A search for soft X-ray emission lines in the afterglow spectrum of GRB 221009A

TL;DR

This study analyzed the X-ray afterglow of the exceptionally bright GRB 221009A using XMM-Newton, searching for emission lines and finding a marginal candidate possibly related to reflection from the central engine.

Contribution

First detailed X-ray spectral analysis of GRB 221009A's afterglow with no significant emission lines detected, identifying a marginal candidate line and discussing its possible origin.

Findings

No bright emission or absorption lines detected.

A marginal candidate Mg XII emission line at 1.455 keV.

Possible reflection origin from the central engine's late-time activity.

Abstract

GRB 221009A was the Brightest gamma-ray burst Of All Time (BOAT), surpassing in prompt brightness all GRBs discovered in ~50 yr and in afterglow brightness in ~20 yr. We observed the BOAT with XMM-Newton 2.3 d after the prompt. The X-ray afterglow was still very bright and we collected the largest number of photons with the Reflection Grating Spectrometers (RGS) on a GRB. We searched the RGS data for narrow emission or absorption features. We did not detect any bright line feature. A candidate narrow feature is identified at a (rest-frame) energy of 1.455+0.006-0.014 keV, consistent with an Mg XII K{\alpha} emission line, slightly redshifted (0.012) with respect to the host galaxy. We assessed a marginal statistical significance of 3.0sigma for this faint feature based on conservative Monte Carlo simulations, which requires caution for any physical interpretation. If this line feature would be for real, we propose that it might originate from the reflection in the innermost regions of the infalling funnel from low-level late-time activity emission of the central engine.