High Energy Astrophysical Phenomena
Discovery of a 9.67-s pulsar in an ultraluminous X-ray source in NGC 4631 with XMM-Newton
L. Ducci, S. Mereghetti, F. Pintore et al. (+4 more)
Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P= 9.6652 +/- 0.0002 s in a new ultraluminous X-ray source (ULX) in the
galaxy NGC 4631. This source, dubbed as X-8, shows one of the largest
spin-up rates ever observed, dP/dt = (-9.6 +/- 0.5)*1E-8 s/s. These findings indicate that the compact object is a
neutron star, and X-8 is a new member of the pulsating ULX class. The 0.3-10 keV luminosity of X-8 is ~3.4E39 erg/s, and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large
spin-up rate: Doppler shift from orbital motion of the
neutron star and intrinsic
spin-up due to
accretion torque. This new ULX
pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.
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All authors L. Ducci, S. Mereghetti, F. Pintore, S. Allak, A. Santangelo, M. Sasaki, P. Kavanagh
Solar and Stellar Astrophysics
The Initial mass function of field stars with mass $\leq$ 1 $M_{\odot}$ varies with metallicity
Dan Qiu, Chao Liu, Jennifer A. Johnson et al. (+2 more)
We investigated a volume-limited sample of LAMOST main-sequence
stars with masses from 0.25 to 1 $M_{\odot}$ and distances of 150-350 pc to explore how the
stellar initial mass function (IMF) varies with
metallicity. We corrected the spectroscopic selection function by comparing the
stellar number densities with the photometric ones at the same colour and magnitude. From these corrected number density distributions, we derived IMFs for each
metallicity sub-samples. Fitting a broken power-law function in each IMF with a fixed break point at 0.525 $M_{\odot}$, we found the power-law indices increase with [Fe/H] for both mass regimes: $\alpha_1$ (mass $\leq$ 0.525 $M_{\odot}$) rises from 0.54 $\pm$ 0.21 to 1.40 $\pm$ 0.07 and $\alpha_2$ (mass>0.525 $M_{\odot}$) grows from 1.40 $\pm$ 0.16 to 1.86 $\pm$ 0.04 as [Fe/H] varies from -1 to +0.5 dex. It demonstrates that low-mass
stars make up a larger fraction in metal-rich environments than in metal-poor ones. We performed simulations to assess the impact of unresolved binaries on the IMF power-law indices. After correction, the binary-adjusted $\alpha$ values retained a similar
metallicity-dependent trend. Furthermore, by examining the IMF of the aggregate sample, we found the corrected indices ($\alpha_{\rm{1,corr}} = 1.48 \pm 0.03$ , $\alpha_{\rm{2,corr}} = 2.17 \pm 0.03$) are consistent with Kroupa's IMF values ($\alpha_1 = 1.3 \pm 0.5$ and $\alpha_2 = 2.3 \pm 0.3$). Finally, we verified the robustness of our results by testing different break points and mass bin sizes, confirming that the IMF's dependence on [Fe/H] remains consistent.
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All authors Dan Qiu, Chao Liu, Jennifer A. Johnson, Jiadong Li, Bo Zhang
Cosmology and Nongalactic Astrophysics
Constraining gravity with the decay rate of cosmological gravitational potential
Xinyi Zhao, Pengjie Zhang and Fuyu Dong
A key task in cosmology is to test the validity of general relativity (GR) at cosmological scales and, therefore, to distinguish between dark energy and modified
gravity (MG) as the driver of the late-time
cosmic acceleration. The decay rate ($DR$) of cosmological gravitational potential, being sensitive to
gravity and being immune to various astrophysical uncertainties, enables GR tests independent to other structure growth probes. Recently we have measured $DR$ at $0.2\leq z\leq 1.4$, combining the DR9
galaxy catalog from the DESI imaging surveys and Planck
cosmic microwave background maps \citep{arXiv:2411.12594}. Here we use this measurement to test
gravity, and restrict the analysis to one-parameter extensions to the standard $\Lambda$CDM cosmology. We consider four one-parameter MG parameterizations. One is $f(a)=\Omega_m^\gamma(a)$. The other three adopt the gravitational slip parameter $\eta=1$ and consider variations in the effective gravitational constant $G_{\rm eff}/G$ with the parameterization $\Sigma(a)=\Sigma_\Lambda \Omega_\Lambda(a)/\Omega_\Lambda$, $\Sigma(a)=\Sigma_1 a$ or $\Sigma(a)=\Sigma_2 a^2$. We find $\gamma=0.47^{+0.22}_{-0.15}$, consistent with the GR prediction $\gamma\simeq 0.55$. We also find $\Sigma_\Lambda=0.018^{+0.052}_{-0.053}$, $\Sigma_1=0.020^{+0.065}_{-0.062}$, and $\Sigma_2=0.027^{+0.067}_{-0.069}$, fully consistent with the GR case of $\Sigma=0$, regardless of parameterizations of $\Sigma(a)$. The constraining power is already competitive, while a factor of 2 further improvement is expected for the upcoming full-sky
galaxy surveys.
