Long-term radio monitoring of the fast X-ray transient EP240315a: evidence for a relativistic jet

TL;DR

This study presents long-term radio observations of a high-redshift fast X-ray transient, revealing evidence of a relativistic jet and suggesting FXRTs can be linked to standard gamma-ray bursts, especially with upcoming sensitive X-ray missions.

Contribution

First detailed radio monitoring of a high-redshift FXRT, demonstrating its association with a relativistic jet similar to GRBs, and highlighting the potential of future X-ray missions to detect such events.

Findings

Radio lightcurve shows steep rise and jet break, indicating collimated outflow.

The transient's properties align with a relativistic jet model typical of GRBs.

FXRTs may often be associated with standard gamma-ray bursts.

Abstract

The recent launch of Einstein Probe (EP) in early 2024 opened up a new window onto the transient X-ray sky, allowing for real-time discovery and follow-up of fast X-ray transients (FXRTs). Multi-wavelength observations of FXRTs and their counterparts are key to characterize the properties of their outflows and, ultimately, identify their progenitors. Here, we report our long-term radio monitoring of EP240315A, a long-lasting ($\sim 1000$ s) high redshift ($z=4.9$) FXRT associated to GRB~240315C. Our campaign, carried out with the Australian Telescope Compact Array (ATCA), followed the transient's evolution at two different frequencies (5.5 GHz and 9~GHz) for three months. In the radio lightcurves we identify an unusual steep rise at 9 GHz, possibly due to a refreshed reverse shock, and a late-time rapid decay of the radio flux, which we interpret as a jet break due to the outflow collimation. We find that the multi-wavelength counterpart of EP240315A is well described by a model of relativistic jet seen close to its axis, with jet half-opening angle $\theta_j \approx 3 ^{\circ}$ and beaming-corrected total energy $E \simeq 4\times 10^{51}$~erg, typical of GRBs. These results show that a substantial fraction of FXRTs may be associated to standard GRBs and that sensitive X-ray monitors, such as Einstein Probe and the proposed HiZ-GUNDAM and Theseus missions, can successfully pinpoint their relativistic outflows up to high-redshifts.