THE VARIABLE ROSAT X-RAY-SPECTRUM OF THE BL-LAC-0716+714

The BLLac object 0716 + 714 is a powerful flat-spectrum radio source d etected by CGRO EGRET at gamma ray energies. It has been observed for about 21 000 s over 2 d by the X-ray telescope on board the ROSAT sate llite, and strong flux and spectral variations have been detected. In the time-scale of 2 d, the source varied by a factor of similar to 7; during a sudden(similar to 1000 s) flux increase, the variation was 70 per cent. Spectral analysis was performed first, using the model-inde pendent hardness ratio, and then the data were fitted using several st andard spectral shapes. Both analyses agree on the conclusion that 071 6 + 714 exhibits spectral variations related to source flux variabilit y, which may be attributed to a 'soft excess' component which is most prominent when the source is in a low state. The spectrum of 0716 + 71 4 steepens in the soft X-ray band when the flux decreases, while in th e hard X-ray band it remains constant. A double power-law model, repre senting steep synchrotron radiation at low energies and flat inverse C ompton radiation above similar to 1 keV, gives the best fit to both th e low and high states.