A Search for Soft X-Ray Emission Lines in the Afterglow Spectrum of GRB 221009A

Abstract GRB 221009A was the brightest gamma-ray burst (GRB) 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 days after the prompt. The X-ray afterglow was still very bright and we collected the largest number of photons with the reflection grating spectrometers (RGSs) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.455</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.014</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.006</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> keV, consistent with an Mg xii K α emission line, slightly redshifted (0.012) with respect to the host galaxy. We assessed a marginal statistical significance of 3.0 σ 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.