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Physics Astrophysics High Energy Astrophysical Phenomena

astro-ph.HE Watch High Energy Astrophysical Phenomena

Wednesday 30 September

High Energy Astrophysical Phenomena

X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A

Genevieve Schroeder, Anna Y. Q. Ho, Gavin P. Lamb et al. (+24 more)

Soft X-ray flashes (XRFs) are predicted from a variety of hypothesized phenomena, including "dirty fireballs" (relativistic jets with significant baryon loading), and gamma-ray bursts viewed marginally off-axis. For a given burst, detailed multiwavelength observations are necessary for distinguishing between possible progenitor scenarios, but the number of XRFs with such data remains small. Here we present X-ray, optical, and radio (mm to cm) data of GRB 250419A, an XRF (with peak energy $E_p<4\,$keV) discovered by the Space-based multi-band astronomical Variable Objects Monitor. We observe a clear achromatic break in the X-ray and optical bands, and likely two spectral components in the radio emission. The afterglow is similar to the long-duration gamma-ray burst population, but slightly underluminous, and the optical afterglow has an unusually shallow decay phase at $\lesssim2\,$d. We find that the event is well explained by a high-Lorentz factor jet with low kinetic energy ($E\sim10^{49}$-$10^{50}\,$erg), with energy injection that could arise from radial stratification and/or the jet being viewed marginally off-axis. We conclude that at least some X-ray flashes are an extension of classical gamma-ray bursts down to lower energies.
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All authors Genevieve Schroeder, Anna Y. Q. Ho, Gavin P. Lamb, Tanmoy Laskar, Daniel A. Perley, Ore Gottlieb, Tomas Ahumada, Igor Andreoni, Aleksandra Bochenek, Michael Bremer, Michael Camilo, Jonathan Carney, S. Bradley Cenko, Alessandra Corsi, Michael W. Coughlin, James Freeburn, Erica Hammerstein, Rahul Jayaraman, Mansi M. Kasliwal, Noel Klingler, Brendan O'Connor, Cassie Sevilla, Gokul P. Srinivasaragavan, Robert Stein, Jada L. Vail, Sylvain Veilleux, Lin Yan

High Energy Astrophysical Phenomena

A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748-676

Amy H. Knight, Lauren Rhodes, Noel Castro Segura et al. (+11 more)

After sixteen years in quiescence, EXO 0748-676 unexpectedly returned to outburst in March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748-676 returned to quiescence in January 2025, after ~ 10 months in outburst. Here, we collate data from nine observing facilities, covering wavelengths from radio to X-ray, to provide the first multi-wavelength study of the 2024/25 outburst. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The light curves do not exhibit the hallmarks of a canonical transient low-mass X-ray binary outburst. Instead, the X-ray and optical fluxes remain reasonably stable during the 2024/25 outburst. The wealth of optical data of EXO 0748-676 reveals a skewed optical phase curve and an irradiation-dominated optical light curve which lasts until the onset of hydrogen recombination in the outer disc at ~11000K, triggering a rapid fade at the end of the outburst. X-ray observations by XMM-Newton and NICER revealed that EXO 0748-676 was heavily obscured early on, with spectra exhibiting a dampened soft X-ray contribution. We find that a spectral model including a clumpy, ionised absorber describes the obscuration, although the exact cause of the early-time obscuration remains uncertain. X-ray colour analysis indicates that EXO 0748-676 spent most of the ~10 month outburst in a faint, hard state. EXO 0748-676 increased marginally in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is characteristic of a failed transition outburst.
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All authors Amy H. Knight, Lauren Rhodes, Noel Castro Segura, Douglas J. K. Buisson, Aru Beri, David M. Russell, Kevin Alabarta, Maria C. Baglio, Jakob van den Eijnden, Matthew J. Middleton, Payaswini Saikia, Chris Done, Rob Fender, Fraser Lewis

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA

The impact of relativistic AGN jets on realistic galaxy cluster environments

K. Hervella-Seoane, M. Perucho, S. Planelles et al. (+2 more)

Context. Low-power FRI-like radio jets dominate the AGN population, yet the main mechanism of heating the intracluster medium (ICM) and its efficiency remain a matter of debate. Aims. We investigate the impact of intermediate-power FRI-like relativistic jets on the inner region of realistic ICM, focusing on the role of weak shocks on heating and the environmental coupling in regulating energy deposition. Methods. We present six three-dimensional relativistic-hydrodynamic simulations of (1e44 - 1e45) erg/s FRI-like jets propagating through the central regions of realistic ICM environments extracted from cosmological GADGET-3 runs. Each simulation tracks 20 Myr of continuous jet activity with radiative cooling included. Results. Despite their intermediate power and mildly relativistic speed at injection, they decelerate rapidly to trans-sonic velocities (average Mach numbers ~ 2.3) and generate weak bow shocks that dominate the energy transfer. Approximately, 80% of the injected kinetic power is converted into internal energy of the ambient ICM, and bremsstrahlung cooling noticeably reduces the temperature of the shocked shells as they expand. Conclusions. Our results demonstrate that heating of the intergalactic and intracluster medium is highly efficient even in the case of weak shocks in FRI-like sources. This finding suggests low-power jets are a viable solution to the long-standing cooling-flow problem and likely regulate star formation across diverse galaxy cluster environments.
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All authors K. Hervella-Seoane, M. Perucho, S. Planelles, J.M. Martí, V. Quilis

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.IM

NASA ASTRA White Paper : A Chandra Successor in the 2030s

C. S. Reynolds, E. Hodges-Kluck, E. A. Kara et al. (+19 more)

Chandra established that soft X-ray spectral imaging with approximately 1 arcsec angular resolution is an essential tool for almost every area of astrophysics, from the early Universe to ice chemistry in protoplanetary disks. While Chandra continues to deliver groundbreaking science, it lacks the sensitivity to be the counterpart to JWST, Roman, Rubin, LISA, ngVLA, ALMA, IceCube, NewAthena, ELT, and other upcoming large facilities. Here we highlight the scientific potential of a Chandra successor with >10x higher sensitivity and >50x survey speed. While the scientific reach of such a mission is very broad, we showcase four high-priority science questions that require these capabilities – How did supermassive black holes form?, How does stellar and AGN feedback work in galaxies?, How does high-energy stellar activity affect exoplanet atmospheres? and, How do black holes merge? A Chandra successor in the 2030s would also demonstrate the X-ray mirror technology for a future flagship and sustain the US high energy astrophysics workforce until that flagship launches. We outline the science-instrument trade space by sketching three missions based on the NASA Probe concept study report for the Advanced X-ray Imaging Satellite (AXIS): AXIS-Deep, AXIS-Classic, and AXIS-Fast, highlighting a breakthrough science case for each and listing others that drive the design.
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All authors C. S. Reynolds, E. Hodges-Kluck, E. A. Kara, M. Koss, K. Madsen, E. Miller, B. Williams, S. Allen, M. Bautz, K.Burdge, N. Cappelluti, B.Cenko, L. Corrales, A.Falcone, A. Foord, D. Haggard, L. Lopez, R. Mushotzky, A. Ptak, H.Russell, S. Safi-Harb, K.Stassun

