arXiv subjects

Antonella Palmese

Publications and source records attributed to Antonella Palmese.

At least 19 recordsLinked to original sources

arXiv2026

Characterizing Gravitational Wave Electromagnetic Counterparts with the Schmidt Observatory System

The emergence of gravitational wave (GW) multimessenger astronomy, marked by the detection of GW170817 and its associated kilonova, has opened a new window into the physics of compact binary mergers. Building a statistical population of kilonovae remains a challenge, limited by the low neutron star merger detection rates and the rapid evolution of these faint transients. The former problem will be overcome with upcoming LIGO/Virgo/KAGRA (LVK) sensitivity improvements leading to the fifth Observing Run (O5), while the latter will be addressed by the Schmidt Observatory System (including the Argus Array, the Deep Synoptic Array (DSA), and the Lazuli Space Observatory), in addition to ongoing and upcoming surveys (including the Rubin Observatory Legacy Survey of Space and Time). Using simulations of expected kilonovae at LVK O5 sensitivity, we find that Argus and Rubin could detect binary neutron star kilonovae at a rate of 0.3-4.2 yr$^{-1}$ and 0.6-7.0 yr$^{-1}$ respectively. For neutron star-black hole mergers, Rubin could detect up to 1 kilonova per year. Lazuli will be able to characterize this kilonova population down to $g \sim 26$ mag as early as 12 hours post merger. Additionally, $\sim35\%$ of these events are expected to exhibit a jet afterglow that will be detectable by DSA. We further characterize kilonova spectra to determine observable features of heavy element production. Finally, we develop an observing strategy that combines high cadence imaging and spectral monitoring, maximizing Lazuli's ability to constrain ejecta properties and composition of GW counterparts with reasonable observing time.

astro-ph.HE↗

arXiv2026

WFST Follow-up of S251112cm: Searching for an Optical Counterpart to a Subsolar-mass Compact-binary Merger Candidate

Electromagnetic counterparts to gravitational-wave (GW) sources probe the properties, environments, and evolution of compact-object mergers. S251112cm belongs to an emerging class of GW candidates with possible subsolar-mass components. We present an optical counterpart search for S251112cm with the Wide Field Survey Telescope (WFST). Observations began 20.2 hr after the GW trigger and lasted three nights, covering approximately 780 deg$^2$ and 51% of the localization probability in the updated skymap. Our search for newly emerging, rapidly evolving, off-nuclear optical transients yielded four candidates with host-galaxy distances broadly consistent with the GW distance estimate. All four evolve substantially more slowly than AT 2017gfo, ruling out an AT 2017gfo-like origin and disfavoring their association with S251112cm as rapidly evolving kilonova counterparts. Combining WFST and DECam increases the localization probability coverage to approximately 67%. Assuming the counterpart lies within this footprint, we constrain a grid of binary-neutron-star kilonova models. For viewing angles $θ_{\rm obs}<60^\circ$, with other parameters fixed to the AT 2017gfo best-fit values, models with dynamical ejecta mass $M_{\rm dyn}\gtrsim0.01 M_\odot$ or wind ejecta mass $M_{\rm wind}\gtrsim0.05 M_\odot$ are disfavored over most of the covered GW probability. Our phenomenological analysis further constrains rapidly evolving optical transients with $T_{\rm rise,1/2}\leq2$ days to peak absolute magnitudes fainter than approximately -13 mag. To our knowledge, WFST provides the strongest optical constraints to date in the northern sky on possible rapidly evolving counterparts to this new class of GW candidates with subsolar-mass components. The comparable depths and complementary sky coverage of WFST and DECam extend these constraints across more of the localization region at similar sensitivity.

astro-ph.HE↗

arXiv2026

The "Rubin 2036: Opportunities for Discovery" Workshop: A Community Roadmap for the Future of the Vera C. Rubin Observatory After the LSST

