Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-07-19
Astrophys.J.629:L17-20,2005
Astronomy and Astrophysics
Astrophysics
13 pages, 3 figures. Accepted for publication in ApJ Letters. Corrected typos & references
Scientific paper
10.1086/444413
We have searched in the XMM-Newton public archive for quasars in the Sloan Digital Sky Survey (SDSS) First Data Release (DR1), and found 55 lying in the field of a XMM-Newton observation with exposure times >20 ksec (as of August, 2004). The 35 quasars which yielded good X-ray spectra span redshifts from 0.5 to 2.5. The large collecting area of XMM-Newton allows us to investigate the dependence of the X-ray spectra of quasars on luminosity, redshift and optical colors. We find: (1) no evolution of X-ray slope Gamma with either redshift or luminosity; (2) no correlation between Gamma or absorbing column density and the optical to X-ray ratio, alpha_{OX}; (4) no relation between alpha_{OX} and optical colors. The two latter results suggest that obscuration is not the dominant cause of the spread in X-ray slope or optical color. We find four unusual quasars, 10% of the sample: three are absorbed (N_H>10^{22} cm^{-2}), of which one has high luminosity (1.5x10$^{44} erg/s); the fourth has Gamma=0.6+-0.2, far flatter than the typical value of 1.8-2.0, and a strong emission line (EW=1.2+-$0.4 keV) which, if Fe-K, implies a redshift of ~1.4.
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Elvis Martin
Risaliti Guido
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