Instrumentation and Methods for Astrophysics
From Hubble to HWO: Bridging the Frontier of White Dwarf Exoplanet Science
Laura K. Rogers, Siyi Xu, Martin Barstow et al. (+20 more)
White
dwarf stars, the endpoint of
stellar evolution for 97% of
stars in our Milky Way, offer a unique and powerful window into the bulk elemental composition of rocky exoplanetary bodies. Up to 50% of single white
dwarfs are observed with photospheric metal lines from accreted exoplanetary bodies (called 'polluted' white
dwarfs), and spectroscopic observations reveal the bulk composition of this material. High-resolution (R>15,000) UV spectra are essential for detecting many elements present in the material, such as the volatile elements imperative for habitability studies (C, N, O, P, S) and key rock-forming elements required to constrain interior structure (e.g. Fe, Si, Mg, Al, Ni). HST, through its COS and STIS spectrographs, remains the only facility capable of performing this science in the near future. Looking to the next decade, the scientific case for continued HST UV observations of polluted white
dwarfs is compelling on three fronts (i) as a standalone to enable the bulk composition of exoplanetary material to be measured in a statistically significant sample, (ii) as essential groundwork for the Habitable Worlds Observatory (HWO), and (iii) in a powerful synergy with JWST, to enable characterization of the bulk mineralogy and bulk elemental composition of exoplanetary material. This white paper argues that continued UV spectroscopic capabilities with HST is a high-return investment for white
dwarf and
exoplanet science, and preserving and prioritizing HST's UV capabilities through at least 2035 is crucial to maximize the scientific return from HST, JWST, and HWO.
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All authors Laura K. Rogers, Siyi Xu, Martin Barstow, Simon Blouin, Amy Bonsor, Andrew M. Buchan, Sarah L. Casewell, Tim Cunningham, John Debes, Patrick Dufour, Boris Gansicke, Joseph Guidry, Ted von Hippel, Mukremin Kilic, Erika Le Bourdais, Carl Melis, Lou Baya Ould Rouis, Judith Provencal, Melinda Soares-Furtado, Andrew Swan, Isabella Trierweiler, Zachary Vanderbosch, Jamie Williams
Instrumentation and Methods for Astrophysics
The role of the Hubble Space Telescope in advancing our understanding of atmospheric escape in exoplanets
Leonardo A. Dos Santos, Arika Egan, Kevin France et al. (+14 more)
An important evolutionary pathway for planetary atmospheres is escape to space, which has been studied on Earth and Mars for several decades and more recently in
exoplanets. A particularly important regime is the hydrodynamic escape, wherein atmospheric mass escapes the
planet at high rates in a collisional fluid
outflow. This process is used to partly explain the early
evolution of rocky
planets in and out of the
Solar System, as well as key aspects of
exoplanet demographics. Hydrodynamic escape is not occurring in the
Solar System
planets, so our only option for such observations is through
exoplanets. The ultraviolet (UV) capabilities of the Hubble Space
Telescope (HST) are fundamental to detect hydrodynamic escape and measure the resulting mass-loss rates for a range of planetary systems and to identify targets for surveys with the Habitable Worlds Observatory. We discuss here what kinds of observations and instrument modes are necessary to continue studying atmospheric escape in
exoplanets for the next decade, as well as how to advance our understanding of planetary
evolution and habitability.
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All authors Leonardo A. Dos Santos, Arika Egan, Kevin France, Eric Gaidos, Antonio García Muñoz, R. O. Parke Loyd, Keighley Rockcliffe, Mercedes López-Morales, P. Christian Schneider, Arif Solmaz, Michael Zhang, Vladimir S. Airapetian, Munazza K. Alam, Laura N. R. do Amaral, Tommi Koskinen, Seth Redfield, Jake D. Turner
Instrumentation and Methods for Astrophysics
Mars, a Post-Habitable Planet?
Matteo Crismani, Richard Cartwright, Michael Chaffin et al. (+3 more)
Mars provides a critical analog to once habitable
exoplanets that have since lost their surface
liquid water. The current atmospheric state of Mars retains the chemical fingerprints of that transition, including isotopic signatures of atmospheric escape and
climate evolution. As the closest accessible example of a terrestrial world with definitive evidence for once supporting
liquid water on its surface, Mars presents a unique opportunity to test hypotheses about planetary habitability and atmospheric
evolution in a spatially and temporally resolved way.
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All authors Matteo Crismani, Richard Cartwright, Michael Chaffin, Sara Faggi, Stephanie Milam, Geronimo Villanueva
Solar and Stellar Astrophysics
Near UV Stellar Activity and Brightness Fluctuations of the Alpha Centauri AB Star System from Weeks to Decades – Inputs for Reflected Light Spectroscopy with HWO
Dolon Bhattacharyya, Kevin France, Soumit Rao et al. (+6 more)
We present the most comprehensive near-ultraviolet (NUV: 2550-3255 Angstrom) activity record to date for the Alpha Centauri AB system, combining archival IUE and HST observations spanning nearly five decades with new high-cadence CUTE measurements. We show that Alpha Centauri A exhibits predominantly quiescent NUV behavior, with the majority of observations remaining within 1 sigma of the median flux and only rare chromospheric flaring events (1
flare every 12 years), consistent with its weak chromospheric activity and 19-year
stellar cycle inferred from X-ray and FUV observations. In contrast, Alpha Centauri B displays a broader variability envelope, characterized by more frequent and higher-amplitude chromospheric excursions that track its well-established 8-year
magnetic activity cycle. Using Lomb-Scargle analysis on the Mg II index derived from CUTE observations, we estimate the rotational period of Alpha Centauri A to be on timescales of 15-20 days. We also confirm the coherence of the
stellar activity cycle of Alpha Centauri B in the NUV with its X-ray activity cycle. These data establish a critical reference framework for interpreting reflected-light observations of terrestrial
exoplanets and for assessing the detectability of ozone and other biosignature-related features at NUV wavelengths with future facilities such as the Habitable Worlds Observatory. These results indicate that HWO observations of terrestrial
exoplanets in reflected light photometry and spectroscopy around magnetically inactive early G-type
stars and early K-type
stars may be expected to show 10-20 percent and 30-40 percent temporal flux variability, respectively, over the course of months to years from the changing
stellar inputs alone.
