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Physics Astrophysics

astro-ph Watch Astrophysics

Monday 20 July

Astrophysics of Galaxies

The Intrinsic Multiphase Gas–Black Hole Connection across Scales in IllustrisTNG

Xiaoxia Zhang, Taotao Fang, Shulan Yan et al. (+2 more)

The relationship between supermassive black holes and the multiphase circumgalactic medium is central to understanding the co-evolution of galaxies and their central black holes. We investigate this relationship using the IllustrisTNG100 simulation with a sample of 5089 central galaxies at $z=0$, measuring the partial correlation between central black hole mass and the mass of cold ($T < 10^4$K), cool ($10^4 \le T < 10^5$K), warm ($10^5 \le T < 10^6$K), and hot ($T \ge 10^6$K) gas within $0.03R_{200}$, $0.15R_{200}$, and $R_{200}$, after accounting for stellar and dark matter halo mass. We find that after removing these confounding factors, black hole mass shows a significant negative partial correlation ($\rho \approx -0.37$) with cold gas within $R_{200}$ and $0.15R_{200}$, whereas warm and hot gas exhibit no substantial intrinsic correlation. The residual plane reveals a threshold pattern: galaxies with over-massive black holes show systematically reduced cold gas, consistent with the cumulative impact of AGN feedback. The anti-correlation persists across environments with a weak trend in local density, and varies with galaxy type (star-forming, green valley, and quenched). These results provide a quantitative multiphase diagnostic of AGN feedback in TNG and support a picture in which feedback progressively removes cold gas, offering testable predictions for future multiwavelength surveys.
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All authors Xiaoxia Zhang, Taotao Fang, Shulan Yan, Si-Yue Yu, Feng Yuan

High Energy Astrophysical Phenomena

Beyond Spin: QCD Magnetars

Rachid Ouyed

We present a unified framework in which anomalous X-ray pulsars (AXPs), soft gamma-ray repeaters (SGRs), superluminous supernovae (SLSNe-I), luminous fast blue optical transients (LFBOTs), and fast radio bursts (FRBs) originate from quark deconfinement in the core of a massive neutron star (NS). Spontaneous ferromagnetism in the deconfined phase generates ~10^18 G core fields, producing a highly magnetized hybrid star (HS) - the "QCD magnetar" - whose surface field is set by NS mass, not birth spin. The Quark-Nova (QN) that forms the HS ejects ~0.01M_sun of neutron-rich outer layers, powering a kilonova and leaving the HS crustless for centuries. During this phase, magnetic instabilities at the hadron-quark interface release rising flux ropes that power X-ray-quiet FRBs; as the crust reforms, the source evolves into an X-ray-loud AXP&SGR. Two parameters govern the model: a critical mass M_dec triggering deconfinement, and a critical period P_fast separating fast and slow rotators. Fast rotators inject spin-down energy into the QN ejecta, producing an LFBOT - directly observable once the SN ejecta is optically thin, or via binary accretion with no preceding SN; otherwise the LFBOT is reprocessed by the SN ejecta into an SLSN-I. A Bayesian Monte Carlo population synthesis reproduces the observed rates of AXPs&SGRs, SLSNe-I, and LFBOTs with M_dec ~2.1M_sun and P_fast ~5.5 ms, and predicts non-merger r-process signatures and kilonovae from isolated NSs, with or without an LFBOT. The QN ejecta also carries its own DM and RM, independent of environment, predicted to appear as excess dispersion and rotation measure in FRBs once Galactic, host, and intergalactic contributions are removed. These provide direct observational tests of the hadron-quark phase transition and ferromagnetic ordering in dense quark matter.
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Solar and Stellar Astrophysics

First double red giant Algol system with active mass transfer

Mikhail Kovalev, Hailiang Chen, Sufen Guo et al. (+7 more)

Double red giant stars are very important for studies of the stability of mass transfer, common-envelope evolution, and the formation of double white dwarfs with short orbital periods. However, no double red giant system undergoing mass transfer has yet been found. We present the discovery of a close Algol-type binary system composed of two red giant stars. This is the first known semi-detached system observed during the very short phase when the accretor has expanded into a red giant just before entering the common envelope phase. The $H_\alpha$ line suggests that the system has recently lost some material, which is now moving toward us. We present a consistent analysis of all the available spectroscopic and photometric observations of this system, constraining its orbital parameters and the fundamental properties of the components. Our findings are supported by a binary evolution model that successfully reproduces the currently observed parameters. The model suggests that the system will eventually merge into a single star.
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All authors Mikhail Kovalev, Hailiang Chen, Sufen Guo, Jiao Li, Hongwei Ge, Dengkai Jiang, Marina Burlak, Natalia Ikonnikova, Xuefei Chen, Zhanwen Han

Astrophysics of Galaxies

Anomalously high deuterium fractionation in a galactic translucent cloud: a challenge to chemical models

Gan Luo, Zhi-Yu Zhang, Thomas G. Bisbas et al. (+8 more)

