The X-ray spectra of high-redshift quasars

Abstract

view Abstract Citations (68) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The X-Ray Spectra of High-Redshift Quasars Canizares, Claude R. ; White, Julia L. Abstract We present mean spectral parameters for various ensembles of quasars observed with the Einstein Observatory Imaging Proportional Counter (IPC). Our sample contains 71 optically or radio-selected quasars with 0.1 < z < 3.5, Galactic N_H_ < 10^21^ cm^-2^, total counts of 30-500, and IPC gain < 19. Data for each object are fitted to a power law with absorption in neutral material. Quasars are grouped into ensembles according to radio properties (flat radio spectrum [FRS], steep radio spectrum [SRS], or radio-quiet [RQ]), and either redshift or X-ray luminosity, l_x_. For each ensemble, composite χ^2^ contours for the two interesting parameters, N_H_ and α (the energy power-law index), are found by adding the individual χ_2_ grids appropriately shifted to remove the Galactic contribution to N_H_ for each object. All ensembles are consistent with little or no additional absorption beyond that expected in the Galaxy, so we have considered the limits on α assuming this is the case. We find a clear correlation between radio properties and α. FRS quasars have α ~ 0.4, SRS quasars have α ~ 0.7, and RQ quasars have α ~ 1-1.4. There is no evidence for a dependence of α on z or, for the FRS and SRS ensembles, on l_x_, over nearly three decades. FRS quasars with 2.0 < z < 3.5 have just as flat mean spectra as those with low z, implying that a single power law, which is flatter than the canonical one with α ~ 0.65, continues into the 1-10 keV band (in which the observed softer X-rays were emitted). Unfortunately, the results for high-redshift and high-l_x_ RQ quasars are ambiguous because of systematic uncertainties in the ensemble means. Thus we cannot test the two-component spectral hypothesis of Wilkes and Elvis for these objects. SRS X-ray spectra could be steeper than FRS spectra because of the mixing of two components, although a single intrinsically steeper spectrum is easier to reconcile with the absence of z-dependence. The uncertainty in α for RQ quasars with high z leaves open the important question of their contribution to the cosmic X-ray background. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/167273 Bibcode: 1989ApJ...339...27C Keywords: Quasars; Red Shift; X Ray Spectra; Absorption Spectra; Heao 2; Luminosity; Proportional Counters; Radio Spectra; Astrophysics; QUASARS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (71)