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All authors Dolon Bhattacharyya, Kevin France, Soumit Rao, Sebastian Escobar, David J. Wilson, Arika Egan, Phillip Chamberlin, A. G. Sreejith, Alexander Brown
High Energy Astrophysical Phenomena
Evidence for mass-dependent spin subpopulations in GWTC-4
Asad Hussain, Maximiliano Isi and Aaron Zimmerman
While the origin of merging black-hole binaries observed in gravitational waves remain uncertain, different formation channels are expected to leave distinct imprints on their observed mass and
spin distributions. In this work, we focus on the mass dependence of the
spin magnitudes $(\chi_1,\chi_2)$, allowing for multiple
spin subpopulations whose relative fraction varies with mass. Using the binaries from the fourth LIGO–Virgo–KAGRA gravitational wave transient catalog (GWTC-4), we find strong evidence for two subpopulations, with a log Bayes factor of $\log_{10} \mathcal B = 4^{+6}_{-2}$. The data support a picture in which low-mass systems are composed almost entirely of slowly spinning black holes ($\chi \lesssim 0.2$), while high-mass systems are dominated by a broader component with moderate to rapid
spins, peaking at $(\chi_1,\chi_2)\approx(0.7,0.7)$ and retaining a non-negligible tail toward asymmetric
spins $(\chi_1,\chi_2)\approx(0.7,0)$. The transition between these regimes spans a broad range from $m_1 \approx 35^{+20}_{-20} \,M_\odot$ to $m_1 \approx 70^{+90}_{-25}\,M_\odot$. For the low-mass, slowly-spinning regime, a flexible model of
spin orientations does not allow us to exclude identically vanishing
spins, $\chi_1 = \chi_2 = 0$. Meanwhile, the high-mass, rapidly-spinning population does not neatly display the
spin-magnitude asymmetry expected from hierarchical mergers, although the data do not exclude that. This kind of localized structure is generally difficult to access with standard Monte-Carlo likelihood
estimators, which we replace with a regularized representation in terms of truncated Gaussian mixtures. Taken together, our results provide a new target for formation models, which should reproduce not only the mass and effective
spin distributions, but also the mass-dependent structure of the individual component-
spin magnitudes.
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Solar and Stellar Astrophysics
Pushing the Limit of Asteroseismic Detection for Cool Dwarfs using TESS and Deep Learning
Waly M Z Karim, Rocio Kiman, Derek Buzasi et al. (+6 more)
Asteroseismology provides a powerful probe of
stellar interiors by detecting
stellar oscillations, including
solar-like oscillations, which are stochastically excited by near-surface convection. While thousands of
solar-like oscillators have been identified in evolved
stars, only a limited number of main-sequence cool
dwarfs have confirmed oscillations due to the low amplitudes of their signals. In this work, we train a convolutional autoencoder on TESS two-minute light curves to automatically identify
solar-like oscillation features in cool
dwarf main sequence and sub-
giant stars. Using catalogs of confirmed oscillators for training and validation, our network achieves a classification accuracy of 99.8% on the test set, along with Precision of 0.945, Recall of 0.998, and F1 Score of 0.971. From the Asteroseismic Target List, our model identifies 3463 potential
solar-like oscillators (probability greater than 0.5). After further analysis, we find a list of 24 candidate
stars that have the potential to exhibit
solar-like oscillations. Notably, several of these candidates occupy regions of the color-magnitude diagram that are accessible only through more resource-intensive radial velocity observations, thereby has the potential of extending the detection frontier of TESS-based asteroseismology. Our candidate catalog provides a valuable foundation for follow-up efforts aimed at expanding the sample of cool-
dwarf solar-like oscillators. This will ultimately improve our understanding of
stellar structure and
evolution across the lower main sequence and strengthen the evidence for using deep learning techniques to study
stellar light curves.
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All authors Waly M Z Karim, Rocio Kiman, Derek Buzasi, Cecilia Garraffo, Joshua D. Wing, Jim Fuller, Benjamin J. Ricketts, Viktor Khalack, Sajia Shahrin Neha
Astrophysics of Galaxies
A Redshift-based Red Selection of Dusty Star-forming Galaxies
A.J. Barger, L.L. Cowie, S.J. McKay et al. (+1 more)
We use JWST observations (1.5 micron to 4.44 micron), together with complete ALMA observations (870 micron and/or 1.2 mm), of the massive
lensing cluster field A2744 to show that
galaxies between z=1.5 and z=5.5 with rest-frame red colors f_J/f_V > 3 correspond to dusty
star-forming
galaxies (DSFGs), little red dots (LRDs), and quiescent
galaxies. The color selection picks out 34 of the 41 >4.5-sigma ALMA sources in the field (83%). We find that the luminous red sources are generally extended, while the less luminous red sources are almost all compact and correspond to the LRD population. We also find that the great majority of the luminous, extended red sources are DSFGs based on the ALMA data, with a small admixture of quiescent
galaxies at z<3-4 that we identify based on their location in the rest-frame U-V versus V-J diagram. We do not detect any LRDs or quiescent
galaxies at the >3-sigma level in the ALMA images. Roughly 10% of the DSFGs have high rest-frame X-ray luminosities and must be AGN dominated. The DSFGs and quiescent
galaxies nearly all have M_
star>10^{10}
solar masses. These massive
galaxies become rare at z>5, paralleling the fall off in the number of detected DSFGs.
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All authors A.J. Barger, L.L. Cowie, S.J. McKay, F.E. Bauer
Earth and Planetary Astrophysics
JWST and Gemini Observations of the Active Centaur 450P/LONEOS: Nucleus and Coma Characterizations
Charles A. Schambeau, Michael S. P. Kelley, Maria Womack et al. (+16 more)
Between 2019 and 2024, we used the Gemini-N and JWST observatories to conduct a detailed case study of the active Centaur 450P/LONEOS, whose orbit was significantly altered by a close Saturn encounter in 1992. Gemini-N GMOS optical images likely captured the first views of 450P's inactive
nucleus, indicating a relatively small radius of $R_N = 1.8\pm0.5$ km and a surface color of $g' - i' = 1.15\pm0.09$. This places 450P on the red end of the neutral/gray Centaur population and may indicate comparatively limited
solar-driven surface processing relative to other known active Centaurs. A coma developed as 450P changed its heliocentric distance, $R_H$, from 7.83 au to 7.24 au, with an estimated low
dust production rate of $\sim$4-8 kg s$^{-1}$. JWST NIRSpec IFU Prism-mode spectra revealed an elongated
dust morphology and a symmetric $CO_2$ gas distribution in the coma but no $H_2O$ or CO emission features, with production rates of $Q_{CO_2} = (6.99\pm0.07)\times10^{24}$ molec. s$^{-1}$, $Q_{H_2O} \leq 1.2\times10^{24}$ molec. s$^{-1}$, and $Q_{CO} \leq 5.2\times10^{24}$ molec. s$^{-1}$. Absorption features at 2.0 and 3.0 $\mu$m indicate the presence of water ice, and a subtle 3.1 $\mu$m feature is consistent with crystalline water ice in larger grains. A Hapke-style model dominated by large ($D_{eff.} = 5.9$ $\mu$m)
dust grains with a volumetric ice fraction of $f_{ice} = 33\%$ fits the spectrum. A thermal model incorporating 450P's orbital history since $\sim$1500 CE aligns with the observed onset of activity driven by $CO_2$ outgassing from amorphous water ice crystallization between 140-160 K.
