The Soft X‐Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results

Abstract

We present the Ðnal results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low-z quasars. This sample includes all 23 quasars from the Bright Quasar Survey with z ¹ 0.400 and cm~2. Pointed ROSAT PSPC observations were made for all N H I Gal \ 1.9 ] 1020 quasars, yielding high signal-to-noise (S/N) spectra for most objects, which allowed an accurate determination of the spectral shape. The following main results were obtained :1. The spectra of 22 of the 23 quasars are consistent, to within D30%, with a single power-law model at rest-frame 0.2È2 keV. There is no evidence for signiÐcant soft excess emission with respect to the bestÐt power law. We place a limit (95% conÐdence) of D5 ] 1019 cm~2 on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are D1 ] 1019 cm~2 in the two highest S/N spectra.2. The mean 0.2È2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050È350 continuum recently determined by et al. for z [ 0.33 quasars. This sug-A Zheng (1996) gests that there is no steep soft component below 0.2 keV.3. SigniÐcant X-ray absorption (q [ 0.3) by partially ionized gas ("" warm absorber ÏÏ) in quasars is rather rare, occurring for of the population, which is in sharp contrast to lower luminosity active [5% galactic nuclei (AGNs), where signiÐcant absorption probably occurs for D50% of the population.4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emissionline parameters indicates that the strongest correlation is between the spectral slope and the Hb a x FWHM. A possible explanation for this remarkably strong correlation is a dependence of on a x L /L Edd , as seen in Galactic black hole candidates.5. The strong correlations between and Fe II/Hb, and the peak [O III] to Hb Ñux ratio are a x L *O III+ , veriÐed. The physical origin of these correlations is still not understood.6. There appears to be a distinct class of "" X-rayÈweak ÏÏ quasars, which form D10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10È30, than expected based on their luminosity at other bands and on their Hb luminosity. These may be quasars in which the direct X-ray source is obscured and only scattered X-rays are observed. 7. Thin accretion disk models cannot reproduce the observed 0.2È2 keV spectral shape, and they also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be maintaining a strong correlation between the optical and soft X-ray emission.8. The H I/He I ratio in the high Galactic latitude ISM must be within 20%, and possibly within 5%, of the total H/He ratio of 10, which indicates that He in the di †use H II gas component of the interstellar medium is mostly ionized to He II or He III.We Ðnally note the intriguing possibility that although in radio-loud quasars ([1.15^0.14) is Sax T signiÐcantly Ñatter than in radio-quiet quasars ([1.72^0.09) the X...

X‐Raying the Ultraluminous Infrared Starburst Galaxy and Broad Absorption Line QSO Markarian 231 withChandra

“…Based on ROSAT data, Boller, Brandt, & Fink (1996) and Laor et al (1997) found a significant correlation between the X-ray photon indexes of radio-quiet AGNs and the value for the FWHM H , where those AGNs with the narrowest broad Balmer lines, narrow-line Seyfert 1 galaxies (NLS1s), tended toward the steep end of the distribution of C. Subsequent work with ASCA in the 2-10 keV band has extended this finding to higher energy, although the spread of C in the 2-10 keV band is much smaller (e.g., Brandt, Mathur, & Elvis 1997;Reeves & Turner 2000). Mrk 231 has relatively narrow Balmer emission lines, FWHM H ¼ 2870 km s À1 (Boroson & Meyers 1992) and FWHM H ¼ 3000 km s À1 (Lípari, Terlevich, & Macchetto 1993), particularly for a QSO of its luminosity.…”

“…A large, negative ox indicates relatively weak soft X-ray emission; the mean value of ox for radio-quiet QSOs is %À1.48 (e.g., Laor et al 1997) with a typical range from À1.7 to À1.3 for objects that do not suffer from X-ray absorption (e.g., Brandt, Laor, & Wills 2000). Weakness in soft X-rays is plausibly explained by intrinsic X-ray absorption, which strongly depresses the observed flux at low-to-moderate energies.…”

Seyfert galaxies and BeppoSAX

“…ASCA observations of iron lines in Seyfert 1s (Nandra et al 1997) indicate that the emitting matter is generally neutral and then that the accretion rate does not exceed about 0.1-0.2 the critical value (according to the model of Matt, Fabian & Ross 1993). On the other hand, high accretion rates have been suggested by (Laor et al 1997) to be the basic explanation for the the Narrow Line Seyfert 1s phenomenon (Boller, Brandt & Fink 1996). In TON S 180 BeppoSAX has actually detected such an ionized line (Comastri et al 1998 and this volume), lending support to this hypothesis.…”

Universal spectral shape of high accretion rate AGN

“…A particularly well-covered broad-band spectrum available is the composite spectrum of quasars (Laor et al 1997) shown in Fig. 1.…”