The average quasar spectrum in the wavelength range 0.1-0.6 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, α, and the equivalent width of the Ly α line, W_Lyα. The 14 strongest quasar emission lines have been included in the spectrum synthesis, the ratios of their intensities to that of Ly α being taken from the work of Netzer & Davidson. Allowing for galactic extinction of the form $$A_\nu = 0.19\enspace \text {cosec}\enspace |b|\centerdot (\nu/\nu_V)$$, we find best fit values of $$\alpha = 0.5\pm 0.1, W_{\text {LY}\alpha} = 90\pm 20$$ Å for the sample. We also present evidence for a steepening of the mean optical spectrum towards shorter wavelengths.