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All authors Charles A. Schambeau, Michael S. P. Kelley, Maria Womack, Eva Lilly, Theodore Kareta, Sara Faggi, Olga Harrington Pinto, Marco Micheli, Dominique Bockelee-Morvan, Yanga R. Fernandez, Adam McKay, Noemi Pinilla-Alonso, Javier Licandro, Aren Beck, Geronimo L. Villanueva, James Bauer, Lori Feaga, Michael A. DiSanti, Kacper Wierzchos
Solar and Stellar Astrophysics
Orbital Parameters of the Unusual WR + O Binary System LS III +44 21
I.I.Antokhin and I.A.Shaposhnikov
We present the results of a spectroscopic study of the recently discovered WR + O binary system LS III +44 21. The system is unusual because, despite having characteristics similar to those of the classical WR + O system V444 Cyg, its X-ray emission is completely absent. We refined the spectral classification of the system components to WN4 + O7III?(f) and obtained their radial velocity curves for the first time. The solution of these curves reveals that the system has a circular orbit. Using photometric observations from ASAS-SN and TESS, we significantly refined the values of the initial epoch T_0 and the orbital period P. The radial velocity curve solution with the updated T_0 and P allowed us to determine the parameters of the orbit and the system components for the first time. A preliminary qualitative analysis of the light curves obtained by the TESS satellite and the ASAS-SN project, combined with the interesting variability of the N IV and N V ion line profiles with the orbital phase that we detected, suggests that the lack of X-ray emission from the system may be due to an unusually weak
stellar wind from the WR
star. The shape of the highly precise mean TESS light curve is extremely unusual, exhibiting non-monotonic behavior near the quadratures and an unusual morphology of the secondary minimum, which shows a nearly total eclipse yet with rounded edges. Such a shape cannot be modeled within the standard Roche geometry, which may provide a direct indication of the influence of the component
stellar winds.
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Instrumentation and Methods for Astrophysics · Cross-listed from physics.ins-det
Wideband Balanced Photodetectors for Classical and Quantum Light Detection from Optical, EUV, to X-rays
Ivan Ryger, Terry Brown, Dina S. Eissa et al. (+1 more)
The rapid development of coherent short-wavelength light sources in the extreme ultraviolet (EUV) and soft X-ray (SXR) regimes has created a growing need for advanced optoelectronic detection capabilities, particularly for
quantum-noise-limited measurements, microelectronics and
semiconductor metrology, and emerging
quantum information applications. However, extending balanced photodetection to these wavelength regimes is severely hindered by a fundamental bandwidth-noise trade-off imposed by the exceptionally large junction capacitance of EUV-SXR silicon photodiodes. Here, we report a novel wideband photoreceiver architecture that overcomes this bottleneck via a bootstrapped transimpedance amplifier design. By leveraging a low-noise junction field-effect transistor interface, we effectively isolate the photodiode capacitance and suppress the apparent input capacitance seen by the core amplifier. Combined with active compensation of parasitic feedback reactance, this architecture mitigates the conventional trade-off between detector active area and signal bandwidth. Experimentally, we achieved a system-level input-referred noise floor of 13 $fA/\sqrt{Hz}$, closely approaching theoretical thermal limits. Furthermore, we achieved a six-fold extension in signal-to-noise limited bandwidth and, through the implementation of a novel grounded field plate, demonstrated a common-mode rejection ratio (CMRR) exceeding 30 dB up to 100 kHz. This highly scalable, silicon-based architecture effectively bridges the short-wavelength detection gap, establishing a robust experimental platform for next-generation
quantum-noise-limited and
quantum-enhanced X-ray measurement, as well as ultra-sensitive inspection and metrology applications in high-numerical-aperture EUV lithography.
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All authors Ivan Ryger, Terry Brown, Dina S. Eissa, Chen-Ting Liao
Instrumentation and Methods for Astrophysics
Hubble Astrometry for the Local Group and Beyond in the 2030s
S. Tony Sohn, Paul Bennet, Kevin Andrew McKinnon et al. (+23 more)
Hubble's long, stable astrometric baseline creates a rare opportunity for discovery in the Local Group and beyond. Many nearby
galaxies, streams, and
star clusters already have archival first-epoch imaging in hand, so future HST observations over the next decade can turn those data into precise proper motions. For many Milky Way satellites, existing measurements already constrain orbital motion at a useful level, but HST still offers a path to full 3D kinematics, internal motions, and more distant systems where current data remain insufficient. That opens the window to dynamical studies inaccessible through line-of-sight velocities alone, revealing orbital histories, internal kinematics, environmental processing, and the dark-matter structure of nearby
galaxies. This white paper identifies HST astrometry as an opportunity to capitalize on archival baselines by completing long-baseline measurements where first epochs already exist, establishing new first epochs where critical gaps remain, and assembling a legacy sample for future JWST, Roman, and HWO-era follow-up. The result will be a transformative
dataset for the Local Group and Local Volume, driving discovery now while laying the groundwork for the next generation of dynamical studies for resolved
stellar populations.