High Energy Astrophysical Phenomena

XClass: An Automated Multiwavelength Machine-Learning Pipeline for Classification of Extragalactic X-ray Sources. I. Pipeline Description

Blagoy Rangelov, Nicholas Moore, Haley Dutton et al. (+1 more)

Most X-ray sources detected in nearby galaxies by the Chandra X-ray Observatory lack astrophysical classifications. We present XClass (X-ray Classifier for extragalactic sources), an end-to-end machine-learning pipeline that classifies extragalactic X-ray point sources into seven classes: AGN, LMXBs, HMXBs, CVs, low-mass and high-mass foreground stars, and supernova remnants. The key challenge is photometric heterogeneity: training sources are predominantly Galactic with photometry from wide-field surveys (PanSTARRS and 2MASS), while extragalactic targets require Hubble Space Telescope (HST) imaging in a disjoint filter system. We address this through a spectral energy distribution (SED) translation that fits class-appropriate spectral models to each training source and convolves the best-fit model through HST filter curves, producing synthetic magnitudes in a common feature space. The classifier uses an asymmetric two-stage Random Forest: Stage 1 separates broad categories (AGN, X-ray binaries, SNRs, stars) and Stage 2 resolves X-ray binaries into LMXBs and HMXBs using an augmented feature vector that includes Stage 1 probabilities. The training set is assembled from ten Galactic catalogs and extragalactic SNR catalogs, cross-matched with the Chandra Source Catalog v2.1. Features include X-ray hardness ratios, SED-translated HST colors, and X-ray-to-optical flux ratios. We restrict the training set to sources with at least one optical magnitude, avoiding imputation artifacts; the pipeline achieves 99.6% accuracy and balanced accuracy of 0.90 on the resulting 11,374-source optical baseline, with excellent calibration (ECE = 0.002). XClass is modular, generalizable to any HST filter configuration, and will be applied to M31 and M33 in a companion paper.
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All authors Blagoy Rangelov, Nicholas Moore, Haley Dutton, Erika Marentes

High Energy Astrophysical Phenomena

Three New Likely Spider Millisecond Pulsar Binaries and Optical Kinematic Tracers of Intrabinary Shocks

Jay Strader, Rebecca Kyer, Laura Chomiuk et al. (+9 more)

We report the discovery of three candidate spider millisecond pulsars via multi-wavelength follow-up of unassociated Fermi Large Area Telescope gamma-ray sources. All have optical spectroscopy from the SOAR 4.1-m telescope and X-ray data from Swift. Two optical/X-ray sources are exceptionally strong redback candidates, sitting close to the centres of the localization ellipses of the gamma-ray sources 4FGL J1020.4-5314 and 4FGL J1408.7-3747. We show that both optical sources are spectroscopic binaries with orbital periods (0.15-0.3 d) and mass functions (0.6-0.9 Msun) typical for redbacks. 4FGL J1020.4-5314 shows bright, very broad (full width at half maximum 2100 km/s) H-alpha emission in all spectra obtained over two years. Remarkably, the H-alpha radial velocities are offset in phase from the absorption-line velocities of the secondary, and match the signal expected for an emission source near the primary. This demonstrates, for the first time for any spider, that the H-alpha emission can be associated with an intrabinary shock that wraps around the pulsar, opening up the possibility of using optical emission lines as kinematic tracers of the intrabinary shock in some redbacks. A third candidate spider is less secure in its association with the gamma-ray source 4FGL J1429.8-0739, but has unusual properties, including a 30.5-hour orbital period, a potential subgiant secondary, and a high X-ray luminosity, making it worthy of further investigation.
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All authors Jay Strader, Rebecca Kyer, Laura Chomiuk, Elias Aydi, Peter Craig, Thomas M. Do, Isabella Molina, Teresa Panurach, Subhroja Roy, Kirill V. Sokolovsky, Samuel J. Swihart, Ryan Urquhart

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

Gaia neutron stars: demographics, birth rates, and connections to other neutron star populations

Cheyanne Shariat, Natsuko Yamaguchi, Kareem El-Badry et al. (+1 more)

Gaia DR3 revealed a new class of neutron stars (NSs) in au-scale orbits with solar-type companions. How these 'Gaia NSs' form remains unclear, and constraining their intrinsic population properties is a necessary step toward understanding their origin. However, the current sample of 27 Gaia NS candidates is strongly biased by Gaia's astrometric sensitivity. In this work, we carefully model the Gaia selection function and incorporate it into a hierarchical Bayesian framework to infer the intrinsic demographics of the Gaia NS population. We find that the population favors a decreasing period distribution with power-law slope $\alpha_P=-0.75 \pm 0.35$, a visible-star mass distribution peaked near $M_1\approx 0.85~{\rm M_\odot}$ ($\sigma_{M_1} \approx 0.2~{\rm M_\odot}$), an NS mass distribution peaked near $M_{\rm NS}\approx 1.3~{\rm M_\odot}$ with a tail towards higher masses, and an eccentricity distribution weighted toward moderate-to-large eccentricities. The intrinsic space density is $n\approx120~{\rm kpc^{-3}}$ within 2 kpc, implying that only $\sim3\%$ of the local Gaia NS population has been discovered so far. Our forecasts predict that Gaia DR4 will increase the sample to $\sim300$-650 NS candidates, including access to longer orbital periods that will provide direct tests of formation models. Finally, we place Gaia NSs in the context of the broader Galactic NS population by comparing their intrinsic birth rate, lifetime, and size to other classes of isolated and binary NSs. Only 0.01% - 1% of all NSs are born into au-scale orbits with solar-type companions, making the birth rate of Gaia NSs 10 - 100 times smaller than that of high-mass X-ray binaries and broadly comparable to symbiotic X-ray binaries.
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All authors Cheyanne Shariat, Natsuko Yamaguchi, Kareem El-Badry, Thomas J. Maccarone

