THE AVERAGE QUASAR SPECTRUM IN THE WAVELENGTH RANGE 0.1-0.6 MU-M

A study has been made of 277 quasars for which U, B, V data are available in order to specify the mean optical spectrum of a quasar in terms of two parameters: the power-law spectral index of the continuum, a, and the equivalent width of the Ly a line, WLya· The 14 strongest quasar emission lines have been included in the spectrum synthesis, the ratios of their intensities to that of Lya being taken from the work of Netzer & Davidson. Allowing for galactic extinction of the form Av= 0.19 cosec Ib I· (v/vv), we find best fit values of a= 0.5 ± 0.1, WLya =90 ± 20 A for the sample. We also present evidence for a steepening of the mean optical spectrum towards shorter wavelengths. © Royal Astronomical Society.