arXiv subjects

Antonella Palmese

Publications and source records attributed to Antonella Palmese.

At least 37 records · Page 2Linked to original sources

arXiv2026

Long GRB 250916A: an Off-axis Powerlaw Jet with Thermal Cocoon

Some gamma-ray bursts (GRBs) exhibit precursor emission episodes preceding the main emission, with a quiescent period in between. The properties of the precursor emission and the duration of the quiescent interval are related to the central engine activity and jet formation processes, thus providing insights into the physics of GRBs. We present a comprehensive analysis of the prompt emission and multi-wavelength afterglow of GRB 250916A. Using detailed afterglow modeling, we find that the broadband data are best described by a powerlaw structured jet with a relatively narrow core ($θ_c \approx 0.8^\circ$), viewed moderately off-axis at a viewing angle $θ_v \approx 2.7^\circ$. The isotropic-equivalent kinetic energy of the jet ($E_{k,iso} \approx 2.4 \times 10^{54}$ erg) is on the higher side for typical GRBs. The precursor emission is well described by a blackbody spectrum with a temperature of kT $\approx$ 13.2 keV and is separated from the main emission by a long quiescent interval of 150 s. Put together, our results indicate that the precursor is likely to be a shock breakout from a cocoon formed by the interaction of the relativistic jet with the progenitor star. The resulting cocoon pressure and shock collimation naturally lead to the launch of a narrowly collimated jet, consistent with the jet geometry inferred from afterglow observations. The long quiescent interval may imply the central engine turn-off in addition to the effect of the off-axis geometry.

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arXiv2026

A GPU-Accelerated Transient Detection Pipeline for DECam Time-Domain Surveys

We present a GPU-accelerated transient detection pipeline developed for time-domain surveys with the Dark Energy Camera (DECam). It enables real-time-capable image processing, incorporating science-driven candidate filtering to support rapid transient identification in time-critical observing programs. The pipeline serves as the core transient discovery engine for multiple long-term DECam programs, including the GW-MMADS gravitational-wave follow-up campaign and the DESIRT survey for intermediate-redshift transients with DESI synergy. The pipeline ingests calibrated imaging products from the DECam Community Pipeline and performs image differencing using the SFFT algorithm, coupled with CNN-based real-bogus classification, to produce science-ready transient alerts and light curves that are delivered to community brokers. We validate the pipeline using archival DECam data from the DESIRT survey. The real-bogus classifier achieves a completeness of $\sim$ 99\% of real transients while rejecting $\sim$ 96\% of subtraction artifacts, and the workflow typically reduces the candidate load to a manageable level for survey operations. With GPU acceleration, the typical processing time per DECam exposure is $\sim$ 50 s from calibrated image processing to alert generation using a modest allocation of computing resources.

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arXiv2026

Multi-messenger constraints on LIGO/Virgo/KAGRA gravitational wave binary black holes merging in AGN disks

While the LIGO/Virgo/KAGRA (LVK) gravitational wave (GW) detectors have detected over 300 binary black hole (BBH) mergers to date, the first confirmation of an electromagnetic (EM) counterpart to such an event remains elusive. Previous works have performed searches for counterpart candidates in transient catalogs and have identified active galactic nuclei (AGN) flares coincident with GW events; existing theory predicts that such flares may arise from the interaction of the merger remnant with the embedding accretion disk environment. We apply a statistical formalism to measure the significance of coincidence for the catalog as a whole, measuring that less than 3\% (90\% credible interval) of LVK BBH mergers give rise to observable AGN flares. This result still allows up to $\sim 40\%$ of BBH mergers to originate in AGN disks. We also examine the individual coincidences of each merger/flare pairing, determining that in all cases the flares are more likely to belong to a background population of flares not associated with GW events. Our results are consistent with theoretical predictions accounting for the observability of EM counterparts in AGN disks, as well as based on the fact that the most massive/luminous AGNs (such as those included in the search) are not expected to harbor the majority of the BBHs. We emphasize that developing both the means to distinguish BBH counterpart flares from background AGN flares and an understanding of which BBHs are most likely to produce AGN flares as counterparts is critical to optimize the use of follow-up resources.