Deuterated (D-) species have long been proposed to diagnose the physical conditions and chemical evolution of cold dense molecular clouds. While deuterium fractionation has been extensively measured in dense cores, observations in diffuse and translucent clouds remain rare. We report here the detection of DCN and DNC toward a translucent cloud ($A_{\rm V} =1.2\pm0.2$ mag, $n_{\rm H_2}$ = $3.9\pm0.2\times10^2$ cm$^{-3}$) through sensitive absorption observations with the IRAM NOrthern Extended Millimeter Array (NOEMA). This detection reaches the lowest column-density and volume-density regime in which deuteration has been observed so far. Interestingly, the observed DCN/HCN and DNC/HNC abundance ratios ($3.3\pm0.6\times10^{-3}$ and $3.6\pm1.2\times10^{-3}$, respectively), which are more than two orders of magnitude higher than the element abundance [D]/[H] (1.5$\times$10$^{-5}$), suggest an unexpected enhancement of deuterium fractionation in the translucent cloud. These results represent a significant departure from established chemical models considering deuterium fractionation, which predict negligible formation of D-molecules in such environments. Although it remains unclear how D-molecules built up their abundances in translucent gas, a dispersed dense core scenario could potentially explain the observed high deuterium fraction. This interpretation is consistent with the idea proposed by Price et al. (2003) more than two decades ago: a translucent cloud may be a transient, dynamically evolving structure formed through the dissipation of a dense molecular cloud.
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All authors Gan Luo, Zhi-Yu Zhang, Thomas G. Bisbas, Di Li, Serena Viti, Roberto Neri, Junzhi Wang, Siyi Feng, Ningyu Tang, Daniel R. Rybarczyk, Lingrui Lin

High Energy Astrophysical Phenomena

Optical observations on the young Type Ia SN 2021fxy with detached high velocity features

Liping Li, Jujia Zhang, Zhenyu Wang et al. (+5 more)

We present optical observations on the young type Ia supernova (SN Ia) SN 2021fxy obtained within a few days after the explosion, with a focus on its prominent high-velocity features (HVFs). It reached a $B$-band maximum of $M_{\rm max}(B) = -19.36\pm0.31$ mag, corresponding to a bolometric luminosity of $\sim 1.3\times10^{43}~\rm{erg~s^{-1}}$ with a synthesized $^{56}$Ni mass of $0.58\pm0.14$ M$_{\odot}$. The early spectra exhibit strong HVFs of intermediate-mass elements that are significantly detached from the photospheric components. In particular, the velocity of the Si II $\lambda6355$ HVFs follows a power-law evolution ($\beta \approx 0.1$), shallower than the expected photospheric velocity evolution expected for an assumed $n=10$ density profile ($\beta \approx 0.22$) under homologous expansion. This behavior is consistent with the HVFs forming in intrinsic ejecta structures at least partially decoupled from the bulk outer ejecta, providing a possible constraint on the explosion physics of SNe Ia.
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All authors Liping Li, Jujia Zhang, Zhenyu Wang, Xiaofeng Wang, Qian Zhai, Shengyu Yan, Bo Wang, Jinming Bai

Cosmology and Nongalactic Astrophysics

Simultaneous inference of isotropic and anisotropic polarization rotation angles for next-generation cosmic microwave background experiments

Yue Zhang, Chang Feng, Sen Li et al. (+1 more)

The ultrahigh-sensitivity polarization imaging of the cosmic microwave background (CMB) is a treasure trove for new physics. Searching for a predicted polarization angle rotation known as the cosmic birefringence effect is a pursuit of the next-generation CMB experiments, and this new polarization signal would be important for testing fundamental physics theories. However, such a delicate rotation effect can be confused by different mechanisms including polarized Galactic foregrounds and instrumental calibration effects. Also, the cosmological rotation effects may arise from both the background evolution and spatial fluctuations of axion-like particles, leaving complicated imprints on CMB polarization fluctuations. In this work, we establish a comprehensive modeling of cosmological rotation effects, the instrumental miscalibration of polarization angles and complex foreground polarization with higher-order fluctuations, and perform a Bayesian analysis to infer both isotropic and anisotropic rotation angles simultaneously for foreground-mitigated polarization datasets from submillimeter to millimeter wavelengths. We find that such a Bayesian formalism can effectively extract the rotation effects for the next-generation CMB experiments and investigate the impact of the known sources including Galactic foregrounds and lensing on the inferred cosmological signals. The method in this work is well suited for future polarization data analyses, and the inferred rotation effects may shed light on the nature of the axion-like particles.
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All authors Yue Zhang, Chang Feng, Sen Li, Filipe B. Abdalla

Solar and Stellar Astrophysics

Surrogate models for type II supernovae: Probing low-energy explosions and interaction-free regimes

Zhengyang Zhang, Shuai Zha, Nikhil Sarin et al. (+3 more)

To address the computational bottleneck of analyzing type II supernova samples from surveys such as the Legacy Survey of Space and Time, we present two STELLA-based neural-network surrogates: an interaction model for low-energy explosions with possible circumstellar-material (CSM) interaction and a photospheric model for standard interaction-free SNe IIP. Each uses an autoencoder to compress spectral energy distributions and an emulator to map physical parameters to the latent space. Latent-mixup regularization improves latent-space continuity, with ResNet blocks used for the interaction model and 2D CNNs for the photospheric model. Their normalized test-set reconstruction MSEs are approximately 9.1e-5 and 1.0e-4, respectively. Applied to SN 2005cs, the interaction model favors a low-mass progenitor, M_ZAMS = 10.40(+0.04/-0.05) M_sun, and confined dense CSM, providing a scenario consistent with direct imaging and helping resolve the historical mass discrepancy. For SN 2012aw, it recovers M_ZAMS = 11.05(+0.06/-0.06) M_sun, consistent with previous studies. For SN 1999em, the photospheric model gives M_ZAMS = 10.05(+0.07/-0.04) M_sun, broadly consistent with preexplosion imaging limits without explicit CSM modeling. These surrogates reduce full Bayesian inference from days to minutes and enable rapid physical characterization of large supernova samples.
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All authors Zhengyang Zhang, Shuai Zha, Nikhil Sarin, Takashi J. Moriya, Chengyuan Wu, Bo Wang

Solar and Stellar Astrophysics

Parker Solar Probe Observations of Preferential Heating of Protons over Alpha Particles near Turbulent Coherent Structures

Jiayang Xi, Tieyan Wang, Daniel Verscharen et al. (+15 more)

