Solar and Stellar Astrophysics
Detection of hot subdwarf binaries and sdB stars using machine learning methods and a large sample of Gaia XP spectra
M. Ambrosch, C. Viscasillas Vázquez, E. Solano et al. (+10 more)
Hot subdwarfs (hot sds) are compact, evolved
stars near the Extreme Horizontal Branch (EHB) and are key to understanding
stellar evolution and the ultraviolet excess in
galaxies. We extend our previous analysis of
Gaia XP spectra of hot subdwarf
stars to a much larger sample, enabling a comprehensive study of their physical and binary properties. Our goal is to identify patterns in
Gaia XP spectra, investigate binarity, and assess the influence of parameters such as temperature, helium abundance, and variability. We analyse approximately 20000 hot subdwarf candidates selected from the literature, combining
Gaia XP data with published parameters. We apply Uniform
Manifold Approximation and Projection (UMAP) to the XP coefficients, which represent the
Gaia XP spectra in a compact, feature-based form, to construct a similarity map. We then use self-organizing maps (SOMs) and convolutional neural networks (CNNs) to classify spectra as binaries or singles, and as cool and helium-poor or hot and helium-rich. The spectra are normalised using asymmetric least squares baseline fitting to emphasise individual spectral features. The BP-RP colour dominates the similarity map, with additional influence from temperature, helium abundance, and variability. Most binaries, identified via the Virtual Observatory SED Analyser (VOSA), cluster in two filaments linked to main sequence companions. CNN classification suggests a strong correlation between variability and binarity, with binary fractions exceeding 60 percent for active hot subdwarfs.
Gaia XP spectra combined with dimensionality reduction and machine learning effectively reveal patterns in hot subdwarf properties. Our findings indicate that binarity and environmental density strongly shape the evolutionary paths of hot subdwarfs, and we identify possible contamination by main sequence and cataclysmic variable
stars in the base sample.
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All authors M. Ambrosch, C. Viscasillas Vázquez, E. Solano, A. Ulla, X. Pérez-Couto, E. Pérez-Fernández, A. Medžiūnas, M. Manteiga, C. Dafonte, A. Drazdauskas, L. Magrini, Š. Mikolaitis, V. Šatas
High Energy Astrophysical Phenomena
Pulse-resolved Classification and Characteristics of Long-duration GRBs with Swift-BAT Data.I. Precursors versus Main Bursts
Liang Li, Yu Wang, Jin-Jun Geng et al. (+2 more)
We present a systematic pulse-by-pulse analysis of 22 long-duration GRBs observed by Swift, each exhibiting a well-separated precursor before the main burst. We compare duration, spectral hardness ratio, minimum variability timescale (MVT), and spectral lag between these components. Both precursors and main bursts have durations and hardness broadly consistent with Type II GRBs. However, precursors show longer MVTs (by factors of 3-10) and diverse lags with near-zero median values, while main bursts display variable MVTs and positive lags. These differences suggest precursors may originate from distinct dissipation conditions, possibly due to cocoon shock breakout or early magnetically dominated
outflows. Despite temporal differences, both episodes are consistent with a single collapsar origin, providing no evidence for dual-progenitor events. Our findings support pulse-resolved classification and show that precursors offer critical insights into jet formation and pre-burst activity.
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All authors Liang Li, Yu Wang, Jin-Jun Geng, Yong-Feng Huang, Rong-Gen Cai
Solar and Stellar Astrophysics
Mixing-induced thermal instabilities and coronal condensations
B. Snow and A. Hillier
Cool, dense material is frequently observed to permeate the hot, tenuous
solar corona in the form of prominences, spicules and coronal rain. Both the cool material and surrounding corona exist at temperatures that are effectively thermally stable, in that their local radiative losses occur on relatively long timescales compared to the dynamics. However, as the
solar atmosphere evolves, driving mixing between the condensations and surrounding hot material, intermediate temperatures form, which can become subject to highly efficient radiative losses. The thermal energy lost due to radiation can far exceed the turbulent heating thus the system undergoes mixing-induced cooling. Here, a 3D radiative MHD simulation is performed of the shear-driven Kelvin-Helmholtz Instability (KHI) occurring between a cool condensation and the hot
solar corona. During the
evolution, thermal instabilities form naturally within the mixing layer, and grow with time to produce long, narrow structures that extend perpendicular to the
magnetic field. The thermal instabilities form self-consistently within the mixing layer as small isolated events, and are then stretched by the background flows to create long structures in relatively narrow planes. The turbulent flows agitate the condensations and cause them to fragment, creating smaller localised clumps of cool, dense (prominence-like) material that can merge and further fragment. In the presented simulation, the thermal instabilities act to replenish the cool, dense material lost due to mixing, with the total mass of cool material being approximately constant through time. By analysing the thermal energy loss due to optically-thin radiation, thermal instabilities are found to account for 15-20% of all radiative losses in the turbulent
plasma.
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High Energy Astrophysical Phenomena
Investigating the emission signatures of pulsar halo candidate HESS J1813-126
Agnibha De Sarkar
Extended gamma-ray sources surrounding middle-aged
pulsars, primarily observed at teraelectronvolt energies, have been interpreted as
pulsar halos, where relativistic $e^\pm$ diffuse into the interstellar medium and produce inverse-Compton (IC) emission. HESS J1813-126, associated with the energetic,
radio-quiet gamma-ray
pulsar PSR J1813-1246, has been suggested as a candidate
pulsar halo, though its nature remains uncertain. We interpreted the high-energy emission of PSR J1813-1246 using the synchro-
curvature (SC) radiation model and tested whether the gamma-ray spectral energy distribution (SED) of HESS J1813-126 can be explained as a
pulsar halo powered by PSR J1813-1246. We explain the X-ray and gamma-ray SEDs of the
pulsar using the SC framework. We further computed the transport and losses of $e^\pm$ injected by the
pulsar through time-dependent
diffusion-loss equations, exploring various common
pulsar halo transport models. The resulting IC emission was compared with
Fermi-LAT, H.E.S.S., HAWC, and LHAASO data. We present predictions for the surface brightness profiles (SBPs) and the aperture-dependent emission for the different transport models, providing key diagnostics for assessing the observability of HESS J1813-126 with current and future instruments. The SC framework successfully reproduces the emission of PSR J1813-1246. The SED of HESS J1813-126 can be consistently reproduced within different
pulsar halo frameworks, albeit with distinct predictions across different transport models. The corresponding SBP predictions and aperture-dependent emission offer testable signatures for future imaging atmospheric Cherenkov
telescopes, which will be crucial for discriminating between the transport models. We further examined the link between the
pulsar central engine and its extended
halo by comparing the pair multiplicities in the magnetospheric and
halo regions.
