5. Summary and conclusions
We have shown that the quiescent X-ray spectrum observed from XTE J0421+560 can be fit with the same model used soon after its outburst, namely a two temperature bremsstrahlung. While the lower temperature, of the order of few hundreds eV, did not change between TOO2 and this observation, the higher temperature decreased considerably. The optical spectrum is very complex, and it is not possible to determine a spectral class for the system components. We discussed the nature of the compact object present in the system, and taking into account the peculiarity of the source, and the difficulty to compare its properties with those of typical systems, we conclude that it is unlikely for the XTE J0421+560/CI Cam system to contain a BH. Both a NS and a WD are possible, but the WD hypothesis is more appealing because it better fits the observational scenario: a two-temperature thermal spectrum for the X-ray emission, the lack of any temporal variability, and the presence of shell like motion observed in radio, that can be explained in terms of ejection of H- and He-rich layers during the outburst due to a thermonuclear runaway. This also fits nicely with the observed reduction of the He I line strengths when compared to observations performed before the outburst.
Finally, we can exclude that the X-ray luminosity observed in quiescence is due to the optical star if it is of spectral type OB. Moreover, the optical emission cannot be due to re-processing of the X-rays because .
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000