Possibility of a White Dwarf as the Accreting Compact Star in CI Camelopardalis (=XTE J0421+560)

Abstract

We present results from ASCA observations of the binary CI Cam both in quiescence and in outburst in order to identify its central accreting object. The quiescence spectrum of CI Cam consists of soft and hard components, which are separated clearly at around 2-3 keV. A large equivalent width of an iron K? emission line prefers an optically thin thermal plasma emission model to a nonthermal power-law model for the hard component, which favors a white dwarf as the accreting object, since optically thin thermal hard X-ray emission is a common characteristic among cataclysmic variables (binaries including an accreting white dwarf). However, since the power-law model, which represents the X-ray spectrum of the soft X-ray transients in quiescence, provides an equally good fit to the hard component statistically, we cannot exclude the possibility of a neutron star or a black hole from the quiescence data. The outburst spectrum, on the other hand, is composed of a hard component represented by multitemperature optically thin thermal plasma emission and an independent soft X-ray component that appears below 1 keV intermittently on a decaying light curve of the hard component. The spectrum of the soft component is represented well by a blackbody with the temperature of 0.07-0.12 keV overlaid with several K edges associated with highly ionized oxygen. This, together with the luminosity as high as ~1 ? 1038 ergs s-1, is similar to a supersoft source. The outburst in the hard X-ray band followed by the appearance of the soft blackbody component reminds us of recent observations of novae in outburst. We thus assume that the outburst of CI Cam is that of a nova and obtain the distance to CI Cam of 5-17 kpc by means of the relation between the optical decay time and the absolute magnitude. This agrees well with a recent estimate of the distance of 5-9 kpc in the optical band. All of these results from the outburst data prefer a white dwarf for the central object of CI Cam.