Davide Lazzati

On 2025 August 18, the LIGO – Virgo – KAGRA collaboration reported S250818k, a subthreshold gravitational-wave (GW) candidate consistent with a binary neutron star (NS) merger potentially involving a subsolar-mass NS. Optical follow-up by the Zwicky Transient Facility identified a transient temporally coinciding with the GW trigger that initially resembled a kilonova but was later classified as a young stripped-envelope Type IIb supernova (SN), dubbed SN 2025ulz. A key question is whether SN 2025ulz harbors fast, possibly collimated, nonthermal ejecta indicative of a central engine, as invoked in “superkilonova” scenarios linking subsolar-mass NSs to accretion-disk fragmentation or core fission. We present early-to-late-time multiband radio observations of SN 2025ulz obtained with the Karl G. Jansky Very Large Array as part of the JAGWAR program, complemented by observations with the upgraded Giant Metrewave Radio Telescope and MeerKAT. We detect a faint but significant radio counterpart to SN 2025ulz at 6−10 GHz. The data are consistent with nonthermal emission from SN ejecta interacting with circumstellar material. Our data are also consistent with emission from an off-axis jet peaking at ∼50−100 days after the GW trigger. While we cannot distinguish between these two interpretations, our radio detection motivates future systematic multiwavelength follow-up of core-collapse events coinciding with subsolar NS GW candidates, should the association between S250818k and SN 2025ulz be supported by offline GW analyses.

The almost simultaneous detection of GRB 170817A and GW170817 ushered in nearly a decade of interest in binary neutron star (BNS) mergers and their multimessenger signals, resulting in a greater understanding of the processes that produce short-duration gamma-ray bursts and gravitational waves. However, open questions remain regarding the emission mechanism of these bursts. In this work, we present results from the first study of an electromagnetic signal produced from a realistic treatment of a BNS merger for both on-axis and off-axis observations. We accomplish this by using the PLUTO hydrodynamical code to inject a relativistic jet into the ejecta of a realistic BNS merger, which was itself obtained from the simulation of a 3D BNS merger. Then, we model the prompt photospheric emission that would emerge from this jet using the Monte Carlo Radiation Transfer radiative transfer code. We find that the resulting photon spectra can peak around ∼1 MeV for on-axis emission and fall off noticeably for off-axis observations. We also find distinctly nonthermal low- and high-energy tails in multiple observations, ranging from shallow to slightly off-axis observations. Our on-axis results are consistent with the Amati correlation for short bursts, with some strain evident at higher observing angles. Finally, we find that the radiative efficiency is much lower than seen in previous studies of the photospheric emission of long-duration gamma-ray bursts.

Population III (Pop III) stars, the first generation of stars formed from primordial gas, played a fundamental role in shaping the early Universe through their influence on cosmic reionization, early chemical enrichment, and the formation of the first galaxies. However, to date, they have eluded direct detection due to their short lifetimes and high redshifts. The launch of the James Webb Space Telescope (JWST) has revolutionized observational capabilities, providing the opportunity to detect Pop III stars via caustic lensing, where strong gravitational lensing magnifies individual stars to observable levels. This prospect makes it compelling to develop accurate models for their spectral characteristics to distinguish them from other stellar populations. Previous studies have focused on computing the spectral properties of nonrotating, zero-age main-sequence (ZAMS) Pop III stars. In this work, we expand upon these efforts by incorporating the effects of stellar rotation and post-ZAMS evolution into spectral calculations. We use the JWST bands and magnitude limits to identify the optimal observing conditions, both for isolated stars, as well as for small star clusters. We find that, while rotation does not appreciably change the observability at ZAMS, the subsequent evolution can significantly brighten the stars, making the most massive ones potentially visible with only moderate lensing.

The almost simultaneous detection of GRB170817A and GW170817 ushered in nearly a decade of interest in binary neutron star mergers and their multi-messenger signals, resulting in a greater understanding of the processes that produce short-duration gamma-ray bursts and gravitational waves. However, open questions remain regarding the emission mechanism of these bursts. In this work we present results from the first study of an electromagnetic signal produced from a realistic treatment of a binary neutron star merger, both for on-axis and off-axis observations. We accomplish this by using the PLUTO hydrodynamical code to inject a relativistic jet into the ejecta of a realistic binary neutron star merger, which was itself obtained from the simulation of a 3D BNS merger. Then, we model the prompt photospheric emission that would emerge from this jet using the MCRaT radiative transfer code. We find that the resulting photon spectra can peak around ~1 MeV for on-axis emission and falls off noticeably for off-axis observations. We also find distinctly non-thermal low and high-energy tails in multiple observations, ranging from shallow to mid-off axis observations. Our on-axis results are consistent with the Amati Correlation for short bursts, with some strain evident at higher observing angles. Finally, we find that the radiative efficiency is much lower than seen in previous studies of the photospheric emission of long-duration gamma-ray bursts.

At least some short gamma-ray bursts (SGRBs) originate from neutron star mergers, systems that release both a relativistic collimated jet and slower, wider ejecta. These jets evolve through a dense, anisotropic, and expanding medium produced during the merger process, resulting in interactions that affect their morphology and observable signatures. We investigate the propagation of SGRB jets through funnel-like structures that can be static or expanding with mildly relativistic speed using two-dimensional axisymmetric relativistic hydrodynamic simulations. Our initial conditions are inspired from radial and angular distributions of density and pressure from general-relativistic magnetohydrodynamic simulations of binary neutron star mergers. We explore different values of the funnel opening angle and density contrast. We find that the polar structure of the ejecta mainly affects the jet evolution in the early stages, whereas the effect of expanding ejecta dominates in later stages. Jets propagating through a low-density polar funnel move initially faster, while the presence of mildly relativistic ejecta maintains the outflow more collimated after the breakout. Despite differences in the external medium, the energy dissipation within the jet and cocoon remains similar across models, while the shocked ambient material shows distinct signatures that could be observationally distinguishable. Our results highlight the importance of the structure and dynamical properties of the ejecta in shaping SGRB jets.

