Item

Abstract

A new component was reported in the X-ray counterpart to the binary
neutron-star merger and gravitational wave event GW170817, exceeding the
afterglow emission from an off-axis structured jet. The afterglow emission from
the kilonova/macronova ejecta may explain the X-ray excess but exceeds the
radio observations if the spectrum is the same. We propose a fallback accretion
model that a part of ejecta from the neutron star merger falls back and forms a
disk around the central compact object. In the super-Eddington accretion phase,
the X-ray luminosity stays near the Eddington limit of a few solar masses and
the radio is weak, as observed. This will be followed by a power law decay with
index $-5/3$. The duration of the constant luminosity phase conveys the initial
fallback timescale $t_0$ in the past. The current multi-year duration requires
$t_0 > 3$--$30$ sec, suggesting that the disk wind rather than the dynamical
ejecta falls back after the jet launch. Future observations in the next decades
will probe the timescale of $t_0 \sim 10$--$10^4$ sec, around the time of
extended emission in short gamma-ray bursts. The fallback accretion has not
been halted by the {\it r}-process heating, implying that fission is weak on
the year scale. We predict that the X-ray counterpart will disappear in a few
decades due to the {\it r}-process halting.