Absorption in X-Ray Spectra of High-Redshift Quasars
Abstract
We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z~3) quasars with the ROSAT PSPC of which four are in directions of low Galactic N_H_. Three out of these four show excess absorption, while only three in ~50 z <~ 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of N_H_ = [0.86(+0.49 -0.28)] x 10^22^ atoms cm^-2^ for PKS 0438-436, and N_H_ = [1.45(+1.20, -0.64)] x 10^22^ atoms cm^-2^ for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636 + 680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyα systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420 - 388. This quasar has two damped Lyα systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) ~> 4 x 10^-3^(3σ), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band ~0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than {DELTA}α_E_ > 0.3(99% confidence) with either redshift or luminosity, for radio-loud quasars.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1994
- DOI:
- 10.1086/173703
- Bibcode:
- 1994ApJ...422...60E
- Keywords:
-
- Absorption Spectra;
- Active Galactic Nuclei;
- Galaxies;
- Quasars;
- Red Shift;
- Spectral Energy Distribution;
- X Ray Astronomy;
- X Ray Spectra;
- Lyman Alpha Radiation;
- Position Sensing;
- Proportional Counters;
- Rosat Mission;
- Astrophysics;
- GALAXIES: QUASARS: ABSORPTION LINES;
- X-RAYS: GALAXIES