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All authors S. Tony Sohn, Paul Bennet, Kevin Andrew McKinnon, Roeland P. van der Marel, Mattia Libralato, Eduardo Vitral, Ekta Patel, Laura L. Watkins, Andres del Pino, Andrea Bellini, Massimo Griggio, Mark A. Fardal, Nitya Kallivayalil, Jack T. Warfield, Karoline M. Gilbert, Puragra Guhathakurta, Daniel Weisz, Andrew Wetzel, Andrew B. Pace, Marcel S. Pawlowski, Joshua D. Simon, Gurtina Besla, Erik Tollerud, Xiaowei Ou, Niusha Ahvazi, Anna Bonaca
Cosmology and Nongalactic Astrophysics
The Sensitivity of Substructure Lensing to SIDM Core-collapse Model Variation
Charlie Mace, Birendra Dhanasingham, Zhichao Carton Zeng et al. (+4 more)
Strong gravitational
lensing has emerged as a powerful probe of dark matter substructure, and shows particularly strong promise as a test of self-interacting dark matter (SIDM). The compact
halos produced by SIDM can leave distinct imprints on
lensing observations, but the core-collapse timeline for subhalos is difficult to model accurately. This difficulty is an obstacle to accurate substructure
lensing predictions, where small variations in core-collapsing subhalos can lead to significant differences in the
lensing power. To quantify this problem and inform future
lensing analyses, we test various methods of modeling core-collapsing
halos and show the effect of each variation on the two-point correlation function of the effective deflection field's divergence and curl. Our tests include smoothly evolving density profiles versus instantaneously collapsing
halos, probabilistic collapse versus individual
halo evolution, and variation of the initial and final density profile parameters. We find that the two-point correlation function is sensitive to most of these variations at small length scales, but the detectability of these differences will depend on the observational probe.
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All authors Charlie Mace, Birendra Dhanasingham, Zhichao Carton Zeng, Francis-Yan Cyr-Racine, Xiaolong Du, Annika H. G. Peter, Andrew Benson
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Leptogenesis without on-shell right-handed neutrinos
Simon Cléry, Alejandro Ibarra and Onur Yonar
We propose a novel mechanism for generating the
baryon asymmetry of the Universe through leptogenesis in a scenario where the right-handed
neutrinos are heavier than the maximal temperature of the Universe, and are never produced on-shell, neither by thermal nor by non-thermal mechanisms. We introduce a new scalar field, $\phi$, lighter than the right-handed
neutrinos, that couples to the latter via a Yukawa coupling, so that it decays into two
lepton doublets and two
higgs doublets via off-shell right-handed
neutrinos. Then, we derive the CP asymmetry arising from the interference between tree-level and loop diagrams in the four-body decay, and we show that the generated
baryon asymmetry can reproduce the observed value both in a scenario where $\phi$ is responsible for the reheating of the Universe, and in a scenario where $\phi$ is a generic scalar that remains in thermal
equilibrium with the
plasma.
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Instrumentation and Methods for Astrophysics · Cross-listed from eess.SY
Efficient Nonlinear Uncertainty Quantification for Spaceflight Leveraging Nonlinear Expansions
Ethan R. Burnett and Spencer Boone
This paper provides a comparative study of modern uncertainty quantification (UQ) methods. To greatly enhance real-time performance, both differential
algebra (DA) and a directional differential
algebra (DDA) approach are employed. This can enable fast UQ in the case of non-Gaussian statistics. Higher-order moments, namely skew and kurtosis, can be computed quickly by several means. This motivates their implementation in an analytic approximation of the confidence bounds for the so-called "banana-shaped" non-Gaussian distributions encountered often in nonlinear astrodynamics problems. This method improves greatly on a linear covariance approach, with only 5x its runtime in numerical tests, even before DA methods are employed. Test problems in this work include a restricted three-body cislunar example and an Earth-return aerocapture example.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
Inferring Neutron-Star Properties from Post-merger Gravitational-wave Spectra with Neural Networks
Dimitrios Pesios and Nikolaos Stergioulas
We present a proof-of-concept study of the inverse problem of inferring
neutron-
star properties directly from the post-merger gravitational-wave spectrum of equal-mass binary
neutron-
star mergers. Using noise-free spectra from numerical-relativity catalogs, we train and compare three artificial-neural-network regression models and two multivariate linear-regression baselines to predict the
stellar mass, $M$, the quadrupolar
tidal deformability, $\kappa_2^\tau$, and the slope of the mass–radius relation, $dR/dM$. Since the inverse mapping is nonlinear and cannot be obtained by analytically inverting the direct neural-network model, we construct inverse surrogates and train the networks with a two-stage procedure in which residuals from an initial pass define sample weights for a second pass, together with regularization via dropout, Gaussian-noise injection, and early stopping. We find that neural networks consistently outperform linear baselines, showing that nonlinear surrogates capture the inverse relation between post-merger spectra and source properties more effectively than algebraic inversion. The best performance is achieved by an ensemble of single-task networks, while a multi-task model gives comparable accuracy for predicting the mass–radius slope, and a mixture-of-experts architecture provides insight into spectral-region importance. We further show that the best model reproduces empirical relations between the dominant post-merger frequency and
tidal deformability, and recovers equation-of-state-dependent mass–
tidal-deformability trends, indicating physical consistency beyond pointwise accuracy. Although restricted to idealized noise-free spectra, the results show that neural-network surrogates provide a promising route for extracting
neutron-
star information from post-merger signals with future third-generation detectors.
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High Energy Astrophysical Phenomena
Pitch-Angle Scattering of Cosmic Rays: Confronting Theory with Observations
Huirong Yan, Siqi Zhao and Ming Zhang
Cosmic ray (CR) propagation is controlled by scattering in turbulent
magnetic fields in space. In general, diffusive propagation is governed by pitch-angle
diffusion in phase space. In this study, pitch-angle
diffusion in the local interstellar medium (LISM) deduced from the analysis of the CR small-scale anisotropy data from the Tibet AS$\gamma$ experiment is compared with theoretical predictions. While it is difficult to reconcile the inferred LISM pitch angle
diffusion coefficient with conventional theoretical results of particle scattering by Alfvénic
turbulence, we find remarkable agreement with the predictions of particle scattering by quasi-slab fast modes whose properties are shaped by damping in the warm ionized medium. These findings offer direct evidence that CR scattering is predominantly governed by fast-mode
turbulence. Furthermore, the comparison between experimental and theoretical results imposes strong constraints on
plasma and
magnetic field parameters within the local bubble, indicating that the LISM is in a low $\beta$ condition. The
turbulence in the LISM should be compressible with an amplitude in the range of $0.3\lesssim \delta B/B_0 <1$.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Tanmoy Kumar and Newton Nath
We investigate the possibility of detecting 14.4 keV and 9.4 keV
solar axions and
axion-like particles that could be produced in the M1
nuclear transitions of $^{57}$Fe and $^{83}$Kr, respectively. To do so, we used data from soft X-ray observations of the quiet Sun collected by the
Solar X-ray Monitor (XSM) on board India's Chandrayaan-2 lunar mission. We observe that although the effective
axion-nucleon couplings for $^{83}$Kr and $^{57}$Fe differ only slightly, their fluxes differ by nearly three orders of magnitude. Consequently, the limit on $|g_{aN}^{\rm eff} \times g_{a\gamma\gamma}|$ and only $g_{a\gamma\gamma}$ vs. $m_a$ provide more than an order-of-magnitude stronger constraint for Fe than for Kr.
