Characterising EP241107a: Multiwavelength Observations of an Einstein Probe-detected Fast X-ray Transient

TL;DR

This paper reports multiwavelength observations of a fast X-ray transient, EP241107a, discovered by Einstein-Probe, and suggests it is likely associated with a gamma-ray burst, providing insights into its jet properties and origin.

Contribution

It presents the first multiwavelength follow-up and modeling of an Einstein-Probe detected FXT, proposing a GRB origin and characterizing its jet parameters.

Findings

EP241107a is likely a gamma-ray burst afterglow.

The jet has an isotropic kinetic energy of ~10^51 erg.

The jet opening angle is approximately 15 degrees.

Abstract

Fast X-ray Transients (FXTs) represent a new class of highly luminous transients in soft X-rays ($\sim$0.3-10 keV) associated with violent astrophysical processes. They manifest as short, singular flashes of X-ray photons with durations lasting from minutes to hours. Their origin remains unclear, and they have been associated with various progenitor mechanisms. The newly launched X-ray survey, Einstein-Probe (EP), is revolutionising this field by enabling the discovery and immediate follow-up of FXTs. Here we present the multiwavelength observations of EP-discovered FXT EP241107a and the discovery of its radio counterpart. Comparison of the optical and radio observations of EP241107a and its host properties with other extragalactic transients suggests a gamma-ray burst (GRB) origin. Through our afterglow modelling, we infer the GRB jet properties for EP241107a, yielding a jet of the isotropic-equivalent kinetic energy $E_{\mathrm{K,iso}} \sim10^{51}$ erg, with a half opening angle $\theta_{c}$ $\approx$15$^{\circ}$, viewed at an angle of $\theta_{\rm obs}$~$\approx$9$^{\circ}$. We also evaluate EP241107a in the landscape of both EP-discovered FXTs as well as the FXTs discovered from Chandra, XMM-Newton, and Swift-XRT.