High Energy Astrophysical Phenomena

A new method for inspiral-to-ringdown waveforms of compact binaries: Helmholtz orbits and spherical-Bessel flux

Hong-Bo Jin

The waveform is obtained from one balance of energy and radiation on Helmholtz orbits of the retarded mass kernel, with a spherical-Bessel flux. That balance follows from the Dirac–Maxwell equation with a modified vertex, the single $U(1)$ coupling being replaced by the eight operators of the source interaction. After contact that balance is the energy loss of one packet: a bounded well for a black-hole remnant and the Coulomb ladder of one star for a neutron-star remnant. Ringdown starts from the spherical projection of that end state, which contains its spin. The comparisons use model waveforms at the published parameters. For GW250114, at the Coulomb-radius contact and after a shift of $-16\,\mathrm{ms}$, the noise-weighted norm ratio against the NRSur7dq4 median in $100$–$200\,\mathrm{Hz}$ stays between $1.36$ and $1.94$ across the published distance interval. Stopping at $2.4$ times that radius lowers the ratio to $1.11$, which the same interval moves from $0.94$ to $1.35$. With contact placed on the published merger time and with no time shift, that ratio is $3.32$ at the Coulomb radius and $2.30$ at $2.4$ times that radius. At the larger radius, the unretarded limit of this balance, with unit spherical-Bessel weights, reaches contact at $94\,\mathrm{Hz}$, and in $20$–$100\,\mathrm{Hz}$ the norm ratio to the Helmholtz balance is $1.12$ after a shift of $+27\,\mathrm{ms}$. For GW170817, $|h_{+}|$ near $100\,\mathrm{Hz}$ is $8.0\times10^{-23}$, and the mismatch against the $2.5$PN inspiral is a difference in phasing.
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High Energy Astrophysical Phenomena

XRISM Resolve Spectroscopy of GX 5-1: Constraints on Iron Spectral Features in a Luminous Neutron-Star Binary

Tasuku Hayashi, Shinya Yamada, Shun Inoue et al. (+11 more)

GX 5-1 is one of the brightest persistent neutron-star low-mass X-ray binaries in the Galaxy and one of the classical Z sources accreting at luminosities close to or above the Eddington limit. We present the first high-resolution X-ray spectroscopic study of GX 5-1 with the Resolve micro-calorimeter onboard the X-Ray Imaging and Spectroscopy Mission (XRISM), based on a 50 ks AO1 observation. With an energy resolution of approximately 5 eV at 6 keV, Resolve enables a sensitive non-dispersive search for weak and narrow spectral features in this luminous source. During the observation, GX 5-1 was located on the horizontal branch of the Z track. Although rapid spectral variability is detected on timescales of tens of ks, the variability is dominated by changes in the continuum spectral shape. No statistically significant emission or absorption features are detected in the time-averaged spectrum. Using matched-filter searches and time-resolved spectroscopy, we place stringent upper limits on the equivalent widths of low-ionization Fe K-alpha emission and highly ionized Fe XXV and Fe XXVI K-alpha transitions of $EW \lesssim$ a few eV. These results reinforce the picture that, in GX 5-1, classical Fe K diagnostics such as reflection- or wind-related features are either intrinsically weak, strongly ionized, geometrically diluted, or suppressed by rapid continuum variability. Placed in the context of the long observational history of GX 5-1 as a continuum-dominated Z source, the present XRISM result shows that this picture continues to hold even at calorimeter resolution. GX 5-1 therefore provides an important benchmark for high-resolution studies of luminous neutron-star accretion flows and demonstrates the capability of XRISM to place meaningful constraints on line formation even when no significant features are detected.
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All authors Tasuku Hayashi, Shinya Yamada, Shun Inoue, Teruaki Enoto, Noriko Yamasaki, Shunji Kitamoto, Shogo Kobayashi, Yusuke Sakai, Shintaro Kaneko, Rin Ebisawa, Haruto Aoki, Kota Miyawaki, Misaki Mizumoto, Hiromitsu Takahashi

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA

Deep radio characterization of the supernova remnant G343.1-00.7

S. Loru, A. Ingallinera, C. Trigilio et al. (+20 more)

Supernova remnants (SNRs) are extended radio sources whose morphology and evolution are shaped by the interaction of strong shocks with the surrounding medium. We combined the 0.944 GHz Australian Square Kilometre Array Pathfinder (ASKAP) image from the Evolutionary Map of the Universe (EMU) survey with 0.088-0.200 GHz MWA-GLEAM data to characterize the integrated radio spectrum of G343.1$-$00.7 and perform its first sensitive spatially resolved spectral analysis. We derived an integrated spectral index of $\alpha=-0.50\pm0.01$, confirming the previously reported value with improved accuracy. We redefined the radio morphology of the remnant, providing the first morphological and spectral evidence that filamentary structures overlapping the nearby \hii\ G343.147$-$00.44 are associated with the SNR. Local brightness-brightness analysis revealed flatter-spectrum filaments along the south-eastern shell, consistent with a possible thermal bremsstrahlung contribution from radiative or partially radiative shocks. Comparison with \hi\ and CO data identified gas components kinematically consistent with the SNR distance, although no clear morphological evidence of interaction was found. The gas distribution around the \hii\ suggests an expanding \hi\ shell partially embedded within its parent molecular cloud. Theoretical models place G343.1$-$00.7 between the late Sedov and early pressure-driven phases, suggesting different evolutionary stages across the remnant. These results highlight the potential of high-resolution ASKAP observations for spatially resolved studies of poorly characterized SNRs in complex Galactic environments.
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All authors S. Loru, A. Ingallinera, C. Trigilio, G. Umana, D. Urošević, B. Arbutina, D. Leahy, C. Bordiu, F. Bufano, I.U. Aalia, C. Buemi, F. Cavallaro, E. Egron, M. D. Filipović, L. Heino, A. M. Hopkins, S. Lazarević, P. Leto, A. Pellizzoni, S. F. Rahman, S. Riggi, A.C. Ruggeri, T. Zafar

High Energy Astrophysical Phenomena

Correlating Surface Magnetic Field Decay with Internal Field Depth of Isolated Neutron Stars

