Late Afterglow Emission Statistics: A Clear Link between GW170817 and Bright Short Gamma-Ray Bursts

GW170817, the first neutron star merger event detected by Advanced LIGO/Virgo detectors, was associated with the underluminous short-duration GRB 170817A. In this Letter we compare the forward shock afterglow emission of GW170817/GRB 170817A to other luminous short gamma-ray bursts (SGRBs) with both a known redshift and an afterglow emission lasting at least one day after the burst. In the rapid decay phase, the afterglow emission of the bright SGRBs and GW170817/GRB 170817A form a natural and continuous sequence, though separated by an observation time gap. If viewed on-axis, in the bursters' frames the forward shock afterglow emission of GW170817/GRB 170817A would be among the brightest detected thus far. This provides strong evidence for the GW170817-like merger origin of bright SGRBs, and suggests that the detection of the forward shock afterglow emission of most neutron star merger events are more challenging than the case of GW170817, as usually the mergers will be more distant and the viewing angles are plausibly higher.