Brno Observatory and Planetarium

Abstract We argue that the outburst of CI Cam (XTE J0421+560), probably containing a black hole, was caused by the thermal instability of the accretion disk. Applying the model of King & Ritter (1998), we obtain a realistic disk mass and radius. The differences from soft X-ray transients belonging to the low-mass X-ray binaries can be explained if the disk in CI Cam heats up an extended envelope and/or a strong jet is formed. We resolve several spectral components in the optical colors in quiescence after the outburst; they vary in a complicated way during a possible 1350 d cycle.We find indications that the variations of the source of the optical light can be related to those of the X-ray source in quiescence. The accretion disk seems to refill at present. Nowadays, only non-periodic intra-night optical (∼0.02 mag) fluctuations are present. As regards the absence of coherent changes in the optical band, we point out the similarities in the situation of CI Cam and the microquasar LS5039/RX J1826–1450.