CI Cam: 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 about 1, 4, 75, 82, 93, 163, and 306 days after the beginning of the 31.64 March 1998 X-ray flare. The first two epochs show a resolved but compact radio source which becomes optically thin at frequencies higher than 5 GHz. 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 fit by an expanding ellipsoid or two expanding rings. The corresponding expansion speed in the plane of the sky was about 12,000 km/s over the first few days, 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 has either been unresolved, or taken the form of highly collimated relativistic jets. We suggest that CI Cam represents a rare case where these jets were smothered early on by the unusually dense circumstellar medium. In this model CI Cam is the analogue to extragalactic supernovae formed by the collapsar mechanism, while the more usual X-ray binaries with relativistic jets are analogous to the jets which escape those supernovae to form a subset of gamma-ray bursts.

17 pages, 10 figures. accepted for publication in Ap.J

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