Solar wind alpha particles exhibit preferential heating and acceleration relative to protons; however, their behavior in the vicinity of turbulent coherent structures remains less understood. We report the first evidence of localized alpha particle and proton heating within coherent structures identified using the Partial Variance of Increments (PVI) method, based on Parker Solar Probe (PSP) observations. Our results show that high-PVI events are associated with significant, species-dependent temperature enhancements: protons undergo a relative larger temperature increase than alpha particles. This preferential proton heating produces a localized decrease in the alpha-to-proton temperature ratio, indicating that the plasma is driven toward thermal equilibration between species. The heating is also anisotropic, being dominated by enhancements in the perpendicular temperature. These temperature-signatures coincide with a pronounced reduction in the normalized alpha-proton differential flow speed and a localized minimum in the Coulomb collision age, suggesting that the relaxation is affected primarily by collisionless kinetic effects. These findings provide new insight into the intermittent energy conversion and ion thermodynamics in the solar wind.
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All authors Jiayang Xi, Tieyan Wang, Daniel Verscharen, Yan Yang, Luca Sorriso-Valvo, Xinyi Wang, Wenhao Chen, Zuzheng Chen, Zeren Zhima, Chao Xiao, Xiangcheng Dong, Jin Liu, Xiang Li, Guoqi Liu, Naifei Gou, Xiaoxiao Qin, Malcolm Dunlop, Jinbin Cao

High Energy Astrophysical Phenomena

Optically Thick Outflow Driven by Supercritical Accretion May Explain Little Red Dots

Jun-Rong Liu, Hua Feng and Luis C. Ho

Recent JWST observations have revealed a population of compact, optically red sources known as Little Red Dots (LRDs). A popular interpretation is that LRDs host massive black holes embedded in dense gaseous envelopes, yet the physical origin of such envelopes remains unclear. We propose that the optically thick outflow driven by supercritical accretion onto black holes may explain the envelope. Based on an analytic radiative hydrodynamic outflow model, we relate the outflow properties to the black hole mass $M$ and dimensionless accretion rate $\dot{m}$. With $M\sim10^5-10^7\,M_\odot$ and $\dot m \sim 1500-5000$, the outflow photosphere reaches luminosities of $10^{43}-10^{45}\,\rm erg\,s^{-1}$ and temperatures of $\sim 3000-6000$ K, effectively matching the red optical continua observed in LRDs. The presence of a thick scattering region beyond the photosphere is central to deciphering the distinctive properties of LRDs. For each object, if one derives $M$ and $\dot{m}$ from the observed luminosity and temperature, while accounting for both kinematic broadening and scattering effects, the model predicts an emission line FWHM consistent with observations within a factor of 1.5 for more than 80% objects. Furthermore, with Cloudy simulations, we find that the partially ionized gas beyond the photosphere produces Balmer breaks broadly consistent with measurements.
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Sunday 19 July

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

The impact of physically motivated calibration errors on search pipeline detection parameters for broadband burst Signals

Milan Wils, Brad Ratto, Jeffrey S. Kissel et al. (+4 more)

Imperfections in the calibration of gravitational wave observatories introduce frequency dependent amplitude and phase errors on the measured GW signal. Previous unmodelled burst searches have approximated these effects using prescriptions such as a uniform amplitude rescaling or a constant time shift, which do not capture the frequency-dependent structure of calibration errors. This limitation is problematic for core-collapse supernovae, whose predicted GW signals occupy a wide frequency band and exhibit complex time-frequency morphology. In this work, we investigate how realistic calibration errors affect burst search pipelines by combining analytical modelling with large-scale injection campaigns. First-order estimates are derived to quantify how frequency-dependent amplitude and phase errors influence detection statistics such as the coherent network SNR and the correlation coefficient. These calculations predict that the relative impact on the coherent network SNR scales with the signal strength until it reaches an asymptotic value. The effect on the correlation coefficient is most pronounced near the detection threshold and is entirely suppressed at high SNR ratio. Injection studies confirm that calibration errors do modify the detection statistics, but show that the dominant contribution arises indirectly through changes in the number of time-frequency pixels selected in an event. Despite these measurable variations, detection efficiencies as a function of distance differ by less than one percent across all tested waveforms, and explosion-energy limits remain dominated by astrophysical uncertainties rather than calibration uncertainty. These results demonstrate that, at current detector sensitivity, realistic calibration errors have minimal impact on the detectability of broadband GW burst signals. The impact of calibration errors on parameter estimation is left for future work.
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All authors Milan Wils, Brad Ratto, Jeffrey S. Kissel, Tjonnie G. F. Li, Marek J. Szczepańczyk, Gabriele Vedovato, Michele Zanolin

Astrophysics of Galaxies

A Rest-frame K-band View of Bulge Growth in Massive Star-forming Galaxies at z~2

Ken-ichi Tadaki

We study bulge formation in 16 massive dusty star-forming galaxies at z~2 by combining JWST MIRI F770W imaging, NIRCam imaging in eight bands, and ALMA 870 micron data. At the median redshift of z=2.18, the F770W band probes the rest-frame 2.4 micron light, close to the rest-frame K band, and traces the stellar mass distribution with much less dust attenuation than the NIRCam bands. The effective radii measured in F770W agree with those of the 870 micron dust emission, with a median ratio of 0.95, and are typically 23% smaller than those in F444W. The rest-frame K-band imaging is therefore essential to measure the true extent of the stellar mass, which is as compact as the dusty star-forming regions. In stacks of the 16 galaxies at a matched resolution, the 870 micron and F770W profiles agree closely out to 8 kpc, and a bulge plus disk decomposition of the F770W stack gives a bulge-to-total ratio of 0.50. Fitting the SEDs in radial bins with CIGALE, we find that the attenuation decreases from A_V = 3.7 mag at the center to 1.9 mag at r >~ 7 kpc, while the specific star formation rate profile is flat. In most of the galaxies, the bulge is already in place inside the dusty core. In two galaxies, by contrast, the 870 micron emission is 3 to 6 times more compact than the F770W light, which suggests that they are still in the bulge-building compaction phase. Our sample may thus catch massive galaxies in several phases of bulge formation.
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Instrumentation and Methods for Astrophysics

