HST observations rule out the association between Cir X-1 and SNR G321.9-0.3

Abstract

Cir X-1 is one of the most intriguing galactic X-ray sources. It is a ~16.6 days variable X/radio source, a type-I X-ray burster and a QPO emitter. In spite of an uncertain optical counterpart classification, all these properties identify the source as an LMXB. The morphology of the surrounding radio nebula has suggested an association with the nearby ( ~ 25 arcmin) SNR G321.9-0.3, implying that Cir X-1 is a runaway binary originated from the supernova explosion {~} 10⁵ years ago. To investigate this hypothesis, we carried out a proper motion measurement of the Cir X-1 m ~ 19 optical counterpart using a set of HST/WFC and WFPC2 observations taken ~ 8.6 years apart. We obtained a 3 sigma upper limit of ~5 mas yr^-1 on the source proper motion. Since the runaway hypothesis would have implied a proper motion due North ranging between 15 and 75 mas yr^-1, depending on the actual age of the SNR, our result definitively rules out the association between Cir X-1 and SNR G321.9-0.3. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under contract No NAS 5-26555.