  The groundbreaking discovery of the optical transient AT2017gfo associated
with GW170817 opens a unique opportunity to study the physics of double neutron
star (NS) mergers. We argue that the standard interpretation of AT2017gfo as
being powered by radioactive decays of r-process elements faces the challenge
of simultaneously accounting for the peak luminosity and peak time of the
event, as it is not easy to achieve the required high mass, and especially the
low opacity of the ejecta required to fit the data. A plausible solution would
be to invoke an additional energy source, which is probably provided by the
merger product. We consider energy injection from two types of the merger
products: (1) a post-merger black hole powered by fallback accretion; and (2) a
long-lived NS remnant. The former case can only account for the early emission
of AT2017gfo, with the late emission still powered by radioactive decay. In the
latter case, both early- and late-emission components can be well interpreted
as due to energy injection from a spinning-down NS, with the required mass and
opacity of the ejecta components well consistent with known numerical
simulation results. We suggest that there is a strong indication that the
merger product of GW170817 is a long-lived (supramassive or even permanently
stable), low magnetic field NS. The result provides a stringent constraint on
the equations of state of NSs.