The NSF-DOE Vera C. Rubin Observatory will complete its ten-year Legacy Survey of Space and Time (LSST) in the mid-2030s. Its 8.4 m aperture and 9.6 square degree field of view give it an étendue unmatched by any facility planned for that era. The Rubin 2036: Opportunities for Discovery workshop (KIPAC/SLAC, 3-5 August 2026) brought together approximately 100 participants to discuss Rubin's future after LSST. This paper summarizes the workshop's structure and discussions across five working groups. The continued-surveying group explored a machine-learning foundation model pretrained on Rubin imaging for detection, deblending, and classification. The cadence group considered allocating post-LSST observing time and strategies for transiting exoplanets, Type Ia supernova cosmology, time-delay cosmography, and second-scale transients. The filter group explored broadband, medium-band, and narrowband filters for photometric redshifts, high-redshift large-scale structure, AGN reverberation mapping, Galactic archaeology, and emission-line imaging. The multi-messenger group explored follow-up of next-generation gravitational-wave and neutrino sources, including standard sirens, kilonovae, and Galactic supernovae. The technology group organized options by cost and lead time, including fast readout, new filters, a faster shutter, and summit edge computing. Cross-cutting topics included AI/ML, Chilean and Latin-American co-leadership, satellite megaconstellations, and weather and climate trends at Cerro Pachón. White papers and workshop talks are available through https://zenodo.org/communities/rubin2036workshop/ and https://rubin-observatory.github.io/Rubin-Beyond-LSST/pages/Rubin2036proceedings/.

astro-ph.IM↗

arXiv2026

A Quiet Night in the Millihertz Sky? LISA Forecasts from LIGO/Virgo/KAGRA Populations

The high-mass black hole population inferred from ground-based observations can substantially affect the number of binaries detectable by the Laser Interferometer Space Antenna (LISA). We use population constraints based on data from the latest LIGO/Virgo/KAGRA Gravitational-Wave Transient Catalog (GWTC-5.0), as well as constraints from the O3 search for intermediate-mass black hole binaries, to forecast the number and masses of binary black holes detectable by LISA. Using the released GWTC-5.0 population posterior, we obtain $6.6^{+5.5}_{-2.2}$ expected detections with signal-to-noise ratio $>8$ in 4 years (median and $90\%$ credible interval). The alternative population models we consider use a broken power law with two or three Gaussian peaks for either the primary-mass distribution or a common mass distribution for both black holes. For the three-peak model with the common mass distribution, extrapolating from $300$ to $1000M_\odot$ roughly doubles the median expected number of LISA detections to $\sim14$. The upper end of its $90\%$ credible interval rises from tens to thousands of expected detections, although approximations may limit the reliability of the most extreme counts. For the reference GWTC-5.0 population, $39^{+22}_{-11}\%$ of the binaries detectable by LISA merge during the mission or within $10$ years after its end and are potential multiband sources. Essentially all of these mergers would be detectable by a reference Cosmic Explorer configuration if it is operating at merger. Ground-based measurements of the high-mass population can therefore inform both LISA forecasts and the prospects for observing the same binaries through inspiral and merger.

astro-ph.HE↗

arXiv2026

Hostbusters: constraining $H_0$ and binary formation channels through gravitational-wave host galaxy associations

Gravitational-wave dark sirens can be combined with galaxy catalogues to provide a measurement of the Hubble constant. This inference requires assigning each potential host galaxy a probability of hosting a merger, but the appropriate weighting is uncertain because the binary merger rate may trace different galaxy properties, such as stellar mass or star formation rate, depending on the underlying formation channel. We introduce a flexible framework in which the dark-siren population is described as a mixture of two host-weighting prescriptions, with the mixing fraction inferred jointly with $H_0$. These components may be interpreted as subpopulations associated with distinct binary-formation environments. Using simulated catalogues, we show that marginalising over uncertainty in the host-population mixture preserves unbiased constraints on $H_0$, while allowing the gravitational-wave data to constrain the relative contribution of the two host populations. In particular, an A#-like detector network with $\mathcal{O}(10^3)$ dark sirens may be able to exclude scenarios in which the host population traces only one of the two weighting prescriptions. Dark-siren analyses may therefore provide robust cosmological measurements and new information about how merging compact binaries trace the galaxy population.

astro-ph.CO↗

arXiv2026

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

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.

