An extremely soft and weak fast X-ray transient associated with a luminous supernova

TL;DR

This paper reports the discovery of a soft, weak, fast X-ray transient associated with a luminous supernova, revealing a new regime of stellar death events accessible through sensitive X-ray observations.

Contribution

It presents the first detection of an extremely soft and weak X-ray transient linked to a supernova, expanding the known diversity of stellar death phenomena.

Findings

The transient has an $E_{peak} \, \lesssim \, 1.8$ keV and $E_{iso} \, \lesssim \, 10^{49}$ erg.

It is associated with a luminous, possibly magnetar-powered Type Ic supernova.

The event suggests a mildly relativistic outflow as the explosion mechanism.

Abstract

Long gamma-ray bursts (LGRBs), including their subclasses of low-luminosity GRBs (LL-GRBs) and X-ray flashes (XRFs) characterized by low spectral peak energies, are known to be associated with broad-lined Type Ic supernovae (SNe Ic-BL), which result from the core collapse of massive stars that lose their outer hydrogen and helium envelopes. However, the soft and weak end of the GRB/XRF population remains largely unexplored, due to the limited sensitivity to soft X-ray emission. Here we report the discovery of a fast X-ray transient, EP250108a, detected by the Einstein Probe (EP) in the soft X-ray band at redshift $z = 0.176$, which was followed up by extensive multiband observations. EP250108a shares similar X-ray luminosity as XRF\,060218, the prototype of XRFs, but it extends GRBs/XRFs down to the unprecedentedly soft and weak regimes, with its $E_{\rm peak} \lesssim 1.8\,\mathrm{keV}$ and $E_{\rm iso} \lesssim 10^{49}\, \mathrm{erg}$, respectively. Meanwhile, EP250108a is found to be associated with SN\,2025kg, one of the most luminous and possibly magnetar-powered SNe Ic-BL detected so far. Modeling of the well-sampled optical light curves favors a mildly relativistic outflow as the origin of this event. This discovery demonstrates that EP, with its unique capability, is opening a new observational window into the diverse outcomes of death of massive stars.