SIMBAD references

ON 231 was observed with BeppoSAX in May and June 1998, following an exceptional optical outburst which occured in April-May. We measured the X-ray spectrum from 0.1 up to 100keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron 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 simultaneously 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 the first and the second observation, and smaller variability amplitude between 4 and 10keV. During the May observation we also detected a fast variability event with the flux below 4keV increasing by about a factor of three in 5 hours. Above 4keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models.