Cosmology and Nongalactic Astrophysics
Dissipative Dark Energy can explain the DESI phantom crossing
Prolay Chanda, Subinoy Das and Suratna Das
DESI results preferring an evolving dark energy component that appears to cross the phantom-divide in the recent past has raised a lot of interest in exploring the nature of dark energy. We present here a simple dissipative dark energy scenario that can explain both the evolving nature of dark energy as well as its crossing of the phantom-divide without invoking any pathological phantom-like dynamics for the quintessence field. We show that even weak dissipation of the quintessence is enough to explain the current DESI observations.
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Instrumentation and Methods for Astrophysics
A Model Selection Criterion for Multidimensional Gaussian Processes: Application to Radial Velocities
Oscar Barragán
Multidimensional Gaussian Process (multi-GP) regression is widely used to disentangle
stellar and planetary signals in radial velocities (RVs) by jointly modelling ancillary activity indicators. However, identifying the combination of indicators that best constrains the
stellar signal in the RVs is non-trivial, as classical model comparison methods are not directly applicable when multi-GPs involve different time series combinations. In this work, we present an information criterion to compare multi-GP models based on their ability to explain the RV component, $\mathrm{MGIC}_{\rm rv}$. This metric combines the conditional RV likelihood with an effective parameter count that accounts for the regularisation imposed by the multi-GP model on the RV component. We demonstrate that $\mathrm{MGIC}_{\rm rv}$ provides a quantitative and robust framework for multi-GP model comparison, identifying the activity indicators that most effectively constrain the RV signal. Although developed in the context of RV analysis, the proposed criterion is general and applicable to multi-GP problems in which the inference focuses on a specific observable.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Constraining Cosmological Parameters From Statistical Superluminal Effects Without a Distance Ladder
Hichem Guergouri, Omar Nemoul and Jamal Mimouni
We employ a statistical approach to study apparent superluminal motion of luminous sources in an expanding flat Friedmann–Lemaître–Robertson–Walker universe, explicitly incorporating cosmological effects through the comoving distance at the emission time. Probability Density Functions (PDFs) of the apparent angular velocity are derived under minimal assumptions regarding source orientations and intrinsic peculiar velocity distributions. We show that the apparent angular velocity distributions and their associated statistical observables are sensitive to cosmological parameters $\Omega_{\Lambda,0}$ and the Hubble parameter $H_0$. Using suitably defined observables, we construct correlated constraints in the $(\Omega_{\Lambda,0}, H_0)$ parameter space and demonstrate that combining measurements at different
redshifts effectively breaks the resulting degeneracy. Apparent superluminal motion thus provides a complementary kinematic consistency test for cosmological models.
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Solar and Stellar Astrophysics
Rediscussion of eclipsing binaries. Paper 31. The slowly-pulsating B-star system CV Velorum in the PLATO southern field
John Southworth
CV Vel is a detached eclipsing binary containing two B2.5 V
stars in a circular orbit of period 6.889 d. Both
stars show line-profile variations arising from g-mode oscillations, characteristic of slowly-pulsating B-
stars, and rotational axes misaligned with the orbital axis. We present the first photometric analysis of the system based on light curves from a space
telescope. By combining our results with published spectroscopic observations we determine the
stars' masses to be 6.065 +/- 0.015 Msun and 5.950 +/- 0.012 Msun, and their radii to be 4.094 +/- 0.056 Rsun and 3.978 +/- 0.049 Rsun. The precision of the radius measurements is limited by the pulsations in the light curves. We identify two confirmed (0.4886 c/d and 0.3695 c/d) and two candidate (0.7468 c/d and 0.7048 c/d) pulsation frequencies but cannot reliably identify further frequencies due to the short duration of the available light curves (approximately 50 d). CV Vel is in the first field to be observed by the PLATO satellite so in future may become the first EB containing an SPB
star for which we have a light curve with a high photometric precision and a duration of over one year.
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Astrophysics of Galaxies
The open cluster NGC 2509. Stellar rotation and main sequence turnoff extension from FLAMES spectroscopy
C. Boeche, J. Alonso-Santiago, A. Bragaglia et al. (+8 more)
NGC 2509 is a distant (~2.5 kpc) and little-studied open cluster located in the third
Galactic quadrant. It is a moderately old cluster, whose age has not yet been precisely determined. The main-sequence
stars in NGC 2509 follow a narrow distribution in the color-magnitude diagram, unlike other clusters of similar age. In addition, its chemical composition has never been investigated. To address these issues and characterize the cluster we performed moderate- and high-resolution spectroscopy with FLAMES@VLT of 132
stars, both
dwarfs and
giants, which represents a significant fraction (~73%) of likely members. We provide atmospheric
stellar parameters and, for the first time, chemical abundances for 21 species with atomic numbers up to 60. In our analysis we followed two different methodologies, both of which will be used for the incoming WEAVE
stellar surveys. We find an average radial velocity for NGC 2509 of 58.6+-1.3 km\s and a mild supersolar
metallicity ([Fe/H]~0.1 dex). This value is slightly higher than expected according to its galactocentric distance, but still compatible with the
Galactic gradient. From the lithium content of the
dwarfs and the isochrone-fitting method we obtain an age for NGC 2509 of 1.26+-0.3 Gyr. The reddening across the cluster field is negligible (A_V=0.25+-0.02 mag). The cluster peculiar main sequence turnoff is due to a narrow distribution of the rotational velocities peaking at vsini~80 km/s, with little dispersion. The chemical pattern of NGC 2509 follows the
Galactic trends shown by other open clusters in the
Galactic thin disk.