Imaging X-ray Polarimetry Explorer

Brian Ramsey, Paolo Soffitta and Martin C. Weisskopf

In 1971 a group from Columbia University detected polarized X rays from the Crab Nebula at about 3 sigma level during a short sounding rocket flight (Novick et al. (1972)). This measurement, later confirmed to high precision (greater than 20 sigma) by the OSO-8 satellite in 1976/77 (Weisskopf et al. (1978)) marks the real beginning of astrophysical X-ray polarimetry. Since then and before the launch of IXPE, surprisingly few additional measurements had been made. X-ray polarimetry is a demanding field of study. To achieve per-cent-level polarization sensitivity, the collection of more than 1E6 photons in a polarization-sensitive detector is required. As most cosmic X-ray sources are much fainter than the Crab, to do a systematic study of the X-ray sky demands a highly-sensitive payload and a dedicated mission. Further, as the expected polarization will vary with location in extended sources, such as supernovae and pulsar wind nebula, it is vital that the polarimeter should be imaging. First proposed in its current form in 2007, and selected for funding in 2017, the Imaging X-ray Polarimetry Explorer (IXPE) fits this bill. In this chapter we describe: the design and construction of the IXPE payload and spacecraft; its testing and launch; commissioning activities; science operations and; a high-level overview of IXPE science to date. Finally, we conclude with the current status of the Observatory (as of 2025) and prospects for continued operations.
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Instrumentation and Methods for Astrophysics

Tolerancing the PIAA-ZWFS: a practical and robust wavefront sensor that approaches the fundamental sensitivity limit

Adam K. Taras, Sebastiaan Y. Haffert and Louis Desdoigts

High-contrast imaging demands extremely sensitive wavefront sensing to correct atmospheric effects and surface errors. While the limits of the sensitivity of a wavefront sensor are well known, a practical, robust design that saturates these limits remains elusive. This work further investigates the PIAA-ZWFS (Phase-Induced Amplitude Apodization-Zernike Wavefront Sensor). In previous work, we developed a framework to optimise its design, maximising Fisher information per frame in the presence of phase aberrations. In these proceedings, we study the effect of various manufacturing and alignment errors in the system on the overall performance. We employ both traditional Monte Carlo sampling and a 2nd order expansion using our auto-differentiable simulator. The performance of the PIAA-ZWFS is not significantly degraded by these errors at values typical for manufacturing.
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Solar and Stellar Astrophysics

A universal scaling between damping time and period of quasi-periodic pulsations from solar EUV brightenings to X-ray stellar flares

Daye Lim, Tom Van Doorsselaere, Valery M. Nakariakov et al. (+5 more)

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2,146 EUV brightenings observed with Solar Orbiter/EUI and 300 EUV solar flares observed with SDO/AIA, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism.
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All authors Daye Lim, Tom Van Doorsselaere, Valery M. Nakariakov, S. Krishna Prasad, David Berghmans, Laura A. Hayes, Kyung-Suk Cho, Sujin Kim

Solar and Stellar Astrophysics

SpecFANN: Spectral Fitting via Artificial Neural Networks I. A deep learning based fastwind emulator and fitting suite

M. Abdul-Masih, C. Hawcroft, T. Lechien et al. (+14 more)

The importance of massive stars cannot be overstated: they are powerful probes of the early universe, play a vital role in the chemical and mechanical evolution of their host environments and their end products allow us to study the most extreme physics in the universe. Obtaining accurate stellar and surface parameters for large samples of massive stars is vital to our understanding of how they evolve, and how their births, lives and deaths affect their surroundings. With the large volume of data expected from upcoming spectroscopic surveys, computational limitations will likely be the most important bottleneck impeding our progress. To address this and dramatically decrease computing times, we aim to develop a robust emulator for the FASTWIND radiative transfer and spectral synthesis code. Additionally, we aim to explore alternative fitting methods that have not been feasible until now due to computational costs. We calculate a set of FASTWIND synthetic spectra of OB-type stars, and we train a collection of neural networks to emulate these models. We also develop the open-source python package SpecFANN, which provides users with a suite of fitting methods that can be used with these or other user-generated neural networks. The majority of the trained neural networks reach average accuracies of better than ~0.01-0.1% for photospheric lines and better than ~0.1-1% for wind lines. SpecFANN is able to obtain robust and accurate stellar parameters that are consistent with the literature for a sample of 52 early-type stars. Using SpecFANN we find that we can achieve the same fit in ~1/360,000 of the time when compared to alternative techniques that rely on on-the-fly FASTWIND computations. We have demonstrated that neural networks offer a viable path forward to address the computational limitations of our current atmosphere analysis and stellar parameter determination methods for hot stars.
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All authors M. Abdul-Masih, C. Hawcroft, T. Lechien, S. Simon-Diaz, H. Sana, K. Deshmukh, J. Vrancken, J. I. Villaseñor, J. Müller-Horn, A. J. Kalita, B. Ludwig, J. Bodensteiner, D. M. Bowman, A. Escorza, G. Holgado, J. Puls, A. de Vicente

Cosmology and Nongalactic Astrophysics

Constraints on Primordial Black Hole Dressed by Dark Matter Halo from Microlensing Effect of Fast Radio Bursts

Hong-Rui Tao, Huan Zhou, Cheng-Gang Shao et al. (+2 more)