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Instrumentation and Methods for Astrophysics · Cross-listed from physics.comp-ph
Novel Numerical Methods for Accurate Space Thermal Analysis: Enforcing View Factors and Modeling Diffuse Reflectivity
Bernat Frangi
Accurate thermal analysis is crucial for modern spacecraft, driving demand for reliable modeling tools. This research advances space thermal modeling by improving the simulation accuracy and efficiency of radiative heat transfer, the dominant mode of heat exchange in space. To this end, we incorporate diffuse reflectivity using the Gebhart method, which computes radiative exchange factors (REFs) from
geometric view factors. The view factors, obtained via Monte Carlo ray tracing (MCRT), require post-processing to mitigate statistical errors. Critically, existing correction schemes cannot simultaneously enforce closure and reciprocity for open systems. This research addresses this gap by proposing two novel enforcement methods: (i) a least-squares optimization with non-negativity rectification (NNR) and small positive value avoidance (SPVA), and (ii) an iterative enforcement
algorithm. To ensure consistency across different discretization levels, this work also introduces the multi-node surface model relations to formalize the connection between sub-face, face, and node representations of view factors and REFs. A simple case study demonstrates a substantial reduction in mean absolute error (MAE): the least-squares method achieves an 81% MAE reduction, while the iterative method offers the best balance of accuracy (56% MAE reduction) and computational efficiency. A second case study shows that including diffuse reflections decreases the steady-state temperature of a plate by $4^{\circ}C$, reinforcing that reflected radiation reduces net absorption. This work introduces and validates computationally efficient methods for integrating diffuse reflectivity into space thermal analyses and for consistently coupling multi-node surface radiative models. The results enable more accurate and robust thermal predictions for spacecraft systems.
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Astrophysics of Galaxies
Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey: VIII: Spectroscopic detection of rare earth dysprosium, erbium, lutetium and thorium in Classical Cepheids
E. Trentin, G. Catanzaro, V. Ripepi et al. (+11 more)
Classical Cepheids are among the most important distance calibrators thanks to the correlation between their period and luminosity, and play a crucial role in the calibration as the first rung of the extragalactic distance ladder. Given their typical age, they also constitute an optimal tracer of the young population in the
Galactic disc. We performed a complete spectroscopic analysis of 136 spectra obtained from three different high-resolution spectrographs, for a total of 60 DCEPs. More than half have pulsational periods longer than 15 days, up to 70 days, doubling the number of
stars in our sample with P>15d. We derived radial velocities, atmospheric parameters, and chemical abundances for up to 33 different species. We present an updated list of trusted spectroscopic lines for the detection and estimation of chemical abundances. We used this new set to revisit the abundances already published in the context of the C-MetaLL survey and increase the number of available chemical species. For the first time (to our knowledge), we present the estimation of abundances for Cepheids for dysprosium, as well as a systematic estimation of erbium, lutetium, and thorium abundances. We calculated a
galactic radial gradient for [Fe/H] with a slope of -0.064 pm 0.002 dex kpc-1, in good agreement with recent literature estimation. The other elements also exhibit a clear negative radial trend, with this effect diminishing and eventually disappearing for heavier
neutron-capture elements. Depending on the proposed spiral arms model present in several literature sources, our most external
stars agree on tracing either the Perseus, the Norma-Outer, or both the Outer and the association Outer-Scutum-Centaurus arms.
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All authors E. Trentin, G. Catanzaro, V. Ripepi, E. Luongo, M. Marconi, I. Musella, F. Cusano, J. Storm, A. Bhardwaj, G. De Somma, S. Leccia, T. Sicignano, R. Molinaro, V. Testa
High Energy Astrophysical Phenomena
Revealing an Oscillating and Contracting Compact Corona near the Event Horizon of the Supermassive Black Hole in 1ES 1927+654
Qing-Cang Shui, Shu Zhang, Shuang-Nan Zhang et al. (+7 more)
Dynamic processes in the
accretion flow near black holes produce X-ray flux variability, sometimes quasi-periodic. Determining its physical origin is key to mapping
accretion geometry but remains unresolved. We perform a novel phase-resolved analysis on a newly discovered quasi-periodic oscillation (QPO) in the active
galactic nucleus 1ES 1927+654. For the first time in a supermassive black hole (SMBH), we detect a unique `U'-shaped QPO lag-energy spectrum and observe coronal spectral variability over the QPO phase. We find that the QPO is adequately explained by
plasma resonant oscillations within a corona. Modeling of QPO spectral properties and reverberation mapping reveal that the corona is contracting and confined to only a few gravitational radii regions near the SMBH, consistent with theoretical predictions for a decreasing QPO period of near 10 minutes. These results present the first observational evidence for an oscillating and contracting compact corona around an SMBH.
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All authors Qing-Cang Shui, Shu Zhang, Shuang-Nan Zhang, Hua Feng, Yu-Peng Chen, Long Ji, Ling-Da Kong, Liang Zhang, Jing-Qiang Peng, Peng-Ju Wang
Solar and Stellar Astrophysics
3D Non-LTE radiation transfer: theory and applications to stars, exoplanets, and kilonovae
Maria Bergemann and Richard Hoppe
Most of the physical information about astrophysical objects is obtained via the analysis of their electromagnetic spectra. Observed data coupled with radiation transfer models in physical conditions representative of
stars,
planets, kilonovae, and ISM, yield constrains on their physical structure, gas flow dynamics at the surface, mass loss, and detailed chemical composition of the systems. All these core astrophysical parameters are just as reliable as the physical quality of the models that are employed for simulations of radiation transfer. Recent advanced in multi-D transfer modelling with Non-Local Thermodynamic
Equilibrium (NLTE) in inhomogeneous time-dependent systems revealed systematic shortcomings of canonical models. Owing to major complexities of solving coupled multi-frequency RT equations in 3D geometry, a number of approximations have been introduced. This review presents an overview of the physical problem, standard solutions, and recent methodological advances. We also provide an overview of main results in the area of 3D NLTE radiation transfer and its applications to modelling diverse astrophysical environments, including FGKM type- and OBA-type
stars, multi-epoch spectra of kilonovae, and atmospheres of rocky and gaseous
exoplanets.