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All authors C. Boeche, J. Alonso-Santiago, A. Bragaglia, A. Frasca, A. Vallenari, I.N. Kallimanis, R. Carrera, D. Bossini, S. Lucatello, V. DOrazi, G. Costa
Cosmology and Nongalactic Astrophysics
Cosmic variance or galaxy bias? Disentangling finite-volume and galaxy formation effects in cosmological analysis
Francesco Sinigaglia and Francisco-Shu Kitaura
Current and forthcoming
galaxy redshift surveys, such as DESI and Euclid, are going to bring cosmological analysis to an unprecedentedly exquisite level of precision in the determination of the cosmological parameters. However, these efforts require a high degree of control over theory and systematics, to obtain unbiased results. In this sense, the
cosmic variance associated to finite-volume effects represents a major challenge and should adequately accounted for. In this work, we revisit the definition of
cosmic variance and develop a novel framework to describe it using a `
galaxy biasing' formalism. In particular, we use
halo/
galaxy Eulerian perturbation theory to relate the density field from an arbitrary
cosmic realization to its counterpart having statistical properties reproducing the ensemble average, introducing a new set of bias parameters. We then apply this idea to the description of the non-linear shift of BAO, disentangling the source of uncertainty from
cosmic variance and
galaxy biasing associated with the measurement of the BAO scale. We finally check our analytical argument against cosmological variance-suppressed $N$-body simulations, finding an expected reduction in the uncertainty on the BAO peak position. We conclude that extra care should be used when inferring cosmological information from perturbative approaches involving the estimation of bias parameters and propose new practical strategies to optimally leverage the novel formulation of
cosmic variance presented herein in cosmological analysis.
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Cosmology and Nongalactic Astrophysics
Steep Redshift Evolution of the Ionizing Escape Fraction at $z = 5$–$12$: Empirical Constraints and Comparison with Simulations
Zihan Wang and Huanyuan Shan
The ionizing
photon escape fraction $f_{\rm esc}$ governs
cosmic reionization yet remains observationally unconstrained as a function of
halo mass. We present the first empirical constraints on $f_{\rm esc}(M_{\rm h},z)$ across the epoch of reionization, using a three-parameter power-law model $f_{\rm esc} = f_0\,(M/10^{10}M_\odot)^{\alpha_M}\,[(1{+}z)/10]^{\alpha_z}$, conditioned on HST and JWST UV luminosity functions at $z=5$–12, the Planck Thomson optical depth, seven neutral-fraction measurements, and one high-
redshift prior. Using Schechter fits to the latest HST and JWST UV luminosity functions, abundance matching to link $M_{\rm UV}$ to
halo mass, and a reionization ODE solver validated against Planck, we constrain the model via a dense grid scan and ensemble MCMC. The profile likelihood yields tight constraints: $f_0=0.061_{-0.023}^{+0.018}$, $\alpha_M=0.18_{-0.30}^{+0.22}$, $\alpha_z=1.98_{-0.42}^{+0.48}$. In contrast, the full marginal posterior is substantially broadened by a strong $f_0$–$\alpha_M$–$\alpha_z$ degeneracy ($\alpha_z = 1.93_{-2.00}^{+2.09}$, $\alpha_M = -0.52_{-0.69}^{+0.69}$). The population-averaged $\langle f_{\rm esc} \rangle(z)$ rises from $\sim$2% at $z=5$ to $\sim$9% at $z=12$, with sub-threshold
halos contributing $>80\%$ of the ionizing budget at $z\geq10$. Comparing with THESAN, we find that the per-
halo median $f_{\rm esc}$ shows steep
evolution consistent with our profile result, while luminosity-weighted averaging systematically flattens the trend because massive
halos dominate the ionizing budget at $z\lesssim7$. Robustness checks confirm $\alpha_z>1.0$ at $>95\%$ confidence; the steep-
evolution model predicts $\tau_e=0.047$, consistent with Planck at $0.7\sigma$. We provide tabulated $f_{\rm esc}(M_{\rm h},z)$ posteriors as empirical inputs for reionization simulations.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Fast gravitational waveform models for quasi-circular coalescences of neutron star–black hole binaries
Felip A. Ramis Vidal, Adrian Abac, Marta Colleoni et al. (+5 more)
We present IMRPhenomXHM_NSBH and SEOBNRv5HM_ROM_NRTidalv3_NSBH, the first two frequency-domain models for gravitational-wave signals from quasi-circular, aligned-
spin neutron star–black hole (NSBH) binaries including higher-order modes beyond the dominant quadrupole. We also present IMRPhenomXPHM_NSBH, an extension of the former model to the
spin-precessing case. These models incorporate
tidal effects in the gravitational-wave phasing and amplitude using a higher-mode extension of the NRTidalv3 model, as well as dedicated amplitude models calibrated to numerical relativity (NR) simulations of NSBH mergers. We test the performance and validity of the new models by comparing them to NR simulations and other existing models for these systems. Finally, we perform parameter estimation studies. The new models show clear improvements over their predecessors in analyses of simulated signals, while yielding results consistent with the literature when applied to real events from the GWTC-3 and GWTC-4 catalogs.
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All authors Felip A. Ramis Vidal, Adrian Abac, Marta Colleoni, Tim Dietrich, Pierre Mourier, Alejandra Gonzalez, Ivan Markin, Anna Puecher
Astrophysics of Galaxies
Cooler Phases of the Circumgalactic Medium Are More Centrally Concentrated: Constraints from Multiphase Absorption Lines
Weiwen Kong, Zeyu Chen, Enci Wang et al. (+7 more)
We present a systematic study of the multiphase circumgalactic medium (CGM) around
galaxies and
quasars, traced by Ca II $\lambda\lambda3934,3969$, Mg II $\lambda\lambda2796,2803$, and C IV $\lambda\lambda1548,1550$, using the Year 1
dataset from the Dark Energy Spectroscopic Instrument. These three doublets trace CGM gas across a range of temperatures, from cold to warm phases, and we employ a stacking technique to measure the corresponding absorption signals using background sources. We show that CGM structure is strongly phase-dependent: ions tracing progressively cooler gas exhibit increasingly steep radial profiles in equivalent width ($W_i$). These trends are broadly consistent with predictions from cosmological simulations, supporting a phase-stratified CGM in which cooler gas is more centrally concentrated. Specifically,
halos of emission-line
galaxies exhibit a strong radial transition from cool to warm gas, whereas
halos of
quasars show a more uniform distribution, likely regulated by active
galactic nuclei feedback; in contrast, the cold gas traced by Ca II in low-
redshift galaxies is tightly confined to inner regions. We further demonstrate that the radial scaling $W_i \propto D^{\alpha}$ is primarily set by host
stellar mass, particularly for the cool-phase medium, suggesting efficient heating processes in massive
halos. By jointly leveraging multiple absorption tracers from observations and simulations, we map the CGM from cold to warm phases and place new constraints on the
baryon cycle governing
galaxy evolution.
