The X-ray quiescence of Swift J195509.6+261406 (GRB 070610): an optical
  bursting X-ray binary?
  We report on a 63ks Chandra observation of the X-ray transient Swift
J195509.6+261406 discovered as the afterglow of what was first believed to be a
long duration Gamma-Ray Burst (GRB 070610). The outburst of this source was
characterized by unique optical flares on timescales of second or less,
morphologically similar to the short X-ray bursts usually observed from
magnetars. Our Chandra observation was performed ~2 years after the discovery
of the optical and X-ray flaring activity of this source, catching it in its
quiescent state. We derive stringent upper limits on the quiescent emission of
Swif J195509.6+261406 which argues against the possibility of this object being
a typical magnetar. Our limits show that the most viable interpretation on the
nature of this peculiar bursting source, is a binary system hosting a black
hole or a neutron star with a low mass companion star (< 0.12 M_{\odot}), and
with an orbital period smaller than a few hours.
