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

The event GW170817/GRB 170817A, discovered via the successful joint observation of its gravitational wave radiation and its multiwavelength electromagnetic counterparts, was the first definite `smoking-gun' from the merger of two neutron stars (NSs). However, the remnant of the merger remains unknown. Piro et al. recently claimed that a low-significance X-ray variability in GRB 170817A. By systematically comparing the properties of variability in the afterglow of GRB 170817A and X-ray flares in GRB afterglows, we find that this X-ray variability seems to share similar statistical correlations with X-ray flares in GRB afterglows. We further investigate several possible merger product scenarios to see whether they can produce the observed X-ray variability in GRB 170817A. The first scenario invokes a stable magnetar as the central engine producing the later X-ray variability via differential rotation or fallback accretion onto the NS. The second scenario invokes a black hole as the central engine with a fallback accretion process. The final scenario is a central engine with a long-lived supra-massive NS. We find that the first two scenarios have difficulty producing the later X-ray variability, which requires either an impractical NS magnetic field or an extraordinarily large stellar envelope and an extremely long accretion time-scale. However, the third scenario seems to be consistent with observations, and the later X-ray variability can be produced by the magnetosphere which is expelled following the collapse of the NS with a B_p ∈ (3.6, 13.5) × 10¹³ G.