Kathleen Sellick and Subharthi Ray

The long-term evolution of neutron star crustal magnetic fields through Ohmic decay is governed by the transport properties of the crust. In this work, we demonstrate that the depth at which the magnetic flux is anchored plays a significant role in the dissipation timescale. We study the evolution of the magnetic field at the surface of the neutron star due to the crustal field evolution, under the assumption of Ohmic diffusion in a curved spacetime. We employ a cumulative distribution function to parameterise the initial current distribution and explore the effects of varying the equation of state and crustal impurity. We compare results across three neutron star models representing varying degrees of compactness. We find that magnetic fields anchored deeper within the inner crust experience negligible decay over timescales up to $10^{10}$ years. Significant field evolution, necessary to explain the deficit of high-field pulsars at advanced ages, is possible if the magnetic currents are confined to the shallow outer crust. This result holds true regardless of the impurity concentration or the specific geometric thickness of the crust determined by the equation of state.
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High Energy Astrophysical Phenomena

Electromagnetic Counterparts of Stellar-mass Binary Black Hole Mergers in AGN Accretion Disks: Theoretical Models and Observational Status

Lei He, Liang-Gui Zhu, Ken Chen et al. (+4 more)

The discovery of the binary neutron star merger GW170817 and its electromagnetic counterparts marked the beginning of the era of multimessenger gravitational-wave astronomy. In contrast, binary black hole (BBH) mergers, which constitute the majority of gravitational-wave sources detected to date, are generally expected to be electromagnetically dark. However, if BBH mergers occur within the dense gaseous environments of active galactic nucleus (AGN) accretion disks, interactions between the merging black holes and surrounding gas may produce electromagnetic emission across multiple wavelengths. AGN disks may therefore provide a natural multimessenger laboratory for studying BBH mergers and their astrophysical environments. A key question is whether such a scenario is supported by current observations and whether the associated electromagnetic signatures are sufficiently distinctive and detectable. In this review, we summarize the current theoretical and observational status of BBH mergers in AGN accretion disks, focusing on the evidence for their occurrence, the physical mechanisms and expected signatures of electromagnetic emission, and searches for candidate counterparts. We also discuss the major challenges in identifying and confirming these associations and the prospects for future multimessenger observations with current and forthcoming multiwavelength facilities.
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All authors Lei He, Liang-Gui Zhu, Ken Chen, Zi-Gao Dai, Ye-Fei Yuan, Jian-Min Wang, Wen Zhao

High Energy Astrophysical Phenomena

Improved Population and EOS Joint Inference for Binary Neutron Star Systems

Marc Ebiri and Richard O'Shaughnessy

The extreme environment within neutron stars presents the opportunity to probe the nuclear equation of state at high densities while studying the properties of these stellar remnants. Historically, the equation of state and neutron star mass distribution have been inferred separately, with the latter often simply assumed to be fixed in studies of the former, but the dependence of the equation of state on neutron star mass indicates they should be inferred together. In this work, we extend the generalized version of the popular RIFT algorithm known as Hyperpipe to interface with flexible, user-provided priors that will facilitate joint inference of equation of state and binary neutron star population hyperparameters. We demonstrate this framework's utility via application to a combination of several existing observations, particularly including massive galactic pulsars, double neutron star systems, millisecond X-ray pulsars, the gravitational wave event GW170817 and the nuclear symmetry energy, within the context of a widely-adopted parametric EOS family and a simple Gaussian population model. We recover parameters consistent with previous analyses of the EOS and binary neutron star population, finding the latter to fit a bivariate normal distribution with mean $(\mu_1,\mu_2) = (1.39,1.27) M_{\odot}$ and width $\sigma = 0.08 M_{\odot}$. Furthermore, we implement novel coordinate transformations in our pipeline, with which we have discovered that the ad hoc prior boundaries used for this EOS family may be too restrictive. We present results with relaxed yet still physical prior boundaries, noting vastly improved hyperparameter posteriors and modestly different equation of state inferences.
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High Energy Astrophysical Phenomena

Pulsar glitches triggered by quakes when stress reaches a threshold

Zixuan Zhou, Ruipeng Lu, Weiyang Wang et al. (+2 more)

The starquake model of pulsar glitches is revisited in light of accumulated observational data. A statistical comparison between the time-predictable and the magnitude-predictable models, borrowed from seismology and grounded in the classical spring-slider framework, is performed against a sample of 245 glitch events from 9 pulsars. It is found that the data supports the time-predictable model, i.e., a quake resulting in a glitch could be triggered when the developed stress reaches a critical threshold, $\tau_{\rm c}$, while the remaining elastic energy (correspondingly the residual stress, $\tau_{\rm r}$) after quakes is not fixed. The critical stress could be the order of $\tau_{\rm c}\simeq (10^{34}-10^{35})\ {\rm dyn\,cm^{-2}}$ if the residual stress after a quake with maximum stress drops could be considered negligible (i.e., $\tau_{\rm r}\simeq 0$), to explain the glitch amplitudes and the intervals. This $\tau_{\rm c}$-value is the order of the shear modulus of strangeon stars, but would be several orders higher than that of conventional neutron stars.
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All authors Zixuan Zhou, Ruipeng Lu, Weiyang Wang, Han Yue, Renxin Xu

High Energy Astrophysical Phenomena

HLX-1 as a Repeating Partial Tidal Disruption Event around an Intermediate-Mass Black Hole

Fangyuan Yu, Andrew Mummery and Muryel Guolo

ESO 243$-$49 HLX–1 is one of the first known compelling intermediate-mass black hole (IMBH) candidates, but the origin of its recurrent X-ray outbursts remains unsettled. In this work we fit the HLX–1 outbursts simultaneously with a relativistic thin-disk time-dependent model. The black-hole and disk-geometry parameters are tied across epochs, while the injected mass and timing parameters are fitted separately for each outburst. We also include a contemporaneous HST UV/optical/IR spectral energy distribution obtained near the peak of one outburst, which constrains the global temperature and radius scales of the disk. The soft-state data are consistent with a common global solution, with black hole mass $\log_{10}(M_{\bullet}/M_\odot)=4.5\pm0.2$ and disk formation radius $\log_{10}(r_0/r_g)=3.8\pm0.3$. The differences between outbursts are mainly described by modest changes in disk mass and viscous timescales. The inferred injected masses are typically $\sim 10^{-3}\,M_\odot$ per outburst, requiring a total fuel supply of at least a few $\times 10^{-3}\,M_\odot$ across the observed sequence. This mass scale, and the disk size, are consistent with repeated partial stripping of a low-mass donor. A simple Keplerian mapping of the flare gaps and fitted disk radius implies a highly eccentric orbit with a pericenter just outside the donor's tidal radius. These results support a repeating partial tidal disruption interpretation for the origin of HLX–1.
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High Energy Astrophysical Phenomena

