CI Camelopardalis: A Shell‐shocked X‐Ray Nova

Abstract

We present radio imaging observations of the 1998 outburst of the peculiar emission-line star CI Cam, taken ~1, 4, 75, 82, 93, 163, and 306 days after the beginning of the 1998 March 31.64 X-ray flare. The first two epochs show a resolved but compact (no larger than 12 mas) radio source that becomes optically thin at frequencies higher than 5 GHz. The spectrum and brightness temperatures are consistent with synchrotron self-absorption, although free-free absorption may also play a role. The later images show a large (120-350 mas) oval-shaped or double-ring remnant. The radio spectrum combined with the high brightness temperature indicates that the emission is synchrotron, while the morphology suggests that this is powered by a decelerating shock moving through dense circumstellar material produced by a strong stellar wind. The radio images of CI Cam are equally well fitted by an expanding ellipsoid or two expanding rings; the former gives Θ ≈ 4.2(t - 50,904.1)0.77, with Θ the major axis in milliaarcseconds and t the Modified Julian Date (MJD). The corresponding expansion speed in the plane of the sky was ~12,000 km s-1 over the first few days (for an assumed distance of 5 kpc), slowing by a factor of ~3 by the time of the last observation almost a year later. The radio emission from all other X-ray binary transients is either unresolved or takes the form of highly collimated relativistic jets. We suggest that CI Cam represents a rare case in which these jets were smothered early on by the unusually dense circumstellar medium. In this model, CI Cam is the analog to extragalactic supernovae formed by the collapsar mechanism, while the more usual X-ray binaries with relativistic jets are analogous to the jets that escape those supernovae to form a subset of γ-ray bursts.