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arXiv2026

Detecting prompt and afterglow jet emission of gravitational wave events from LIGO/Virgo/KAGRA and next generation detectors

Following the wealth of new results enabled by multimessenger observations of the binary neutron star (BNS) merger GW170817, the next goal is increasing the number of detections of electromagnetic (EM) counterparts to gravitational wave (GW) events. We study the detectability of the prompt emission and afterglows produced by the relativistic jets launched by BNS mergers that will be detected by LIGO-Virgo-KAGRA during their fifth observing run (O5), and by next generation (XG) GW detectors (Einstein Telescope and Cosmic Explorer). We quantify the impact of various BNS merger and jet afterglow parameters on the likelihood of detection, focusing on the impact of the observer's viewing angle and the jet's core half-opening angle. We explore detectability over a wide range of current state-of-the-art facilities (e.g., the James Webb Space Telescope, Chandra X-ray Observatory) as well as upcoming next-generation facilities (e.g., AXIS, NewAthena, ngVLA, SKA). We find that a few GW events (~0-4) per year may have a detectable afterglow component in O5, with the largest detection rates expected with SKA in the radio and JWST in the near-infrared. In the XG era, hundreds of multimessenger detections of afterglows per year may be possible with a range of instruments, such as NewAthena in the X-ray and ngVLA in the radio. While zero to a few GW events per year are expected to be accompanied by a detectable prompt emission in O5, dozens per year may be detectable in XG.

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arXiv2026

The DESI Transients Survey: Legacy Classifications and Methodology

We present the first systematic spectroscopic observations of extragalactic transients from the Dark Energy Spectroscopic Instrument (DESI), as part of the DESI Transients Survey program. With 5,000 fibers and an ${\sim} 8$ deg$^2$ field of view, we exploit DESI as a machine for the discovery and classification of transients. We present transient classifications from archival DESI data in Data Releases 1 and 2, relying on a combination of a secondary target program and serendipitous observations. We also present observations from the first 6 months of the DESI spare fiber program dedicated to transients. The program is run in coordination with a dedicated DECam time-domain survey, serving as a pathfinder for what we will be able to achieve in conjunction with the Rubin Observatory Legacy Survey of Space and Time (LSST). We classify over 250 transients, of which the majority were previously unclassified. The sample comprises thermonuclear and core-collapse supernovae and tidal disruption events (TDEs), including a TDE observed before its discovery in imaging. We demonstrate DESI's ability to classify a population of faint transients down to $r\sim 22.5$ mag during main survey operations, with negligible impacts on DESI's main observations.

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arXiv2026

Detecting electromagnetic counterparts to LIGO/Virgo/KAGRA gravitational wave events with DECam: Neutron Star Mergers

With GW170817 being the only multimessenger gravitational wave (GW) event with an associated kilonova detected so far, there exists a pressing need for realistic estimation of the GW localization uncertainties and rates, as well as optimization of available telescope time to enable the detection of new kilonovae. We simulate GW events assuming a data-driven distribution of binary parameters for the LIGO/Virgo/KAGRA (LVK) fourth and fifth observing runs (O4 and O5). We map the binary neutron star (BNS) and neutron star-black hole (NSBH) properties to the kilonova optical light curves. We use the simulated population of kilonovae to generate follow-up observing plans, with the primary goal of optimizing detection with the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS). We explore the dependence of kilonova detectability on the mass, distance, inclination, and spin of the binaries. Assuming that no BNS was detected during O4 until the end of 2024, we present updated GW BNS (NSBH) merger detection rates. We expect to detect BNS (NSBH) kilonovae with DECam at a per-year rate of: $0$-$2.0$ ($0$) in O4, and $2.0$-$19$ ($0$-$1.0$) in O5. We expect the majority of BNS detections and also those accompanied by a detectable kilonova to produce a hypermassive NS remnant, with a significant fraction of the remaining BNSs promptly collapsing to a BH. We release GW simulations and depths required to detect kilonovae based on our predictions to support the astronomical community in their multimessenger follow-up campaigns and analyses.

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arXiv2025

Connecting current and future dual AGN searches to LISA and PTA gravitational wave detections