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Earth and Planetary Astrophysics
Gaussian perturbation interpretability for regression techniques to understand retrieval black-boxes
Jools D. Clarke, Kai Hou Yip and Nikolaos Nikolaou
Characterizing the atmospheres of large numbers of
exoplanets from their observed spectra can answer key questions about planetary populations. Traditional
retrievals are resource-intensive; machine-learning methods offer a fast alternative, but their complexity renders them black-boxes. Understanding how they reach their predictions allows us to inspect them for biases and verify predictions are based on physically plausible spectral features. Commonly used perturbation-based methods for explaining machine learning struggle with the strong correlations between inputs in spectra. We introduce Gaussian perturbation methodology for explaining black-box regression models with one-dimensional structured inputs, which addresses these issues by leveraging physical knowledge of spectral correlation. We compare it with naive perturbation of one wavelength bin at a time. Although traditional
retrievals are built on physical forward models, their mapping from spectrum to retrieved parameters is a black-box. We therefore first demonstrate this methodology using TauREx-III as a proxy black-box
retrieval on HST observations of HD 209458 b and JWST observations of HAT-P-26 b, showing the efficacy of Gaussian perturbation-based interpretability against a well-established, physically motivated
retrieval framework, before applying it to a convolutional neural network
retrieval of HAT-P-26 b. For the gases the
retrievals constrain, the importance from Gaussian perturbation agrees with the expected importance, including at the core of the $CO_2$ band of HAT-P-26 b, where a degeneracy inverts the response. The method shows which parts of a spectrum a
retrieval relies on, which the physics of its forward model alone does not, and shows the neural network has learned the same degeneracy.
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Astrophysics of Galaxies
Little Red Dots and Supermassive Black Hole Seed Formation in Ultralight Dark Matter Halos
Dongsu Bak and Jae-Weon Lee
We explore a possible mechanism for the formation of supermassive black hole (SMBH) seeds at the centers of ultralight dark matter (ULDM)
halos in the early Universe. We investigate the conditions under which high-
redshift baryonic gas, strongly confined by central solitonic cores of the
halos, undergoes direct and monolithic collapse. The solitonic core sets characteristic mass and length scales for the confined
baryons. Once the confined gas becomes self-gravitating, rapid inflow and shock heating may drive it into a high-temperature and high-density regime favorable for suppressing molecular cooling, without requiring a strong external UV background. We present semi-analytic scaling relations for the
halo mass,
soliton mass, baryonic core radius, and characteristic thermodynamic state of the gas, parametrizing the possible effects of baryonic contraction. These relations provide order-of-magnitude estimates of the characteristic range of SMBH seed masses as a function of
redshift. In this framework, pristine gas clouds satisfying the adopted thermal criterion may avoid efficient fragmentation and undergo rapid central collapse, potentially forming massive black hole seeds with characteristic masses of order $10^5 M_\odot$, while systems below the threshold may form compact
star clusters instead. The ULDM particle mass required to reproduce the inferred seed mass scale, $m \simeq O(10^{-22}){\rm eV}$, lies in a range favored by
galactic-scale observations, suggesting a possible connection between the characteristic scales of
galactic cores and early SMBH seeds. Our estimates indicate that favorable conditions for SMBH seed formation may arise at
redshifts $z \gtrsim 10$. Such conditions may be relevant to the young SMBHs inferred in some little red dots, which appear to be embedded in compact, dense, ionized gas.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Perturbative analysis of singularity-free cosmological solutions in unimodular Kaluza-Klein theory
J.C. Fabris, S. Faller and R. Kerner
The unimodular version of the Kaluza-Klein theory is briefly recalled, and its projection on the $4$-dimensional spacetime is constructed. Imposing unimodularity condition on the $5$-dimensional Kaluza-Klein metric, det$g_{AB}=1$ is equivalent with introducing cosmological term in Einstein's equations in $4$ dimensions, and with scalar field of the Brans-Dicke type. Singularity-free cosmological solutions with scalar field and with matter sources are constructed, and their basic properties analyzed, along the results obtained in previous publications. In the present paper,
attention is focussed on the perturbative analysis of cosmological solutions, providing a clue concerning their stability against small fluctuations.
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Earth and Planetary Astrophysics
Eta-Virginids: another asteroidal meteoroid stream
Jiri Borovicka, Pavel Spurny, Pavel Koten et al. (+5 more)
Eta-Virginids is a less-known meteor shower active in March. We investigated the meteoroids of this shower using fireball data from the European Fireball Network supplemented by
video data of faint meteors. We first derived the criteria for assigning meteors to this shower. A fragmentation model was then applied to selected shower fireballs with good deceleration data and light curves. Meteoroid fragmentation strengths and bulk densities were derived and compared with three other showers. We have confirmed the four year periodicity in the activity of eta-Virginids and their presence in the 3:1 mean motion resonance with Jupiter. The orbital period of four years was directly measured for the fireballs. Fainter meteors showed somewhat longer periods but the shower is poor in faint meteors. No member fainter than magnitude +1 was observed instrumentally. The physical properties of meteoroids are different from cometary streams and are similar to the Geminids. The limiting fragmentation strength of 0.5 MPa and typical bulk density of cm-sized meteoroids of 1500 kg/m3 suggest that the parent body is a carbonaceous asteroid. Besides Geminids, eta-Virginids is another stream of asteroidal origin. Some small meteoroids have densities around 2500 kg/m3. Three asteroids of the sizes between 40 - 120 meters have been found to have similar orbits but their relation to eta-Virginids remains uncertain.