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Astrophysics of Galaxies
The Lumina Project: The Demographics of Active Galactic Nuclei from Quasars to Little Red Dots at $z\geq 3$
Xuejian Shen, Oliver Zier, Aaron Smith et al. (+16 more)
High-
redshift active
galactic nuclei (AGN) serve as powerful probes of early black-hole growth,
galaxy formation, and the evolving intergalactic medium (IGM). In this work, we use Lumina, a cosmological radiation-hydrodynamic simulation spanning the epochs of hydrogen and helium reionization, which combines a large $(500\,{\rm cMpc})^3$ volume with $2\times 6000^3$ resolution elements, to explore high-
redshift AGN. The simulation self-consistently follows hundreds of millions of
galaxies and supermassive black holes (SMBHs), together with their impact on the ionization and thermal state of the IGM. We exploit this uniquely large dynamic range to predict multi-band AGN luminosity functions (LFs) at $z \geq 3$, from hard X-rays to the mid-infrared. These predictions encompass both moderately luminous
quasars and the faint “Little Red Dots” (LRDs) uncovered by JWST. We develop an empirical model that maps simulated SMBHs onto observed AGN using bolometric and extinction/absorption corrections for canonical AGN and LRDs, and in which SMBHs with $M_{\rm BH}\leq 10\,M_{\rm seed} \sim 10^{7}\,{\rm M}_{\odot}$ stay in the LRD phase with a duty cycle of $30\%$. This simple framework reproduces the observed LFs and clustering of LRDs. Meanwhile, the pre-JWST
quasar LF constraints are recovered, although we find that a $\sim 0.3$ dex log-normal scatter in bolometric luminosity is required to reproduce the bright end. We place the simulated AGN population in the cosmological context by quantifying the
redshift evolution of AGN and LRD number densities, and their contributions to the integrated BH mass densities. The same AGN population is the dominant driver for the HeII reionization modelled self-consistently in Lumina. This empirical AGN model paves the way for general population-synthesis models of high-
redshift AGN, including LRDs, in a unified cosmological framework.
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All authors Xuejian Shen, Oliver Zier, Aaron Smith, Rongrong Liu, Rahul Kannan, Teodora-Elena Bulichi, Sonja M. Koehler, Volker Springel, Mark Vogelsberger, Lars Hernquist, Rohan P. Naidu, Anna de Graaff, Elia Pizzati, David M. Alexander, Luis C. Ho, Vasily Kokorev, Gene Leung, Anna-Christina Eilers, Ryan C. Hickox
High Energy Astrophysical Phenomena
On the maximum neutrino flux of blazars in the one-zone leptohadronic model
Wei-Jian Li, Rui Xue, Ze-Rui Wang et al. (+1 more)
The origin of extragalactic high-energy
neutrinos remains a major mystery in astrophysics, with blazars as leading candidate sources. The widely adopted one-zone leptohadronic jet model, however, faces severe challenges from stringent X-ray observational constraints. In this work, we present an analytical approach that derives the maximum
neutrino flux as a function of the observed X-ray flux and the corresponding physical parameters attainable within the one-zone leptohadronic framework. Applying this approach to a sample of
neutrino candidate blazars, we further perform numerical modeling and find agreement between analytical and numerical results. Both approaches consistently show that the model-predicted
neutrino fluxes do not significantly exceed those obtained in previous one-zone studies and remain below the flux levels inferred from IceCube observations, suggesting that the one-zone scenario alone is unlikely to fully account for high-energy
neutrino-blazar associations. This highlights the importance of considering multi-zone models or alternative production sites (e.g., jet base, hot corona) to better explain high-energy
neutrino origins in blazars.
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All authors Wei-Jian Li, Rui Xue, Ze-Rui Wang, Dingrong Xiong
Astrophysics of Galaxies
Resolving galaxy formation in the early Universe with BonFIRE and CampFIRE
Jenna Samuel, Michael Boylan-Kolchin, Robert Feldmann et al. (+16 more)
The abundance and rapid growth of
galaxies at
cosmic dawn revealed by the James Webb Space
Telescope challenges models of
galaxy formation, motivating new simulations to uncover the processes driving early
galaxy assembly. We present the first results from BonFIRE ($L\approx40$ cMpc, $m_{\rm baryon}\approx5\times10^4~\rm{M}_{\odot}$) and CampFIRE ($L\approx5$ cMpc, at both $m_{\rm baryon}\approx800~\rm{M}_{\odot}$ and $\approx6\times10^3~\rm{M}_{\odot}$), a suite of cosmological hydrodynamic simulations of early
galaxy formation ($z\gtrsim6$) from the Feedback In Realistic Environments (FIRE) project, using the FIRE-3 model. We use a resampling procedure to combine the large statistics of BonFIRE with the higher resolution of CampFIRE and robustly predict
galaxy properties over a wide dynamic range ($M_{\star}\sim10^4-10^{10}~\rm{M}_{\odot}$).
Galaxy formation in this suite emerges through clustered, bursty
star formation, with
halo-scale
star formation efficiencies reaching $10-30\%$ in high-mass
halos. A subset of low-mass
halos also have surprisingly high efficiencies of $\gtrsim1\%$ and host ultra-compact
galaxies with narrow age spreads. We predict
galaxy UV luminosity functions at $9\lesssim~z\lesssim25$ in broad agreement with observations at $M_{\rm UV}\gtrsim-19$, with a faint-end turnover at $M_{\rm UV}\approx-14$, but we slightly overpredict the abundance of brighter
galaxies. We find that UV luminosity variability in early
galaxies is strongly mass-dependent, with
halo-to-
halo scatter dominating at low masses and contributing comparably to rapid temporal burstiness at $M_{\rm halo}\gtrsim10^{10}~\rm{M}_{\odot}$. We also present first results from a simple Pop~III model with a top-heavy IMF, demonstrating broad agreement with independent Pop~III predictions and observational constraints.