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All authors Weiwen Kong, Zeyu Chen, Enci Wang, Haoran Yu, Kai Wang, Dongdong Shi, Cheqiu Lyu, Yuxuan Zhang, Haoyi Zhang, Haowen Guan
High Energy Astrophysical Phenomena
Granular mass perturbations on the pulsar - supermassive black hole system
Zexin Hu and Lijing Shao
Discovery and timing observations of a
radio pulsar orbiting around Sagittarius A*, the supermassive black hole (SMBH) in our
Galactic Centre (GC), will provide unprecedented opportunities of studying the SMBH spacetime, testing
gravity theories, and probing the astrophysical environment in the GC. However, unknown mass distributions might cause timing residuals that are much larger than the timing precision. With extensive numerical simulations, for the first time we find that the perturbations caused by a granular cusp of
stellar-mass black holes in the GC lead to post-fit timing residuals of 10-100 s–contrary to traditional wisdom–even for a
pulsar in a tight orbit with an orbital period $P_b=0.5\,{\rm yr}$. Such a large timing residual can lead to significant measurement bias or even prevent construction of a phase-connected timing solution for the full orbit. We revisit the idea of extracting SMBH parameters only with data around periastron where the perturbation is small. Under the realistic phase-disconnected assumption, we point out that it is vital to consider the frame-dragging effect in the light propagation, which breaks parameter degeneracy and leads to an order of magnitude improvement for the measurement precision of the SMBH
spin.
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High Energy Astrophysical Phenomena
Galactic absorption measured by X-ray observations of clusters of galaxies at the low Galactic latitude
Yumiko Anraku, Shigeo Yamauchi, Anje Yoshimoto et al. (+3 more)
The amount of the interstellar gas in the
Galaxy has been conventionally estimated through observations at various wavelengths. The estimation of the total hydrogen column density (N_H) depends on assumptions such as temperature. The X-ray absorption process is the photoelectric absorption, which depends on the number of atoms to encounter X-ray
photons, and hence X-ray observations would be able to derive the N_H values independently on the condition of the interstellar matter. We measured the
Galactic absorption using clusters of
galaxies at the low
Galactic latitude. Comparing the observed N_H with the calculated N_H} values from HI and CO intensities indicates that the observed values are systematically larger than the calculated values. The observed $N_{\rm H}$ values at high
Galactic latitude (N_H<10^{22} cm^{-2}) are comparable to those estimated from N_HI} and optical reddening values using the method by Willingale et al. (2013, MNRAS, 431, 394), but the values near to the
Galactic plane (N_H >10^{22} cm^{-2}) are larger than the estimated ones. The
dust optical depth at 353 GHz, tau_{353}, and the observed N_H values are expressed by a linear function of N_H=(1.01-1.59)x10^{26} tau_{353} cm^{-2} even at N_H >10^{23} cm^{-2}. We also confirmed a linear correlation between the optical reddening, E(B-V), and the N_H values expressed by N_H=(6.3-9.5)x10^{21} E(B-V) cm^{-2}. This work is an additional and independent test of the relation among the amount of interstellar gas, the optical depth, and the optical reddening.
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All authors Yumiko Anraku, Shigeo Yamauchi, Anje Yoshimoto, Masayoshi Nobukawa, Kumiko K. Nobukawa, Hideki Uchiyama
High Energy Astrophysical Phenomena
TDEs on FIRE: Illuminating the Cosmic Evolution of Tidal Disruption Rates
Rudrani Kar Chowdhury, Lixin Dai, Janet N.Y. Chang et al. (+1 more)
Tidal disruption events have been extensively studied in the local universe, but their prevalence at high
redshifts remains largely unexplored. Using the FIRE-2 cosmological zoom-in simulations, we compute the per-
galaxy tidal disruption rate (TDR) over $z=1-10$, covering black holes from IMBHs to SMBHs. The averaged TDR rises from the early universe, peaks at $\sim 4 \times 10^{-4} \, \text{yr}^{-1}$ near $z \sim 2.5$, and declines to $\sim 10^{-5} \, \text{yr}^{-1}$ at $z=1$. The TDR correlates strongly with host
galaxy star formation rate and central
stellar density at all
redshifts. Qualitatively, the TDR trends with the $M_{\rm BH}$ and $M_{\rm gal}$ persist from high
redshift to the local universe, suggesting similar BH-
galaxy scaling across
cosmic time. Satellite
galaxies exhibit comparably high TDRs, with their fractional contribution increasing significantly at high
redshifts, highlighting their potential for probing IMBHs and early
galaxy assembly. This work demonstrates that cosmological simulations offer a promising avenue for constraining the
cosmic evolution of the TDR, paving the way for future comparisons with next-generation observations.
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All authors Rudrani Kar Chowdhury, Lixin Dai, Janet N.Y. Chang, Tsang Keung Chan
High Energy Astrophysical Phenomena
A candidate cyclotron line at 1.89 keV in the ultraluminous X-ray source NGC 4861 X-2
Sinan Allak, Lorenzo Ducci, Valery F. Suleimanov et al. (+6 more)
In this Letter, we report the detection of an absorption-like feature at ~1.89 keV in Chandra/ACIS spectra of the ultraluminous X-ray source NGC 4861 X-2, based on the deepest observation (ObsID 20992; ~58 ks). The feature is consistently recovered across independent continuum models and significantly improves the fit statistics. Monte Carlo simulations yield a detection significance of ~3.5-4.1 sigma, depending on the adopted continuum, and a blind line scan reveals a single, localized peak at the same energy. The observed properties are consistent with a
proton cyclotron resonant scattering feature (CRSF), implying a
magnetic field strength of B ~(3-4) x 10^14 G. The spectrum is well described by a multicolor disk blackbody (diskbb) with kTin ~0.8 keV or a strongly curved continuum with a low cutoff energy (cutoffpl; Ecut ~1.3 keV). The source shows variability confined to the soft X-ray band in the two Chandra observations where the absorption-like feature is detected. In these observations, a candidate periodic signal at P ~7.4 s is also detected, with a global significance of ~2.5 sigma.
