The high energy X-ray spectrum of 3C 273

The quasar 3C 273 was observed in the hard X-ray range (18-240 keV) with a balloon-borne detector on 1981 September 28. The source was found to be quite strong and to emit a very hard spectrum. The photon number spectrum is well described by a single power law of index alpha = - 1.2, with no apparent break or cut-off. The integrated hard X-ray luminosity (20-200 keV) is 1.2 10 to the 47th erg/s. Compared to other observations in 1978/79 this corresponds to an increase in luminosity (18-240 keV) by a factor of three, representing the first observation of time variability of 3C 273 in this energy range. This behavior is briefly discussed with respect to Synchrotron-self-Compton and thermal Comptonization models.