New Constraints on $r$-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-Phase Spectra

Salma Rahmouni, Masaomi Tanaka, Daiji Kato et al. (+2 more)

The neutron star merger event GW170817 provided the first direct evidence of $r$-process nucleosynthesis. Observed spectra of its electromagnetic counterpart AT2017gfo exhibited several features that encode information on the nature and abundance of the synthesized $r$-process elements. In this study, we investigate the late nebular-phase spectral features of AT2017gfo, which provide important probes of the elemental abundance of the ejecta. We construct an analytic spectral model that computes emission features produced by the radiative decay of collisionally excited ions through allowed and forbidden transitions. By comparing our model to AT2017gfo, we identify La III and Ce III as the main contributors to the emission features at $1.4\,\mu{\rm m}$ and $1.6\,\mu{\rm m}$, respectively. We also confirm Te III as the dominant contributor to the $2.1\,\mu{\rm m}$ feature proposed in previous works. We infer mass fractions of $X({\rm La})\approx 0.025-0.05$, $X({\rm Ce})\approx 0.05-0.1$, and $X({\rm Te})\approx 0.04-0.08$, although the La and Ce abundance estimates remain tentative due to uncertainties in the radiation field. From the non-detections of Kr and Sb lines, we derive upper limits of $X({\rm Kr})\lesssim 0.03$ and $X({\rm Sb})\lesssim 0.003$. These results suggest that nucleosynthesis in the inner ejecta of GW170817 produced a suppressed first $r$-process peak and an enhanced heavy-element abundance compared to the solar $r$-process pattern, with an estimated lanthanide fraction of $X_{\rm LN}\approx (3-6) \times 10^{-2}$. Our conclusions are consistent with the apparent universality of heavy $r$-process elements and the lanthanide fraction inferred from observations of $r$-enhanced metal-poor stars.
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All authors Salma Rahmouni, Masaomi Tanaka, Daiji Kato, Gediminas Gaigalas, Kenta Hotokezaka

Tuesday 29 September

High Energy Astrophysical Phenomena

Chandra X-ray Spectral Evidence of the Dual AGN Nature of the Gravitational lens MG B2016+112 at z=3.273 with 175 pc separation

Daniel A. Schwartz, Julia M. Sisk-Reynes and Anna Barnacka

We use a new 175 ks Chandra X-ray observation of the z=3.273 radio-loud gravitational lens system MG B2016+112 to present definitive X-ray spectral evidence that it contains a dual active galactic nucleus (AGN). With a projected separation of 175 parsecs, this is the tightest separation confirmed AGN pair at high redshift. Prior multi-epoch very long baseline interferometry (VLBI) and archival Chandra data had revealed two distinct emitters, but could not rule out a single AGN with a complex, highly magnified jet. Our new Chandra observations allow a separate spectral analysis of each source. We find strong intrinsic absorption (N_H > 4 x 10^22 cm^-2, 3 sigma confidence) at the source redshift for the highly magnified source inside the caustic. This identifies it as an obscured AGN. In addition to the previously known type II AGN outside the caustic, this confirms a compact dual AGN system in MG B2016+112. On-going and future surveys using facilities such as Euclid, SKA, and the Rubin and Roman observatories will uncover more such systems, probing SMBH evolution and gravitational wave precursors.
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High Energy Astrophysical Phenomena

Angular structure of finite-field vacuum birefringence in magnetars

Shahram Abbassi and S. R. Valluri

Vacuum birefringence in a strong magnetic field is often modelled by combining the finite-field splitting for perpendicular propagation with the familiar $\sin^2\theta$ angular factor. That separation is exact only at leading response order. Continuing our previous study of the field-strength dependence of the birefringence, we examine the angular dependence of the full one-loop refractive indices in the local Heisenberg–Euler theory. We show analytically that, on the positive-birefringence branch, the usual factorized form systematically overestimates the exact splitting away from perpendicular propagation. The relative correction is largest close to the magnetic-field direction, where the absolute birefringence itself vanishes. Compact analytic expansions expose the sign, scaling, and angular location of the correction throughout the magnetar-relevant range $B/B_{\rm cr}\le100$, while an independent proper-time calculation verifies the underlying one-loop coefficients. In centred-dipole applications the correction is sub-percent for moderate magnetar fields but reaches several percent for the strongest sources; surface averaging and radial phase accumulation reduce its physical weight as the field decreases away from the stellar surface. The result separates finite-field amplitude and angular corrections and provides controlled analytic benchmarks for precision polarization transport in strongly magnetized environments.
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High Energy Astrophysical Phenomena

Nonthermal hotspot orbiting the supermassive black hole Sgr A*

Alejandro Cruz-Osorio, David Barrero-González, Juan J. Zaldívar et al. (+1 more)

Recent millimeter wavelength observations at 230 GHz by the Event Horizon Telescope have resolved the innermost structure of the Galactic Center, Sgr A*, revealing a ring-like structure consistent with synchrotron emission from a magnetized torus surrounding a supermassive black hole. These observations reveal two key features: three prominent hotspots near the photon ring and low light-curve variability, with a modulation index of $\bar{\sigma}/\bar{\mu} \lesssim 0.1$. State-of-the-art general relativistic magnetohydrodynamic and radiative transfer simulations cannot simultaneously reproduce the three hotspots and low light-curve variability observed in Sgr A*. We propose that an orbiting hot blob, formed in the accretion disk, could account for hotspots. We systematically explore its physical properties, including size, emission amplitude, orbital radius, eccentricity, velocity, and orbital plane orientation. The results – applying two observational constraints–show that the preferred model corresponds to a black hole with dimensionless spin $a_{\star}=0.5$, a magnetically arrested disk configuration for the magnetic field, and the presence of a hotspot with size $\sigma_{\rm hs}=2\,M$ and intensity amplitude $A=10$, orbiting circularly at $r_{\rm orb}=6 M$ with Keplerian velocity in the black holes equatorial plane, and an inclination view angle of $i^\circ=20$.
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All authors Alejandro Cruz-Osorio, David Barrero-González, Juan J. Zaldívar, Yosuke Mizuno