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All authors Sinan Allak, Lorenzo Ducci, Valery F. Suleimanov, Andrea Santangelo, Aysun Akyuz, Santina Piraino, Faruk Soydugan, Amar Deo Chandra, Wei Yu
Solar and Stellar Astrophysics
Recurrent Coronal Jets and QPPs: Periodic Reconnection and Localized Heating Across Quiet-Sun to Active Regions
Sudheer K. Mishra, Kartika Sangal, Balveer Singh et al. (+3 more)
We analyze quasi-periodic pulsations (QPPs) in recurrent coronal jets driven by periodic
magnetic reconnection associated with successive flux emergence in the fan-spine
magnetic topologies. Using the Atmospheric Imaging Assembly (AIA) onboard the
Solar Dynamics Observatory (SDO), we investigate three long-lived recurring jets spanning quiet Sun to moderate-field-strength regions, each exhibiting recurrent eruptions linked to episodic reconnection. Wavelet analysis of multithermal AIA EUV jet-base light curves detects QPPs with periods of 6-13 min, exceeding typical p-mode oscillation periods. Distance-time analysis reveals quasi-periodic propagating ridges, interpreted as recurrent field-aligned
plasma ejections, and morphological similarities to slow magnetoacoustic waves, which cannot be entirely excluded. However, the dominant photospheric unsigned flux periodicities of 10-32 min at the jet source regions favor the reconnection-driven interpretation. DEM analysis confirms multithermal
plasma with the hottest emission concentrated near the jet base, and the QPP periods fall well below both radiative and conductive cooling timescales, implying persistent localized heating within the fan-spine configuration. These results demonstrate that periodic reconnection in fan-spine
topologies drives recurrent jet eruptions and contributes to localized coronal heating across the quiet Sun, moderate-field strength regions, and active regions.
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All authors Sudheer K. Mishra, Kartika Sangal, Balveer Singh, Ayumi Asai, A. K. Srivastava, Ding Yuan
Astrophysics of Galaxies
Statistical analysis of the relative orientations between filaments and magnetic fields using Herschel and Planck data in star-forming regions
Jonathan Oers, Isabelle Ristorcelli, Katia Ferrière et al. (+4 more)
Observations and simulations of the interstellar medium both suggest that
magnetic fields play a key role in the formation and
evolution of filaments and in the process of
star formation, yet their exact role is still poorly understood. Here, we aim to statistically examine the relative orientations between filaments and
magnetic fields in various
star-forming regions with different physical properties and
Galactic environments. We used a dedicated method, FilDReaMS, to detect and extract filaments at multiple scales, and we applied it to the 116 fields of the Herschel "
Galactic Cold Cores" key project (18"-36" resolution). We then compared the filament orientations to the orientation of the plane-of-sky (PoS)
magnetic field (B_PoS), inferred from Planck observations (7' resolution) using histograms of relative orientations (HROs). We find that low-N_H2 filaments tend to be roughly parallel to B_PoS at all scales, while narrow high-N_H2 filaments do not have any preferred orientations and wide high-N_H2 filaments tend to be roughly perpendicular. This change in preferred orientations occurs at a transition column density typically in the range [0.8, 8] x 10^{21} cm^{-2}, a range consistent with results of previous Planck studies. We also analyzed the HROs for filaments with embedded cores and find them to be consistent with HROs for high-N_H2 filaments. However, several fields do not follow the general trends, with a variety of behaviors that can be due to factors such as projection effects, confusion along the line of sight (LoS), or
magnetic field tangling. Our analysis of projection effects shows that, statistically, preferred orientations in the PoS are indicative of true preferred orientations in 3D. Our results suggest that higher polarization fractions, p, entail weaker projection effects, consistent with the presumed link between p and the
magnetic field inclination to the LoS.
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All authors Jonathan Oers, Isabelle Ristorcelli, Katia Ferrière, Mika Juvela, Ludovic Montier, Dana Alina, Julien Montillaud
Astrophysics of Galaxies
The Extreme Rarity and Physical Properties of Low-redshift AGNs with Balmer Absorption
Jinyi Shangguan, Chang-Hao Chen, Luis C. Ho et al. (+6 more)
Balmer absorption lines are increasingly observed in the little red dots (LRDs) discovered by the James Webb Space
Telescope, potentially tracing dense circumnuclear gas around rapidly accreting black holes. Motivated by this connection, we search for Balmer absorption using homogeneously analyzed spectra of a representative parent sample of 14,584 low-
redshift ($z<0.35$) type 1 active
galactic nuclei selected from the Sloan Digital Sky Survey. We identify seven sources with robust Balmer absorption (occurrence $\sim 0.05\%$) and model them with a partially covering absorber model, accounting for the spectral resolution. By fitting H$\alpha$, H$\beta$, and H$\gamma$ simultaneously and tying their optical-depth ratios to theoretical values, we constrain optical depth at the line center ($\tau_0$) and the covering factor ($C_f$). All sources with robust modeling require optically thick H$\alpha$ absorption and typically moderate covering factors ($C_f\approx 0.2-0.6$), while the LRD analog J1025 shows $C_f \gtrsim 0.8$ consistent with recent measurements of high-
redshift LRDs. The absorbers have modest velocity offsets ($\sim 150-850\,\mathrm{km\,s^{-1}}$) and narrow intrinsic widths ($\sim 20-200\,\mathrm{km\,s^{-1}}$). Multi-epoch spectroscopy of three sources reveals Balmer-absorption variability on both year and month timescales. Three objects exhibit exceptionally weak Fe II emission, high Eddington ratio, and low gas-phase
metallicity, an atypically rare combination of properties that might elevate the incidence of Balmer-absorption to $\sim$10%. We argue that low-
metallicity conditions may suppress disk winds and help retain dense neutral gas along the line-of-sight in systems of high
accretion rate.
