  Because supernova remnants are short lived, studies of neutron star X-ray
binaries within supernova remnants probe the earliest stages in the life of
accreting neutron stars. However, such objects are exceedingly rare: none were
known to exist in our Galaxy. We report the discovery of the natal supernova
remnant of the accreting neutron star Circinus X-1, which places an upper limit
of t < 4, 600 years on its age, making it the youngest known X-ray binary and a
unique tool to study accretion, neutron star evolution, and core collapse
supernovae. This discovery is based on a deep 2009 Chandra X-ray observation
and new radio observations of Circinus X-1. Circinus X-1 produces type I X-ray
bursts on the surface of the neutron star, indicating that the magnetic field
of the neutron star is small. Thus, the young age implies either that neutron
stars can be born with low magnetic fields or that they can rapidly become
de-magnetized by accretion. Circinus X-1 is a microquasar, creating
relativistic jets which were thought to power the arcminute scale radio nebula
surrounding the source. Instead, this nebula can now be attributed to
non-thermal synchrotron emission from the forward shock of the supernova
remnant. The young age is consistent with the observed rapid orbital evolution
and the highly eccentric orbit of the system and offers the chance to test the
physics of post-supernova orbital evolution in X-ray binaries in detail for the
first time.
