Possibility of a White Dwarf as the Accreting Compact Star in CI Cam (= 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 aound 2-3keV. A large equivalent width of an iron Ka emission line prefers an optically thin thermal plasma emission model to a non-thermal power-law model for the hard component, which favors a white dwarf as the accreting object, since the optically thin thermal hard X-ray emission is a common characteristic among cataclysmic variables (binaries including an accreting white dwarf). The outburst spectrum, on the other hand, is composed of a hard component represented by a multi-temperature optically thin thermal plasma emission and of 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 0.07-0.12keV overlaid with several K-edges associated with highly ionized oxygen. This, together with the luminosity as high as ~1E38 erg/s is similar to a super-soft source (SSS). 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 the outburst of CI Cam is that of a nova, and obtain the distance to CI Cam to be 5-17kpc 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 5-9kpc in the optical band. All of these results from the outburst data prefer a white dwarf for the central object of CI Cam.

arXiv comment20 pages, 5 figures, to appear in the Astrophysical Journal v601, n2, February 1, 2004 issue

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Cited by in corpus (10)

• WATCHDOG: A Comprehensive All-Sky Database of Galactic Black Hole X-ray Binaries
• X-ray Emissions from Accreting White Dwarfs: a Review
• A Systematic View of Ten New Black Hole Spins
• Spectroscopic survey of emission-line stars. I. B[e] stars
• Timing and Spectral Analysis of the Unusual X-Ray Transient XTE J0421+560/CI Camelopardalis
• Diagnostics of the Early Explosion Phase of a Classical Nova Using Its X-ray Emission: A Model for the X-ray Outburst of CI Camelopardalis in 1998
• Investigation of the Nature of the B[e] Star CI Cam in the Optical Range
• X-ray evolution of the nova V959 Mon suggests a delayed ejection and a non-radiative shock
• Optical Spectroscopic Observations of CI Camelopardalis
• Unified Model of Radio and X-Ray Emission of Nova CI Cam 1998