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