Incidence of afterglow plateaus in gamma-ray bursts associated with binary neutron star mergers

TL;DR

This study analyzes the occurrence of X-ray afterglow plateaus in short gamma-ray bursts associated with binary neutron star mergers, revealing a lower incidence compared to long GRBs and implications for neutron star remnants.

Contribution

It provides the first extensive analysis of plateau incidence in short GRBs with known redshift, linking observations to neutron star remnant properties.

Findings

Plateau incidence in short GRBs is 18-37%.

Short GRB plateaus can be explained by magnetar energy injection.

Results suggest a maximum neutron star mass of 2.3-2.35 solar masses.

Abstract

One of the most surprising gamma-ray burst (GRB) features discovered with the Swift-X ray telescope (XRT) is a plateau phase in the early X-ray afterglow light curves. These plateaus are observed in the majority of long GRBs, while their incidence in short GRBs (SGRBs) is still uncertain due to their fainter X-ray afterglow luminosity with respect to long GRBs. An accurate estimate of the fraction of SGRBs with plateaus is of utmost relevance given the implications that the plateau may have for our understanding of the jet structure and possibly of the nature of the binary neutron star (BNS) merger remnant. This work presents the results of an extensive data analysis of the largest and most up-to-date sample of short GRBs observed with the XRT, and for which the redshift has been measured. We find a plateau incidence of 18-37% in short GRBs, which is a significantly lower fraction than that measured in long GRBs (>50%). Although still debated, the plateau phase could be explained as energy injection from the spin-down power of a newly born magnetized neutron star (NS; magnetar). We show that this scenario can nicely reproduce the observed short GRB plateaus, while at the same time providing a natural explanation for the different plateau fractions between short and long GRBs. In particular, our findings may imply that only a minority of BNS mergers generating short GRBs leave behind a sufficiently stable or long-lived neutron star (NS) to form a plateau. From the probability distribution of the BNS remnant mass, a fraction 18-37% of short GRB plateaus implies a maximum NS mass in the range $\sim$ 2.3 - 2.35 M$_{\odot}$.