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Solar and Stellar Astrophysics
Partial Null Point Reconnection of an Eruptive Filament
Pooja Devi, Cristina H. Mandrini, Ramesh Chandra et al. (+4 more)
Solar filaments are cool and dense
plasma structures suspended in the
solar corona against
gravity. We present observations of a quiescent filament eruption that occurs on 13 July 2015. The eruption is associated with a two-ribbon GOES B8.9 class
flare. Photospheric
magnetic flux cancellation is present below the filament during days. This builds up a flux rope which progressively rises until it gets unstable, first leading to a confined eruption and pre-
flare brightenings, then to an ejection which starts $\approx$ 20 min later with the
flare onset. An interesting feature of this event is the presence of a large circular brightening formed around the erupting region. This brightening is produced due to interchange reconnection of the ejected
magnetic configuration with the surrounding open
magnetic field. This null-point
topology is confirmed by a potential-field extrapolation. The EUV loops located on the southern side of the filament eruption first contract during the null-point reconnection, then expand as the flux rope is ejected. The associated CME has both a classical flux rope shape and
plasma ejected along open field lines on the flux rope side (a trace of interchange reconnection). Finally, we set all this disparate observations within a coherent framework where
magnetic reconnection occurs both below and above the erupting filament.
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All authors Pooja Devi, Cristina H. Mandrini, Ramesh Chandra, Germán D. Cristiani, Pascal Démoulin, Cecilia Mac Cormack, Diego G. Lloveras
Instrumentation and Methods for Astrophysics · Cross-listed from hep-ex
Accelerated Sequential Posterior Inference via Reuse for Gravitational-Wave Analyses
Michael J. Williams
We introduce Accelerated Sequential Posterior Inference via Reuse (ASPIRE), a broadly applicable framework that transforms existing posterior samples and
Bayesian evidence estimates into unbiased results under alternative models without rerunning the original analysis. ASPIRE combines normalizing flows with a generalized Sequential Monte Carlo (SMC) scheme, enabling efficient updates of existing results and reducing total likelihood evaluations and wall times by factors of up to 5.8 and 5.5, respectively, with larger gains per posterior sample. This addresses a growing problem in gravitational-wave astronomy, where events must be repeatedly reanalyzed under different models or physical hypotheses. We show that ASPIRE reproduces full
Bayesian results when switching waveform models or adding physical effects such as
spin precession and orbital eccentricity. With this statistical robustness, ASPIRE turns repeated reanalyses into fast, reliable updates-paving the way for systematic studies of waveform systematics, scalable reanalyses across large event catalogs, and broadly applicable
Bayesian reanalysis across other scientific domains.
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Cosmology and Nongalactic Astrophysics
NANOGrav 15-year gravitational-wave signals from binary supermassive black-holes seeded by primordial black holes, and implications for the origins of Little Red Dots
Mikage U. Kobayashi and Kazunori Kohri
In this paper, we explain the recently reported a nHz-band gravitational-wave background from NANOGrav 15-year through the merger of binary super-massive black holes with masses of $10^9 M_{\odot}$ formed by the growth of
primordial black holes. When a
primordial black hole accretes at a high
accretion rate, it emits a large number of high-energy
photons. These heat the
plasma, causing high-
redshift cosmological 21cm line emission. Since this has not been detected, there is a strict upper bound on the
accretion rate. We have found that with the
primordial black hole abundance $10^{-14} \lesssim f_{\rm PBH} \lesssim 10^{-12}$ and the mass $1 M_{\odot} \lesssim m_{\rm PBH} \lesssim 10^3 M_{\odot}$, we successfully fit the nHz band gravitational wave background from NANOGrav 15-year while avoiding the 21 cm line emission. In addition, we also discuss the implication for the origins of the Little Red Dots. We propose that future observations of the gravitational wave background and the cosmological 21cm line can test this scenario.
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High Energy Astrophysical Phenomena
Hadronic Processes in Advection-Dominated Accretion Flow as the Origin of TeV Excesses in BL Lac Objects
Ji-Shun Lian, Ze-Rui Wang and Jin Zhang
The spectral energy distributions (SEDs) of certain BL Lac objects (BL Lacs) exhibit an additional hard $\gamma$-ray component in the TeV energy range that surpasses the predictions of the one-zone leptonic jet model. The origin of this excess emission remains unclear. In this study, we selected five BL Lacs whose SEDs display a very hard intrinsic spectrum in the TeV band and successfully reproduced their broadband SEDs using a two-zone lepto-hadronic model. Within this framework, the emission observed in the optical, X-ray, GeV $\gamma$-ray, and sub-TeV $\gamma$-ray bands is modeled using the synchrotron and synchrotron self-Compton radiation processes of the relativistic electrons in the jets. Meanwhile, the TeV excess is attributed to $\gamma$-ray emission resulting from the photomeson ($p\gamma$) process via $\pi^0$ decay occurring within advection-dominated
accretion flows (ADAFs). This scenario requires a hard
proton spectrum with a spectral index of $p \sim 1.6-1.7$ and a cutoff energy ranging from 30 to 90 TeV, as well as a relatively large ADAF radius. Such hard
proton spectra suggest that the dominant acceleration mechanisms are likely
magnetic reconnection and/or
stochastic acceleration processes within ADAFs. Additionally, the emission from the cascaded electrons results in a bump in the keV–MeV band; however, it is overwhelmed by the jet emission. Although the hadronuclear ($pp$) process cannot be entirely ruled out, it would necessitate an even harder
proton spectrum and a higher cutoff energy compared to the $p\gamma$ process, making it a less favorable explanation for the observed TeV excess.
