EP241217a: a likely Type II GRB with an achromatic bump at z = 4.59

TL;DR

This paper reports the discovery of a high-redshift, likely Type II gamma-ray burst with an unusual achromatic bump in its X-ray afterglow, challenging existing models and prompting further theoretical investigation.

Contribution

It presents the first detailed analysis of EP241217a, a candidate high-redshift Type II GRB with unique afterglow features, and discusses the limitations of current models in explaining its properties.

Findings

EP241217a is a high-redshift (z=4.59) X-ray transient without gamma-ray detection.

The X-ray afterglow shows a plateau and an achromatic bump, suggesting complex circumburst environment interactions.

Standard models of a relativistic jet in a wind medium cannot fully explain the observed features.

Abstract

EP241217a is an X-ray transient detected by the Einstein Probe (EP) lasting for about 100 seconds and without accompanying $\gamma$-ray detection. The optical spectroscopy reveals the redshift of EP241217a is 4.59. By combining the $\gamma$-ray upper limit provided by GECAM-C, there is a considerable possibility that EP241217a is a typical Type II gamma-ray burst (GRB), but it is fainter than the detection threshold of any available $\gamma$-ray monitors (i.e., $E_{\gamma,{\rm iso}}\lesssim10^{53}$ erg). The X-ray light curve exhibits a plateau lasting for $\sim5\times10^4$ seconds. However, the joint analysis with optical data suggests the presence of an achromatic bump peaking at $\sim3\times10^4$ s after the trigger, indicating the actual duration of the X-ray plateau may be significantly shorter than it appears. To interpret the achromatic bump, we adopt the scenario of a mildly relativistic jet coasting in a wind-like medium and encountering a rapid density enhancement of the circumburst medium, which is likely induced by the the interaction of the progenitor's stellar wind and the interstellar medium. However, this model cannot fully explain observed data, and some issues do exist, e.g., the observed spectrum is harder than the model prediction. Consequently, we conclude that the scenario of a mildly relativistic jet coasting in the wind-like medium cannot explain all observed features of EP241217a. In addition, some alternative models commonly invoked to explain X-ray plateaus are discussed, but there are more or less issues when they are applied to EP241217a. Therefore, further theoretical modeling is encouraged to explore the origin of EP241217a.