astro-ph.HE↗

arXiv2026

Normalizing Flow Emulator for Gravitational Wave Detection Probability in the LIGO/Virgo/KAGRA O4 Observing Run

Efficient and accurate estimation of the detection probability of a gravitational wave event given its source parameters remains a challenging problem because it conventionally requires expensive Monte Carlo injection campaigns. We present a machine learning-based approach using Normalizing Flows (NFs) to estimate the detection probability of gravitational wave events for a LIGO/Virgo/KAGRA O4-like detector network configuration. We utilize the publicly available injection dataset as training and test data. We test our method and implementation by evaluating the raw NF performance as a generative model based on its ability to recover the correct parameter distributions of detected events. In addition, we compare the detection efficiencies predicted by the NF emulator with those from Monte Carlo importance sampling and find good agreement. Furthermore, estimating the detection efficiency for a wide variety of population models, including those with sharp features, can be more precise than estimates obtained via Monte Carlo importance sampling. We show that the NF emulator can be used in population analysis of gravitational wave sources, similar to previous neural network approaches. The training code and trained models from this work are publicly available.

gr-qc↗

arXiv2026

The host galaxies and merger environments of short and long gamma-ray bursts producing kilonovae

Gamma-ray bursts (GRBs) have traditionally been classified by their prompt emission duration and spectral hardness, with short GRBs (sGRB; $\lesssim2 \ \rm{s}$) originating from compact object mergers and long GRBs (LGRB; $\gtrsim2 \ \rm{s}$) from massive star core-collapse. Recent kilonova (KN) associations with long-duration GRBs have challenged this standard picture. We analyze the host galaxies of nine GRBs with associated kilonova candidates at $z<0.6$, including five sGRB-KNe and four LGRB-KNe. Using both parametric and non-parametric modeling of the host light distributions, we investigate the progenitor environments of these events and test whether their hosts show evidence for recent galaxy interactions that could favor dynamical formation channels or isolated pathways following merger-driven star formation episodes for neutron star binaries. We find that five of the nine hosts display tidal features that show they have likely undergone recent mergers, suggesting that merger-driven, dynamical formation pathways may contribute in some systems. We find no clear morphological distinction between sGRB-KN and LGRB-KN hosts as both populations span a wide range of morphologies, including ellipticals, spirals, and interacting systems with tidal features. Multi-Sérsic modeling of the host light profiles further shows that host-normalized offsets inferred from single-Sérsic fits can be overestimated when the transient is associated with a specific subcomponent of a complex host light profile. These results highlight the importance of decomposing host morphology into physically relevant components when interpreting GRB environments and galactocentric offsets.

astro-ph.HE↗

arXiv2026

The Lazuli Space Observatory: Architecture & Capabilities

The Lazuli Space Observatory is a 3-meter aperture astronomical facility designed for rapid-response observations and precision astrophysics across visible to near-infrared wavelengths (400-1700 nm bandpass). An off-axis, freeform telescope delivers diffraction-limited image quality (Strehl $>$0.8 at 633 nm) to three instruments across a wide, flat focal plane. The three instruments provide complementary capabilities: a Wide-field Context Camera (WCC) delivers multi-band imaging over a 35' $\times$ 12' footprint with high-cadence photometry; an Integral Field Spectrograph (IFS) provides continuous 400-1700 nm spectroscopy at R $\sim$ 100-500 for stable spectrophotometry; and an ExtraSolar Coronagraph (ESC) enables high-contrast imaging expected to reach raw contrasts of $10^{-8}$ and post-processed contrasts approaching $10^{-9}$. Operating from a 3:1 lunar-resonant orbit, Lazuli will respond to targets of opportunity in under four hours--a programmatic requirement designed to enable routine temporal responsiveness that is unprecedented for a space telescope of this size. Lazuli's technical capabilities are shaped around three broad science areas: (1) time-domain and multi-messenger astronomy, (2) stars and planets, and (3) cosmology. These capabilities enable a potent mix of science spanning gravitational wave counterpart characterization, fast-evolving transients, Type Ia supernova cosmology, high-contrast exoplanet imaging, and spectroscopy of exoplanet atmospheres. While these areas guide the observatory design, Lazuli is conceived as a general-purpose facility capable of supporting a wide range of astrophysical investigations, with open time for the global community. We describe the observatory architecture and capabilities in the preliminary design phase, with science operations anticipated following a rapid development cycle from concept to launch. [Abstract abridged]

