Probing the Birth of Post-merger Millisecond Magnetars by X-ray and Gamma-ray Emission

TL;DR

This paper explores the potential detection of post-merger millisecond magnetars through their X-ray and gamma-ray emissions, particularly focusing on SSC emission peaking around 1 GeV, with implications for current and future observatories.

Contribution

It introduces a model for SSC emission from reverse shocks powered by post-merger magnetars, extending observational prospects to gamma-ray and X-ray bands.

Findings

SSC emission peaks at 1 GeV and extends beyond 10 TeV.

Detectability up to 1 Gpc with Fermi/LAT and CTA.

Strong high-energy emission only for ejecta masses below 10^{-3} solar masses.

Abstract

There is growing evidence that a stable magnetar could be formed from the coalescence of double neutron stars. In previous papers, we investigated the signature of formation of stable millisecond magnetars in radio and optical/ultraviolet bands by assuming that the central rapidly rotating magnetar deposits its rotational energy in the form of a relativistic leptonized wind. We found that the optical transient PTF11agg could be the first evidence for the formation of post-merger millisecond magnetars. To enhance the probability of finding more evidence for the post-merger magnetar formation, it is better to extend the observational channel to other photon energy bands. In this paper we propose to search the signature of post-merger magnetar formation in X-ray and especially gamma-ray bands. We calculate the SSC emission of the reverse shock powered by post-merger millisecond magnetars. We find that the SSC component peaks at $1\,{\rm GeV}$ in the spectral energy distribution and extends to $\gtrsim 10\,{\rm TeV}$ for typical parameters. These energy bands are quite suitable for Fermi/LAT and CTA, which, with their current observational sensitivities, can detect the SSC emission powered by post-merger magnetars up to $1\,{\rm Gpc}$. NuSTAR, sensible in X-ray bands, can detect the formation of post-merger millisecond magnetars at redshift $z\sim 1$. Future improvement in sensitivity of CTA can also probe the birth of post-merger millisecond magnetars at redshift $z\sim 1$. However, because of the $\gamma$-$\gamma$ collisions, strong high-energy emission is clearly predicted only for ejecta masses lower than $10^{-3}M_\odot$.