Primordial black holes (PBHs) are not only considered as a candidate for dark matter, but also as potential sources of gravitational waves from binary black hole mergers by the LIGO-Virgo-KAGRA and as seeds for the supermassive black holes observed by the James-Webb Space Telescope, thereby remaining intense interest in cosmology and astrophysics. Fast radio bursts (FRBs) are bright millisecond-duration radio transients whose physical origin remains elusive, which have rapidly developed into one of the most active and rapidly evolving fields in astronomy. The microlensing effect of FRBs offers a clean and powerful probe of PBHs, especially in the mass range above stellar-mass window. In this work, we derive a complete transformation that converts any upper limit on the abundance of PBHs originally derived for `bare' PBHs with monochromatic mass distribution, into the corresponding constraint on `dressed' PBHs with arbitrary extended mass distributions. Based on this framework, we estimate the future constraints on the dressed PBH abundance \(f_{\mathrm{PBH}}\) from FRB observations assuming an expected sample of \(10^5\) FRBs accumulated over the next decade well within the projected detection capabilities of SKA. Our results indicate that including halo enhancement tightens the upper limits on \(f_{\mathrm{PBH}}\) by approximately one order of magnitude, with the most stringent constraint reaching \(\sim10^{-4}\) for the typical mass range from stellar-mass to intermediate-mass black holes.
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All authors Hong-Rui Tao, Huan Zhou, Cheng-Gang Shao, Xiao-Long Gong, Zheng-Xiang Li

Instrumentation and Methods for Astrophysics

Calibration database design for the wide-field X-ray telescope on board the Einstein Probe

Huaqing Cheng, Hai-Wu Pan, Yuan Liu et al. (+6 more)

The wide-field X-ray telescope (WXT) is the primary payload of the Einstein Probe (EP) mission. Utilizing innovative lobster-eye micro-pore optics (MPO), it achieves an unprecedented combination of a large instantaneous field-of-view (FoV) and high detection sensitivity. Precise scientific data analysis relies fundamentally on a standardized and robust calibration database (CALDB). In this paper, we present the design and implementation of the WXT CALDB. The database is designed to manage calibration data for 12 independent WXT modules and 48 CMOS sensors, incorporating both extensive on-ground calibration results and dynamic in-orbit updates. We detail the directory hierarchy, naming conventions, functionality, and the specific formats of key calibration products. The WXT CALDB is seamlessly integrated into the WXT data analysis pipeline, ensuring high data integrity and maximizing the scientific return of the mission.
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All authors Huaqing Cheng, Hai-Wu Pan, Yuan Liu, Donghua Zhao, Wenxin Wang, He-Yang Liu, Chen Zhang, Zhixing Ling, Weimin Yuan

Instrumentation and Methods for Astrophysics

Onboard catalog of known X-ray sources for EP-WXT

He-Yang Liu, Haiwu Pan, Yuan Liu et al. (+14 more)

The Einstein Probe (EP) is dedicated to explore the dynamic X-ray universe and capture transient events in real time with its Wide-field X-ray Telescope (WXT). However, WXT's unprecedentedly large instantaneous field of view, exceeding 3,600 square degrees, simultaneously encompasses numerous known X-ray emitters. Distinguishing genuine novel transients from these persistent sources is a critical observational challenge. To resolve this, EP-WXT incorporates a dedicated reference catalog of known X-ray sources directly into its onboard data processing and triggering system. In this paper, we detail the compilation of this onboard catalog. By merging data from the ROSAT All Sky Survey, the MAXI source list, and a curated stellar flare candidate catalog, we constructed a robust baseline database of 9,000 sources. This catalog provides coordinates, baseline count rates, and spatial veto radii for exceptionally bright emitters. Real-time cross-matching against this database effectively decouples known background sources from the transient alert stream. Recent in-orbit operations validate the high stability and efficiency of this catalog-driven trigger system. Notably, the catalog successfully masked over 6,100 potential triggers from known active stars. This proves its essential role in ensuring EP's rapid and accurate response to genuine astrophysical discoveries.
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All authors He-Yang Liu, Haiwu Pan, Yuan Liu, Hong-Bo Cai, Yuyin Tan, Haitao Xu, Changbin Xue, Xiaojin Sun, Yifan Chen, Zhiwei Cheng, Ying Li, Yulong Xue, Huaqing Cheng, Xuan Mao, Zhixing Ling, Chen Zhang, Weimin Yuan

Astrophysics of Galaxies

The initial conditions and initial mass functions of Alpha Persei, Pleiades and Praesepe

L. Hobart, H. Baumgardt and S. Sweet

We have determined the initial mass function (IMF) and structural properties of the open clusters Alpha Persei, Pleiades and Praesepe using Gaia DR3 astrometry and photometry. Cluster members were identified using primarily Gaia astrometry, supplemented with near-infrared UKIDSS and optical HIPPARCOS survey data with stellar masses down to $0.10-0.17$ MSun. We measure and correct for unresolved binaries in each cluster using photometry and Monte Carlo simulations in a Bayesian framework, finding present-day fractions between ($20.0\pm0.8$)% and ($23.8\pm1.2$)%. Through a novel approach that combines N-body simulations with machine learning emulators and MCMC algorithms, we have also determined the most probable initial number of stars, binary fraction, half-mass radius and mass function under the assumption that early gas removal does not significantly influence the subsequent cluster evolution. We find a best-fitting initial mass function described by a three-stage broken power-law distribution with break masses in the ranges $0.24-0.50$ MSun and $0.91-1.20$ MSun and average slopes of $\alpha_{\rm med}=1.72\pm0.09$ and $\alpha_{\rm high}=2.98\pm0.22$. While the low-mass slope remains sensitive to the adopted mass-luminosity relation, this IMF reproduces the present-day clusters well once their dynamical evolution is taken into account. We find evidence of scatter in the high mass IMF between the individual clusters, as described by the measured dispersions in mass function slopes of ${\sigma}_{\rm high}=0.29\pm0.16$, whereas the intermediate mass slope shows no significant variation with ${\sigma}_{\rm med}=0.00\pm0.14$. Our findings provide additional evidence for a (compared to a Salpeter MF) top-light IMF and a cluster-to-cluster variation of the IMF. They therefore provide constraints on the universality of the IMF and its dependence on environmental conditions.
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Cosmology and Nongalactic Astrophysics

