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 $L_{\mathrm{peak}}\sim 2\times 10^{39}{\mathrm{erg}\mathrm{s}}^{-1}$ 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 ${\dot{M}}_{\max }$ 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 ${\dot{M}}_{\max }$ is in the range of $\sim 4{\dot{M}}_{\mathrm{Edd}}$ and $\sim 21{\dot{M}}_{\mathrm{Edd}}$ when the angular velocity of the slim disc is between $(1/5{)}^{1/2}{\mathrm{\Omega }}_{\mathrm{K}}$ and ${\mathrm{\Omega }}_{\mathrm{K}}$ (where ${\dot{M}}_{\mathrm{Edd}}$ is the Eddington accretion rate and ${\Omega }_K$ is the Keplerian angular velocity). With ${\dot{M}}_{\max }$, the slim disc can provide a luminosity $\sim L_{\mathrm{peak}}$ 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 $L_{\mathrm{peak}}$.

5 pages, 2 figures,accepted for publication in MNRAS

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Cited by in corpus (1)

• Diagnosing the remnants of binary neutron star merger from GW170817/GRB170817A event