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All authors Jiri Borovicka, Pavel Spurny, Pavel Koten, Gabriel Borderes Motta, Lenka Kotkova, Rostislav Stork, Dusan Tomko, Thomas Weiland
Astrophysics of Galaxies
A Low-mass Model of The Milky Way: The Disk Warp Resulting from A Galaxy Merger
Mingji Deng, Cuihua Du, Jian Zhang et al. (+2 more)
Previous studies have shown that disk warps can result from
galaxy mergers. Recent research indicates a noticeable decline in the rotation curve (RC) of the Milky Way (MW), suggesting the need for a new low-mass model to describe its dynamical features. This study constructs a new
Gaia-Sausage-Enceladus (GSE) merger model to characterize the RC features of our
galaxy. We use the GIZMO code to simulate mergers with various orbital parameters to investigate how the disk warp evolves under different conditions. This simulation demonstrates the evolutionary mechanism of disk warp, which arises due to the asymmetric gravitational potential of the dark matter (DM)
halo generated universally by
galaxy mergers. The results indicate that the tilt angle of the DM
halo partly reflects the gravitational strength at the $Z=0$ plane, while the gravitational strength on the disk plane reflects the amplitude of disk warp. We identify a dual-regime interaction mechanism driven by the asymmetric
halo potential. On short timescales, we find a distinct anti-correlation between the
halo's tilt angle and the disk's warp amplitude, indicating a `seesaw' mechanism of angular momentum exchange. On secular timescales, however, dynamical friction drives a global
alignment, causing both the
halo tilt and the warp amplitude to decay simultaneously. Furthermore, we demonstrate that high-inclination mergers can sustain long-lived prograde precession, where the persistent yet decaying gravitational torque maintains the prograde bending mode against differential wind-up.
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All authors Mingji Deng, Cuihua Du, Jian Zhang, Haoyang Liu, Zhongbcheng Li
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Leveraging rapid parameter estimates for efficient gravitational-wave Bayesian inference via posterior repartitioning
Metha Prathaban, Charlie Hoy and Michael J. Williams
Gravitational-wave astronomy typically relies on rigorous, computationally expensive
Bayesian analyses. Several methods have also been developed to perform rapid, approximate
Bayesian inference. We present a novel approach to leverage the results of these low-latency analyses to accelerate the final inference, whilst ensuring that the
Bayesian prior remains independent of the data. By combining the fast constraints from the simple-pe
algorithm with the nested sampling acceleration technique of posterior repartitioning, we demonstrate that our method can guide the nested sampler towards the most probable regions of parameter space more efficiently for signal-to-noise ratios (SNR) greater than 20, while mathematically guaranteeing that the final inference is identical to that of a standard, uninformed analysis. We validate the method through an injection study on signals with SNR $<$ 150, demonstrating that it produces statistically robust and unbiased results whilst providing speedups of up to 210%, with a mean speedup of 34% for SNRs $>$ 20. Importantly, we show that the performance gain provided by our method scales with SNR, establishing it as a powerful technique to mitigate the cost of analysing signals from current and future gravitational-wave observatories.
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Astrophysics of Galaxies
Galaxy-Galaxy Blending in SPHEREx Survey Data
Kim Dachan, Hyunmi Song, Yigon Kim et al. (+8 more)
The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) will provide all-sky spectral survey data covering optical to mid-infrared wavelengths with a spatial resolution of 6\farcs2, which can be widely used to study
galaxy formation and
evolution. We investigate the
galaxy-
galaxy blending in SPHEREx
datasets using the mock
galaxy catalogs generated from cosmological simulations and observational data. Only $\sim0.7\%$ of the
galaxies will be blended with other
galaxies in all-sky survey data with a limiting magnitude of 19 AB mag. However, the fraction of blended
galaxies dramatically increases to $\sim7$–$9\%$ in the deep survey area around the ecliptic poles, where the depth reaches $\sim22$ AB mag. We examine the impact of the blending in the number count and luminosity function analyses using the SPHEREx data. We find that the number count can be overestimated by up to $10$–$20\%$ in the deep regions due to the flux boosting, suggesting that the impact of
galaxy-
galaxy blending on the number count is moderate. However,
galaxy-
galaxy blending can marginally change the luminosity function by up to 50%\ over a wide range of
redshifts. As we only employ the magnitude limit at $K_s$-band for the source detection, the blending fractions determined in this study should be regarded as lower limits.
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All authors Kim Dachan, Hyunmi Song, Yigon Kim, Minjin Kim, Hyunjin Shim, Dohyeong Kim, Yongjung Kim, Bomee Lee, Jeong Hwan Lee, Woong-Seob Jeong, Yujin Yang
Instrumentation and Methods for Astrophysics
DESHIMA 2.0: A 200-400 GHz Ultra-wideband Integrated Superconducting Spectrometer
K. Karatsu, A. Endo, A. Moerman et al. (+26 more)
DESHIMA (Deep Spectroscopic HIgh-
redshift MApper) is a broadband integrated superconducting spectrometer (ISS) for millimeter (mm) / sub-millimeter (sub-mm) wave astronomy based on Kinetic Inductance Detectors (KIDs). This paper describes characterization of DESHIMA 2.0 in laboratory settings. The instrument features NbTiN superconducting microstrip (MS) filters with low-loss a-SiC:H dielectric and an ultra-wideband leaky-wave
antenna. A laboratory setup was designed, incorporating the cryostat housing cryogenic optics and ISS chip comprising 339 KIDs connected to MS filters tuned for (sub-)mm wave frequencies. Room-temperature mirrors on a hexapod stage allowed precise positioning and
alignment of optical elements. The sky-position chopper was positioned on a motor-controlled stage for fine-tuned control over its position and
alignment. Thanks to the multiplexing capability of KIDs, we could simultaneously measure multiple performance metrics across the entire frequency range. We showed that DESHIMA 2.0 achieved significant improvements in performance compared to its predecessor (DESHIMA 1.0): measured instantaneous frequency coverage was 200$-$400 GHz with a mean filter $Q_{filter}$ of $340 \pm 50$; instrument efficiency reached $\sim8$ %, indicating 4 times wider band coverage and 4 times higher sensitivity. The yield rate for MS filters exceeded 98 %. The estimated aperture efficiency from measured beam patterns agreed well with the designed value of approximately 70 %. The
telescope far-field beam patterns calculated from measured beam patterns also exhibited good agreement with design specifications. We also demonstrated validity of a new method of absolute frequency calibration using the data from beam pattern measurement.
