THE YOUNGEST KNOWN X-RAY BINARY: CIRCINUS X-1 AND ITS NATAL SUPERNOVA REMNANT

Abstract

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