Could an X-ray Flare after GRB 170817A Originate from a Post-merger Slim Accretion Disc?

Abstract

GRB 170817A, detected by Fermi-GBM 1.7\,s after the merger of a neutron star (NS) binary, provides the first direct evidence for a link between such a merger and a short-duration gamma-ray burst. The X-ray observations after GRB 170817A indicate a possible X-ray flare with a peak luminosity $L_{\rm peak} \sim 2\times 10^{39}\,{\rm erg\,s}^{-1}$ near day 156. Here we show that this X-ray flare may be understood based on a slim disc around a compact object. On the one hand, there exists the maximal accretion rate $\dot M_{\rm max}$ for the slim disc, above which an optically thick outflow is significant and radiation from the disc is obscured. Based on the energy balance analysis, we find that $\dot M_{\rm max}$ is in the range of $\sim 4\dot M_{\rm Edd}$ and $\sim 21\dot M_{\rm Edd}$ when the angular velocity of the slim disc is between $\rm (1/5)^{1/2}Ω_K$ and $\rm Ω_K$ (where $\dot M_{\rm Edd}$ is the Eddington accretion rate and $Ω_K$ is the Keplerian angular velocity). With $\dot M_{\rm max}$, the slim disc can provide a luminosity $\sim L_{\rm peak}$ for a compact object of $2.5 M_{\sun}$. On the other hand, if the merger of two NSs forms a typical neutrino-dominated accretion disc whose accretion rate $\dot M$ follows a power-law decline with an index $-1.8$ , then the system must pass through the outflow regime and enter the slim disc in $\sim 11-355$ days. These results imply that a post-merger slim accretion disc could account for the observed late-time $L_{\rm peak}$.