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA

A triple-induced merger in T Ori C, solves the proplyd lifetime problem and the curiously young shell of M42

Maite Wilhelm, Alessandro Alberto Trani and Simon Portegies Zwart

There has long been a tension between the stellar ages in the Orion Nebula Cluster and the rapid external photoevaporation of its protoplanetary discs. Recently, the shell of the Orion Nebula has similarly been found to be dynamically younger than the stars. We propose a new scenario to resolve both age problems. In our scenario the ONC's most massive star, $\theta^1$~Ori~C1, formed from the merger between two binary stars induced by vZLK oscillations due to the present-day binary companion $\theta^1$~Ori~C2. This merger, happening between the last episode of star formation and today, resulted in an increased luminosity, leading to stronger external photoevaporation and accelerated expansion of the shell. {We use Monte Carlo simulations to explore both whether a sudden brightening of $\theta^1$~Ori~C1 could reconcile the stellar ages and shell expansion, and whether triple progenitors to $\theta^1$~Ori~C\, could result in stellar mergers on the relevant timescale. A sudden brightening of $\theta^1$~Ori~C1 a few hundred thousand years ago can reproduce today's shell while maintaining a system age close to the stellar age. However, the required increase in stellar wind luminosity is greater than a simple merger can explain. At lower luminosity, the mixing of a relatively greater amount of cool photoevaporative wind material into the hot bubble could result in the required increase, and we identify a lower limit on the required strength of this effect. Additionally, we find that progenitor triples merge readily, on timescales even shorter than the scenario would require. These mergers could be deferred by increased pre-main sequence tidal effects, rotation, and perturbing encounters with other cluster members. Our proposed scenario with a merger driven by von Zeipel-Lidov-Kozai evolution solves two problems; the Orion Nebula's proplyd problem and the shell lifetime problems.
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High Energy Astrophysical Phenomena

Relativistic Spherical Accretion in Nonrotating Black Hole Spacetimes: Astrophysical Implications

David Barrero-González and Alejandro Cruz-Osorio

We present a systematic analysis of spherically symmetric accretion flows onto thirteen non-rotating black hole spacetimes, including charged and regular black holes, scalar and dilaton fields, dark energy, cosmological constants, modified gravity, and parametrized metrics. We generalize the Michel solution to arbitrary diagonal spacetimes and investigate the density, temperature, velocity, Mach number, mass accretion rate, and bremsstrahlung luminosity equilibrium profiles. Using the Schwarzschild solution as a reference, we find significant deviations in the accretion flow, with cosmological-constant and dark-energy models strongly suppressing accretion, while regular and Yukawa-like spacetimes generally enhance it. We provide fitting formulas for the accretion rates and luminosities as functions of the spacetime parameters. We then explore two astrophysical applications: accretion onto supermassive black holes, such as Sgr A* and M87*, and the inferred progenitor masses of the intermediate-mass black holes associated with the GW190521 gravitational wave event. We find that observational uncertainties may allow us to probe signatures of deviations from the Schwarzschild spacetime.
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High Energy Astrophysical Phenomena

NEXUS: Discovery of a Luminous Young Transient at $z=5$

Ming-Yang Zhuang, Yue Shen, Lei Hu et al. (+8 more)

We report the discovery of AT 2026abck, a young transient in a star-forming galaxy at $z=5.017\pm0.001$, identified from Year-2 observations of the NEXUS JWST treasury program. AT 2026abck was detected on 2026 Aug 7 in NIRCam F200W imaging, with contemporaneous detection from parallel NIRCam F150W+F277W imaging. It brightened by $>1.4$ mag in F200W over $\lesssim 12$ rest-frame days, as inferred from the last non-detection. AT 2026abck has nearly identical magnitudes of $\sim27.4$ mag in F150W, F200W, and F277W, and is undetected in F444W ($>27.9$ mag, 3$\sigma$). It is the first robustly detected $z\gtrsim 5$ transient in a non-lensing field. A blackbody fit yields a hot photosphere ($\sim1.7\times10^4\, \mathrm{K}$) with a radius of $\sim7\times10^{14}$ cm ($\sim46$ au) and a bolometric luminosity of $\sim3\times10^{43}\, {\rm erg\, s^{-1}}$. Its off-nucleus location and hot, UV-bright SED favor a core-collapse supernova origin, while its temperature and luminosity exceed those of normal SNe II at comparable early phases. We estimate that AT 2026abck was detected $\sim4$-10 rest-frame days after explosion. The host is an intermediate-mass ($M_*\approx10^{9.8}\,M_\odot$), star-forming (${\rm SFR}\approx20\,M_\odot\,{\rm yr^{-1}}$) galaxy on the $z\approx5$ main sequence, $\sim3.3$ dex more massive than the ultra-faint host of the lensed SN Eos at $z=5.13$. Together, the two events show that CCSNe at $z>5$ occur across a wide range of host environments, and AT 2026abck demonstrates that high-cadence blank-field JWST surveys can capture high-redshift SNe during their early, UV-bright phase.
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All authors Ming-Yang Zhuang, Yue Shen, Lei Hu, Justin D. R. Pierel, Zhiwei Pan, Zachary Stone, Adam J. Burgasser, Xiaohui Fan, Jenny E. Greene, Junyao Li, Feige Wang

High Energy Astrophysical Phenomena

VLBI Astrometric Procedure and Performance with CHIME/FRB Outriggers

Shion Andrew, Mattias Lazda, Calvin Leung et al. (+15 more)

Precise localizations of fast radio bursts (FRBs) to specific environments within their host galaxies with very-long-baseline interferometry (VLBI) have provided valuable insights into the nature of their progenitors. To date, such localizations have relied primarily on traditional VLBI facilities, whose observational capabilities limit them to targeted follow-up on a small subset of repeating FRBs that are observable at frequencies above $\sim$1 GHz. CHIME/FRB Outriggers is a low-frequency (400–800 MHz), wide-field VLBI network designed to overcome these limitations, aiming to localize a substantial fraction of CHIME-detected FRBs to $\sim$50 mas precision. In this work, we detail the analysis procedure used to localize FRBs with the CHIME/FRB Outriggers VLBI array. With over a thousand test localizations consisting of continuum calibrators and pulsars with known VLBI positions, we provide a comprehensive demonstration of the VLBI array's ability to robustly meet, and under some conditions exceed, the nominal astrometric specification required for the central science goals of CHIME/FRB Outriggers. By demonstrating that a wide-field survey instrument can deliver routine $\sim$50 mas astrometry on one-off FRBs, this work lays the foundation for the upcoming generation of wide-field VLBI FRB surveys and the population-scale FRB samples they will enable.
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All authors Shion Andrew, Mattias Lazda, Calvin Leung, Kiyoshi W. Masui, Danté M. Hewitt, Jacob Willis, Alyssa Atkinson, Kevin Bandura, Shami Chatterjee, Nina V. Gusinskaia, Adam E. Lanman, Juan Mena-Parra, Kenzie Nimmo, Aaron B. Pearlman, Ziggy Pleunis, Alexander W. Pollak, Gurman Sachdeva, Vishwangi Shah

