Professor Stephen Smartt CBE FRS MRIA

Abstract:

Most stripped-envelope supernova progenitors are thought to be formed through binary interaction, losing hydrogen and/or helium from their outer layers. Ultrastripped supernovae are an emerging class of transient that are expected to be produced through envelope stripping by a neutron star companion. However, relatively few examples are known, and the outcomes of such systems can be diverse and are poorly understood at present. Here we present spectroscopic observations and high-cadence, multiband photometry of SN 2023zaw, a rapidly evolving supernova with a low ejecta mass. SN 2023zaw was discovered in a nearby spiral galaxy at D = 39.7 Mpc. It has significant Milky Way extinction, E(B − V)MW = 0.21, and significant (but uncertain) host extinction. Bayesian evidence comparison reveals that nickel is not the only power source and that an additional energy source is required to explain our observations. Our models suggest that an ejecta mass of Mej ∼ 0.07 M⊙ and a synthesised nickel mass of MNi ∼ 0.007 M⊙ are required to explain the observations. We find that additional heating from a central engine, or interaction with circumstellar material, can power the early light curve.

Abstract:

Fast X-ray transients (FXTs) are extragalactic bursts of X-rays first identified in archival X-ray data and are now routinely discovered in real time by the Einstein Probe, which is continuously surveying the night sky in the soft (0.5–4 keV) X-ray regime. In this Letter, we report the discovery of the second optical counterpart (AT 2024gsa) to an FXT (EP 240414a). EP 240414a is located at a projected radial separation of 27 kpc from its likely host galaxy at z = 0.4018 ± 0.0010. The optical light curve of AT 2024gsa displays three distinct components. The initial decay from our first observation is followed by a rebrightening episode, displaying a rapid rise in luminosity to an absolute magnitude Mr ∼ −21 after two rest-frame days. While the early optical luminosity and decline rate are similar to those of luminous fast blue optical transients, the color temperature of AT 2024gsa is distinctly red and we show that the peak flux is inconsistent with a thermal origin. The third component peaks at Mi ∼ −19 at ≳16 rest-frame days post-FXT, and is compatible with an emerging supernova. We fit the riz-band data with a series of power laws and find that the decaying components are in agreement with gamma-ray burst afterglow models, and that the rebrightening may originate from refreshed shocks. By considering EP 240414a in context with all previously reported known-redshift FXT events, we propose that Einstein Probe FXT discoveries may predominantly result from (high-redshift) gamma-ray bursts, and thus appear to be distinct from the previously discovered lower-redshift, lower-luminosity population of FXTs.