GRB 210217A: A short or a long GRB?

TL;DR

This study analyzes GRB 210217A, which exhibits ambiguous characteristics of both short and long gamma-ray bursts, using multi-wavelength data and various classification criteria to determine its true nature.

Contribution

The paper presents a comprehensive multi-wavelength analysis of an ambiguous GRB, combining different classification methods to assess its true short or long nature.

Findings

GRB 210217A shows properties of both short and long GRBs.

Multi-wavelength analysis suggests ambiguity in its classification.

The burst's placement in the Amati plane supports its ambiguous nature.

Abstract

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{\rm 90}$ value (the time interval during which an instrument observes $5\%$ to $95\%$ of gamma-ray/hard X-ray fluence). However, $T_{\rm 90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{\rm 90}$ for a burst. GRB 210217A is detected with different duration by {\it Swift} and {\it Fermi}. It is classified as a long/soft GRB by {\it Swift}-BAT with a $T_{\rm 90}$ value of 3.76 sec. On the other hand, the sub-threshold detection by {\it Fermi}-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 sec. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{\rm 90}$-hardness ratio, $T_{\rm 90}$-\Ep, and $T_{\rm 90}$-$t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and place the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.