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All authors Jinyi Shangguan, Chang-Hao Chen, Luis C. Ho, Jiwei Liao, Yanqing Liu, Chengzhou Wu, Ruancun Li, Kohei Inayoshi, Linhua Jiang
Astrophysics of Galaxies
ABCD: The Nuclear Structure of the Little Red Dots Revealted through Absorption, Break, Continuum, and Decrement
Chang-Hao Chen, Jinyi Shangguan, Luis C. Ho et al. (+3 more)
We present a spectroscopic analysis of 14 little red dots (LRDs) at
redshifts $2.2 < z < 6.7$ using NIRSpec/MSA prism and medium-resolution grating observations, aiming to constrain the
nuclear gas structure through Balmer emission-line profiles, absorption features, relative line intensities, and continuum properties. We simultaneously decompose the broad, narrow, and absorption components of ${\rm H \alpha}$, ${\rm H \beta}$, and ${\rm H \gamma}$, and measure both integrated line ratios and velocity-resolved Balmer decrements. The narrow-line Balmer decrements are broadly consistent with Case~B recombination modified by mild
dust attenuation, while the broad-line decrements are elevated to levels consistent with photoionization models of high-density gas at $n_{\rm H} \gtrsim 10^9\ {\rm cm^{-3}}$. Velocity-resolved Balmer decrements in five sources with highest signal-to-noise ratio are centrally peaked. Assuming virialized broad-line region dynamics, our model can reproduce the Balmer decrement profiles in three sources using a radial density profile with a power-law index $\beta<2$. The Balmer absorption lines detected in six sources yield absorber covering factors exceeding $50\%$. Sources with blueshifted absorption lines tend to have elevated narrow-line Balmer decrement, suggesting a connection between
dust content and the presence of
outflow. Comparing the incident luminosity inferred from broad and narrow ${\rm H \alpha}$ emission with the continuum suggests that both the UV and optical continuum and the line emission are linked by photoionization. We propose that the distinctive spectral and continuum properties of LRDs can be explained via a viewing angle-dependent
nuclear structure in which an optically thick, clumpy gaseous
torus surrounds the central
accretion disk, with broad-line clouds and absorbers distributed along the less-obscured polar directions.
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All authors Chang-Hao Chen, Jinyi Shangguan, Luis C. Ho, Zijian Zhang, Kohei Inayoshi, Ruancun Li
Cosmology and Nongalactic Astrophysics
BBN constraints on primordial black holes with a continuous memory-burden crossover
Xuan-Yu Zhang, Mei-Ting Yang and Hong-Bo Jin
Light
primordial black holes (PBHs) are disfavored as dark matter if they evaporate through standard Hawking radiation alone. The
memory-burden effect can extend their lifetimes by suppressing emission after roughly half the mass is lost. Existing cosmological studies often model the onset of this phase as an instantaneous transition between semi-classical and burden-dominated evaporation. We instead treat the crossover as continuous and compare additive versus multiplicative combinations of the two rates, using a smoothed tanh profile with parameters $(q,\delta)$. Monochromatic PBHs are mapped to a decaying scalar field and evolved with Modified AlterBBN during Big Bang nucleosynthesis (BBN). The two prescriptions yield distinct exclusion curves: the additive crossover always gives weaker bounds than the multiplicative one, while both are tighter than the instantaneous transition. For $10^{5}\,\mathrm{g}\lesssim M_i\lesssim 10^{10}\,\mathrm{g}$, the additive case can permit $f_{\mathrm{PBH},0}\sim 10^{-1}$ where the multiplicative case gives $f_{\mathrm{PBH},0}\lesssim 10^{-2}$. Specifying the rate-combination rule is therefore essential when translating
memory-burden models into BBN constraints on PBH dark matter.
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Solar and Stellar Astrophysics
Long-term investigation of gamma Cas analogs
Yael Naze, Gregor Rauw, Robbie Webbe et al. (+2 more)
The subcategory of gamma Cas analogs gathers Be
stars with bright and hard X-ray emission. Long-term variations are expected in such objects for two reasons: their Be disk builds and dissipates, and such
stars are suspected long-period binaries. Seven targets are analysed in this paper: five of them benefit from a spectroscopic monitoring in the visible (ESO, TIGRE, and amateur data) and three of them have been repeatedly observed at X-ray wavelengths (using XMM-Newton, Chandra, and Swift). Broad-band photometric data are also examined. We confirm the binary status of five targets (HD44458, HD110432, HD119682, HD161103, and HD162718) and propose first orbital solutions for all of them (they remain preliminary for two cases). Their long periods (59-322 d) and small velocity amplitudes (K~5km/s) imply low-mass (~1 M_sol) companions, as in other Be binaries and in agreement with expectations from binary interaction models. In parallel, variations of the X-ray flux are detected in all three targets with a large
dataset of X-ray observations. For NGC 6649 9 and HD162718, these changes are modest (a factor of three) and uncorrelated to simultaneous optical broad-band photometry (which remains rather stable). In contrast, SS397 varies by nearly one dex and the largest and best monitored X-ray changes correlate well with optical variations. At minimum flux, SS397 keeps a hard X-ray spectrum despite a nearly normal L_X/L_BOL ratio, which has not been seen yet among gamma Cas analogs. Finally, the photometric behaviours on short timescales of HD161103, SS397, and NGC 6649 9 appear linked to broad frequency groups, as typically found for Be
stars. The frequency spectrum of HD162718 displays a complex mix of (isolated) periodicities with the main one at 6.658/d. This target is thus one of the rare gamma Cas analogs to display a strong high-frequency signal typical of beta Cep activity. [summarized]
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All authors Yael Naze, Gregor Rauw, Robbie Webbe, Myron A. Smith, Christian Motch
Earth and Planetary Astrophysics
Upper limits on exosatellites around $\beta$ Pictoris b
M.A. Kenworthy, R. Landman, A. Vanderburg et al. (+8 more)
$\beta$ Pictoris b is one of the closest known directly-imaged gas
giant exoplanets with an orbit that is almost edge-on to our line of sight, making it an ideal target for radial velocity monitoring to search for massive exomoons. We measure the radial velocity of $\beta$ Pictoris b over several epochs between October 2024 and March 2025 by using the cross-correlation of a template spectrum with absorption lines in the
planet's atmosphere, giving a mean precision of 160 m s$^{-1}$. The resultant set of radial velocities is analysed with a periodogram to search for candidate RV signals indicating a massive exomoon. Although we do not detect an exomoon signal in our data, our detection limits for a single moon are 80 Earth masses at P=1 day and 1 Jupiter at P=200 days, comparable to RV exomoon searches around other substellar companions. The RV limit is comparable with the astrometric exomoon limit at a period of 7 days and a mass of 150 Earth masses, where for longer periods the astrometric searches have lower mass limits. With an additional observing season, CRIRES+ can detect a
planet/moon mass ratio of $10^{-3}$ (4 Earth masses) with a period of up to one day, and can detect a Neptune-mass moon at hundreds of Jupiter radii.