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All authors K. Karatsu, A. Endo, A. Moerman, S. J. C. Yates, R. Huiting, A. Pascual Laguna, S. Dabironezare, V. Murugesan, D. J. Thoen, B. T. Buijtendorp, S. Cray, K. Fujita, S. Hähnle, S. Hanany, R. Kawabe, K. Kohno, L. H. Marting, T. Matsumura, S. Nakatsubo, L. G. G. Olde Scholtenhuis, T. Oshima, M. Rybak, F. Steenvoorde, R. Takaku, T. Takekoshi, Y. Tamura, A. Taniguchi, P. P. van der Werf, J. J. A. Baselmans
Earth and Planetary Astrophysics
Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by Hubble Space Telescope
Man-To Hui, David Jewitt, Max J. Mutchler et al. (+2 more)
We report the detection of the
nucleus of interstellar object 3I/ATLAS, using a
nucleus extraction technique on Hubble Space
Telescope (HST) observations taken between December 2025 and January 2026. The product of the V-band
geometric albedo, $p_V$, with the physical cross-section of the
nucleus is $0.22 \pm 0.07$ km$^{2}$, which corresponds to an effective radius of $1.3 \pm 0.2$ km if assuming a comet-like albedo $p_{V} = 0.04$. This size is in agreement with an independent estimate based on the reported nongravitational acceleration and activity of the interstellar object. If the measured photometric variations are solely due to the rotation of an aspherical
nucleus, the axis ratio must be $2:1$ or greater, and the rotation period $\gtrsim\!1$ hr. Leveraging the range of covered phase angles, we identified a significant opposition surge of $\sim\!0.2$ mag with a width of $3^{\circ} \pm 1^{\circ}$, which may include concurrent contributions from orbital plane crossing and tail projection, and determined a linear phase slope of $0.026 \pm 0.006$ mag degree$^{-1}$ for the coma
dust. Compared to the preperihelion brightening trend, 3I faded more rapidly on the outbound leg, following an activity index of $4.5 \pm 0.3$, not unusual in the context of
solar system comets. This activity asymmetry is further corroborated by a postperihelion coma surface brightness profile that is significantly shallower than its preperihelion counterpart. From discovery statistics, we infer that multiple interstellar objects resembling 3I probably went undetected prior to the discovery of 1I/`Oumuamua, unless the overall population possesses a steep size distribution.
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All authors Man-To Hui, David Jewitt, Max J. Mutchler, Jessica Agarwal, Yoonyoung Kim
Earth and Planetary Astrophysics
The dynamically hazardous asteroid 2025 TV$_{10}$. A new co-orbital asteroid of Venus
V. Carruba, R. Sfair and O. C. Winter
Twenty co-orbital asteroids of Venus are currently known, several of which may evolve into potentially hazardous asteroids (PHAs) over timescales of thousands of years. We report the identification and first dynamical characterization of 2025 TV$_{10}$, a newly discovered Venus co-orbital asteroid, and assess its potential collisional hazard to Earth. We performed numerical simulations of a large number of asteroid clones, and we studied their close encounters with Venus and Earth. The asteroid may leave its co-orbital orbit on timescales of 1200 yr. The orbit of 2025 TV$_{10}$ is one of the closest of the absolute minimum of the "minimum orbital intersection distance" (MOID) with Earth for the Venus co-orbital asteroids known to date. Owing to its orbital parameters, 2025 TV$_{10}$ represents one of the most dynamically hazardous members of the Venus co-orbital population identified to date. Its faint magnitude and restricted observability windows make future observations challenging but essential for constraining its orbit.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Primordial black holes and Scalar-Induced Gravitational Waves formed by inflation potential with non-trivial characteristics
Ruifeng Zheng and Yanqing Xu
The formation of
primordial black holes (PBHs) generally requires large density perturbations, which is widely supported by researchers. This paper studies the local coupling properties of the Starobinsky and Kachru-Kallosh-Linde-Trivedi (KKLT) potentials by introducing a linear Lorentzian-type coupling, which locally breaks the slow-roll conditions. We find that both positive and negative couplings can generate a considerable abundance of PBH. Additionally, we study the scalar-induced gravitational waves (SIGWs) generated by this model.
