The variable ROSAT X-ray spectrum of the BL Lac 0716+714

The BL Lac object 0716+714 is a powerful flat spectrum radio source detected by CGRO-EGRET at gamma-ray energies. It has been observed for about 21000 s over two days by the X-ray telescope onboard the ROSAT satellite and strong flux and spectral variations have been detected. In the time scale of two days the source varied by a factor of $$\sim 7$$; during a sudden ($$\sim 1000$$ s) flux increase, the variation was 70 per cent. 0716+714 exhibits spectral variations related to source flux variability which may be attributed to a "soft excess" component most prominent when the source was in a low state. The spectrum of 0716+714 steepened in the soft X-ray band when the flux decreased while in the hard X-ray band remained constant. A double power law model, representing steep synchrotron radiation at low energies and flat inverse Compton radiation above $$\sim$$ 1 keV, best fits both the low and high state.