MULTIFREQUENCY OBSERVATIONS OF XTE J0421+560 CI CAM IN OUTBURST/

We report on two X-ray observations of the transient source XTE J0421560 performed by BeppoSAX, and on a series of observations performed b y the 0.7 m Teramo-Normale Telescope. Outburst peak occurrence time an d duration depend on photon energy: the outburst peak is achieved firs t in the X-pay band, then in the optical and finally in the radio. An exponential decay law fits well the X-ray data except in the TOO2 0.5- 1.0 keV band, where erratic time variability is detected. During TOO1 the e-folding time scale decreases with energy up to -20 keV, when it achieves a saturation; during TOO2 it decreases up to similar to 2 keV and then increases. This change is correlated with a spectral change, characterized by the onset of a soft (less than or similar to 2 keV) component in TOO2 (Orr et al. 1998). This component might originate fr om the relativistic jets, while the hard component is more likely asso ciated to processes occurring in the circumstellar matter and/or near the compact object. Optical observations show that the object appears intrinsically red even during the outburst. The nature of the compact object is discussed.