One Year and One Night to 1% in $\mathbf{H_0}$: Efficient Spectroscopic Strategy for Dark Siren Cosmology

Yixuan Dang, Ariel J. Amsellem, Ignacio Magaña Hernandez et al. (+2 more)

Gravitational-wave events from compact binary coalescences (CBCs) can be used as standard sirens: they encode the luminosity distance to their sources, which yields a measurement of the Hubble constant ($H_0$) if the source's redshift is known. In the absence of an electromagnetic counterpart, $H_0$ can still be inferred statistically from the cataloged galaxies within the event's sky localization volume via the dark siren, or galaxy catalog method. In this work we examine how the depth of a volume-limited galaxy catalog affects this inference and identify the most efficient spectroscopic strategy for an unbiased measurement of $H_0$ at a median precision of $0.98\%$. The strategy is to carry out spectroscopic surveys to a depth of $r \sim 19$ for the ten best-localized events observed in one year by the LIGO Hanford, LIGO Livingston, and LIGO-India network at A$^\#$ sensitivity. Such a campaign requires a minimum of one night of electromagnetic follow-up observations, or five nights under the most conservative assumptions. For the Hanford, Livingston, and Virgo network at A$+$ sensitivity, the same ten-event strategy yields a median precision of $2.63\%$.
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All authors Yixuan Dang, Ariel J. Amsellem, Ignacio Magaña Hernandez, Antonella Palmese, B. S. Sathyaprakash

Astrophysics of Galaxies

Ongoing Morphological Transformation from Elliptical to Spiral Galaxies Induced by Gas Accretion from Neighboring Galaxies

Gene Yun, Sang Hyeok Im, Ho Seong Hwang et al. (+2 more)

We present results from spectroscopic observations of three interacting galaxy systems, each associated with a primary galaxy - VCC 1748, SDSS J1106, and 2MASX J0834 - which have been proposed as potential examples of reverse morphological transformation from elliptical to spiral types driven by external gas accretion. Using Gemini/GMOS longslit spectroscopy, we obtain spectra along longslits connecting the regions of interest between the two galaxies in each pair. We model the spatially binned spectra with the Penalized Pixel-Fitting (pPXF) method to derive velocity fields and radial profiles of gas-phase metallicity, stellar metallicity, and stellar population age. Among the three systems, only the VCC 1748 system provides spectra with sufficiently high signal-to-noise ratios (S/N) to reveal coherent velocity and metallicity structures extending between the companion spiral galaxy and its elliptical target. The velocity profile along the gas-bridge of the system is smooth and continuous from the companion spiral arm to the target galaxy, and the elliptical target, VCC 1748, exhibits younger stellar populations and a high star formation rate (SFR) in its newly formed disk regions compared to its central parts. These findings indicate in-situ star formation at the newly-formed spiral arm of the elliptical target, consistent with ongoing gas inflow from the spiral companion into the elliptical target. Altogether, the results capture an intermediate stage of morphological evolution in which an originally quiescent elliptical galaxy begins to assemble a rotationally supported disk through interaction-driven gas accretion.
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All authors Gene Yun, Sang Hyeok Im, Ho Seong Hwang, Changbom Park, Hyeonguk Bahk

Astrophysics of Galaxies

Super-Jeans Fragmentation and Supply-Limited Accretion: Environment-Dependent Co-Evolution of Low- and High-Mass Cores

Dan Miao, Guang-Xing Li and Patricio Sanhueza

Protostellar core formation and growth in high-mass star-forming regions remain key to understanding massive star birth. We analyze the masses of 839 cores (resolved at scales of a few thousand au) from the ASHES project targeting 39 massive infrared dark cloud clumps. The masses of the three most massive cores scale linearly with the total core mass. They maintain a constant mass fraction of ~25%, 16%, and 10% along the mass growth sequence. These fractions reveal that the progenitor seeds destined to become high-mass cores establish their mass dominance very early. Additionally, the Gini coefficient (a statistical measure of inequality) of the core mass distributions increases along the mass growth sequence, confirming that the relative population of low-mass cores builds up toward later stages. This points to an environment-dependent fragmentation picture: central prestellar seeds rapidly evolve into high-mass cores via transport-driven super-Jeans fragmentation under rapid, non-stationary mass accumulation in high-density, turbulent hubs, subsequently maintain their dominance through supply-limited synchronized growth (at R<1 pc, n_H2>10^5 cm^-3), while the formation of the surrounding low-mass cores is relatively delayed due to their lower gas densities and the lack of non-stationary inflow acceleration effect, resulting in their continuous emergence through Jeans-like fragmentation in lower-density envelopes. This picture is consistent with a gravity-driven scenario where the local free-fall time is the controlling factor. Our analysis suggests that non-stationary density-regulated fragmentation and supply-limited accretion jointly drive the synchronized co-evolution of the core cluster, seamlessly linking small-scale core growth with large-scale reservoir regulation.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Probing Nonlinear Logarithmic Kalb-Ramond Black Holes: Particle Dynamics, Epicyclic Oscillations and Thermodynamic Signatures