astro-ph.IM↗

arXiv2026

The Cross-Survey Decade: A Call to Action

By 2027, three flagship wide-field surveys will be operating simultaneously from ground and space, observing overlapping sky and representing more than $6 billion in US and European public investment. Together they will produce overlapping petabyte-scale datasets across thousands of square degrees. This is a different class of challenge: the observations are no longer the bottleneck; realizing their joint scientific return now depends on shared computational infrastructure and coordination. Decades of community studies show that combining these datasets does more than improve precision. For science ranging from weak lensing to transient discovery and Galactic-plane astronomy, joint processing and analysis can unlock capabilities no single survey provides alone. Yet the required infrastructure -- joint pixel-level processing, cross-calibration and validation, interoperable data access, and the people to build and sustain it -- falls outside any single mission or institution's mandate. We issue a call to action for cross-survey science infrastructure, built around four pillars: (1) joint pixel-level processing and validation; (2) an AI-ready data substrate for scientific foundation models; (3) standardized, interoperable data access across surveys, democratizing participation in astrophysical discovery; and (4) dedicated personnel and career pathways. We outline concrete steps for policymakers, agencies, observatories, universities, the research community, and philanthropy, and argue that the moment to act is now, while foundational technical choices can still be aligned at a fraction of the cost of reconciling them later.

astro-ph.IM↗

arXiv2026

Cosmic Pairs: A DESI Census of Dual and Offset AGN as Precursors to Massive Black Hole Binaries

We present a systematic census of dual and offset active galactic nuclei (AGN) using spectroscopic data from the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI). After correcting for observational systematics, our final sample contains $>7,000$ dual AGN and 27,000 galaxy pairs containing one AGN over the redshift range $0 \lesssim z \lesssim 3.6$. This sample expands the known dual AGN sample by $\sim 1-2$ orders of magnitude at $0.2 \lesssim z \lesssim 0.4$, includes $\sim 50$ dwarf dual AGN candidates in a regime where only a handful were previously known, and triples the census at $z>2$. Dual AGN are preferentially found at small separations, consistent with merger-driven triggering of AGN activity. The two members of a pair differ in their star formation response: the more massive (primary) host changes little with separation, while the less massive (secondary) lies $\sim 0.3$ dex above matched inactive and one-AGN companions at the same projected separation in main-sequence offset. Using ASTRID simulations, we predict that the fraction of DESI dual AGN whose central black holes will merge by $z \sim 0$ increases with redshift, reaching $\sim 76\%$ by $z \sim 2$, while the fraction producing LISA-detectable mergers peaks at $\sim 37\%$ near $z \sim 0.9$. These results provide the largest uniformly selected spectroscopic sample of kpc-scale dual and offset AGN candidates from a single survey, connecting their host-galaxy and AGN demographics to the progenitor population of massive black hole mergers detectable by LISA.

astro-ph.GA↗

arXiv2026

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

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

astro-ph.CO↗

arXiv2026

GRB 260310A/SN 2026fgk: Photometric and Spectroscopic Evolution of a Nearby GRB-Supernova and an Exceptionally Bright Afterglow at z=0.153

The association of broad-lined Type Ic supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 years. However, only about seventy gamma-ray burst supernovae (GRB-SNe) have been identified, of which only half have spectroscopic classifications. At $z=0.153$, GRB 260310A is the 12th spectroscopically confirmed GRB-SN discovered within 1 Gpc, offering a critical opportunity to follow one of these rare supernovae in detail. We present optical to near-infrared imaging and spectroscopy of GRB 260310A and SN 2026fgk out to 65 d after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than two weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova. Modeling of the multi-wavelength ($grizJK_s$) lightcurve shows that the supernova is approximately half the luminosity ($k_\textrm{98bw}=0.4-0.6$) of the canonical GRB-SN 1998bw. We derive a nickel mass of $M_\textrm{Ni}=0.4-0.5$ $M_\odot$ with a total ejected mass of $M_\textrm{ej}\approx4-6 $ $M_\odot$ and kinetic energy $E_\textrm{K}=(3-8)\times10^{51}$ erg. The GRB exploded at an extremely large offset of 15 kpc from its host galaxy. Long-slit spectra reveal a ``bridge'' of nebular emission extending along the galaxy's disk to the GRB location, which has a sub-solar metallicity ($\sim$\,$0.4Z_\odot$), compared to a near solar metallicity for the host galaxy. This indicates that the large offset arises from the galaxy's extended light profile rather than an isolated environment.