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High Energy Astrophysical Phenomena
Compton hump reverberation lag in the bright Seyfert 1 galaxy IC 4329A with NuSTAR
Samuzal Barua, Hengxiao Guo, Minfeng Gu et al. (+1 more)
Recent reverberation delay measurements have moved beyond the 10 keV X-ray range, providing evidence for the Compton hump (a.k.a. reflection hump) in the lag spectra. We report the relativistic reverberation of the reflection hump in the bright Seyfert\,1
galaxy IC\,4329A based on a long NuSTAR observation. We find a delayed response of the 20–30 keV X-ray band, with a lag time of $\sim1825$ s at frequencies $0.5-1.5 \times 10^{-4}$ Hz. The lag amplitude drops to $\sim195$ s as the frequencies increase to $(1.5-10)\times10^{-4}$ Hz. Including IC\,4329A, so far five sources have been explored for reflection hump reverberation. We perform reverberation modelling of the 3–50 keV lag-energy spectra using the general relativistic transfer function code, which provides independent timing-based measurements of the black hole mass $M_{\rm BH}=1.37_{-0.36}^{+0.33}\times10^8~M_{\odot}$ and the coronal height $h=2.45_{-2.36}^{+1.92}~R_{\rm g}$ (with uncertainties at 90% confidence). Within the uncertainties, the measured mass is found to be consistent with the previous finding. Furthermore, we undertake reflection spectroscopy to account for the hump feature and the associated relativistic effect using the time-averaged flux spectrum. Further sampling of the NuSTAR data (with a bin width of 0.2/0.4 keV below and above 10 keV) that reshapes the spectral resolution allows us to constrain the coronal temperature at $50.26_{-4.03}^{+5.58}$ keV – consistent with the previous result from the combined Suzaku and NuSTAR data.
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All authors Samuzal Barua, Hengxiao Guo, Minfeng Gu, Wenwen Zuo
Solar and Stellar Astrophysics
How large spectroscopic surveys are shaping our understanding of massive stars
S. Simón-Díaz
We present an overview of the main characteristics of several spectroscopic surveys designed to advance our understanding of the physical properties and
evolution of massive
stars. We also summarize key results obtained from the analysis of these
datasets, highlighting how the interpretation of some observables in the framework of massive-
star evolution is considerably more complex than previously anticipated.
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Solar and Stellar Astrophysics
Wave generation via oscillatory reconnection at a three-dimensional magnetic null point
Luiz A. C. A. Schiavo, Gert J. J. Botha and James A. McLaughlin
This work conducts a three-dimensional (3D), nonlinear magnetohydrodynamic (MHD) simulation to investigate wave generating, time-dependent reconnection around a
magnetic null point. A non-periodic perturbation (in the $xz$-plane) triggers oscillatory reconnection (OR) at the 3D null, resulting in a self-sustained oscillation with a constant period $P$. We investigate the response of the system using three distinct wave proxies (compressible parallel, compressible transverse and incompressible parallel) as well as Spectral Proper Orthogonal Decomposition for decoupling and analyzing the resultant MHD wave behavior. We find that OR generates a slow magnetoacoustic wave of period $P$ that propagates outwards in all directions along the spine and fan plane of the 3D null point. We also find the generation of a propagating Alfvén wave of period $P$, exclusively along the $y$-axis in the fan plane, i.e. in the direction perpendicular to the spine motion. These findings provide new insights into waves generated from a 3D null point and their implications for coronal seismology.
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Solar and Stellar Astrophysics
MgAl burning chain in M 54: the globular cluster-like properties of a nuclear star cluster
D. A. Alvarez Garay, A. Mucciarelli, P. Ventura et al. (+2 more)
In this study, we present the chemical abundances of Fe, Mg, Al, Si, and K for a sample of 233 likely member
stars of M 54. All the
stars were observed with the FLAMES high-resolution multi-object spectrograph mounted at the VLT. Our analysis confirmed the presence of a large
metallicity range in M 54, with the majority of the
stars having -1.8 < [Fe/H] < -1.0 dex and few
stars with [Fe/H] > -1.0 dex. The mean value of the total sample is [Fe/H] = -1.40 ({\sigma} = 0.22 dex). A Markov Chain Monte Carlo analysis revealed that the observed spread in [Fe/H] is compatible with a non-null intrinsic iron dispersion. We also found that the
metallicity distribution function and the broadening of the red
giant branch of M 54 are not compatible with a single age, but instead they suggest a wide age range from ~ 13 Gyr to ~ 1 - 2 Gyr or a smaller age range if a significant He enhancement (Y ~ 0.35/0.40) is present in the most metal-rich
stars. We identified among the
stars in M 54 the entire pattern of anticorrelations linked to the MgAl burning cycle. In particular, the metal-rich component displays a higher level of H-burning with the presence of more extended anticorrelations than the metal-poor component. No Mg-poor ([Mg/Fe]<0.0 dex)
stars are identified in M 54. The evidence collected so far cannot be explained neither with a globular cluster-like scenario nor with a
galactic chemical
evolution. The chemical properties of M 54 can be explained within a scenario where this system formed through the merging of two globular clusters, the metal-poor one with standard characteristics and the more metal-rich one with more pronounced chemical anomalies, a possibly younger than the first one. M 54 is confirmed as a key
stellar system for explaining the chemical
evolution of a
nuclear star cluster.
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All authors D. A. Alvarez Garay, A. Mucciarelli, P. Ventura, M. Bellazzini, S. Covella
Cosmology and Nongalactic Astrophysics
Cosmological analysis of the DESI DR1 Lyman alpha 1D power spectrum
J. Chaves-Montero, A. Font-Ribera, P. McDonald et al. (+63 more)
We present the cosmological analysis of the one-dimensional Lyman-$\alpha$ flux power spectrum from the first data release of the Dark Energy Spectroscopic Instrument (DESI). We capture the dependence of the signal on cosmology and intergalactic medium physics using an emulator trained on a cosmological suite of hydrodynamical simulations, and we correct its predictions for the impact of astrophysical contaminants and systematics, many of these not considered in previous analyses. We employ this framework to constrain the amplitude and logarithmic slope of the linear matter power spectrum at $k_\star=0.009\,\mathrm{km^{-1}s}$ and
redshift $z=3$, obtaining $\Delta^2_\star=0.379\pm0.032$ and $n_\star=-2.309\pm0.019$. The robustness of these constraints is validated through the analysis of mocks and a large number of alternative data analysis variations, with cosmological parameters kept blinded throughout the validation process. We then combine our results with constraints from DESI BAO and temperature, polarization, and
lensing measurements from Planck, ACT, and SPT-3G to set constraints on $\Lambda$CDM extensions. While our measurements do not significantly tighten the limits on the sum of
neutrino masses from the combination of these probes, they sharpen the constraints on the effective number of relativistic species, $N_\mathrm{eff}=3.02\pm0.10$, the running of the spectral index, $\alpha_\mathrm{s}=0.0014\pm0.0041$, and the running of the running, $\beta_\mathrm{s}=-0.0006\pm0.0048$, by a factor of 1.18, 1.27, and 1.90, respectively. We conclude by outlining the improvements needed to fully reach the level of confidence implied by these uncertainties.
