2000A&A...356..163O

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ∼150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10keV flux of 6.7x10^–12erg/cm²/s and was still remarkably soft with most of the emission below 2keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ∼7keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L_x/L_opt∼10^–3 the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.