Misidentification of Short GRBs as Magnetars in Nearby Galaxies

Abstract

Context. Recent observations of GRB 200415A, a short and very bright pulse of $\gamma$-rays, have been claimed to be an extragalactic magnetar giant flare (MGF) whose proposed host galaxy is the nearby $\mathrm{NGC}253$. However, as the redshift of the transient object was not measured, it is possible that the measured location of the transient on the celestial sphere and the location of the local galaxy merely coincided. Thus, its real progenitor could have been arbitrarily far away, leading possibly to a much larger luminosity of the transient, and leaving the standard model of short gamma-ray bursts (sGRBs), the merger of two compact objects, as an explanation for the observations. Aims. In this study, our aim is to compute the false-alarm rate for the misinterpretation of sGRBs as magnetars in a given observation period. Methods. We simulate synthetic surveys of sGRB observations in a time period of 14 years corresponding to the operation period of the Gamma-ray Burst Monitor (GBM) detector. For all sGRBs that align on the sky with a nearby Local Volume galaxy, we generate realistic data which is folded through the response of the GBM. To identify candidates of sGRBs that may be misinterpreted as magnetars, six selections (spatial, star formation rate, GBM trigger, duration, isotropic energy release, and fluence) are applied to the simulated surveys. Results. In a non-negligible fraction, 15.7 %, of the simulated surveys, we identify at least one sGRB that has the same characteristics as a magnetar giant flare and could be thus misinterpreted as magnetar. Thus, we conclude that the selections that were proposed in previous work to unambiguously identify an extragalactic magnetar giant flare are not sufficient.

14 pages, 15 figures. Submitted to A&A

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