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All authors J. Chaves-Montero, A. Font-Ribera, P. McDonald, E. Armengaud, D. Chebat, C. Garcia-Quintero, N. G. Karaçayl{\i}, C. Ravoux, S. Satyavolu, N. Schöneberg, M. Walther, J. Aguilar, S. Ahlen, S. Bailey, D. Bianchi, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, P. Doel, S. Ferraro, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, A. X. Gonzalez-Morales, G. Gutierrez, J. Guy, C. Hahn, H. K. Herrera-Alcantar, K. Honscheid, M. Ishak, R. Joyce, S. Juneau, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, J.M. Le Goff, L. Le Guillou, A. Leauthaud, M. E. Levi, M. Manera, P. Martini, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, G. Niz, N. Palanque-Delabrouille, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, H. Seo, J. Silber, D. Sprayberry, T. Tan, G. Tarlé, B. A. Weaver, C. Yèche, R. Zhou, H. Zou
High Energy Astrophysical Phenomena
TeV Gamma-Rays from the Low-Luminosity Active Galactic Nucleus NGC 4278: Implications for the Diffuse Neutrino Background
Chengchao Yuan and Ruo-Yu Liu
This work investigates the origin of the TeV emission detected by the Large High Altitude Air Shower Observatory (LHAASO) from NGC~4278, a
galaxy hosting a low-luminosity active
galactic nucleus (LLAGN). Considering two plausible scenarios, AGN jets and winds, we model the X-ray, GeV, and TeV emission during both TeV-low (quasi-quiet) and TeV-high (active) states. The spectral energy distributions can be explained either by single-zone leptonic emission from moderately relativistic jets or by lepto-hadronic emission from sub-relativistic winds. The best-fit parameters suggest that the transition from the quasi-quiet to the active state may be driven jointly by an enhanced
accretion rate and the jet deceleration or wind expansion. We further show that future MeV and very-high-energy $\gamma$-ray observations can discriminate between the leptonic and lepto-hadronic scenarios. Although the
neutrino flux from NGC 4278 predicted by the wind model is too low to be detected with current
neutrino observatories, a lepto-hadronic wind scenario can account for the PeV diffuse
neutrino background when adopting a local LLAGN density ($n_{\rm L,0}$) corrected for the TeV duty cycle ($\Delta T_{\rm TeV}/T$, the fraction of a LLAGN's lifetime spent in a TeV-emitting phase), $n_{\rm L,0}(\Delta T_{\rm TeV}/T) \sim 10^{-5}~\rm Mpc^{-3}$, as inferred from the LHAASO detection.
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Earth and Planetary Astrophysics · Cross-listed from cond-mat.mtrl-sci
High-Pressure Torsion-Induced Transformation of Adenosine Monophosphate: Insights into Prebiotic Chemistry of RNA by Astronomical Impacts
Kaveh Edalati, Jacqueline Hidalgo-Jimenez and Thanh Tam Nguyen
The origin of life is yet a compelling scientific mystery that has sometimes been attributed to high-pressure impacts by small
solar system bodies such as comets, meteoroids, asteroids, and transitional objects. High-pressure torsion (HPT) is an innovative method with which to simulate the extreme conditions of astronomical impacts and offers insights relevant to prebiotic chemistry. In the present study, we investigated the polymerization and stability of adenosine monophosphate (AMP), a key precursor to ribonucleic acid (RNA), in dry and hydrated conditions (10 wt% water) under 6 GPa at ambient and boiling water temperatures. Comprehensive analyses with the use of X-ray diffraction, Raman spectroscopy,
Fourier-transform infrared spectroscopy,
nuclear magnetic resonance, scanning electron microscopy, and
matrix-assisted
laser desorption/ionization time-of-flight mass spectrometry revealed no evidence of polymerization, while AMP partly transformed to other organic compounds such as nucleobase-derived fragments of adenine, phosphoribose fragments, dehydrated adenosine, protonated adenosine, and oxidized adenosine. The torque measurements during HPT further highlight the mechanical behavior of AMP under extreme conditions. These findings suggest that, while HPT under the conditions tested does not facilitate polymerization, the formation of various compounds from AMP confirms the significance of astronomical impacts on the prebiotic chemistry of RNA on early Earth. Keywords: Ribonucleic acid (RNA), Origin of life; Phase transformations; Chemical reactions, Small
solar system bodies
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Earth and Planetary Astrophysics
Coupled Thermal-Chemical Evolution Models of Sub-Neptunes Reveal Atmospheric Signatures of Their Formation Location
Marie-Luise Steinmeyer, Caroline Dorn, Aaron Werlen et al. (+1 more)
The observed masses and radii of sub-Neptunes are typically explained by the gas
dwarf and the water world scenarios. While their evolutionary history on a population level has been proposed as a method to distinguish between these compositions, previous evolutionary models, neglected the crucial role of atmosphere-interior chemical interaction. We present a novel
evolution framework for sub-Neptunes that combines the thermal
evolution with the chemical coupling of the atmosphere and interior. Using this model, we examine how
planets formed inside and outside the water-ice line can be observationally distinguished, with an emphasis on their atmospheric properties. Young sub-Neptunes store the majority of their volatile budget in the interior, regardless of formation location. Nevertheless, the atmospheric
metallicity is a factor 4 higher for the
planet formed outside the water-ice line. During cooling, hydrogen and oxygen exsolve from the interior, increasing the atmospheric mass fraction and counteracting the thermal contraction. Consequently, radius
evolution alone cannot distinguish between the two formation scenarios. Instead, the primary discriminators are the abundance of carbon-bearing species and the resulting atmospheric C/O ratio. For sub-Neptunes formed beyond the water-ice line, nearly all carbon resides in the gaseous phase. We find that high molar fractions of CH$_4$ ($>10^{-2}$) and H$_2$O ($> 5\times10^{-2}$), and a high C/O ratio $(> 5\times10^{-1})$ are indicative of formation outside the water-ice line. In contrast, sub-Neptunes formed inside the water-ice line exhibit strongly suppressed CH$_4$ abundances, yielding C/O ratios ranging from $10^{-7}$ to $10^{-1}$.