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All authors M.A. Kenworthy, R. Landman, A. Vanderburg, J.E. Rodriguez, J.L. Birkby, I. Macias, D. González Picos, S.A. Jenkins, E. Kleisioti, T. Stolker, I. Koutalios
High Energy Astrophysical Phenomena
NICER detection of a new candidate cyclotron line in the bursting X-ray pulsar GRO J1744-28
Qi Liu, Lingda Kong, Long Ji et al. (+15 more)
We report the detection of cyclotron resonant scattering features (CRSFs) in the spectrum of the unique bursting
pulsar GRO J1744-28, observed during its recent outburst in 2021 with the
Neutron Star Interior Composition Explorer (NICER). Clear pulsations at a frequency of 2.141128 Hz as well as Type II X-ray bursts were observed. The pulse profile exhibits a single-peaked shape in all energy bands, with the pulse fraction showing a positive correlation with energy. We find that the persistent X-ray continuum of the accreting
pulsar is well described by typical phenomenological models, and we confirm the presence of the cyclotron line at $\sim$5 keV as reported in previous studies. In addition, we detect a candidate absorption feature with a centroid energy of 2 keV. If confirmed, this feature could be interpreted as a CRSF, which would correspond to a
magnetic field of $\sim$1.8 $\times 10^{11}$ G. Pulse-phase-resolved analysis also reveals this absorption line around the peak pulse phases. These NICER observations provide tentative evidence for the cyclotron line candidate, establishing GRO J1744-28 as a key laboratory for studying
accretion physics in an intermediate-strength
magnetic field.
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All authors Qi Liu, Lingda Kong, Long Ji, Lorenzo Ducci, Xiaohang Dai, Andrea Santangelo, Ce Cai, Shuwang Cui, Mingyu Ge, Shu Zhang, Shuang-Nan Zhang, Lian Tao, Hua Feng, Wei Wang, Valery Suleimanov, Sergey Tsygankov, Juri Poutanen, Alexander Mushtukov
Astrophysics of Galaxies
Environmental Dependence of Star Formation and Galaxy Colors around Abell 2029
Xiaolan Hou and Heng Yu
Environmental processes drive
galaxy evolution, with the impact varying significantly across different
stellar masses. We present a comprehensive environmental analysis of the
galaxies within a $10^\circ \times 10^\circ$ field around A2029, utilizing high-density spectroscopic data from the DESI and SDSS surveys. We investigate the quenched fraction ($f_Q$) and red fraction ($f_{red}$) as functions of local surface density ($\log_{10} \Sigma_5$) across three
stellar mass intervals (low-mass: $9.5 \le \log M_\star/M_\odot < 10.0$; medium-mass: $10.0 \le \log M_\star/M_\odot < 10.5$; high-mass: $\log M_\star/M_\odot \geq 10.5$ ). Our results show that, for
galaxies of all masses, both
star formation activity and
galaxy color are strongly correlated with the local density. Although the environmental dependence of both the quenched and red fractions is somewhat weaker in low-mass
galaxies than in their high-mass counterparts, the variations remain significant. This suggests that
galaxy colors, even for low-mass systems, can serve as effective tracers of large-scale structure.
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Solar and Stellar Astrophysics
Blue Straggler Stars in Berkeley 18: A Multiwavelength Study of Their Physical Properties and Dynamical Evolution
D. Bisht, Ing-Guey Jiang, K. Belwal et al. (+7 more)
Berkeley~18 is an old open cluster in the outer
Galactic disk that hosts a population of blue straggler
stars (BSSs). We present a comprehensive multiwavelength analysis of its BSS population using
Gaia DR3 astrometry, optical–infrared photometry, and time-domain TESS observations. Using a Gaussian Mixture Model (GMM) in astrometric space, we identify 798 high-probability cluster members ($p > 0.7$). Isochrone fitting yields an age of $3.2 \pm 0.2$ Gyr and a heliocentric distance of $5.01^{+0.75}_{-0.55}$ kpc. We identify 24 BSS candidates above the main-sequence turn-off. Spectral energy distribution (SED) modelling reveals effective temperatures of $6000$–$8500$ K, radii of $1.4$–$5.7\,R_\odot$, and luminosities of $3.3$–$38\,L_\odot$, indicating a heterogeneous population spanning multiple evolutionary stages. The BSS population exhibits only a mild central concentration, with a low $A^{+}$ parameter and an extremely low
stellar collision-rate proxy, implying weak mass segregation and an inefficient collisional channel. We find no significant photometric variability among the BSS candidates within TESS's sensitivity limits. Although WISE W3/W4 data initially suggested possible mid-infrared excesses, detailed image inspection and SPHEREx spectrophotometry indicate that these are caused by background contamination and blending, with no clear evidence of circumstellar
dust. The structural parameters derived from King-profile fitting ($r_c = 6.91^{+0.91}_{-0.73}$ arcmin, $r_t = 13.23^{+0.44}_{-0.43}$ arcmin) indicate a dynamically evolved, low-density system. Together, these results suggest that dynamical interactions are inefficient in Berkeley~18 and that binary
evolution is likely the dominant formation channel of BSSs.
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All authors D. Bisht, Ing-Guey Jiang, K. Belwal, D. C. Ç{\i}nar, Alok Durgapal, Shraddha Biswas, A. Raj, Geeta Rangwal, M. S. Bisht, M. Manu
High Energy Astrophysical Phenomena
Earliest simultaneous multi-color optical observations of GRB 230328B: from 41 seconds to the host-galaxy identification
T. Komesh, A. Pozanenko, N. Pankov et al. (+36 more)
We present a multiwavelength study of the long-duration gamma-ray burst GRB 230328B, combining
prompt gamma-ray observations with exceptionally early simultaneous optical photometry and extensive X-ray,
radio, and host-
galaxy follow-up. NUTTELA- TAO began simultaneous g'r'i' observations only 41 s after the Swift/BAT trigger, corresponding to approximately 16 s in the rest frame for the adopted host-
galaxy photometric
redshift of z_phot = 1.54+-0.06. Unlike sequential multiband measurements, these observations provide an instantaneous optical spectral slope without temporal-interpolation uncertainties. The early optical continuum remains statistically consistent with a constant slope from approximately 10^2 to 7*10^3 s, providing no evidence for rapid
dust destruction. The absence of significant color
evolution across the pronounced optical rebrightening at approximately 4*10^3 s demonstrates that it is achromatic and favours a dynamical rather than spectral origin. Broadband modelling shows that the optical, X-ray, and
radio evolution can be broadly explained by forward-shock emission with late energy injection, although alternative scenarios cannot be excluded. The red optical continuum is consistent with substantial line-of-sight extinction in a massive, dusty host
galaxy. Late-time observations are insufficient to constrain a typical GRB-associated
supernova at the adopted
redshift. GRB 230328B therefore provides a
benchmark for connecting the earliest simultaneous optical colors with afterglow dynamics and the host environment of a representative long GRB.