High Energy Astrophysical Phenomena

HESS J1507-622: A Plausible Young Galactic Kilonova Remnant

Prantik Sarmah, Xilu Wang, Myles Carley et al. (+3 more)

HESS J1507$-$622 is an unusual TeV gamma-ray source in the H.E.S.S. Galactic Plane Survey, distinguished by its off-plane location ($b \simeq -3.5^{\circ}$) and compact angular extent ($\sim 0.3^{\circ}$). Its location and morphology challenge conventional pulsar wind nebula and supernova remnant interpretations, and the absence of a detected shell or associated pulsar leaves its origin unresolved. Here, we propose that HESS J1507$-$622 is a young Galactic kilonova remnant with gamma-ray emission of leptonic origin. In this scenario, the source's off-plane location is naturally explained by the natal kick imparted to the progenitor binary neutron star system. We demonstrate that a leptonic kilonova remnant model can successfully reproduce the observed GeV-TeV flux via inverse-Compton scattering from shock-accelerated electrons, while the corresponding synchrotron emission remains below current X-ray and radio upper limits. Based on the source's energetics and angular size, we infer a distance of 3.8-14.3 kpc and an age of 0.2-3.0 kyr. Importantly, our analysis favors a kilonova remnant younger than 1 kyr. The lack of a historical naked-eye record can be explained by the transient's rapid fading, far-southern declination, and potential line-of-sight extinction. We further assess the detectability of unique multi-wavelength signatures that could confirm this hypothesis. We find promising prospects for detecting the unique MeV gamma rays from the decay of $r$-process radioisotopes using future observatories, alongside complementary low-energy signatures from thermal dust emission and a possible optical light echo. These distinct signals offer definitive tests of the kilonova remnant interpretation and motivate targeted observations with current and future facilities.
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All authors Prantik Sarmah, Xilu Wang, Myles Carley, Shu-Xu Yi, Ming-Yu Ge, Rebecca Surman

High Energy Astrophysical Phenomena

Cosmic-neighbor-associated distances to VHE and candidate VHE BL Lacs via SDSS photometric redshifts

Karri I. I. Koljonen and Elina Lindfors

BL Lac objects have distances that are difficult to estimate because their optical spectra rarely show identifiable emission or absorption lines. We present a method to constrain BL Lac distances by associating them with their cosmic neighborhoods (local overdensities in the cosmic web) identified in optical fields. Distances to galaxies in these neighborhoods are estimated using the Sloan Digital Sky Survey (SDSS) photometric redshifts. Using cosmic-neighborhood associations previously established from multi-object spectroscopy and BL Lacs with spectroscopically measured redshifts as control samples, we find that the redshifts of the associated neighborhoods can be recovered with an accuracy of $\Delta z \sim 0.02-0.04$. When multiple candidate neighborhoods are present in the field, we rank the candidate associations using a central-location criterion. In 50% of cases, the BL Lac is associated with the correct neighborhood. We provide distance estimates for BL Lac objects without secure redshift measurements, focusing on sources detected at very high energies (VHE; $E>100$,GeV) and on candidate VHE emitters. Combining intervening absorption systems, host-galaxy imaging, $\gamma$-ray constraints, and our SDSS group-association method, we propose likely redshifts for TXS 0141$+$268 ($z\sim0.93-0.95$), PKS 0735$+$178 ($z=0.64$), 3FHL J0905.5$+$1357 ($z=0.644$), B2 0912$+$29 ($z = 0.53$), 3FHL J1120.8$+$4212 ($z=0.363$), BZB J1243$+$3627 ($z=0.485$), 3FHL J1253.1$+$5300 ($z=0.664$), and RX J2156.0$+$1818 ($z=0.635$). Finally, we discuss how forthcoming narrow-band photometric-redshift surveys can improve blazar distance estimates via group-association techniques.
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High Energy Astrophysical Phenomena

Revealing the accretion disc structure in Changing State AGN: NGC1566

Huimei Wang, Chris Done, Riccardo Middei et al. (+1 more)

NGC1566 is a low mass ($\sim 6\times 10^6M_\odot$) Active Galactic Nuclei (AGN) with unusual large amplitude variabilty in its accretion flow. In the optical it cycles between recurring short bright episodes with strong blue/UV continuum and clear broad line emission, and a much weaker persistent level without strong broad lines, making it a repeating changing-state (CS) AGN. We re-analyse the intensive optical/UV and X-ray monitoring during the 2018 bright flare, showing for the first time the corresponding evolution of the (host galaxy subtracted) optical/UV AGN spectrum as well as the X-ray emission. As seen in other CS-AGN, the strongest spectral evolution is in a UV/soft X-ray component which likely disappears below $0.01L_{\rm Edd}$ while the hot corona and a redder optical/UV component show weaker variability. We show that the red optical emission is likely a composite of the intrinsic and reprocessed emission from the outer disc, together with diffuse continuum from the BLR, and discuss the different signatures predicted by this for continuum reverberation across the changing-state event. The historical lightcurves show repeated changing-state events tied to unusually large amplitude discrete flares in the lightcurve. While these do not appear to be periodic, the behaviour does support previous suggestions that the accretion flow is repeatedly perturbed, perhaps by an intermediate mass black hole companion.
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All authors Huimei Wang, Chris Done, Riccardo Middei, Xue-Bing Wu

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.IM

An X-ray Extremely wide-field Spectroscopic Surveyor (XESS, or "chess")

