The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lacertae objects

ON 231 was observed with BeppoSAX in May and June 1998, following an except ional optical outburst which occured in April-May. We measured the X-ray sp ectrum from 0.1 up to 100 keV, In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We inte rpret the steeper component at energies below the break as due to synchrotr on emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simulta neously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between t he first and the second observation, and smaller variability amplitude betw een 4 and 10 keV. During the May observation we also detected a fast variab ility event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these resu lts in the framework of synchrotron self-Compton models.