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Solar and Stellar Astrophysics
TESS and ground-based observations of WZ Sge-type dwarf novae in outburst
Y. Tampo, N. Kojiguchi, K. Isogai et al. (+30 more)
Dwarf nova (DN) superoutbursts are accompanied by superhumps, which change their periods and profiles over a superoutburst. We present the TESS and ground-based observations of nine WZ Sge-type DNe and candidates in superoutburst. In TCP J23580961$+$5502508, ASASSN-23ba, PNV J19030433$-$3102187, V748 Hya, and ASASSN-25ci, we confirmed double-peaked oscillations called early superhumps, which are regarded as the unambiguous feature of WZ Sge-type DNe. On the other hand, the superhump and outburst properties of MO Psc and V1676 Her suggest that they may not be a member of WZ Sge-type DNe. The 2022 superoutburst of a confirmed WZ Sge-type DN TCP J05515391$+$6504346, however, lacked an early superhump phase. We find superhumps in a WZ Sge-type DN ASASSN-20mq during its rebrightening outburst. Thanks to the continuous coverage of TESS, we find the broken-powerlaw rise of the outburst light curve in V748 Hya and PNV J19030433$-$3102187, previously found in only one WZ Sge-type DN observed by Kepler. Early superhumps appeared when the system reached $\simeq40$% of the outburst peak flux. No orbital modulation from a hot spot is detected before and after this. This non-detection of orbital humps on the early rise of V748 Hya constrains that the corresponding mass transfer rate should be below $\simeq1\times10^{16}$ g s$^{-1}$, disfavouring an enhancement of a mass transfer rate by an order of magnitude or larger, even if it occurs. The contentious TESS observations also confirm the coexistence of early and ordinary superhumps during their transition and $\leq$2-cycle duration of stage A–B superhump transition in V748 Hya.
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All authors Y. Tampo, N. Kojiguchi, K. Isogai, D. Nogami, H. Itoh, F.-J. Hambsch, K. Matsumoto, R. Matsumura, D. Fujii, T. Tordai, Y. Sano, B. Monard, P.A. Dubovsky, T. Medulka, D.A.H. Buckley, N. Rawat, S.B. Potter, A. van Dyk, P.J. Groot, P. Woudt, S. Kiyota, G. Bolt, T. Vanmunster, J. Pietz, P. Starr, S.Y. Shugarov, K. Kasai, K. Menzies, S.M. Brincat, E.P. Pavlenko, A. Baklanov, J. Ito, T. Kato
Astrophysics of Galaxies · Cross-listed from gr-qc
Observational Constrains on the Sgr A$^*$ Black Hole Immersed in a Dark Matter Halo: Shadow and S2 Star Orbit
Zhen Li
It is widely believed that Sgr A$^*$, located at the center of our
Galaxy, is a supermassive black hole. Recent observations of its shadow and long-term monitoring of the S2
star have provided compelling evidence supporting this hypothesis. These observational advancements also offer valuable opportunities to explore the physical properties of the black hole and its surrounding environment. Since a dark matter
halo is expected to exist in the Milky Way and around Sgr A$^*$, investigating the behavior of the
Galactic Center black hole embedded in such a
halo provides a crucial means to simultaneously probe both black hole physics and dark matter properties. In this work, We develop a black hole metric that incorporates a generalized double power law dark matter
halo, and analyze the corresponding null and timelike geodesics to investigate how the
halo parameters affect the black hole shadow and the motion of the S2
star. Furthermore, by comparing our theoretical predictions with observational data of the shadow and the S2 orbit, we constrained the dark matter
halo parameters. The results of this study provide both theoretical and phenomenological insights into the nature of Sgr A$^*$ and the distribution of dark matter in our
Galaxy.
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Earth and Planetary Astrophysics
Limiting Eccentricity in Restricted Hierarchical Three-Body Systems with Short-Range Forces
Xiumin Huang, Dong Lai and Bin Liu
A hierarchical three-body model can be widely applied to diverse astrophysical settings, from satellite-
planet-
star systems to binaries around supermassive black holes. The octupole-order perturbation on the inner binary from the tertiary can induce extreme eccentricities and cause orbital flips of the binary, but short-range forces such as those due to General Relativity (GR) may suppress extreme eccentricity excitations. In this paper, we consider restricted hierarchical three-body systems, where the inner binary has a test-mass component. We investigate the maximum possible eccentricity (called "limiting eccentricity") attainable by the inner binary under the influence of the tertiary perturbations and GR effect. In systems with sufficiently high hierarchy, the double averaging (DA) model is a good approximation; we show that the orbits which can flip under the octupole-order perturbation reach the same limiting eccentricity, which can be calculated analytically using the quadrupole-order
Hamiltonian. In systems with moderate hierarchy, DA breaks down and the so-called Brown
Hamiltonian is often introduced as a correction term; we show that this does not change the limiting eccentricity. Finally, we employ the single averaging (SA) model and find that the limiting eccentricity in the SA model is higher than the one in the DA model. We derive an analytical scaling for the modified limiting eccentricity in the SA model.