Dual active galactic nuclei (DAGN) mark an observable stage of massive black hole (MBH) pairing in galaxy mergers and are precursors to the MBH binaries that generate low-frequency gravitational waves. Using the large-volume ASTRID cosmological simulation, we construct DAGN catalogs matched to current (COSMOS-Web, DESI) and forthcoming (AXIS, Roman) searches. With realistic selection functions applied, ASTRID reproduces observed dual fractions, separations, and host-galaxy properties across redshifts. We predict a substantial population of small-separation (<5 kpc) duals that current surveys fail to capture, indicating that the apparent paucity of sub-kpc systems in COSMOS-Web is driven primarily by selection effects rather than a physical deficit. By following each simulated dual forward in time, we show that dual AGN are robust tracers of MBH mergers: ~30-70% coalesce within $\lesssim 1$ Gyr, and 20-60% of these mergers produce gravitational-wave signals detectable by LISA. Duals accessible to AXIS and Roman are the progenitors of ~10% of low-redshift LISA events and ~30% of the PTA-band stochastic background. Massive green-valley galaxies with moderate-luminosity AGN, together with massive star-forming hosts containing bright quasars at $z>1$, emerge as the most likely environments for imminent MBH binaries. These results provide a unified cosmological framework linking dual AGN demographics, MBH binary formation, and gravitational-wave emission, and they identify concrete, high-priority targets for coordinated electromagnetic and GW searches in upcoming multi-messenger surveys.

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arXiv2025

ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger

On August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a sub-threshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF25abjmnps (AT2025ulz), that was spatially and temporally coincident with the gravitational wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF25abjmnps and S250818k. Theoretical models have been proposed wherein sub-solar neutron star(s) may form (and subsequently merge) via accretion disk fragmentation or core fission inside a core-collapse supernova i.e. a ``superkilonova". Here, we qualitatively discuss our multi-wavelength dataset in the context of the superkilonova picture. Future higher significance gravitational wave detections of sub-solar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multi-messenger association.

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arXiv2025

Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors

GRB 250702B was the longest gamma-ray burst ever detected, with a duration that challenges standard collapsar models and suggests an exotic progenitor. We collected a rich set of optical and infrared follow-up observations of its rapidly fading afterglow using a suite of telescopes including the W. M. Keck Observatory, the Gemini telescopes, the Magellan Baade Telescope, the Victor M. Blanco 4-meter telescope, and the Fraunhofer Telescope at Wendelstein Observatory. Our analysis reveals that the afterglow emission is well described by forward shock emission from a highly obscured relativistic jet. Deep photometric observations of the host galaxy reveal a massive (10^10.66 solar masses), dusty, and extremely asymmetric system that is consistent with two galaxies undergoing a major merger. The galactocentric offset, host galaxy properties, and jet characteristics disfavor a jetted TDE around a supermassive black hole but do not definitively distinguish between competing progenitor scenarios. We find that the afterglow and host are consistent with a range of progenitors including an atypical collapsar, a merger between a helium star and a stellar mass black hole, the disruption of a star by a stellar mass compact object (micro-TDE), and the tidal disruption of a star by an off-nuclear intermediate mass black hole.

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arXiv2025

AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery

The first joint electromagentic (EM) and gravitational wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 years since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported a low-significance (high false alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ($z=0.08484$) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with \textit{Swift}, \textit{XMM-Newton}, \textit{Chandra}, and the Very Large Array to search for non-thermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz ($\approx 400$ Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of $θ_\textrm{v}\approx 12.5$ degrees. We briefly discuss the prospects for the future discovery of off-axis afterglows.

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arXiv2025

AT2025ulz and S250818k: Investigating early time observations of a subsolar mass gravitational-wave binary neutron star merger candidate

Over the past LIGO--Virgo--KAGRA (LVK) observing runs, it has become increasingly clear that identifying the next electromagnetic counterparts to gravitational-wave (GW) neutron star mergers will likely be more challenging compared to the case of GW170817. The rarity of these GW events, and their electromagnetic counterparts, motivates rapid searches of any candidate binary neutron star (BNS) merger detected by the LVK. We present our extensive photometric and spectroscopic campaign of the candidate counterpart AT2025ulz to the low-significance GW event S250818k, which had a ${\sim} 29\%$ probability of being a BNS merger. We demonstrate that during the first five days, the luminosity and color evolution of AT2025ulz are consistent with both kilonova and shock cooling models, although a Bayesian model comparison shows preference for the shock cooling model, underscoring the ambiguity inherent to early data obtained over only a few days. Continued monitoring beyond this window reveals a rise and color evolution incompatible with kilonova models and instead consistent with a supernova. This event emphasizes the difficulty in identifying the electromagnetic counterparts to BNS mergers and the significant allotment of observing time necessary to robustly differentiate kilonovae from impostors.

