Detectability of "Merger-nova" emission from a long-lived magnetar in short gamma-ray bursts

TL;DR

This paper investigates the potential detectability of merger-nova emissions from long-lived magnetar remnants in short gamma-ray bursts, analyzing candidate events and modeling their optical signals with current and future telescopes.

Contribution

It systematically searches for signatures of supramassive neutron star engines in short GRBs and models the resulting merger-nova brightness for various parameters, assessing detectability with upcoming telescopes.

Findings

Five short GRBs show signs of a supramassive NS engine.

Merger-nova emissions are generally faint and hard to detect with current telescopes.

GRB 160821B's merger-nova is bright enough for detection and matches observations.

Abstract

One possible progenitor of short gamma-ray bursts (GRBs) is thought to be from a double neutron star (NS) merger, and the remnant of such a merger may be a supramassive NS, which is supported by rigid rotation and through its survival of hundreds of seconds before collapsing into a black hole (BH). If this is the case, an optical/infrared transient (namely merger-nova) is generated from the ejected materials and it is powered by radioactive decay from $r$-process, spin-down energy from a supramassive NS, as well as the magnetic wind from a newborn BH. In this paper, we systematically search for the signature of a supramassive NS central engine by analyzing the X-ray emission of short GRBs with internal plateau observed by {\em Swift}, and we find that five candidates of short GRBs have such feature with redshift measurement. Then, we calculate the possible merger-nova emission from those candidates for given the typical model parameters by considering the above three energy sources, and compare its brightness with the sensitivity of some optical telescopes. We find that the merger-nova emission of GRB 060801 in K-, r-, and U-bands with variations of $M_{\rm ej}$ ($10^{-4}-10^{-2} M_{\odot}$), $\kappa$ ($0.1-10 ~\rm cm^{2}~g^{-1}$), and $\beta$ ($0.1-0.3$) is very difficult to detect using the Vera C. Rubin, Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Zwicky Transient Facility, and the Roman Space Telescope (Roman), except for the case of large ejecta mass $M_{\rm ej}=10^{-2} M_{\odot}$. However, we are very hopeful to detect the merger-nova emission of GRBs 090515, 100625A, and 101219A using more sensitive instruments, such as Vera C. Rubin, Pan-STARRS, and Roman. Moreover, the merger-nova emission of GRB 160821B is bright enough to detect in our calculations, and it is also consistent with current real observations of merger-nova emission.