As a class, they are interesting because:
nThey imply the
existence of a fair number
of neutron stars
different from standard radio pulsars and
X-ray binaries
n Accreting from ISM?
Unlikely: high proper motion
n Cooling NS or descendant from AXP, SGRs (old
magnetars?)
n Standard radio pulsars beamed away from the Earth? (however:
they are relatively numerous and all
close-by)
n Genuinely radio-quiet? (as Geminga, SGRs, AXPs)? Population
synthesis models