arXiv subjects

Isabelle Worssam

Publications and source records attributed to Isabelle Worssam.

4 recordsLinked to original sources

arXiv2026

Waiting in the wings: extended emission GRBs from high latitude emission and jet expansion

The process powering extended emission (EE) Gamma Ray-Bursts (GRBs) from compact object mergers is a debated problem due to their observed prompt emission timescales of 10 to 100s, far in excess of the expected accretion timescales following a binary neutron star (BNS) merger. Here, we present an analytical thin shell structured jet model designed to alleviate this tension. We show that a changing jet structure in which early emission is produced by a high Lorentz factor narrow, structured jet and later emission comes from a wider, lower Lorentz factor tophat-like jet can successfully reproduce the EE GRB phenomenology. All emitting shells are powered by a $0.1$ $M_{\odot}$ accretion torus and launched within a time window of 2s after merger. Spatially-resolved opacity checks confirm that our emission sites remain optically thin, in part due to the spectral softness associated with EE. We present light curves from our model compared to representative EE bursts detected by Swift: GRBs 211211A, 211227A and 060614. While our light curves provide good matches to the data, our spectra do not evolve fast enough when compared to the well-measured spectral evolution observed in GRB 211211A. We discuss an update to the model incorporating inherent spectral evolution in the shells as future work. We also show a parameter exploration of the model and identify a parameter range within which standard short GRB-like light curves could be produced, showcasing the ability of the model to produce short bursts with and without EE.

astro-ph.HE↗

arXiv2026

GRB 220706A: a gamma-ray burst with a month-long engine and a luminous supernova

While the progenitors of many long gamma-ray bursts (GRBs) are well established as the core collapse of very massive, envelope-stripped, rapidly-rotating stars, it has been suggested that bursts at the extremely long end of the duration distribution may be a separate population of `ultra-long' GRBs. With durations of thousands of seconds or more, these bursts are difficult to reconcile with the engine timescales expected from the compact Wolf-Rayet stars that are typically assumed to produce more `standard' long GRBs. Here, we present observations of GRB~220706A, where X-ray follow-up reveals flaring episodes that last until $\approx 51$ days after trigger. At the measured redshift of $z = 0.8577$, this corresponds to 27 days in the GRB rest frame, and represents the latest central engine activity ever observed in a GRB by a margin of $\approx 21$ rest-frame days. We also identify a likely supernova (SN) which, when accounting for the inferred optical extinction of $0.9 \leq A_V \leq 3.6$ mag (constrained by indirect arguments), has a peak absolute magnitude of $M_r \leq -20.25$, resembling the energetic SN\,2011kl found accompanying ultra-long GRB\,111209A, and consistent with super-luminous SNe. We discuss the implications GRB~220706A has for ultra-long GRB progenitor models and possible powering mechanisms for the extremely late central engine activity.

astro-ph.HE↗

arXiv2025

Binary Neutron Star Mergers: Multi-Messenger Systematics and Prospects with Next-Generation Facilities

Multi-messenger astronomy was galvanized by the detection of gravitational waves (GWs) from the binary neutron star (BNS) merger GW170817 and electromagnetic (EM) emission from the subsequent kilonova and short gamma ray burst. Maximizing multi-messenger constraints on these systems requires combining models of the progenitors and products of BNS mergers within a single framework. Motivated by GW170817, we create a combined model that relate the progenitor astrophysics of a BNS population with their GW observability and localizability, kilonova light curves, gamma-ray burst afterglow flux, and kilonova remnant evolution. We compute the BNS merger rate by convolving metallicity-dependent star-formation history with population-synthesis predictions, and we sample realistic populations to evaluate their GW and EM observables and joint detection rates. We find that next-generation detectors will typically observe BNS mergers with GW network signal-to-noise ratios of $\sim$ 10 to 20, 90th-percentile sky areas of order $\sim$ 10 deg$^2$, and kilonova $i$-band magnitudes spanning $\sim$ 23 to 33. The variation of the merger rate with respect to the common-envelope efficiency is shown in the GW and EM observables and the resulting multi-messenger detection yield, demonstrating how uncertainties propagate into all stages of joint GW+EM forecasting. Across the models examined, no more than $\sim$ 4% of BNS mergers are detectable simultaneously by a two-Cosmic-Explorer plus one-Einstein-Telescope network and by both Roman (in a $K$-like band) and Rubin ($i$ and $g$ bands). These results show that assumptions underlying the combination of progenitor evolution and source observables will constitute key multi-messenger modeling systematics for inference of astrophysical, nuclear, and fundamental physics from future datasets.

astro-ph.HE↗

arXiv2024

The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation.

astro-ph.HE↗