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Instrumentation and Methods for Astrophysics
Neural Network identification of Dark Star Candidates. II. Spectroscopy
Adiba Amira Siddiqa, Sayed Shafaat Mahmud and Cosmin Ilie
Some of the first
stars in the Universe might be powered by Dark Matter (DM) annihilations, rather than
nuclear fusion. Those objects, i.e. Dark
stars (DS), offer a unique window into understanding DM via the observational study of the formation and
evolution of the first
stars and their Black Hole (BH) remnants. In \cite{NNSMDSPhot} (Paper~I) we introduced a feedforward neural network (FFNN) trained on synthetic DS photometry in order to detect and characterize dark
star photometric candidates in the early universe based on data taken with the NIRCam instrument onboard the James Webb Space
Telescope (JWST). In this work we develop a FFNN trained on synthetic DS spectra in order to identify spectroscopic dark
star candidates in the data taken with JWST's NIRSpec instrument. In order to validate our FFNN model we apply it to real data for the four spectroscopic Supermassive Dark
Star (SMDS) candidates recently identified in \cite{ilie2025spectroscopicsupermassivedarkstar} and reconfirm that indeed \JADESeleven, \JADESzthirteen, \JADESfz, and \JADESfo have spectra that are consistent with those of Supermassive Dark
Stars. The main advantage of our FFNN model, in comparison to the Nedleaer-Mead Monte Carlo parameter
estimator used in \cite{ilie2025spectroscopicsupermassivedarkstar}, is that the approach introduced here predicts parameters in milliseconds, over 10,000 times faster than the traditional method used in \cite{ilie2025spectroscopicsupermassivedarkstar}. With this in mind, the FFNN model we developed and validated in this work will be adapted for
Bayesian uncertainty analyses and automatic analyses of NIRSpec publicly available data for high
redshift objects. This study establishes a robust and efficient tool for probing Dark
Stars and understanding their role in
cosmic evolution.
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Cosmology and Nongalactic Astrophysics
Neural Network identification of Dark Star Candidates. I. Photometry
Sayed Shafaat Mahmud, Adiba Amira Siddiqa and Cosmin Ilie
The formation of the first
stars in the universe could be significantly impacted by the effects of Dark Matter (DM). Namely, if DM is in the form of Weakly Interacting Massive Particles (WIMPs), it could lead to the formation (at $z\sim 25-10$) of
stars that are powered by DM annihilations alone, i.e. Dark
Stars (DSs). Those objects can grow to become supermassive ($M\sim 10^6 M_\odot$) and shine as bright as a
galaxy ($L\sim 10^8 L_\odot)$. Using a simple $\chi^2$ minimization, the first three DSs photometric candidates (i.e. JADES-GS-z11, JADES-GS-z12, and JADES-GS-z13) were identified by Ilie et al. (2023). Our goal is to develop tools to streamline the identification of such candidates within the rather large publicly available high
redshift JWST data sets. We present here the key first step in achieving this goal: the development and implementation of a feed-forward neural network (FFNN) search for Dark
Star candidates, using data from the JWST Advanced Deep Extragalactic Survey (JADES) photometric catalog. Our method reconfirms JADES-GS-z13 and JADES-GS-z11 as dark
star candidates, based on the chi-squared goodness of fit test, yet they are $\sim10^4$ times faster than the Neadler-Mead $\chi^2$ minimization method used in Ilie et al. (2023). We further identify six new photometric Dark
Star candidates across
redshifts $z \sim 9$ to $z \sim 14$. These findings underscore the power of neural networks in modeling non-linear relationships and efficiently analyzing large-scale photometric surveys, advancing the search for Dark
Stars.
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Astrophysics of Galaxies
Machine-Learning Estimation of Energy Fractions in MHD Turbulence Modes
Jiyao Zhang and Yue Hu
Magnetohydrodynamic (MHD)
turbulence plays a central role in many astrophysical processes in the interstellar medium (ISM), including
star formation and
cosmic-ray transport and acceleration. MHD
turbulence can be decomposed into three fundamental modes-fast, slow, and Alfvén-each contributing differently to the dynamics of the medium. However, characterizing and separating the energy fractions of these modes was challenging due to the limited 2D information available from observations. To address this difficulty, we use 3D isothermal and multiphase MHD
turbulence simulations to examine how mode energy fractions vary under different physical conditions. Overall, we find that the Alfvén and slow modes carry comparable kinetic-energy fractions and together dominate the turbulent energy budget in multiphase media, while the fast mode contributes the smallest fraction. Relative to isothermal conditions, multiphase simulations exhibit an enhanced fast-mode energy fraction. We further introduce a machine-learning-based approach that employs a conditional Residual Neural Network to infer these fractions directly from spectroscopic data. The method leverages the fact that the three MHD modes imprint distinct morphological signatures in spectroscopic maps owing to their differing contributions to density and velocity fluctuations. Our model is trained on a suite of isothermal and multiphase simulations covering typical ISM conditions. We demonstrate that our machine learning model can recover the mode fractions from spectroscopic observables, achieving mean relative normalized errors of approximately 0 and standard deviation of 0.01 - 0.02 for seen data and 0.1 - 1.8 for unseen data.
