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

摘要

We present results from ASCA observations of the binary CI Cam both inquiescence and in outburst in order to identify its central accreting object.The quiescence spectrum of CI Cam consists of soft and hard components whichare separated clearly at aound 2-3keV. A large equivalent width of an iron Kaemission line prefers an optically thin thermal plasma emission model to anon-thermal power-law model for the hard component, which favors a white dwarfas the accreting object, since the optically thin thermal hard X-ray emissionis a common characteristic among cataclysmic variables (binaries including anaccreting white dwarf). The outburst spectrum, on the other hand, is composed of a hard componentrepresented by a multi-temperature optically thin thermal plasma emission andof an independent soft X-ray component that appears below 1 keV intermittentlyon a decaying light curve of the hard component. The spectrum of the softcomponent is represented well by a blackbody with the temperature of0.07-0.12keV overlaid with several K-edges associated with highly ionizedoxygen. This, together with the luminosity as high as ~1E38 erg/s is similar toa super-soft source (SSS). The outburst in the hard X-ray band followed by theappearance of the soft blackbody component reminds us of recent observations ofnovae in outburst. We thus assume the outburst of CI Cam is that of a nova, andobtain the distance to CI Cam to be 5-17kpc by means of the relation betweenthe optical decay time and the absolute magnitude. This agrees well with arecent estimate of the distance of 5-9kpc in the optical band. All of theseresults from the outburst data prefer a white dwarf for the central object ofCI Cam.