Interpretation of the 1998 outburst of the unique X-ray transient CI Camelopardalis (XTE J0421+560)

Abstract

We present an analysis of the 1998 outburst of the peculiar X-ray binary and X-ray transient CI Cam (XTE J0421+560). We discuss the observations in the framework of several possible models and argue that this outburst can be explained by the thermal instability of the accretion disc, analogous to the outbursts of soft X-ray transients. Applying the model by King & Ritter and Shahbaz, Charles & King on the X-ray light curve, we obtain a realistic mass of the disc at the peak of outburst to be M h (0) ≈ 1.5 x 10 23 g (the distance d = 5 kpc) or 3.8 × 10 22 g (d = 2.5 kpc). The disc radius at this moment is then R h (0) ≈ 2.5 x 1010 cm (d = 5 kpc) or 1.6 x 10 10 cm (d = 2.5 kpc), provided that the factor / (the ratio of the mass of the hot disc at that moment with respect to its maximum possible mass) is close to unity. Even if we take a quite low f = 0.05, we still obtain R h (0) by only 2.7 times larger. The reddening in the outburst maximum and brighter peak absolute magnitude of CI Cam with respect to those of soft X-ray transients in outbursts can be explained if the disc in CI Cam heats up an extended envelope and/or a strong jet is formed. We thus bring firm arguments for Robinson, Ivans & Welsh's hypothesis. On the other hand, we bring the arguments against the mass transfer burst from the donor and the periastron passage of the compact object.