  Recent observations indicate that in a large fraction of binary neutron star
(BNS) mergers a long-lived neutron star (NS) may be formed rather than a black
hole. Unambiguous electromagnetic (EM) signatures of such a scenario would
strongly impact our knowledge on how short gamma-ray bursts (SGRBs) and their
afterglow radiation are generated. Furthermore, such EM signals would have
profound implications for multimessenger astronomy with joint EM and
gravitational-wave (GW) observations of BNS mergers, which will soon become
reality with the ground-based advanced LIGO/Virgo GW detector network starting
its first science run this year. Here we explore such EM signatures based on
the model presented in a companion paper, which provides a self-consistent
evolution of the post-merger system and its EM emission starting from an early
baryonic wind phase and resulting in a final pulsar wind nebula that is
confined by the previously ejected material. Lightcurves and spectra are
computed for a wide range of post-merger physical properties and particular
attention is paid to the emission in the X-ray band. In the context of SGRB
afterglow modeling, we present X-ray lightcurves corresponding to the
'standard' and the recently proposed 'time-reversal' scenario (SGRB prompt
emission produced at the time of merger or at the time of collapse of the
long-lived NS). The resulting afterglow lightcurve morphologies include, in
particular, single and two-plateau features with timescales and luminosities
that are in good agreement with the observations by the Swift satellite.
Furthermore, we compute the X-ray signal that should precede the SGRB in the
time-reversal scenario. If found, such a signal would represent smoking-gun
evidence for this scenario. Finally, we find a bright, highly isotropic EM
transient signal peaking in the X-ray band ...