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Cosmology and Nongalactic Astrophysics
Exploring Cosmological Constraints of the Void-Lensing Cross-Correlation in the CSST Photometric Survey
Qi Xiong, Yan Gong, Junhui Yan et al. (+8 more)
We investigate the cosmological constraints from the void-
lensing cross-correlation assuming the $w$CDM model for the Chinese Space Station Survey
Telescope (CSST) photometric survey. Using Jiutian simulations, we construct a mock
galaxy catalog to $z=3$ covering 100 deg$^2$, which incorporates the instrumental and observational effects of the CSST. We divide the
galaxy sample into seven photometric-
redshift (photo-$z$) tomographic bins and identify 2D voids within each bin using the Voronoi tessellation and watershed
algorithm. We measure the angular cross-power spectrum between the void distribution and the weak
lensing signal, and estimate the covariance
matrix via jackknife resampling combined with pseudo-$C_{\ell}$ approach to account for the partial sky correction. We employ the
Halo Void
Dust Model (HVDM) to model the void-matter cross-power spectrum and adopt the Markov Chain Monte Carlo (MCMC) technique to implement the constraints on the cosmological and void parameters. We find that our method can accurately extract the cosmological information, and the constraint accuracies of some cosmological parameters from the void-
lensing analysis are comparable or even tighter than the weak
lensing only case. This demonstrates that the void-
lensing serves as an effective cosmological probe and a valuable complement to
galaxy photometric surveys, particularly for the Stage-IV surveys targeting the high-
redshift Universe.
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All authors Qi Xiong, Yan Gong, Junhui Yan, Furen Deng, Hengjie Lin, Xingchen Zhou, Xuelei Chen, Qi Guo, Ming Li, Yun Liu, Wenxiang Pei
Earth and Planetary Astrophysics
Early evidence for isotropic planetary obliquities in young super-Jupiter systems
Michael Poon, Marta L. Bryan, Hanno Rein et al. (+3 more)
This decade has seen the first measurements of extrasolar planetary obliquities, characterizing how an
exoplanet's
spin axis is oriented relative to its orbital axis. These measurements are enabled by combining projected rotational velocities, planetary rotation periods, and astrometric orbits for directly-imaged super-Jupiters. This approach constrains both the
spin axis and orbital inclination relative to the line of sight, allowing obliquity measurements for individual systems and offering new insights into their formation. To test whether these super-Jupiters form more like scaled-up
planets or scaled-down
stars, we develop a hierarchical
Bayesian framework to infer their population-level obliquity distribution. Using a single-parameter Fisher distribution, we compare two models: a
planet-like formation scenario ($\kappa=5$) predicting moderate
alignment, versus a brown
dwarf-like formation scenario ($\kappa=0$) predicting isotropic obliquities. Based on a sample of four young super-Jupiter systems, we find early evidence favoring the isotropic case with a Bayes factor of 15, consistent with turbulent fragmentation.
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All authors Michael Poon, Marta L. Bryan, Hanno Rein, Jiayin Dong, Joshua S. Speagle, Dang Pham
Earth and Planetary Astrophysics
Super amplification of lunar response to gravitational waves driven by thick crust
Lei Zhang, Jinhai Zhang, Han Yan et al. (+1 more)
The Moon has been long regarded as a natural resonator of gravitational waves (GWs) since 1960, showing great potential to fill the frequency gap left behind GW detections by ground- or space-based
laser interferometry. However, the spatial variation of this amplification capacity on the Moon remains unclear. Here, we numerically simulate the lunar response to GWs by fully considering the fluctuant topography and laterally heterogeneous interior structures. Our results show that most regions on the Moon can amplify GWs with a ratio over 2, a finding significantly higher than previous estimations. Particularly, the amplification ratio can even reach factors of tens at the resonant frequency of ~0.015 Hz on the highlands surrounding the South Pole-Aitken (SPA) basin, where the regional crust is the thickest. Our findings establish the thick-crust regions as critical zones of GW amplification, which is essential for future landing site selection and instrumental setting for GW detection on the Moon.
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All authors Lei Zhang, Jinhai Zhang, Han Yan, Xian Chen
Cosmology and Nongalactic Astrophysics
Low redshift observational constraints on dark energy models using ANN - CosmicANNEstimator
Ashly Joseph, Albin Joseph, Christina Terese Joseph et al. (+3 more)
We present CosmicANNEstimator (Cosmological Parameters Artificial Neural Network
Estimator), a machine learning approach for constraining cosmological parameters within the Lambda Cold Dark Matter ($\Lambda$CDM) framework. Our methodology employs two specialized artificial neural networks (ANNs) designed to analyze Hubble parameter and
Supernova data independently. The
estimator is trained on synthetic data covering broad parameter ranges, with Gaussian random noise incorporated to simulate observational uncertainties. Our results demonstrate parameter estimates and associated uncertainties comparable to traditional Markov Chain Monte Carlo (MCMC) methods, establishing machine learning as an efficient alternative for cosmological parameter estimation. This work underscores the potential of neural network-based inference to complement traditional
Bayesian methods and accelerate future cosmological analyses.
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All authors Ashly Joseph, Albin Joseph, Christina Terese Joseph, John Paul Martin, Sunil Kumar PV, Sarthak Giri
Astrophysics of Galaxies · Cross-listed from gr-qc
Probing Gravitational Wave Speed and Dispersion with LISA Observations of Supermassive Black Hole Binary Populations
Tian-Yong Cao and Shu-Xu Yi
According to General Relativity (GR), gravitational waves (GWs) should travel at the speed of light $c$. However, some theories beyond GR predict deviations of the velocity of GWs $c_{\rm gw}$ from $c$, and some of those expect
vacuum dispersion. Therefore, probing the propagation effects of GWs by comparing the wave format detectors against the one at emission excepted from GR. Since such propagation effects accumulate through larger distance, it is expected that super-massive black holes binary (SMBHB) mergers serve as better targets than their stellarmass equivalent. In this paper, we study with simulations on how observations on a population of SMBHs can help to study this topic. We simulate LISA observations on three possible SMBHB merger populations, namely Pop\MakeUppercase{\romannumeral 3}, Q3-nod and Q3-d over a 5-year mission. The resulting constraints on the graviton mass are \(9.50\), \(9.33\), and \(9.05 \times 10^{-27} \, \mathrm{eV}/c^2\), respectively. We also obtain the corresponding constraints on the dispersion coefficients assuming different dispersion scenarios. If the electromagnetic wave counterparts of SMBHB merger can be detected simultaneously, the $c_{\rm gw}$ can be constrained waveform-independently to \(\Delta c/c\) to \(10^{-13}-10^{-12}\), corresponding to graviton mass constraints of \(10^{-26}-10^{-24} \mathrm{eV}/c^2\).