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arXiv2025

AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate

On August 18th, 2025, the LIGO--Virgo--KAGRA collaboration reported a sub-threshold gravitational wave candidate detection consistent with a sub-solar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova (KN) emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after $\sim$\,$5$ days and developed spectral features characteristic of a Type IIb supernova. In this work, we analyze the observations of the host galaxy of AT2025ulz obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift of $z = 0.084840 \pm 0.000006$, which places the transient within $2σ$ of the gravitational wave distance and results in an integral overlap between the gravitational wave alert and the transient location of $\log_{10}\mathcal{I} \approx 3.9-4.2$. Our analysis of the host galaxy's spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass ${\sim} 10^{10}~M_\odot$, broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational wave alerts. These results emphasize the value of DESI's extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up.

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arXiv2025

Comprehensive X-ray Observations of the Exceptional Ultra-long X-ray and Gamma-ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-ray Afterglow Properties

GRB 250702B is an exceptional transient that produced multiple episodes of luminous gamma-ray radiation lasting for $>25$ ks, placing it among the class of ultra-long gamma-ray bursts (GRBs). However, unlike any known GRB, the \textit{Einstein Probe} detected soft X-ray emission up to 24 hours before the gamma-ray triggers. We present comprehensive X-ray observations of the transient's afterglow obtained with the Neil Gehrels Swift Observatory, the Nuclear Spectroscopic Telescope Array, and the Chandra X-ray Observatory between 0.5 to 65 days (observer frame) after the initial high-energy trigger. The X-ray emission decays steeply as $\sim t^{-1.9}$, and shows short timescale X-ray variability ($ΔT/T < 0.03$) in both Swift and NuSTAR, consistent with flares superposed on an external shock continuum. Serendipitous detections by the Swift Burst Alert Telescope (BAT) out to $\sim$0.3 days and continued NuSTAR variability to $\sim$2 days imply sustained central engine activity; including the precursor, the required engine duration is $\gtrsim 3$ days. Afterglow modeling favors the combination of forward and reverse shock emission in a wind-like ($k \approx 2$) environment. These properties, especially the long-lived engine and early soft X-ray emission, are difficult to reconcile with a collapsar origin, and GRB 250702B does not fit neatly with canonical ultra-long GRBs or relativistic tidal disruption events (TDEs). A hybrid scenario in which a star is disrupted by a stellar-mass black hole (a micro-TDE) provides a plausible explanation, although a relativistic TDE from an intermediate-mass black hole remains viable.

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arXiv2025

The redshift distribution of Einstein Probe transients supports their relation to gamma-ray bursts

The launch of the \textit{Einstein Probe} unleashed a new era of high-energy transient discovery in the largely unexplored soft X-ray band. The \textit{Einstein Probe} has detected a significant number of fast X-ray transients that display no gamma-ray emission, complicating their robust association to more common gamma-ray bursts. To explore their possible connection, we analyzed the redshift distribution of both \textit{Einstein Probe} fast X-ray transients and long duration gamma-ray bursts. A comparative analysis of their cumulative redshift distributions using non-parametric two-sample tests, namely the Kolmogorov-Smirnov and Anderson-Darling tests, finds no statistically significant difference. These tests favor that their redshifts are drawn from the same underlying distribution. This empirical connection between \textit{Einstein Probe} transients and long gamma-ray bursts is further supported by their agreement with the so-called ``Amati relation'' between the spectral peak energy and the isotropic-equivalent energy. Together, these results indicate that most extragalactic \textit{Einstein Probe} fast X-ray transients are closely related to long gamma-ray bursts and originate from a massive star (collapsar) progenitor channel. Our findings highlight the role of the \textit{Einstein Probe} in uncovering the missing population of failed jets and dirty fireballs that emit primarily at soft X-ray wavelengths.

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arXiv2025

Kilonova emission from GW230529 and mass gap neutron star-black hole mergers

The detection of the gravitational wave event GW230529, presumably a neutron star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA (LVK) Collaboration marks an exciting discovery for multimessenger astronomy. The black hole (BH) has a high probability of falling within the "mass gap" (mg) between the neutron star (NS) and the BH mass distributions. Because of the relatively low primary mass, this system has a higher likelihood of producing an electromagnetic counterpart than previously detected NSBH mergers. We analyze the potential kilonova (KN) emission from GW230529 and find that, if the source was an NSBH merger, there is a $\sim $2-$28\%$ probability (depending on the assumed equation of state) that it produced a KN peaking at $\sim 1$ day post-merger with $g \lesssim 23.5$ and $i < 23$. Hence, it could have been detected by ground-based telescopes. If instead the event was a binary neutron star (BNS) merger, the probability of KN production drops to $\sim $0-$10\%$. Motivated by these results, we simulate a broader population of mgNSBH mergers expected during the fifth LIGO/Virgo/KAGRA observing run (O5) and find a $2$-$3\%$ chance of KN production per event. Such KNe would typically be fainter than GW230529, with $g \lesssim 26$ and $i \lesssim 25$. Based on these findings, DECam-like instruments may be able to detect up to $\sim 70\%$ of future mgNSBH KNe, corresponding to $1-2$ multimessenger mgNSBH per year in O5.