astro-ph.HE↗

arXiv2026

EP260321a/SN 2026gzf: The Faintest Shock Breakout Associated with a Broad-Lined Supernova

The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae. Here, we report on our multi-wavelength observing campaign of the closest Einstein Probe fast X-ray transient EP260321a at $z=0.0344$. The thermal ($kT=130$ eV) X-ray emission with peak luminosity $1.0\times10^{45}$ erg s$^{-1}$ points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope supernovae associated with gamma-ray bursts. However, deep X-ray upper limits obtained with the \textit{Chandra X-ray Observatory} do not detect an X-ray afterglow, and instead exclude the afterglow of known gamma-ray bursts or fast X-ray transients. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor $Γ_0$\,$<$\,$30$ and a kinetic energy $E_\textrm{kin}$\,$<$\,$10^{49}$ erg for a stellar wind density of $A_*$\,$\gtrsim$\,$1$. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.

astro-ph.HE↗

arXiv2026

First Results from the LSST Shadow Survey: The Restless Luminous Blue Variable AT2017des in the Virgo-Cluster Galaxy, NGC4532

The Legacy Survey of Space and Time (LSST) will start in late-summer 2026, revolutionizing transient astronomy. Here, we present the Dark Energy Camera (DECam) Shadow Survey, which is designed to maximize the science potential of LSST by shadowing LSST observations of local galaxy-cluster fields, producing a nightly cadence of these fields. The Shadow Survey will discover extremely young supernovae (SNe), SN precursors, as well as other explosive transients and exotic phenomena, helping to characterize such transients at unprecedented cadence and depth when combined with LSST. We describe our workflow, pipeline, public data releases, and candidate vetting. As an early result of Shadow, we present the fitful luminous blue variable (LBV) eruptions of AT2017des in the Virgo-Cluster galaxy NGC4532. AT2017des has short-timescale variability (of order 10 days), peaking at around $M_r=-12.5$mag, brighter than normal LBVs, and similar to the more extreme flaring of hot LBVs/SN impostors such as SN2000ch, AT2016blu, and the precursor activity of SN2009ip. Our spectral time-series reveals features typical of these hot LBVs and SN impostors/precursors. Combining our data with long-baseline photometry from additional observatories, we find that the peaks of the outbursts of AT2017des are getting brighter over time, with 2026 peak fluxes being up to 5 times greater than in 2023 and an average brightening of $\sim0.05$ mag yr$^{-1}$. The peaks of AT2017des are more luminous than those of most other LBVs, only being fainter than bright precursors such as SN2009ip, and extreme SN impostors such as AT2016blu. AT2017des may therefore be ``ramping up'' to a terminal explosion.

astro-ph.HE↗

arXiv2026

Discovery of a Supernova Following the Einstein Probe Transient EP250302a at z = 1.131

We present a multi-wavelength analysis of the Einstein Probe (EP) fast X-ray transient (FXT) EP250302a located at redshift $z=1.131$. Despite its luminous prompt X-ray emission, the event was not detected in gamma-rays. Multi-wavelength follow-up identified a bright optical and X-ray source that displayed rapid chromatic flaring before returning to the standard decay of a gamma-ray burst afterglow. We interpret the chromatic flare as either due to a refreshed shock caused by a discrete shell collision or as reverse shock emission. Using the early optical data, we place constraints on the Lorentz factor of the outflow, requiring an ultrarelativistic jet with $Γ_0>25$. We additionally obtained deep late-time imaging with the Gemini North Telescope that reveals the presence of an optical excess at $20-30$ d post-explosion. We interpret this as supernova (SN) emission and find good agreement with the canonical broad-lined Ic SN 1998bw with a flux-scaling factor of $k_\textrm{98bw}>0.3$. This adds to the growing evidence that the majority of EP FXTs are associated with the deaths of massive stars.

