Session 5 - High Luminosity AGN.
Display session, Monday, June 10
Tripp Commons,

[5.03] A HST Composite Quasar Spectrum

W. Zheng, G. A. Kriss, R. C. Telfer, J. P. Grimes, A. F. Davidsen (Johns Hopkins University)

We construct a composite quasar spectrum from 284 HST FOS spectra of 101 quasars with a mean redshift z \approx 1. The spectrum covers the wavelengths between 350 Å\ and 3000 Å\ in the rest frame, with a peak S/N level of \sim 150 per Å\ at \sim 1100 ÅSince \sim 90% of the sample quasars have redshift z < 1.5, the spectrum is suitable for studying the wavelength region shortward of Ly\alpha without any significant effect of intervening Ly\alpha forest absorption. Data in the waveband between 350 Å\ and 600 Å\ are mainly from the spectra of z > 1.5 quasars, for which significant corrections for the accumulated Lyman--series line and continuum absorption have been applied.

There is a significant steepening of the continuum slope around 1000 ÅThe continuum longward of 1000 Å\ can be modeled as a power law f_\nu \propto \nu^\alpha with \alpha \approx -1. The power--law index between 350 Å\ and 1000 Å\ is \alpha \approx -2. The break in the power--law index is a feature expected in Comptonized accretion--disk spectra. Electron scattering in a corona above the disk produces a power--law tail in the wavelength band shortward of \sim 1000 ÅComptonization also smears out the Lyman--limit edge of the intrinsic accretion disk spectrum.