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Astrophysics of Galaxies
Ultra-Diffuse, Ultra-Different: Observed vs. Simulated Ultra-Diffuse Galaxies Live in Fundamentally Different Halos
Jonah S. Gannon, Arianna Di Cintio, Duncan A. Forbes et al. (+5 more)
In this work, we compare
galaxies from the NIHAO and HESTIA simulation suites to ultra-diffuse
galaxies (UDGs) with spectroscopically measured dynamical masses. For each observed UDG, we identify the simulated dark matter
halo that best matches its dynamical mass. In general, observed UDGs are matched to simulated
galaxies with lower
stellar masses than they are observed to have. These simulated
galaxies also have
halo masses much less than would be expected given the observed UDG's
stellar mass and the
stellar mass –
halo mass relationship. We use the recently established relation between globular cluster (GC) number and
halo mass, which has been shown to be applicable to UDGs, to better constrain their observed
halo masses. This method indicates that observed UDGs reside in relatively massive dark matter
halos. This creates a striking discrepancy: the simulated UDGs are matched to the dynamical masses of observed ones, but not their total
halo masses. In other words, simulations can produce UDGs in
halos with the correct inner dynamics, but not with the massive
halos implied by GC counts. We explore several possible explanations for this tension, from both the observational and theoretical sides. We propose that the most likely resolution is that observed UDGs may have fundamentally different dark matter
halo profiles than those produced in NIHAO and HESTIA. This highlights the need for a simulation that self-consistently produces
galaxies of a
stellar mass of $\sim 10^8 M_\odot$ in dark matter
halos that exhibit the full range of large dark matter cores to cuspy NFW-like
halos.
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All authors Jonah S. Gannon, Arianna Di Cintio, Duncan A. Forbes, Guacimara García-Bethencourt, Jean P Brodie, Noam Libeskind, Warrick J. Couch, Johanna Hartke
Cosmology and Nongalactic Astrophysics
Cosmic voids and the kinetic analysis. V. Hubble tension, the cosmological constant and aperiodic filaments
V.G. Gurzadyan, N.N. Fimin and V.M. Chechetkin
We study the appearance and specific properties of the structures in the local Universe by means of the Vlasov kinetic technique. We consider the role of the cosmological constant in local structure formation via the
theorem on the general function that satisfies the identity of the
gravity of the sphere and of the point mass. Then, the Hubble tension is naturally explained as a result of two flows, a local and a global one, with non-coinciding Hubble parameters. The linearized Vlasov-Poisson equation with the cosmological term is shown to lead to van Kampen's waves, Landau damping, and then to aperiodic structures. Aperiodicity thereby emerges as a intrinsic feature of the filamentary and void structure of the local Universe, and reveals the self-consistent field mechanism of its formation. The damping of the aperiodicity is then predicted and can be observationally traced upon the increase in the scale of the filaments.
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Astrophysics of Galaxies
SOFIA FEEDBACK Survey: The Eagle Nebula in [C II] and Molecular Lines
Ramsey L. Karim, Marc W. Pound, Alexander G.G.M. Tielens et al. (+15 more)
We characterize the physical conditions and energy budget of the M16 H II region using SOFIA FEEDBACK observations of the [C II] 158 $\mu$m line. The O
stars in the $\sim 10^{4}~{\rm M}_{\odot}$ NGC 6611 cluster powering this H II region have blown at least 2 cavities into the
giant molecular cloud: the large M16 cavity and the small N19 bubble. We detect the spectroscopic signature of an expanding photodissociation region shell towards N19, and traces of a thin, fragmented expanding shell towards M16. Our [C II] observations are resolved to 0.5 km s$^{-1}$ and 15.5$^{\prime\prime}$ and analyzed alongside similarly resolved CO J=3$-$2 observations as well as archival data ranging from the
radio to X-ray tracing a variety of gas phases spanning dense $\sim$10 K molecular gas, $10^{4}$ K photoionized gas, and million-K collisionally ionized
plasma. With this
dataset, we evaluate the coupling of energetic feedback from NGC 6611 and the O9 V
star within N19 to the surrounding gas. Winds from NGC 6611 have blown a 20 pc radius cavity constrained in size along the major axis of the natal
giant molecular filament, and much of the mechanical wind energy ($>$90%) has escaped through breaches in the $\lesssim 10^{4}~{\rm M}_{\odot}$ shell. Reservoirs of dense gas remain within a few parsecs of the cluster. N19, younger than M16 by $\gtrsim 10^6$ yr, is driven by a combination of mechanical wind energy and thermal pressure from photoionized gas and has swept up $\sim 10^{3}~{\rm M}_{\odot}$ into neutral atomic and molecular shells.