W. P. Maksym, M. Ulmer, S. Friedrich et al. (+20 more)

We propose an X-ray observatory, the X-ray Extremely wide-field Spectroscopic Surveyor (XESS, pronounced "Chess") to bring high energy resolution X-ray spectroscopy into the era of synoptic surveys and of time domain and multi-messenger astrophysics (TDAMM). Such an observatory is optimally suited to addressing key elements of the 2020 Decadal Survey on Astronomy and Astrophysics.
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All authors W. P. Maksym, M. Ulmer, S. Friedrich, F. Tombesi, R. Arcodia, S. P. Singam, S. Bongiorno, N. Thomas, M. Elvis, Ming Sun, John ZuHone, M. Lynne Saade, Philip Kaaret, Doug Swartz, Erin Hicks, Ming-Yi Lin, Pallavi Patil, Jimmy Irwin, Shrabani Kumar, Kirtan Dixit, Sudip Chakraborty, Andrea Gnarini, Steven Ehlert

High Energy Astrophysical Phenomena · Cross-listed from hep-ph

How collective modes terminate in the dispersion relation of neutrino plasmas

Damiano F. G. Fiorillo

Neutrino plasmas support flavor waves, collective oscillations of flavor, that neutrinos can resonantly emit or absorb when they fulfill the Cherenkov condition. For massless neutrinos, this condition leads to singular features in the dispersion relation: Landau-damped modes, for example, disappear abruptly as their phase velocity approaches the speed of light. We investigate how these singularities change when nonzero neutrino masses introduce a spread in velocities. We classify singular behavior of the frequency as a function of wavevector into i) degeneracy points, where two modes merge, and ii) abrupt mode termination, when the frequency approaches a branch cut of the flavor dielectric function. For each class, we derive a local expansion and determine where such behavior can occur. Degeneracy points, frequently encountered in axially symmetric dispersion relations in the fast limit, are not generic. Nonzero neutrino masses lift the degeneracy along the symmetry axis; in three-dimensional wavevector space, there might still be degeneracy lines, not necessarily intersecting the symmetry axis. Abrupt mode termination, instead, persists generically, except for special energy distributions which are entire functions. As a byproduct of this study, we show that a collisionless plasma with a Lorentzian neutrino-frequency distribution is exactly equivalent to a monochromatic one with an effective collision rate. Approximating a general frequency distribution by a sum of Lorentzians therefore substantially simplifies the dispersion relation.
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High Energy Astrophysical Phenomena

Nearby Supernova Science in the Era of High Cadence, Deep Time Domain Surveys

David J. Sand, Bhagya Subrayan, Conor L. Ransome et al. (+41 more)

We discuss science opportunities for nearby supernovae (SNe) in the era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), emphasizing the youngest phases of SN evolution and the value of contemporaneous high-cadence shadowing surveys. LSST's Wide Fast Deep survey will discover enormous numbers of SNe, but its $\sim 3$ day cadence will miss signatures that evolve on timescales of hours to days, requiring cadence augmentation by other programs. We present simple depth and cadence metrics for early SN signatures and use empirical nearby SN discovery rates to estimate the resulting science opportunities. LSST will detect or set limits on precursor emission from hundreds of core-collapse SNe, including $\gtrsim 50$-100 Type IIn and normal Type II SNe annually, as well as $\gtrsim$5 Type Ibn. Within $\sim 200$ Mpc, roughly 250 Type Ia SNe are classified annually, providing the potential to identify hundreds of early light-curve excesses, and a robust measurement of the incidence of extreme high-velocity features ($>$25,000 km s$^{-1}$). A shadowing survey with a depth of $\sim 22.5$ mag could search for early light-curve features, akin to the light-curve excess seen in SN2023ixf, across dozens of nearby core-collapse SNe every year. Rapid spectroscopy of $\gtrsim$150 Type II SNe per year within $\sim$160 Mpc could constrain the incidence of short-lived flash features to $\lesssim$10% within a few years, while also tying this to the rate of precursor emission in normal Type II SNe. We also discuss the challenges posed by early classification ambiguity and faint nonterminal transients, and we request that the 80 hour embargo on LSST images be waived for nearby galaxy fields. Finally, we advocate for a Nearby Galaxy Transient Broker that integrates transient streams, archival data, forced photometry, and follow-up triggering in a single platform.
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All authors David J. Sand, Bhagya Subrayan, Conor L. Ransome, Griffin Hosseinzadeh, K. Azalee Bostroem, Jennifer E. Andrews, Jeniveve Pearson, Yize Dong, Stefano Valenti, Saurabh W. Jha, Brian Hsu, Aravind P. Ravi, Manisha Shrestha, Charles D. Kilpatrick, Noah Franz, Mojgan Aghakhanloo, Igor Andreoni, Moira Andrews, Katie Auchettl, Federica B. Bianco, Régis Cartier, Siddharth Chaini, Diva Chaudhary, Analía V. Smith Castelli, Shar Daniels, Alexei V. Filippenko, Melissa L. Graham, Xander J. Hall, Daichi Hiramatsu, D. Andrew Howell, Lindsey A. Kwok, Amanda R. Lopes, Thomas Matheson, Curtis McCully, Maryam Modjaz, Phillip Noel, Gregory S. H. Paek, Antonella Palmese, Anthony L. Piro, Monika Soraisam, Sergiy Vasylyev, József Vinkó, J. Craig Wheeler, Samuel D. Wyatt

High Energy Astrophysical Phenomena

Not So Heavy Metal I: Low Metallicity Enhances the Rate of high-redshift Superluminous Supernovae

Ben Margalit, Allison L. Strom, Cristina Andrade et al. (+4 more)

Type-I superluminous supernovae (SLSNe) are extremely luminous stellar explosions originating from the core collapse of massive stars. Unlike normal core-collapse supernovae, however, SLSNe occur almost exclusively in metal-poor environments. As a result, their volumetric rate should not simply trace the cosmic star-formation history, but instead evolve with the metallicity distribution of galaxies across cosmic time. In this work, we introduce a new theoretical framework for modeling the volumetric rate of astrophysical sources as a function of redshift and metallicity. Our formalism combines metallicity scaling relations, stellar-mass distribution functions, and metallicity-dependent production efficiencies. Importantly, it also distinguishes between metallicity measures based on O/H and Fe/H abundances. Applying our new formalism to SLSNe, we predict volumetric rates that are nearly an order of magnitude higher at $z \gtrsim 2$ compared to models in which the rate traces the cosmic star-formation rate alone. This has direct implications on the detection prospects of high-$z$ SLSNe with the Vera C. Rubin Observatory and other time-domain surveys.
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All authors Ben Margalit, Allison L. Strom, Cristina Andrade, Michael W. Coughlin, Adam A. Miller, Steve Schulze, Ved G. Shah