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All authors Marie-Luise Steinmeyer, Caroline Dorn, Aaron Werlen, Simon L. Grimm
Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
$\mathcal{R}^2$-corrected Tachyon Scalar Field Inflation, the ACT Data, and Phantom Transition
S.D. Odintsov and V.K. Oikonomou
Phantom divide line transitions are not possible in the context of single scalar field scalar-tensor theories. In this article we study a combined framework of a tachyonic minimally coupled single scalar field theory in the presence of an $\mathcal{R}^2$ correction term and with a rescaled Einstein-
Hilbert term of the form $\sim \lambda \frac{\mathcal{R}}{16\pi G}$. Such terms can be part of an $f(\mathcal{R})$
gravity which in the large
curvature regime yields such correction terms effectively. Alternatively, such terms can simply be
quantum corrections to the scalar field action. We aim to answer two questions, firstly if this framework can lead to phantom divide line transitions and secondly whether the resulting model can be compatible with the ACT data. The model we studied is an inverse square power-law model, well known from tachyon inflation models. As we show, the field equations can be cast in terms of the scalar field solely, however the resulting theory is distinct from a single scalar field theory, because the phantom divide line is crossed during inflation. Thus initially the tachyonic nature of the scalar field generates a phantom equation of state parameter, and during inflation the phantom divide line is crossed, with the effective equation of state parameter at the end of inflation being $w=-1/3$ which corresponds to the non-accelerating state of the Universe. The model is proved to be compatible with the ACT data, only when the
gravity during inflation is stronger than Einstein-
Hilbert gravity, with the effective gravitational constant during inflation being $\frac{G}{\lambda}$. The effective theory is valid only during inflation, thus Big-Bang nucleosynthesis is not affected by the rescaling of the Einstein-
Hilbert gravity. The feature of a phantom crossing in $f(\mathcal{R},\phi)$ frameworks is new in the literature.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Is Phantom Divide Crossing in General Relativity Completely Impossible? Shortcomings in canonical and minimally coupled scalar field and Possible Solutions in $k$-essence Models
Shin'ichi Nojiri, S.D. Odintsov and V.K. Oikonomou
General relativity has its successes at the local astrophysical level, however, it seems to be insufficient in describing the Universe at large scales. In this work we investigate how the most general field theories in the context of general relativity can accomodate a phantom-to-quintessence transition which may be essential element of realistic Dark Energy scenarios in the late Universe. As we demonstrate in a very detailed manner, this is impossible for a canonical and minimally coupled single scalar field theory, but it may be possible for ghost condensate theories like $k$-essence theories. We point out how the ghost instabilities may be eliminated, and we analyze the quantitative features of a $k$-essence theory that may realize a phantom-to-quintessence transition in the late Universe. We also qualitatively compare the difficulties and fine-tunings required for $k$-essence theories to realize a phantom-to-quintessence transition, and how such a transition is naturally realized in modified
gravity, without unnecessary fine-tunings and ghost eliminations.
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Cosmology and Nongalactic Astrophysics
Cross-correlating galaxies and cosmic dispersion measures: Constraints on the gas-to-halo mass relation from 2MASS galaxies and 133 localized fast radio bursts
Masato Shirasaki, Ryuichi Takahashi, Ken Osato et al. (+1 more)
We conduct a cross-correlation analysis between large-scale structures traced by the Two Micron All Sky Survey (2MASS)
galaxy catalog and the
cosmic dispersion measures of 133 localized fast
radio bursts (FRBs). The cross-correlation signal is measured as a function of the comoving separation $R$ between 2MASS
galaxies and background FRB sightlines, making full use of the available
redshift information for both
datasets. Our measurements are consistent with a null detection over the range $0.01 < R\, [h^{-1}\mathrm{Mpc}] < 1$. Using a
halo-based model in which free-electron density profiles are drawn from the hydrodynamical simulation IllustrisTNG-300 (TNG300), we show that the null signal at $R \sim 0.01\, h^{-1}\mathrm{Mpc}$ is inconsistent with the TNG300 prediction. This discrepancy indicates that the hot-gas mass fraction in
halos with masses of $10^{12-13}\, M_\odot$ hosting 2MASS
galaxies must be lower than that predicted by TNG300. A simple phenomenological modification of the TNG300 model suggests that the hot-gas mass fraction in
halos of $10^{12-13}\, M_\odot$ should be below $\sim 10\%$ of the global
baryon fraction in the nearby universe, implying the need for stronger feedback in this mass range. Our constraints are consistent with those inferred from X-ray emission and Sunyaev-Zel'dovich measurements in
galaxies, while providing a direct estimate of the hot-gas mass fraction that does not rely on electron-temperature measurements. These results demonstrate that
galaxy-FRB cross correlations offer a powerful probe of feedback processes in
galaxy formation.