Aftab Ansari, Rajesh Kumar and Praveen Kumar Dhankar

In contrast to conventional linear coupling frameworks, the proposed work investigated the nonlinear effects that become more significant in the strong field regime of a black hole. We have investigated a new class of Kalb Ramond black holes generated by a nonlinear logarithmic coupling of the field, referred to as a Logarithmic Kalb Ramond black hole. The logarithmic coupling introduces strong-field modifications to the spacetime geometry, leading to significant departures from the Schwarzschild and Reissner Nordstrom BHs. We analyze the motion of test particles using the effective potential formalism and derive the conserved energy and angular momentum for circular equatorial geodesics. The stability of circular orbits and the location of the innermost stable circular orbit are examined, revealing a strong dependence on the model parameters Q, l, and $\beta$. The epicyclic frequencies (radial, vertical, and azimuthal) together with the associated periastron precession demonstrate that nonlinear logarithmic corrections can substantially modify quasi periodic oscillation observables. We further investigate the Hawking temperature and energy emission rate, which show that the nonlinear coupling also impacts distinct imprints on the thermodynamic behavior and evaporation characteristics of the black hole. Our results also identify the BH parameters as key regulators of the orbital dynamics, oscillatory properties, and thermal evolution, providing a unified framework for probing nonlinear KR gravity through strong-field astrophysical phenomena.
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Solar and Stellar Astrophysics

Magnetic fields in Massive Star-Forming Regions (MagMaR). VIII. Magnetic field overrun by gravity in GGD 27's accretion streamers

M. Fernández-López, J. A. López-Vázquez, J. M. Girart et al. (+20 more)

Context. Accretion streamers connected to protostellar disks and/or envelopes are thought to transport material across several thousand of au. Whether the motions of the gas comprising these streamers are dominated by gravity, large scale external turbulence or the action of magnetic fields is still under scrutiny. Aims. The aim of this work is to understand the role of the magnetic fields in the star-formation processes, in particular the role that magnetic forces have in potentially leading flows of gas and the accretion onto the envelopes and disks orbiting protostars. Methods. First, we try to identify the large-scale accretion streamers toward the high-mass Young Stellar Object GGD 27-MM1 and fit their trajectories using the so-called Mendoza's model, a modification of the classical model of pure gravitational infalling motion of fluid particles in a potential well. Second, we estimate the strength of the magnetic field associated with the streamers. Then, we determine if the streamers are dominated by magnetic or centrifugal forces. Results. Inspecting the Atacama Large Millimeter/submillimeter Array (ALMA) H$_2$CO cube we were able to identify four accretion streamers spreading up to $\sim$7,000 au and fit their trajectories in the position-position-velocity space. The polarized continuum emission reveals a good alignment of the magnetic field and the trajectory of the streamers. Using the Davis-Chandrasekhar-Fermi method, we derive estimates for the magnetic field strength, find that the streamers are sub-alfvénic, and discuss (after estimating energy terms for turbulence, ordered motions, magnetic forces and gravity) a possible qualitative scenario in which, the gravitational well of the GGD 27-MM1 protostar dominates streamer gas motions over turbulence and magnetic forces at distances of $\sim 3,000$ au.
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All authors M. Fernández-López, J. A. López-Vázquez, J. M. Girart, P. Sanhueza, L. A. Zapata, P. C. Cortés, H. Beuther, G. Busquet, I. W. Stephens, K. Morii, N. Añez-López, C.-F. Lee, Q. Zhang, M. T. Beltrán, S. Curiel, F. A. Olguin, E. J. Chung, P. Saha, S. Li, P. M. Koch, J. Hwang, C.-Y. Law, J.-H. Kang

Cosmology and Nongalactic Astrophysics

Microlensing of Microlensing: Effects of Random Stars on the Double-Source-Plane Gravitational Lens

Nada Salama, Daniel J. Ballard, Huimin Qu et al. (+2 more)

Microlensing, the influence of stars within a galactic gravitational lens, has emerged as a powerful probe of compact mass and, through differential magnification, sub-parsec scale sources at cosmological distances. The recent discovery of a double-source-plane gravitational lens system in which the most distant source is a quasar offers the prospect of compound microlensing, in which quasar light rays are influenced by compact masses within the two foreground lensing galaxies. Here, we present the first numerical simulations of this "microlensing of microlensing". We consider the recently discovered "Einstein zig-zag" lens, J1721+8842, as a fiducial case, and construct microlensing magnification maps for each of the six quasar images in this system. Due to the secondary microlensing effects of the myriad of initial microimages, the resulting maps contain more complex caustic features than seen in the case of single plane microlensing. This is reflected in the expected lightcurves seen for each of the images.
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All authors Nada Salama, Daniel J. Ballard, Huimin Qu, Geraint F. Lewis, Karl Glazebrook

Cosmology and Nongalactic Astrophysics

Unveiling dark energy properties with high-sensitivity cross-correlations of neutral hydrogen intensity mapping and galaxy surveys

Alessandro Marins, Chang Feng and Filipe B. Abdalla

The redshifted 21 cm line from the hyperfine structure of neutral hydrogen atoms is a promising tracer for the three-dimensional evolution of our universe. Its broad spatial and temporal coverage is crucial for understanding the complex nature of dark matter and dark energy. However, it is very challenging to directly detect the 21 cm signal due to the existence of radio foreground contaminants that are orders of magnitude brighter. Therefore, mitigating the foreground contamination becomes an indispensable task for detecting the 21 cm signal, which is also expected to be correlated with the dark-matter-dominated large-scale structure. The cross-correlations between the neutral hydrogen intensity mapping and galaxy surveys in the future can not only confirm a detection of the 21 cm signal but can also be a complementary probe for 21 cm cosmology. To meet the precision requirements for cosmological studies, it is important to investigate the complex features of the estimated cross-correlations using numerical simulations. In this work, we simulate the low-redshift HI observations and narrow-band optical surveys in the future and obtain the foreground- and bias-mitigated HI-galaxy cross-power spectra, with which we perform a Bayesian analysis to infer cosmological parameters of dynamical dark energy model. The method developed in this work will be important for cosmological studies with the 21 cm intensity mapping.
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Astrophysics of Galaxies