Population III (Pop III) stars, the first generation of stars formed from primordial gas, played a fundamental role in shaping the early universe through their influence on cosmic reionization, early chemical enrichment, and the formation of the first galaxies. However, to date they have eluded direct detection due to their short lifetimes and high redshifts. The launch of the James Webb Space Telescope (JWST) has revolutionized observational capabilities, providing the opportunity to detect Pop~III stars via caustic lensing, where strong gravitational lensing magnifies individual stars to observable levels. This prospect makes it compelling to develop accurate models for their spectral characteristics to distinguish them from other stellar populations. Previous studies have focused on computing the spectral properties of non-rotating, zero-age main sequence (ZAMS) Pop III stars. In this work, we expand upon these efforts by incorporating the effects of stellar rotation and post-ZAMS evolution into spectral calculations. We use the JWST bands and magnitude limits to identify the optimal observing conditions, both for isolated stars, as well as for small star clusters. We find that, while rotation does not appreciably change the observability at ZAMS, the subsequent evolution can significantly brighten the stars, making the most massive ones potentially visible with only moderate lensing.

While gamma-ray bursts (GRBs) have the potential to shed light on the astrophysics of jets, compact objects, and cosmology, a major set back in their use as probes of these phenomena stems from our incomplete knowledge surrounding their prompt emission. There are numerous models that can account for various observations of GRBs in the gamma-ray and X-ray energy ranges, due to the flexibility in the number of parameters that can be tuned to increase agreement with data. Furthermore, these models lack predictive power that can test future spectropolarimetric observations of GRBs across the electromagnetic spectrum. In this work, we use the MCRaT radiative transfer code to calculate the X-ray spectropolarimetric signatures expected from the photospheric model for two unique hydrodynamic simulations of long GRBs. We make time-resolved and time-integrated comparisons between the X-ray and gamma-ray mock observations, shedding light on the information that can be obtained from X-ray prompt emission signatures. Our results show that the T ₉₀ derived from the X-ray light curve is the best diagnostic for the time that the central engine is active. We also find that our simulations reproduce the observed characteristics of the Einstein Probe–detected GRB 240315C. Based on our simulations, we are also able to make predictions for future X-ray spectropolarimetric measurements. Our results show the importance of conducting global radiative transfer calculations of GRB jets to better contextualize the prompt emission observations and constrain the mechanisms that produce the prompt emission.

At least three members of the recently identified class of fast luminous blue optical transients show evidence of late-time electromagnetic activity in great excess of what was predicted by an extrapolation of the early time emission. In particular, AT2022tsd displays fast, bright optical fluctuations approximately a month after the initial detection. Here we propose that these transients are produced by exploding stars in black hole binary systems and that the late-time activity is due to the accretion of clumpy ejecta onto the companion black hole. We derive the energetics and timescales involved, compute the emission spectrum, and discuss whether the ensuing emission is diffused or not in the remnant. We find that this model can explain the observed range of behaviors for reasonable ranges of the orbital separation and the ejecta velocity and clumpiness. Close separation and clumpy, high-velocity ejecta result in bright variable emission, as seen in AT2022tsd. A wider separation and smaller ejecta velocity, conversely, give rise to fairly constant emission at a lower luminosity. We suggest that high-cadence, simultaneous, panchromatic monitoring of future transients should be carried out to better understand the origin of the late emission and the role of binarity in the diversity of explosive stellar transients.

We analyze the spectral evolution of 62 bright Fermi gamma-ray bursts with large enough signal-to-noise to allow for time-resolved spectral analysis. We develop a new algorithm to test for single-pulse morphology that is insensitive to the specific shape of pulses. Instead, it only checks whether or not there are multiple, isolated, or statistical significant peaks in the light curve. In addition, we carry out a citizen science test to assess light-curve morphology and spectral evolution. We find that, no matter the adopted assessment method, bursts characterized by single-peaked prompt emission light curves have a greater tendency to also have a consistently decaying peak energy or hard-to-soft spectral evolution. This contrasts with the behavior of multipeaked bursts, for which the tendency is to have a peak frequency that is not monotonically decreasing. We discuss this finding in the theoretical framework of internal/external shocks and find it to be consistent with at least some single-pulse bursts associated with particularly high-density environments.

GRB 221009A was the brightest gamma-ray burst (GRB) of all time (BOAT), surpassing in prompt brightness all GRBs discovered in ∼50 yr and in afterglow brightness in ∼20 yr. We observed the BOAT with XMM-Newton 2.3 days after the prompt. The X-ray afterglow was still very bright and we collected the largest number of photons with the reflection grating spectrometers (RGSs) on a GRB. We searched the RGS data for narrow emission or absorption features. We did not detect any bright line feature. A candidate narrow feature is identified at a (rest-frame) energy of 1.455 − 0.014 + 0.006 keV, consistent with an Mg xii K _α emission line, slightly redshifted (0.012) with respect to the host galaxy. We assessed a marginal statistical significance of 3.0σ for this faint feature based on conservative Monte Carlo simulations, which requires caution for any physical interpretation. If this line feature would be for real, we propose that it might originate from the reflection in the innermost regions of the infalling funnel from low-level late-time activity emission of the central engine.