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All authors T. Komesh, A. Pozanenko, N. Pankov, A. Volnova, P. Minaev, R. Gill, D. Berdikhan, B. Grossan, Z. Maksut, Z. Abdullayev, S. Belkin, M. Krugov, A. Moskvitin, K. Baigarin, A. Tursynkan, E. Klunko, A. Tatarnikov, S. Zheltoukhov, V. Rumyantsev, A. Volvach, L. Volvach, O. A. Burkhonov, S.A. Ehgamberdiev, R. Inasaridze, L. Elenin, A. Krylov, M. Zheltobryukhov, S. B. Pandey, A. K. Ror, R. Gupta, V. Swain, V. Bhalerao, G. C. Anupama, S. Barway, R. Sánchez-Ramírez, A. J. Castro-Tirado, S. Antier, P. Beniamini, E. Abdikamalov
Cosmology and Nongalactic Astrophysics
Cosmography of the Sloan Basin of Attraction and Neighborhood
Daniel Pomarede, R. Brent Tully, Aurelien Valade et al. (+2 more)
The Sloan Great Wall is a dominant structure that is relatively nearby. As well as evident in
redshift survey maps, its presence is manifested in distortions to
cosmic expansion. Here,
Hamiltonian Monte Carlo forward reconstruction in a {\Lambda}CDM framework gives probabilistic density and velocity fields constrained by the Cosmicflows-4 compendium of
galaxy distances and radial velocities. Streamlines of the reconstructed velocity field started from arbitrary points in space can be followed to sinks, i.e. the minima of the gravitational potential, due to the distribution of mass. A basin of attraction encompasses the volume of all streamlines ending at the same sink. The solution can be assigned probabilities, with uncertainties associated with the imperfect data and the random nature of the {\Lambda}CDM model. The Sloan basin of attraction is by far the largest basin in the study region, extending across a diameter of ~0.13c. It can be described by velocity streamlines that converge on the Sloan Great Wall, by the reconstructed density field, and by the network of filaments of the V-web, formulated by shear in the velocity field. The discussion of these elements is augmented by a
video and interactive models. It is of interest to see the relationship of the Ho`oleilana
baryon acoustic oscillation feature with the Sloan basin of attraction.
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All authors Daniel Pomarede, R. Brent Tully, Aurelien Valade, Noam Libeskind, Yehuda Hoffman
Astrophysics of Galaxies
The GALAH Survey: Neutron-Capture Elemental Abundances for 350,000 Gaia-RVS spectra and the Chemodynamics of Accreted Structures
Pradosh Barun Das, Daniel B. Zucker, Aldo Mura-Guzmán et al. (+15 more)
We present a comprehensive data-driven spectroscopic analysis of 357,415 red
giant stars using
Gaia DR3 Radial Velocity Spectrometer (RVS) spectra (8460-8700 A; $R\approx11,500$), aimed at deriving homogenous
stellar parameters and elemental abundances (collectively referred to as
stellar labels). We employ The Cannon, a generative model based on 2747
giants in common with
GALactic Archaeology with HERMES (GALAH) DR4, adopting GALAH labels ($R\approx28,000$) for training. The resulting model predicts 11
stellar labels for RVS
giants: effective temperature ($T_{\rm eff}$), surface
gravity ($\log g$), projected rotational velocity ($v\sin i$), and abundances of [Fe/H], [Ca/Fe], [Si/Fe], [Ni/Fe], [Ti/Fe], as well as the
neutron-capture elements [Zr/Fe], [Ce/Fe], and [Nd/Fe]. Building on these results, we develop a probabilistic framework to chemically identify debris from the
Gaia-Sausage-Enceladus (GSE)
accretion event. A logistic regression classifier, optimized via Markov chain Monte Carlo sampling and trained on a small reference sample of GSE members and comparison
stars, identifies
stars with high GSE membership probabilities based solely on their chemical abundances, with the resulting candidates exhibiting distinctive abundance-ratio patterns, including [Ca/Ti], [Ti/Ce], and [Nd/Zr]. Applying independent kinematic constraints yields a robust sample of GSE candidates, demonstrating that the characteristic chemical signatures remain consistent after applying these constraints. This work demonstrates the potential of data-driven analysis techniques to extract detailed chemical information from medium-resolution spectra and establishes a framework for tracing
Galactic accretion events using chemical abundances.
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All authors Pradosh Barun Das, Daniel B. Zucker, Aldo Mura-Guzmán, Nicholas W. Borsato, Gayandhi M. De Silva, Sven Buder, Diane Feuillet, Thomas Nordlander, Melissa K. Ness, Sarah L. Martell, Janez Kos, Joss Bland-Hawthorn, Ken C. Freeman, Andrew R. Casey, Geraint F. Lewis, Dennis Stello, Richard de Grijs, the GALAH Collaboration
Earth and Planetary Astrophysics
Late infall of molecular cloud material reshaped the outer Solar System
Elishevah van Kooten, Sebastian Sjørring Lodal, Isaac Onyett et al. (+5 more)
Understanding the physicochemical
evolution of the outer protoplanetary disk is critical because it governed the distribution and delivery of key volatiles such as water and organic compounds to the inner, initially hot and volatile-poor terrestrial
planet-forming region. These materials are essential for establishing potentially habitable environments and directly influence the emergence of life on rocky
planets. Here, we move beyond the traditional building blocks of the outer disk, the carbonaceous chondrite groups, and examine their ungrouped counterparts, the anomalous chondrites, to constrain a coherent model of disk
evolution using Si, Mg, Fe, and Cr nucleosynthetic isotope systematics. Our results show that the outer disk was replenished through the addition of isotopically distinct molecular cloud material that contributed a significant fraction of mass (>30%) to the building blocks of the gas
giant accretion region. This late infalling material did not contribute to the main
accretion phase of the terrestrial
planets, which instead derived their volatile inventory solely from Ivuna-type planetesimals such as Ryugu and Bennu. In our model, these bodies formed at the inward-migrating water ice line. The inferred
accretion of these icy planetesimals in the inner disk represents a fundamental shift in our understanding of the
evolution of the
Solar System.
