Testing the blazar paradigm: ASCA observations of flat-spectrum radio quasars with steep soft X-ray spectra.
SAMBRUNA R.M., CHOU L.L. and URRY C.M.
Abstract (from CDS):
We present the first observations at medium-hard X-rays with ASCA in 1998 August-November of four flat-spectrum radio quasars (FSRQs), which are characterized by unusually steep soft X-ray spectra (photon index, Γ0.2–2.4keV∼2-2.5), as previously measured with ROSAT. Such steep X-ray slopes are similar to those observed in synchrotron-dominated BL Lac objects and are unexpected in the context of the recent blazar paradigm, where sources with strong emission lines (such as FSRQs) are dominated in soft X-rays by a flat inverse Compton tail. We find that the ASCA spectra of the four FSRQs are consistent with a power law model with Γ2–10keV∼1.8, flatter than their ROSAT spectra. This indicates the onset of an inverse Compton component at energies ≳2 keV, in agreement with the blazar unification scheme. However, these objects are still anomalous within the blazar class for their steep soft X-ray continua, which, together with nonsimultaneous data at longer wavelengths, hint at the possibility that the synchrotron emission extends to soft X-rays. This would imply an anomalously high synchotron peak frequency for a quasar with luminous broad lines, challenging current blazar unification schemes. Alternatively, a plausible explanation for the steep optical to soft X-ray continua of the four FSRQs is thermal emission from the accretion disk, similar to the blazars 3C 273 and 3C 345. In the Appendix, we present fits to the SIS data in an effort to contribute to the ongoing calibration of the time dependence of the SIS response at low energies.
Galaxies: Quasars: General - X-Rays: General