Galactic massive
stars with the code STAREVOL. During the advanced phases of
evolution, we applied outer boundary conditions interpolated within a grid of CMFGEN model atmospheres at each time step. The effective temperature and effective
gravity were extracted from the atmosphere models. We then compared the resulting evolutionary tracks with classical calculations assuming Eddington gray atmospheres. We find that including detailed model atmospheres has a significant impact on the effective radius and temperature of the models during the later stages of the
evolution. The effective temperatures of the
evolution models computed with detailed model atmospheres are greatly reduced and in better agreement with observations of Wolf-Rayet
stars. On the other hand, the internal structure of the models is barely affected by the choice of the atmosphere. We show that applying post-processing corrections on effective temperature and
gravity is a method equivalent to our direct inclusion of atmosphere models in evolutionary calculations. The inclusion of detailed atmosphere models in the computation of evolutionary models is necessary to correctly reproduce the position of evolved massive
stars in the Hertzsprung-Russell diagram. However, this has no impact on the internal and chemical
evolution.
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Instrumentation and Methods for Astrophysics
Photometric classification of quasars from DES and photo-$z$ estimation with Machine Learning
Pablo Motta, Filipe B. Abdalla, Elcio Abdalla et al. (+2 more)
This paper presents a comprehensive study of
quasar photometric classification and
redshift estimation using machine learning techniques. We cross-matched photometric data from the Dark Energy Survey Data Release 2 (DES DR2) with spectroscopic classifications from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16), yielding an initial sample of 168,738 point-like objects. Using a K-Nearest Neighbors (KNN)
algorithm with PSF magnitudes in the $g$, $r$, $i$, and $z$ bands, we achieved high-precision
quasar/
galaxy classification against
stellar contaminants, reaching a recall of 0.77 at 0.99 precision. Photometric
redshifts were subsequently estimated using a hybrid machine learning approach combining a Boosted Decision Tree from ANNz and a Decision Tree Regressor from scikit-learn. The resulting catalog spans
redshifts from $z \approx 0.5$ to $z > 3$, with a distinct population recovered at $z \approx 4$. A stacked outlier classifier was developed to mitigate catastrophic
redshift errors. The full photometric
redshift sample contains 872,372 objects and remains reliable for cosmological applications at $z \approx 4$. The cleaned catalog contains 675,683 objects and is suitable for large-scale structure studies in the range $0 < z < 3$. This robustly characterized
quasar catalog provides a valuable resource for future cosmological investigations.
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All authors Pablo Motta, Filipe B. Abdalla, Elcio Abdalla, Gabriel S. Costa, Camila Cardoso
Cosmology and Nongalactic Astrophysics
Gravitational lensing time delay beyond the Shapiro/geometry split
Luca Teodori, Kfir Blum and Zhaoyu Bai
Time delays are a key observable in strong gravitational
lensing systems. Their theoretical expression is usually written as a sum of a geometrical delay and a Shapiro delay, with cosmology entering through angular diameter distances and a
redshift prefactor. In this work we derive this structure from the exact null geodesics of the Schwarzschild-de Sitter metric. The standard formula is recovered as the leading term in a small-angle expansion, and we identify the first correction to the usual geometrical-plus-Shapiro split. Such correction does not introduce any new cosmological dependence: it corresponds instead to a higher-order correction intrinsic to the Schwarzschild part of the metric. As a consequence, up to this order, the cosmological constant enters only through the unlensed angular diameter distances and the unlensed lens-
redshift prefactor.
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Astrophysics of Galaxies
SDSS-V: Revealing a weak accretion state in X-ray selected red quasars
Paloma Guetzoyan, James Aird, Amy L. Rankine et al. (+17 more)
Red
quasars (rQSOs) have been recognized as a short-lived, early stage in the evolutionary cycle of Active
Galactic Nuclei (AGN), with fundamental differences in their intrinsic properties compared to blue
quasars (bQSOs). In this work, we present the first large X-ray sample of 380 rQSOs, selected from the eROSITA/SDSS-V collaboration, providing uniform X-ray detection with optical spectroscopy accros half the sky, in the German hemisphere of eROSITA. We combine X-ray imaging, optical spectroscopy, and multi-wavelength photometry to fully probe the
accretion, absorption and host properties of rQSOs. Independent Component Analysis is used to reconstruct optical spectra in a data-driven and non-parametric approach, while accounting for
dust reddening and host contamination. rQSOs are intrinsically X-ray weak compared to bQSOs, with a higher fraction found at low X-ray luminosities (over 50$\%$ of the rQSO sample have $L_X < 10^{43.5} \, \rm erg \, s^{-1}$). We investigate the relative X-ray strength of rQSOs by measuring the spectral slope indicator $\alpha_{OX}$. Despite their suppressed X-ray emission, rQSOs are not optically faint, but show low $\alpha_{OX}$ values, indicating weak X-ray emission relative to their bright optical continua. X-ray spectral measurements reveal large gas column densities relative to optical reddening due to
dust, implying that X-ray absorption could arise from
dust-free gas close to the supermassive Black Hole (BH) rather than a classical dusty
torus, while the
dust responsible for optical reddening likely resides on larger host-
galaxy scales or is associated with dusty gas carried in disc winds. rQSOs trace a phase of suppressed BH assembly relative to
stellar mass growth, suggesting that they represent a distinct evolutionary stage where BH
accretion is suppressed while the host
galaxy continues to grow.