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All authors Jenna Samuel, Michael Boylan-Kolchin, Robert Feldmann, Philip Hopkins, Guochao Sun, Pratik Gandhi, Alessandra Venditti, Xuejian Shen, Andrew Wetzel, Jorge Moreno, Julian Munoz, Rachel Cochrane, Claude-Andre Faucher-Giguere, Volker Bromm, Steven Finkelstein, Maria Straight, Connor Painter, Jonathan Stern, James Bullock
Astrophysics of Galaxies
Dark matter-deficient twins: FCC 224 and FCC 240 as possible analogues of NGC 1052-DF2 and DF4
Maria Luisa Buzzo, Pieter van Dokkum, Michael Hilker et al. (+3 more)
The recent "bullet-
dwarf" model proposes that high-velocity collisions between
dwarf galaxies can produce
stellar systems with overluminous globular clusters (GCs) and a deficiency of dark matter, as observed in the NGC 1052 group
galaxies NGC 1052-DF2 and NGC 1052-DF4. We present a possible analogue system in the outskirts of the Fornax cluster: the ultra-diffuse
galaxy FCC 224 and its close companion FCC 240. Using deep VLT/MUSE integral-field spectroscopy, we characterize their
stellar populations, internal kinematics, and GC systems to test this formation scenario. Both
galaxies exhibit low velocity dispersions. Interpreted with a standard mass
estimator at the half light radius, and allowing for the known limitations associated with flattened systems, their inner dynamics are more naturally explained by
stars alone than by either cuspy or cored dark matter
halos. Both systems host unusually luminous GCs, closely resembling the top-heavy GCLF of the NGC 1052 pair. Moreover, FCC 224 and FCC 240 are coeval with each other, with mass-weighted
stellar ages of ~10 Gyr, and their GC populations share similarly old ages, in agreement with predictions of the formation scenario. Despite these similarities, FCC 224 and FCC 240 form a much tighter system than DF2 and DF4, with a projected separation of 75 kpc (compared to 240 kpc) and a relative velocity of only 16 km/s (compared to 358 km/s). This distinct configuration may suggest a different present-day manifestation of the same general class of
galaxies and provides additional observational constraints on models of their formation and
evolution.
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All authors Maria Luisa Buzzo, Pieter van Dokkum, Michael Hilker, Duncan A. Forbes, Aaron J. Romanowsky, Yimeng Tang
Astrophysics of Galaxies
The AGORA High-resolution Galaxy Simulations Comparison Project. XI: Solving the Non-Spherical Morphology and Evolution of Dark Matter Halos with Haskap Pie
Kirk S. S. Barrow, Thinh Huu Nguyen, Santi Roca-Fàbrega et al. (+26 more)
We introduce a
halo solving and tracking procedure that intrinsically treats dark matter
halos as non-spherical objects by leveraging the bound particle searching techniques used in Haskap Pie. The AGORA Collaboration's hydrodynamic simulation CosmoRun}project provides a useful laboratory to explore trends in dark matter
halo morphology that are revealed by our new procedure in the context of any dispersions or similarities between the codes. We find that several morphological and shape measures were very responsive to high mass ratio mergers. The greatest difference in these measures between the simulation codes were related to timing discrepancies and the dynamical state of the
halos prior to the mergers. Most other quantities were similar across codes, including several secular and
redshift-dependent trends in various dynamical quantities that showed a departure from Virial
Theorem (e.g., overdensity and
halo mass). We find that
halo spin and the ratio between the semi-major and the semi-minor axis peaked at 4>z>2 before declining at low
redshift. Also,
halo overdensity is both mass-dependent and
redshift-dependent, diverging for low mass
halos at low
redshift. Our method contributes a new perspective on these trends that have not been fully replicated in other works due to our emphasis on fundamentally non-spherical
halos and measures of morphology that correspondingly do not assume spherical
symmetry.
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All authors Kirk S. S. Barrow, Thinh Huu Nguyen, Santi Roca-Fàbrega, Ji-hoon Kim, Varun Satish, Kentaro Nagamine, Saulius Matusaitis, Eduárd Illes, Ramón Rodríguez-Cardoso, Minyong Jung, Hyeonyong Kim, Anna Genina, Pablo Granizo, Alessandro Lupi, Johnny W. Powell, Héctor Velázquez, Tom Abel, Oscar Agertz, Renyue Cen, Daniel Ceverino, Boon Kiat Oh, Yuri Oku, Joel R. Primack, Thomas R. Quinn, Yves Revaz, Alvaro Segovia-Otero, Ikkoh Shimizu, Edward Skrabacz, Romain Teyssier
Instrumentation and Methods for Astrophysics
Fast and Flexible Characterisation of Astronomical Light Curves Using Multi-Time Attention
Yash Gondhalekar, Anais Möller and Paula Sánchez-Sáez
We present an unsupervised, data-driven framework for rapid characterisation of astronomical photometric time series using a Multi-Time
Attention Network. The model learns time-aware latent representations directly from irregular, partial light curves without heavy preprocessing. Through application on ZTF alert data retrieved with Fink, a community alert broker for Rubin LSST, we demonstrate that the model: (i) produces accurate interpolations with small bias (0.01 mag) and scatter (0.1 mag) even for sparse light curves, (ii) learns a temporally distributed latent space correlating with physically meaningful properties (duration, peak time, variability, color) while being robust to unimportant properties such as observed magnitude and number of observations, (iii) separates general SN and AGN samples despite data being heavily dominated by AGNs, and (iv) generalises to unseen classes: The long-period variable and TDE show good interpolation and sensible latent space placement; however, the model cannot capture RRLyrae's $\sim$0.4-0.5 day pulsation period, which is far below our model's chosen two-day temporal resolution.