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All authors Ramsey L. Karim, Marc W. Pound, Alexander G.G.M. Tielens, Jelle S. Kaastra, Leisa K. Townsley, Patrick S. Broos, Maitraiyee Tiwari, Lars Bonne, Ümit Kavak, Mark G. Wolfire, Nicola Schneider, Robert Simon, Rolf Güsten, Jürgen Stutzki, Marc Mertens, Oliver Ricken, Friedrich Wyrowski, Lee G. Mundy
Solar and Stellar Astrophysics
Non ideal Transport Processes in the Solar Atmosphere
B P Pandey and Mark Wardle
Transport coefficients are calculated for a partially ionized
plasma consisting of approximately 90% hydrogen and 10% helium, representative of a model
solar atmosphere with an assumed
magnetic field profile. The ion
Hall parameter, defined as the ratio of ion cyclotron to ion collision frequency, is determined by considering dominant resonance charge exchange processes alongside less significant nonresonant ion neutral collisions. Based on these calculations, we derive profiles for various transport coefficients. Our results demonstrate that thermal conductivity in partially ionized media, both parallel and perpendicular to the ambient
magnetic field, is dominated by neutral particles. The perpendicular thermal conductivity components show weak dependence on the ion
Hall parameter and remain comparable in magnitude to their parallel counterparts. Wave damping through neutral thermal conductivity may contribute significantly to
solar atmospheric heating. These findings indicate that perpendicular thermal conductivity components are essential for accurate modelling of partially ionized regions, including photosphere-chromosphere transition layers, spicules, and coronal prominences.
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High Energy Astrophysical Phenomena
Origin and Evolution of the $\Omega$ Structure in the Head-Tail Radio Galaxy of Abell 3322
Kohei Kurahara, Takuya Akahori, Takumi Ohmura et al. (+8 more)
A head-tail
galaxy is thought to be a
radio galaxy with bent active
galactic nuclei (AGN) jets interacting with the intracluster medium (ICM). Study of head-tail
galaxies provides us with fruitful insights into the mechanisms of shock waves and
turbulence, as well as
magnetic-field amplification and
cosmic-ray acceleration. A recent MeerKAT observation revealed that a head-tail
galaxy in the
galaxy cluster, Abell 3322, exhibits a peculiar “Omega" structure in its shape. In this paper, we investigated this Omega-tail
galaxy using the upgraded
Giant Meterwave
Radio Telescope (GMRT) and the Australia
Telescope Compact Array (ATCA). We found that the southern jet tends to be brighter than the northern jet, with a brightness ratio of about 2. This can be attributed to Doppler boost and the inclination of the jets. Our broadband data suggest that the
radio spectrum becomes steeper along the jet propagation direction, and the
cosmic-ray aging model with a weak reacceleration of
cosmic rays is preferable to explain the index profile. We further found a gradient of the spectral index perpendicular to the jet propagation. We discussed the origin of the gradient and suggested that a shock wave along one side of the jets is present. The resultant ram pressure as well as the backflow made at the early stage of the jet may produce the tail component of this Omega-tail
galaxy, while the observed Omega-shape structure is more likely due to a twin
vortex seen in the low Reynolds number flow.
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All authors Kohei Kurahara, Takuya Akahori, Takumi Ohmura, Shintaro Yoshiura, Daisuke Ito, Yik Ki Ma, Kazuhiro Nakazawa, Yuki Omiya, Kosei Sakai, Haruka Sakemi, Motokazu Takizawa
High Energy Astrophysical Phenomena
Spectral Diversity in Type Ibn Supernovae and the Large Host Offset of SN2024acyl
Yize Dong, V. Ashley Villar, Anya Nugent et al. (+35 more)
In this paper, we first present observations of SN~2024acyl, a normal Type Ibn
supernova with a large projected offset ($\sim$35~kpc) from its host
galaxy. The low
star-formation rate measured at the explosion site raises the possibility that the progenitor of SN~2024acyl may not have been a massive
star. We then examine, more broadly, the spectral diversity of Type Ibn
supernovae around 20–35 days after peak brightness and identify two distinct groups: Group I, which shows bluer rest-frame optical color and narrower He~I emission lines; and Group II, which shows redder rest-frame optical color and broader He~I lines. Group~I also tends to show higher peak luminosities. The diversity we identify appears to be closely connected to the diversity observed around peak and to persist into late phases ($>80$ days after peak). Given its redder color and broader He~I lines, we classify SN~2024acyl as belonging to Group II. Based on the current
dataset, we find no clear connection between this spectral diversity and either the host environments of Type Ibn SNe or their pre-explosion activity. The observed diversity in Type Ibn SNe likely reflects differences in circumstellar material properties and/or explosion energetics. These differences could result from a range of progenitor properties, such as different helium
star mass, orbital period and companion type if they are in binary systems, and may indicate fundamentally diverse progenitors. Whether a continuous distribution exists between the two groups remains to be determined and will require further data to explore.
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All authors Yize Dong, V. Ashley Villar, Anya Nugent, Griffin Hosseinzadeh, Ryan J. Foley, Christa Gall, Monica Gallegos-Garcia, Conor Ransome, Aidan Sedgewick, Daichi Tsuna, Stefano Valenti, Henna Abunemeh, Moira Andrews, Katie Auchettl, K. Azalee Bostroem, David A. Coulter, Thomas de Boer, Kaylee de Soto, Diego A. Farias, Joseph Farah, Danielle Frostig, Hua Gao, Alex Gagliano, Emily Hoang, D. Andrew Howell, Willem B. Hoogendam, Mark E. Huber, David O. Jones, Chien-Cheng Lin, Michael Lundquist, Curtis McCully, Darshana Mehta, Anthony L. Piro, Aravind P. Ravi, Nicolás Meza Retamal, César Rojas-Bravo, S. Karthik Yadavalli, Qinan Wang