Disentangling the Morphology of Palomar~5: Effects of the Bar, Spiral Arms, LMC, and Halo Flattening

Zhenghao He, Long Wang, Zi-yi Zhou et al. (+2 more)

The Palomar~5 (Pal~5) globular cluster and its tidal tails provide a sensitive probe of globular-cluster evolution in the time-dependent Milky Way potential. We study the past 3~Gyr evolution of Pal~5 using collisional direct \(N\)-body simulations with PeTar, adopting Galactic potential models that include spiral arms, the Galactic bar, halo flattening, the Large Magellanic Cloud (LMC), and bar deceleration. We find that halo shape strongly influences the projected stream track, reflecting Pal~5's sensitivity to Galactic force-field flattening. The LMC causes only modest direct changes to the present-day projected stream morphology, but can alter the pericentric distance and hence the progenitor's mass evolution. The Galactic bar strongly affects stream length and debris redistribution along the tails, producing model-dependent density structures and leading–trailing asymmetries. Comparison with observations from Erkal et al. (2017) and Xiao et al. (2025) shows that no single model simultaneously reproduces all observed properties of Pal~5, including cluster evolution, stream length, track, width, and line-density profile. Although our simulations capture several global properties, the remaining discrepancies indicate that a more precise match likely requires better constraints on the initial properties of the Pal~5 progenitor and a more complex Galactic potential, including perturbations from small-scale perturbers such as dark matter subhalos and giant molecular clouds. Future work may combine self-consistent direct \(N\)-body simulations with particle-spray methods to investigate these discrepancies more efficiently.
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All authors Zhenghao He, Long Wang, Zi-yi Zhou, Yang Huang, Eugene Vasiliev

Saturday 18 July

Earth and Planetary Astrophysics · Cross-listed from physics.ins-det

Unveiling neutron and gamma spectrum at Martian surface in presence and absence of hydrogen: A computational study based on GEANT4 simulations

A. Ilker Topuz

Motivated by the interactions of the GCR protons with the Martian atmosphere and regolith in the absence and presence of hydrogen, a series of GEANT4 simulations are employed in this study to reveal the influence of hydrogen on the energy spectra of secondary and albedo neutrons together with the associated gamma rays at the Martian surface. Following irradiation of the Martian atmosphere with a planar vertical PAMELA proton beam, the attenuation of the primary proton spectrum through the atmosphere and the range and energy deposition of protons within the regolith are first obtained. The depth profile of the generated neutrons and gamma rays, along with the kinetic energy spectra of the atmospherically produced neutrons and gammas reaching the surface, is then acquired in the absence and presence of 0.11 wt.% hydrogen by voxelizing the regolith into 100 cells of 2-cm thickness. Finally, a surface detector is placed atop the regolith, and the atmospherically produced neutron and gamma spectra are compared with the total neutron and gamma spectra recorded at the surface detector to assess the relative contribution of the atmosphere and the regolith to the overall radiation field reaching the Martian surface. The energy spectra of the albedo neutrons are also obtained in terms of thermal, epithermal, and fast neutrons. The GEANT4 simulations show that the Martian regolith, rather than the atmosphere alone, constitutes the dominant source of the neutron and gamma population arriving at the surface, and that the presence of 0.11 wt.% hydrogen is observable in each energy regime of the albedo neutrons at the Martian surface, while proton transport and the overall neutron and gamma production depth profile remain comparatively insensitive to hydrogen, thereby indicating the elemental variation of the Martian regolith.
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High Energy Astrophysical Phenomena · Cross-listed from nucl-th

The equation of state for neutron stars

Veronica Dexheimer

This chapter is intended as an introduction to dense matter and the equation of state of neutron stars and their mergers. It begins with a brief description of neutron star interiors, followed by a historical overview of the theoretical frameworks used to describe them focusing on relativistic formalisms, including different degrees of freedom, models, symmetries, and phases. It also provides an overview of our current understanding of dense matter (including theory, experiments, and observations) and discusses the advances we expect to see in the field over the next decade.
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Solar and Stellar Astrophysics

Constraining the radial decay timescale of solar surface magnetic field through a comparative study of data-assimilative 2D surface flux transport and 3D dynamo models

Soumyadeep Chatterjee and Gopal Hazra

The polar magnetic field is the most reliable precursor for predicting the amplitude of the solar cycle, and the 2D surface flux transport (SFT) model is widely used to reconstruct its evolution. Traditional 2D SFT models can not capture the surface-interior coupling of the surface field, causing delays in polar field reversals. This deficiency is conventionally corrected by adding a decay term $-B_r/\tau$ with a poorly constrained radial decay timescale $\tau$. Here, we present a self-consistent estimate of $\tau$ through a comparative study of radial flux transport in the 2D SFT model and the 3D kinematic dynamo model, STABLE. By keeping the same transport parameters for both models and assuming surface-interior coupling is diffusive, the poloidal field evolution equation reduces to an eigenvalue problem, which yields a spectrum of $\tau$ that decrease with increasing angular modes $l$. To capture realistic surface-interior coupling, we further perform data-assimilated 2D SFT simulations with real magnetograms and compare those results with that of the data-assimilated 3D STABLE model to constrain $\tau$ and effective decay modes. With our choice of transport parameters, a value of $\tau=~2~\text{yr}$ keeps the surface dynamics of the two models consistent, and this timescale corresponds to the angular mode $l=8$ from our self-consistent estimate. We also perform 2D SFT simulations with only large-scale active regions, as the source. We find that $\tau=7~\text{yr}$ accurately captures the radial decay of the dipole mode ($l=1$) and removes the secular drift in the polar fields.
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