The X-ray spectrum of the superluminal quasar 1928 + 73

Abstract

Exosat observations (0.1-10 keV) of the radio-loud quasar 1928 + 73, which displays superluminal motion, were made at four epochs between 1983/283 and 1984/009, and spectra of this source are obtained from the Exosat database. The X-ray spectrum (0.1-10 keV) can be explained by a power law and uniform absorption model. No significant low-energy absorption or soft excess were detected. Both soft (0.1-2 keV) and hard (2-10 keV) X-ray-flux variations of this quasar were found on a time-scale of one month. Spectral-index values obtained from the power-law model suggest that they exceed the 'canonical' value. A thermal bremsstrahlung model also fits well. Uncorrelated variations of the soft and hard fluxes are found in this source which indicate that they have different origins. A soft excess is apparent when the spectra are fitted with a canonical photon index of about 1.7. The spectra presented here are the first in the X-ray region for 1928 + 73