Attention map analysis reveals the capability of multi-time
attention to capture local structure. The model is extremely lightweight (a few hundred kilobytes) and has fast inference ($\sim$0.01 and $\sim$$3\times10^{-4}$ s per light curve on CPU and GPU, respectively) that is independent of the number of observations, unlike GP regression. Our approach offers flexible and scalable characterisation, with high relevance in the Rubin LSST era. We discuss future possibilities to incorporate observational uncertainties and
symmetries for robustness and
forecasting applications for real-time follow-up.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Neutron Star Bounds on Muonic Fifth Forces from Picometer to Meter Scales
Damiano F. G. Fiorillo, Alessandro Lella, Georg G. Raffelt et al. (+2 more)
Testing muonic fifth forces is difficult because
muons are scarce in ordinary matter. However, they are abundant in
neutron stars. We show that this population responds coherently to electromagnetic fluctuations induced by
proton and electron collisions, emitting the fifth-force mediator and cooling the
star. This energy loss constrains $g_{X\mu}\lesssim 10^{-13}$ for scalars with $m_\phi\lesssim 100$ keV and vectors with $10^{-5}\,\mathrm{eV}\lesssim m_V\lesssim 100$ keV, far more strongly than SN 1987A cooling. Vector forces at smaller $m_V$ deplete
muons, suppressing the coherent response. Cooling from incoherent
muon scattering still constrains $g_{V \mu}\lesssim 2.5\times 10^{-12}$ for $1.3\times 10^{-7}\,\mathrm{eV}\lesssim m_V\lesssim 10^{-5}$ eV. Together, these bounds provide the most sensitive probes of muonic interactions across a broad range of parameters.
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All authors Damiano F. G. Fiorillo, Alessandro Lella, Georg G. Raffelt, Nudzeim Selimovic, Edoardo Vitagliano
Astrophysics of Galaxies
FASTAR – I. Continuous and differentiable evolutionary stellar population models
Ignacio Martín-Navarro, Alexandre Vazdekis, Luis Peralta de Arriba et al. (+11 more)
The development of evolutionary
stellar population models is central to interpreting observations of
galaxies in terms of astrophysical quantities.
Stellar population models must therefore be both accurate and compatible with inversion
algorithms in order to extract meaningful information from the observed data. Here we present FASTAR, a fully differentiable
stellar population synthesis code. Contrary to traditional, grid-based single
stellar population models, FASTAR can be continuously evaluated at any age (between 20 Myr and 14 Gyr),
metallicity (-2.5 < [M/H] < +0.3), and initial mass function (IMF). Changes in the IMF parameterization are straightforward, allowing for consistent conversions of colors, magnitudes, and mass-to-light ratios, as well as the synthesis of models under the assumption of arbitrary IMF functional forms. FASTAR provides detailed spectroscopic predictions over the MILES wavelength range (3,540-7,400 A) as well as more coarsely sampled spectral energy distributions across a wider 2,000-to-12,000 A, which can be directly convolved with any arbitrary set of photometric filters. FASTAR performs at the same level of state-of-the-art simple
stellar population models benchmarked against observations of globular clusters and high signal-to-noise spectra of early-type
galaxies, but it is faster, lighter, and more flexible. Moreover, its differentiable nature allows for a quantitative understanding of model behavior and uncertainties, as well as a natural framework for gradient descent inference
algorithms.
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All authors Ignacio Martín-Navarro, Alexandre Vazdekis, Luis Peralta de Arriba, Isaac Alonso Asensio, Eirini Angeloudi, Patricia Iglesias Navarro, Francesco La Barbera, Katja Fahrion, Tereza Jerabkova, Michael A. Beasley, Jesús Falcón-Barroso, Marc Huertas-Company, Sebastián F. Sánchez, Prashin Jethwa
Cosmology and Nongalactic Astrophysics
Exploring the Dark Sector: Interacting Radiation in Light of Modern Cosmological Probes
Matteo Mescia and Héctor Gil-Marín
[abridged] We constrain a phenomenological dark radiation (DR) framework consisting of free-streaming and fluid-like components, providing a model-independent extension of the standard radiation sector. Using Planck CMB data, DESI DR2 BAO measurements, and Pantheon+ and DES Y5 (Dovekie)
supernova samples, we derive constraints on additional relativistic degrees of freedom and assess their impact on cosmological tensions. We obtain $N_{\rm fld}<0.66$ (95% C.L.) from CMB data alone, while the combination with BAO yields $N_{\rm fs}=2.93\pm0.23$ and $N_{\rm fld}=0.36^{+0.16}_{-0.21}$ (68% C.L.), consistent with Standard Model expectations for free-streaming radiation. The DR framework significantly alleviates the Hubble tension through an enhanced early-time expansion rate, which reduces the sound horizon scale. The tension with SH0ES is reduced from highly significant in $\Lambda$CDM to statistically non-significant for CMB+BAO data according to the $\mathcal{T}$-statistic.
Bayesian model comparison shows no decisive preference for DR over $\Lambda$CDM when SH0ES is excluded, with results in the regime of weakly disfavoured. However, including SH0ES data leads to decisive
Bayesian evidence in favour of the DR scenario. Overall, DR provides a compelling framework for resolving the Hubble tension. When CMB, BAO, Pantheon$+$ and SH0ES data are considered, we find an increased effective radiation content, $N_{\rm tot}=3.63^{+0.13}_{-0.15}$, with a fraction of free-streaming radiation, $f_{\rm fs}=0.392\pm 0.026$, a reduced sound horizon scale, $r_d = 141.8^{+1.3}_{-1.2}\,\mathrm{Mpc}$, and a higher
primordial helium fraction, $Y_{\rm He}=0.2530 \pm 0.0017$, which lies at the level of approximately $\sim 2$-$2.5\sigma$ above direct determinations from metal-poor H II regions, while remaining broadly consistent with other abundance measurements within current uncertainties.
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High Energy Astrophysical Phenomena
Discovery and Analysis of a Type II Supernova Candidate at z = 3.19 from JWST's COSMOS-Web Survey
Valeria Aparicio, David O. Jones, Willem B. Hoogendam et al. (+21 more)
The launch of the James Webb Space
Telescope (JWST) has enabled the discovery of a small but increasing sample of high-
redshift core-collapse
supernovae (CC SNe), which provide new tests of massive
star evolution in the early Universe. In this study, we report the discovery of SN 2023aeaf in COSMOS-Web survey observations, which at $z = 3.195$ has one of the highest SN spectroscopic
redshifts to date. Using two epochs of JWST photometry separated by $\sim$1 month in the rest frame, we photometrically classify SN 2023aeaf by comparing the JWST photometry to spectrophotometric CC SN and Type Ia (SN Ia) models and UV observations of SNe from the Swift
telescope, finding that SN 2023aeaf is highly likely to be a Type II SN. A spectrum of the SN$+$host
galaxy was also obtained $\sim$30 rest-frame days after discovery but shows no clearly identifiable SN features, with H$\alpha$ emission from the host potentially masking emission from the SN. Although the limited photometric coverage prevents strong constraints on the explosion properties, we find that the data are most consistent with a $\sim$12$M_\odot$ progenitor with $\sim$0.5$M_{\odot}$ of circumstellar material. We next use the host-
galaxy spectrum and photometry to model the host spectral energy distribution (SED) using the Prospector
Bayesian inference framework. We find that the host is a
star-forming
galaxy with a sSFR of $ \log_{10}(\rm sSFR/yr^{-1})= -10.17^{+0.13}_{-0.10}$, a
stellar mass of $\log(M_\star/M_\odot) = 9.04^{+0.03}_{-0.04}$, and a gas-phase
metallicity of $12 +{\rm log_{10}}({\rm O/H}) = 7.82\pm0.02$. SN 2023aeaf joins a growing sample of early Universe CC SNe with high luminosities, dense CSM, and low-
metallicity environments.