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All authors Masato Shirasaki, Ryuichi Takahashi, Ken Osato, Kunihito Ioka
Cosmology and Nongalactic Astrophysics
A redshift survey of the nearby galaxy cluster Abell 2199 : No upturn of the faint-end slope of galaxy luminosity function
Jong-In Park, Hyunmi Song and Ho Seong Hwang
We determine the
galaxy luminosity function of cluster
galaxies in the nearby
galaxy cluster Abell 2199 (A2199), focusing on the faint-end slope down to $M_r \sim -14.5$. To achieve this, we augment the existing
dataset by adding
redshift data from our deep MMT/Hectospec survey and from the Dark Energy Spectroscopic Instrument (DESI), significantly improving the spectroscopic completeness down to $r_{\mathrm{petro},0} = 20.8$ within the central $30^\prime$ region. The resulting luminosity function is well described by a Schechter function with a characteristic magnitude $M^* = -21.30 \pm 0.27$ and a faint-end slope $\alpha = -1.23 \pm 0.05$. This faint-end slope is consistent with those measured in the nearby Coma and Virgo clusters and in a cluster from the TNG50 cosmological simulation, and is slightly shallower than that of field
galaxies. These findings indicate that the previously claimed steep faint-end upturn (with $\alpha \sim -2$) in nearby
galaxy clusters is not supported. Instead, they indicate that environmental processes in dense cluster cores does not seem to trigger the formation or survival of low-mass
galaxies, thereby preventing a steep faint-end upturn in the luminosity function.
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Solar and Stellar Astrophysics
Evaluating Classifications of Extremely Metal-poor Candidates Selected from Gaia XP Spectra
Riley Thai, Andrew R. Casey, Alexander Ji et al. (+2 more)
Extremely metal-poor
stars are intrinsically rare, but emerging methods exist to accurately classify them from all-sky
Gaia XP low-resolution spectra. To assess their overall accuracy for targeting metal-poor
stars, we present a high-resolution spectroscopic followup of 75 very metal-poor candidates selected from the catalog by R. Andrae, V. Chandra, and H. W. Rix. We discover 2 new extremely metal-poor ($\rm{[Fe / H]}<-3$)
stars and 20 new very metal-poor ($\rm{[Fe/H]} < -2$)
stars. Abundances of up to 22 elements are derived from 1D local thermodynamic
equilibrium analysis and kinematic parameters are derived using
Gaia astrometry and spectroscopic radial velocities. The chemodynamical properties are mostly consistent with expectations for
halo stars, but we discover an Mg-enhanced CEMP
star ($\mathrm{[Mg/Fe]} = 0.89$) and an Mg-poor
star from an accreted ultra-faint
dwarf galaxy. The
Gaia XP
metallicity estimates are consistent with our $\rm{[Fe/H]}$ measurements down to $\rm{[Fe/H]}\sim -3.0$, but estimates worsen in highly extincted regions. We find that 4 other XP-based
metallicity catalogs succeed in mitigating contaminants and can also classify metal-poor
stars robustly to $\rm{[Fe/H]}\sim -3.0$. Our results demonstrate the utility of
Gaia XP spectra for identifying the most metal-poor
stars across the
Galaxy.
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All authors Riley Thai, Andrew R. Casey, Alexander Ji, Vedant Chandra, Hans-Walter Rix
Cosmology and Nongalactic Astrophysics
Realizing the phantom-divide crossing with vector and scalar fields
Shinji Tsujikawa
In generalized Proca theories, characterized by a vector field with broken $U(1)$ gauge invariance, late-time
cosmic acceleration can be realized with a dark energy equation of state in the regime $w_{\rm DE} < -1$. In such scenarios, however, a phantom-divide crossing, as recently suggested by DESI observations, is not achieved without encountering theoretical inconsistencies. We incorporate a canonical scalar field with a potential, in addition to the vector field, and show that the phantom-divide crossing from $w_{\rm DE} < -1$ to $w_{\rm DE} > -1$ can occur at low
redshifts. We propose a minimal model that admits such a transition and identify the region of parameter space in which all dynamical degrees of freedom in the scalar, vector, and tensor sectors are free from ghost and
Laplacian instabilities. We further investigate the
evolution of linear cosmological perturbations by applying the quasi-static approximation to modes well inside the Hubble radius. The dimensionless quantities $\mu$ and $\Sigma$, which characterize the growth of matter perturbations and the bending of light rays, respectively, depend on the sound speed $c_\psi$ of the longitudinal scalar perturbation associated with the vector field. Since $c_\psi$ is influenced by the transverse vector mode, the model exhibits sufficient flexibility to yield values of $\mu$ and $\Sigma$ close to 1. Consequently, unlike theories such as scalar Galileons, the present model can be consistent with observations of
redshift-space distortions and integrated Sachs-Wolfe-
galaxy cross-correlations.
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High Energy Astrophysical Phenomena
Data-Driven Generation of Neutron Star Equations of State Using Variational Autoencoders
Alex Ross, Tianqi Zhao and Sanjay Reddy
We develop a machine learning model based on a structured variational autoencoder (VAE) framework to reconstruct and generate
neutron star (NS) equations of state (EOS). The VAE consists of an encoder network that maps high-dimensional EOS data into a lower-dimensional latent space and a decoder network that reconstructs the full EOS from the latent representation. The latent space includes supervised NS observables derived from the training EOS data, as well as latent random variables corresponding to additional unspecified EOS features learned automatically. Sampling the latent space enables the generation of new, causal, and stable EOS models that satisfy astronomical constraints on the supervised NS observables, while allowing
Bayesian inference of the EOS incorporating additional multimessenger data, including gravitational waves from LIGO/Virgo and mass and radius measurements of
pulsars. Based on a VAE trained on a Skyrme EOS
dataset, we find that a latent space with two supervised NS observables, the maximum mass $(M_{\max})$ and the canonical radius $(R_{1.4})$, together with one latent random variable controlling the EOS near the crust–core transition, can already reconstruct Skyrme EOSs with high fidelity, achieving mean absolute percentage errors of approximately $(0.15\%)$ for $(M_{\max})$ and $(R_{1.4})$ derived from the decoder-reconstructed EOS.
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