X-ray observation of the super-luminal quasar 4C +73.18 (1928+738) by ASCA

The results of an X-ray observation of the nearby core-dominated, super-luminal quasar 4C +73.18 (1928+738) with ASCA are reported. Variations of the 1–10 keV flux on time-scales of the order of 10 hours were found while the 0.5– 1 keV flux remained almost unchanged. The measured flux at 1 keV had increased by ∼ 50% compared with previous observations 18 years ago. The quasar shows a curved spectrum in the 0.8–10 keV band, which can be modeled as a power law plus a soft component. Both, the spectrum and the uncorrelated flux variations indicate the presence of a soft X-ray excess over the extrapolation of the hard band power law, which might be the high-energy extension of the intense UV bump emission found in this quasar. The hard X-ray emission is dominated by the flat power law (photon index 1.1–1.5). The results are consistent with models in which the hard X-rays are emitted from the relativistic jet. An iron line emission as reported in a previous Ginga observation (Lawson & Turner 1997) was not detected. We suggest that the line detected by Ginga might be not associated with the quasar but, most likely, with a distant cluster of galaxies. There are indications for a weak emission line-like feature around 1.25 keV (1.63 keV in the quasar frame) in both, the SIS and GIS spectra. The energy is surprisingly close to that of an unexplained line found in the flat-spectrum quasar PKS 0637-752 by Yaqoob et al. (1998). No evidence for the proposed binary black holes (Roos et al. 1993) is found in the current X-ray data.