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All authors Elishevah van Kooten, Sebastian Sjørring Lodal, Isaac Onyett, Lasse Rasmus Pohlmann, Jean Bollard, Courtney Rundhaug, Martin Schiller, Martin Bizzarro
Earth and Planetary Astrophysics
Detection of stratospheric HCN and tropospheric CO in Uranus and the implication for their sources
T. Cavalié, R. Moreno, C. Lefour et al. (+16 more)
Uranus belongs to the category of ice
giants that are common in our
Galaxy. However, Uranus is one of the least explored and understood
planets in our
Solar System. This work aims to constrain the deep oxygen abundance of Uranus to better understand its formation. Another goal concerns the origin of exogeneous species, such as CO and HCN, found in the upper stratospheres of
giant planets. We used spectral mapping observations of the CO (J=3-2) and HCN (J=4-3) rotational submillimeter lines obtained with ALMA in 2022 and 2024. We combined them with radiative transfer and thermochemical modeling to determine the tropospheric abundance of CO and the deep O/H ratio of Uranus. We used radiative transfer simulations with physical models of various sources of external CO and HCN to constrain the vertical and meridional distributions of these species and narrow down the nature of their external sources. We also applied a wind
retrieval algorithm to search for zonal winds in the stratosphere of Uranus at the levels probed by the CO and HCN lines. We unambiguously detect tropospheric CO for the first time with a mole fraction of 5.8$\pm$0.3 ppb and stratospheric HCN with a mole faction of (1.8$\pm$0.2)$\times10^{-11}$ restricted to pressures lower than 0.2 mbar. Thermochemical calculations suggest that the deep interior of Uranus is enriched in oxygen with respect to the protoplanetary nebula by at least a factor of 52$^{+30}_{-20}$. We also find that the stratospheric CO is rather uniform over the observed latitudes and that the CO lines are best fit by an old comet impact model, in which a large comet hit the
planet several centuries ago. CO therefore has a dual origin in Uranus. Finally, we do not detect stratospheric winds from these data, but the CO data indicate that zonal winds in the 10$^\circ$S-10$^\circ$N latitudinal range are likely retrograde at submillibar pressures.
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All authors T. Cavalié, R. Moreno, C. Lefour, S. H. Luszcz-Cook, T. Fouchet, E. Lellouch, O. Carrión-González, B. Benmahi, S. Guerlet, G. Milcareck, N. Clément, M. Dobrijevic, I. de Pater, A. Moullet, J. I. Moses, J. Leconte, A. Spiga, M. T. Roman, L. N. Fletcher
Astrophysics of Galaxies
Primordial Binary Stars, Mass segregation and Fractality Effects on the Early Evolution of Young Open Clusters
Vahid Amiri, Francesco Flammini Dotti, Xiaoying Pang et al. (+4 more)
We want to understand how the combined effect of initial substructure,
primordial mass segregation, and
primordial binaries affects the dynamical
evolution of the cluster, and which one of these features is the most important to agree with observations. Methods. We use Nbody6++GPU to simulate the dynamics of
star clusters with initial substructure,
primordial mass segregation, and
primordial binaries, and we also study the relative importance of the processes. Initial models were generated by a modified version of McLuster, and we compared our results with observational data from Pang et al. 2022 database of open clusters. Our results show that
primordial mass segregation and binaries do not change the result already obtained in previous works, as the time scale on which initial substructure disappears is of the order of few Myrs. However, we also find that in the presence of initial substructure,
primordial mass segregation does not lead to an early expansion of the cluster. The processes in the core, discussed in previous works, lead to a loss of low mass
stars and early expansion, are postponed in the presence of initial substructure. Finally, we find from comparison with observed clusters that
primordial mass segregation is not a fundamental process to reproduce observational data.
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All authors Vahid Amiri, Francesco Flammini Dotti, Xiaoying Pang, A.W.H. Kamlah, Peter Berczik, Bekdaulet Shukirgaliyev, Rainer Spurzem
Astrophysics of Galaxies
A More Complex Than Expected Formation History of the Milky Way's Last Major Merger
Hai-Feng Wang, Guan-Yu Wang, Giovanni Carraro et al. (+3 more)
The
Gaia$-$Sausage$-$Enceladus (GSE) structure, widely recognized as the most recent major
accretion event experienced by our
Galaxy, is traditionally interpreted as the remnant of a single ancient merger that played a significant role in building the Milky Way's inner
halo. Most previous studies have characterized the GSE as a kinematically coherent population that originated from either a single progenitor or a recent infall event. Here, we present evidence for a more complex origin, based on data from the DESI and a novel unsupervised clustering
algorithm, GS$^3$ Hunter. Applying this method to local
halo stars near the
solar neighborhood, we identify 17 structures, including known systems such as Sequoia and GSE, as well as several previously unrecognized structures/
stellar streams. A more detailed analysis incorporating chronological, dynamical, and chemical dimensions reveals four distinct substructures within the GSE region, herein designated GSE$-$GSH1 (12 Gyr), GSE$-$GSH2 (10 Gyr), GSE$-$GSH3 (8 Gyr), and GSE$-$GSH4 (7 Gyr). Although all four are broadly consistent with the overall phase$-$space distribution and abundance patterns of the GSE, they display markedly distinct orbital actions and chemical abundances relative to previously reported results. This finding reveals an unprecedented level of internal complexity in the GSE's formation history and supports a scenario in which the GSE is not the remnant of a single
accretion event, but rather a composite structure assembled through multiple, sequential merger episodes during the early Milky Way.
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All authors Hai-Feng Wang, Guan-Yu Wang, Giovanni Carraro, Gražina Tautvaišienė, Joss Bland-Hawthorn, Thor Tepper-García