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arXiv2025

GRB 250704B: An Off-axis Short GRB with a Long-Lived Afterglow Plateau

We present a detailed multi-wavelength afterglow study of the short GRB 250704B, extensively monitored in optical and near-infrared bands. Its afterglow displays an unusually long-duration plateau followed by an achromatic break and a steep decline, deviating from canonical GRB afterglows. While long plateaus are often explained by central engine activity, we find that for GRB 250704B, an energy injection model requires unreasonable parameters. The afterglow is better explained by an off-axis power-law structured jet with a narrow core ($θ_c \approx 0.7^{\circ}$) viewed at a modest angle ($θ_v \approx 1.9^{\circ}$). A comparison with GRB 170817A shows that both events are consistent with the off-axis structured jet scenario, where the shape of the light curve is governed primarily by the geometry of the jet and the viewing angle rather than the energetics, microphysical parameters, or external density. Our results underscore the importance of incorporating the jet structure in GRB modeling.

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arXiv2025

Spectral siren cosmology from gravitational-wave observations in GWTC-4.0

Gravitational wave standard sirens offer a promising avenue for cosmological inference, particularly in measuring the expansion history of the universe. Traditionally, bright sirens require an electromagnetic counterpart to determine the redshift of the emission source while dark sirens rely on the presence of complete galaxy catalogs over large sky regions. Spectral sirens, using GW data alone, can circumvent these limitations by leveraging features in the mass distribution of compact binaries. With the recent release of the Gravitational-Wave Transient Catalog 4 (GWTC-4.0), the number of significant binary black hole (BBH) merger candidates has increased to 153, enabling more robust population studies and cosmological constraints. This work builds upon previous spectral siren analyses by analyzing the latest BBH observations with parametric and non-parametric models. In particular, we consider a parametric approach using the Powerlaw + Peak and Broken Powerlaw + 2 Peaks models as well as a more flexible non-parametric model based on Gaussian processes. We find broad consistency in the inferred Hubble constant $H_0$ constraints across models. Our most constraining result is from the Gaussian Process model, which, combined with the GW170817 bright siren measurement, results in $H_0 = 69^{+7}_{-6} \ \mathrm{km\,s^{-1}\,Mpc^{-1}}$, a 10% precision measurement. For the Powerlaw + Peak and Broken Powerlaw + 2 Peaks we find fractional uncertainties of 17% and 13% respectively.

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arXiv2025

Astrophysics informed Gaussian processes for gravitational-wave populations: Evidence for the onset of the pair-instability supernova mass gap

We analyze binary black hole (BBH) mergers from the latest Gravitational Wave Transient Catalog (GWTC-3) using a flexible, non-parametric framework to infer the underlying black hole mass distribution. Our model employs Gaussian Processes (GPs) with astrophysically motivated priors to represent the mass distribution, assuming both component masses are independently drawn from a common mass function. This approach enables us to capture complex features in the population without relying on rigid parametric forms. Motivated by predictions from binary stellar evolution, we focus on the presence of a mass gap in the range $40-120~ M_\odot$, attributed to the pair-instability supernova (PISN) and pulsational PISN (PPISN) processes. Using our GP-based model, we find for the first time, strong evidence for the onset of this mass gap, locating its lower edge at approximately $45-60 ~M_\odot$. We observe a suppression in the BBH merger rate when comparing the latest constraints against our GP model, by a factor of $10-60$ at $\sim60~M_\odot$, corresponding to the minimum of the mass gap. Additionally, we find evidence of a subpopulation of mergers populating the mass gap around $70 ~M_\odot$, which we argue is due to hierarchical mergers, as well as of a feature in the $40-50~ M_\odot$ range, albeit with low significance, which we attribute to the predicted PPISN build up. We discuss the astrophysical implications of this result in light of GW231123, a recently reported BBH merger with a total mass potentially above the PISN gap, suggesting the need for revised models of massive stellar evolution or alternative formation channels.

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