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All authors Valeria Aparicio, David O. Jones, Willem B. Hoogendam, Takashi J. Moriya, David A. Coulter, Justin D. R. Pierel, Matthew Siebert, Bingjie Wang, Hollis B. Akins, Caitlin M. Casey, Nicole E. Drakos, Andreas L. Faisst, Ori D. Fox, Aryana Haghjoo, Michaela Hirschmann, Olivier Ilbert, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Henry Joy McCracken, Bahram Mobasher, Armin Rest, Jason Rhodes, Brant E. Robertson, Marko Shuntov
High Energy Astrophysical Phenomena
A formation scenario of black hole-envelope systems –viscous hydrodynamics simulation in general relativity–
Alan Tsz Lok Lam, Masaru Shibata, Kenta Hotokezaka et al. (+1 more)
By performing a viscous hydrodynamics simulation in general relativity for super-Eddington
accretion flows onto massive black holes of mass $M=10^5$–$10^7M_\odot$, we discuss a formation scenario for black hole-envelope systems. We consider the mass
accretion rate of $a^3/G \approx 1.5 \times 10^{25} (a/10\,\mathrm{km\,s^{-1}})^3$\,g/s, comparable to the Eddington mass
accretion rate of a $10^7M_\odot$ black hole, assuming that the gas temperature of the infalling matter is $\lesssim 10^4$\,K. Here, $a$ and $G$ denote the sound speed and gravitational constant. For the
accretion flow, we set up a quasi-spherical Bondi-type flow in which radial inflow dominates over angular momentum in the distant region. It is found that (i) for low-mass black holes with $M \lesssim 10^6M_\odot$, a
photon-trapped region forms in the inner region, and a significant viscous
outflow driven near the polar region overcomes the ram pressure of the mass inflow, leading to an inflow-
outflow structure; (ii) for massive black holes of $M \gtrsim 3 \times 10^6M_\odot$, the
outflow is not launched, and a convective envelope around the black hole gradually develops; and (iii) irrespective of the black-hole mass, the mass
accretion rate onto the black hole is of order 10% of the Eddington
accretion rate for reasonable values of the viscous coefficient. As the mass
accretion rate onto the black holes is much lower than the mass growth rate of the envelope for low-mass black holes with $M\lesssim 10^6M_\odot$, the envelope mass is likely to increase until the total viscous heating rate exceeds the Eddington luminosity of the system, if the mass
accretion rate is preserved to be high for $\gtrsim 10^8 (M/10^7M_\odot)$\,yrs.
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All authors Alan Tsz Lok Lam, Masaru Shibata, Kenta Hotokezaka, Carlo Musolino
High Energy Astrophysical Phenomena
Rostom Mbarek, Jiaxi Wu and Elias R. Most
Binary
neutron star mergers can form short-lived magnetar-like remnants whose magnetically dominated polar towers reach $B\sim10^{15}$–$10^{16}\,\mathrm{G}$, but the microphysical composition of these
outflows remains poorly understood. Combining tower geometries from GRMHD simulations with an analytic treatment of QED and hadronic processes, we argue that
magnetic reconnection is the most viable particle acceleration channel in this strongly radiative regime, where the current sheets thin to collisionless scales. Purely leptonic pair loading – including resonant inverse Compton scattering of soft
photons – is bottlenecked by rapid pitch-angle damping and the tendency of one-
photon magnetic conversion to populate low Landau levels. Once
protons reach mildly relativistic energies ($\gamma_p\gtrsim1.3$), however, inelastic
proton-
proton ($pp$) collisions inject large-pitch-angle pions that drive $\pi^0\to2\gamma\to e^\pm$ cascades with multiplicity $\mathcal{M}_{\rm cas}\simeq4$ at $B=10^{15}\,\mathrm{G}$, supplying the perpendicular momentum the leptonic channel cannot maintain. This hadronic route dominates pair loading and channels most of the dissipated
magnetic energy into the $e^\pm$ population that could power the nonthermal emission emerging at larger radii. Charged-pion decay, modulated by $\pi^\pm$ synchrotron cooling, further seeds a nonthermal
neutrino tail up to $\sim 300\,(\sigma_p/5)\,\mathrm{MeV}$, spectrally distinct from the thermal cooling burst and detectable from sources within $\sim 100\,\mathrm{kpc}$
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Cosmology and Nongalactic Astrophysics
Detecting Gravitational-Wave Anisotropies with Simulation-Based Inference
Anna-Malin Lemke, Andrea Mitridate, Thomas Konstandin et al. (+2 more)
Over the last five years, multiple
Pulsar Timing Array (PTA) collaborations have reported mounting evidence for a gravitational-wave background (GWB) at nanohertz frequencies. Measuring anisotropies in the sky distribution of the GWB power is one of the most promising ways to identify and characterize its source. These anisotropies are expected to manifest as deviations from the Hellings-Downs (HD) correlations between the timing residuals of different
pulsars. Current search strategies include
Bayesian methods, which model anisotropies in the timing residuals likelihood, and faster frequentist approaches, which construct correlation
estimators from timing residuals and use these to test the isotropic assumption. However, frequentist methods rely on the assumption that correlation
estimators are Gaussian-distributed, an assumption that is not justified and that – as we will show – severely limits detection sensitivity. In this work, we present a Simulation-Based Inference (SBI) framework that replaces the analytic Gaussian likelihood used in frequentist searches with a neural network classifier trained on synthetic data. This approach captures the non-Gaussian structure of the data and significantly improves performance. Specifically, we find that the probability of $3\sigma$ detection increases by approximately 90% for single-hotspot scenarios and by 200% for double-hotspot scenarios compared to standard frequentist methods.
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All authors Anna-Malin Lemke, Andrea Mitridate, Thomas Konstandin, Mauro Pieroni, James Alvey