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All authors Paloma Guetzoyan, James Aird, Amy L. Rankine, Stephanie M. LaMassa, Peter Breiding, Mara Salvato, Johannes Buchner, Zsofi Igo, Roberto J. Assef, Hector Ibarra-Medel, Catarina Aydar, Castalia Alenka Negrete, Claudio Ricci, W. N. Brandt, Dong-Woo Kim, Dominika Wylezalek, Scott F. Anderson, Donald P. Schneider, Delvin Demke, Anton M. Koekemoer
Solar and Stellar Astrophysics
Testing the reliability of magnetic field strength measurements for M dwarfs
I. Amateis, O. Kochukhov and A. Hahlin
M
dwarfs host strong
magnetic fields that can be measured using several complementary techniques. However, the impact of key methodological choices on Zeeman broadening diagnostics has not been systematically quantified. We assess the reliability of different approaches for inferring
magnetic fields in M
dwarfs using synthetic Stokes $I$ spectra of Ti I lines generated from three-dimensional magnetohydrodynamic simulations combined with MARCS model atmospheres. Synthetic observations were produced for different surface
magnetic field strengths, projected rotational velocities, and inclination angles. Zeeman broadening and intensification were analysed using polarised radiative transfer calculations coupled with Markov chain Monte Carlo inference. We evaluated several statistical criteria (BIC, AIC, and WAIC) for selecting the number of
magnetic filling factors and compared alternative strategies for treating line strengths. In most cases, BIC, AIC, and WAIC favoured the same number of components. For some active and rapidly rotating models, BIC and AIC selected fewer components and yielded lower field estimates, while WAIC generally produced closer agreement with the input field strengths. Treating individual line strengths as free parameters underestimated the field strength by 30–50%, whereas fitting a joint element abundance together with continuum scaling recovered the input field more reliably. Our results show that Zeeman broadening diagnostics can robustly recover
magnetic fields in M
dwarfs, but their accuracy depends strongly on methodological choices.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
The impact of seasonality over the sensitivity of Einstein Telescope and the SNR of CBC signals at the Sardinia candidate site
Matteo Di Giovanni, Davide Rozza, Giovanni Diaferia et al. (+14 more)
This work presents the first investigation of the impact of seasonal variations in seismic noise on the low-frequency performance of the Einstein
Telescope (ET) at the Sardinia candidate site, focusing on their effect on compact binary coalescence signals. Using seismic data collected between 2022 and 2025 from a deep borehole, we characterize the seasonal variability of the seismic noise and identify the months of July and December as representative best- and worst-case conditions. The measured seismic spectra are used to estimate the Newtonian noise contribution in the 2–10 Hz frequency range and to derive corresponding modifications to the ET design sensitivity. These sensitivity curves are then used in simulations of binary
neutron star and intermediate-mass black hole signals in the triangular ET configuration to quantify the resulting changes in signal-to-noise ratio. We find that median seasonal conditions produce only minor changes in the expected SNR, with deviations from the design case typically below 2$\%$. Larger effects are found for the upper tail of the December noise distribution, where the average SNR loss reaches approximately 20–25$\%$. These results indicate that seasonal seismic variability at the Sardinia candidate site has a limited impact on the low-frequency performance of ET under typical conditions.
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All authors Matteo Di Giovanni, Davide Rozza, Giovanni Diaferia, Andrea Contu, Rosario De Rosa, Carlo Giunchi, Luca Naticchioni, Marco Olivieri, Annalisa Allocca, Enrico Calloni, Giovanni Luca Cardello, Luciano Errico, Giovanni Losurdo, Irene Molinari, Lucia Trozzo, Alessandro Cardini, Domenico D'Urso
Earth and Planetary Astrophysics
Inferring stellar metallicity and elemental abundances from kinematic and spectroscopic data using machine learning – Implications for exoplanet host stars
V. Adibekyan, B.M.T.B. Soares, S.G.Sousa et al. (+7 more)
(abridged) Elemental abundances of FGK
stars can be derived routinely from high-resolution optical spectra, but this remains considerably more difficult for cooler
stars. Machine-learning methods offer a practical route to infer otherwise inaccessible abundances from more widely available
stellar data. We use a large APOGEE DR17 sample of red
giant stars as the main training set and an independent HARPS sample of nearby FGK
dwarfs for external validation. We
benchmark several machine-learning regressors, optimise the strongest models, and analyse feature importance using gain-based metrics, permutation importance, single-feature models, and SHAP values. We also explored the prediction of C and O from Mg, Si, and [Fe/H], and derived simple empirical relations between selected abundance ratios (Fe/Si, Mg/Si, C/O, and Fe/O) and
metallicity. Kinematic information alone recovers only a limited fraction of the variance in
stellar metallicity, with a clear performance ceiling at RMSE $\sim$0.20 dex. The most informative predictor is the maximum vertical orbital excursion, $Z_{\max}$, followed by radial orbital parameters. When [Fe/H] is combined with kinematic information, the abundances of C, O, Mg, and Si are predicted significantly more accurately than with the baseline approximation $\mathrm{[X/H]}=\mathrm{[Fe/H]}$. In contrast, when predicting C and O from Mg, Si, and [Fe/H], most of the predictive power is already contained in the elemental abundances themselves, with Mg being the dominant contributor, and the addition of kinematic information provides little improvement. The trained models reproduce the main abundance trends associated with
Galactic chemical
evolution. We find that the slopes of the relations between Fe/Si, Mg/Si, C/O, and Fe/O and
metallicity differ slightly between the HARPS and APOGEE samples, with fractional differences generally below 17%.
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All authors V. Adibekyan, B.M.T.B. Soares, S.G.Sousa, N.C. Santos, E. Delgado-Mena, I. Minchev, R. Chertovskih, Zh. Martirosyan, G. Israelian, A.A. Hakobyan