astro-ph.HE↗

arXiv2026

SN 2025adpq: A Type Ia supernova in a collisional ring formed during a major galaxy merger

Galaxy mergers can both trigger star formation and rearrange where stars live, producing long-lived tidal structures and collisionally driven density waves (known as collisional rings) that can extend for tens of kpc from their host galaxy centers. Here we report the discovery of SN 2025adpq, a Type Ia supernova at $z=0.1540$, found within a collisional ring, which we call Pika's Halo, with circumference $\sim$\,70 kpc that was produced by a major merger between two comparable mass galaxies ($\log(M_*/M_\odot)\approx10.5)$. The supernova lies along the ring at a projected offset of $\sim$11.4 kpc from the nucleus of the primary galaxy (hereafter G1). Optical spectroscopy obtained with the Southern African Large Telescope (SALT) and Gemini South reveal signatures consistent with merger induced ongoing star formation, while prominent Calcium H and K absorption indicates a substantial old stellar population within the ring. Therefore, we propose that SN 2025adpq may have been produced by an old progenitor system that was displaced from G1 during the head-on encounter. In this scenario, the progenitor was stripped from its parent galaxy by the collisionally induced pressure wave and exploded far from its birthplace. However, given the broad diversity in SN Ia delay times, we cannot conclusively demonstrate that the progenitor was not formed in a more recent burst of star formation triggered by the expanding pressure wave. Regardless, SN 2025adpq highlights collisional rings as a path to large offset SNe Ia, and it motivates targeted searches for faint, dynamically displaced old populations in seemingly hostless environments. We additionally identify other supernovae, including supernova siblings, in the low redshift sample of collisional ring galaxies, and find that SN 2025adpq is one of only a handful of classified supernova identified in the expanding ring of a collisional ring complex.

astro-ph.GA↗

arXiv2026

EP250827b/SN 2025wkm: An X-ray Flash-Supernova Powered by a Central Engine and Circumstellar Interaction

We present the discovery of EP250827b/SN 2025wkm, an X-ray Flash (XRF) discovered by the Einstein Probe (EP), accompanied by a broad-line Type Ic supernova (SN Ic-BL) at $z = 0.1194$. EP250827b possesses a prompt X-ray luminosity of $\sim 10^{45} \, \rm{erg \, s^{-1}}$, lasts over 1000 seconds, and has a peak energy $E_{\rm{p}} < 1.5$ keV at 90\% confidence. SN 2025wkm possesses a double-peaked optical light curve (LC), though its bolometric luminosity plateaus after its initial peak for $\sim 20$ days, consistent with a central engine injecting additional energy into the explosion. Its spectrum transitions from a blue to red continuum with clear blueshifted broad absorption features consistent with a SN Ic-BL classification. We do not detect any transient radio emission and rule out the existence of an on-axis, energetic jet $\gtrsim 10^{50}~$erg assuming a typical LGRB circumburst constant density ($n \approx 10^{-3}$--$10^{-1}~{\rm cm}^{-3}$) and microphysical parameters ($ε_{\rm e} = 0.1$ and $ε_{\rm B} = 0.01$). In the model we invoke, the collapse gives rise to a long-lived magnetar, potentially surrounded by an accretion disk. Magnetically--driven winds from the magnetar and the disk mix together and break out with a velocity $\sim 0.35c$ and interact with an extended circumstellar medium with radius $\sim 10^{13}$ cm, generating X-ray breakout emission through non-thermal free-free processes. The disk outflows and magnetar winds power blackbody photospheric emission as they cool adiabatically and thermalize, producing the first SN peak. The spin-down luminosity of the magnetar and radioactive decay of $^{56}$Ni powers the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP's recent discoveries.

astro-ph.HE↗