The average quasar spectrum in the wavelength range 0.1 - 0.6 μm

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, Wsub(Lyα). The 14 strongest quasar emission lines have been included in the spectrum synthesis, the ratios of their intensities to that of Lyα being taken from the work of others. Allowing for galactic extinction of the form Asub(ν) 0.19 cosecb.(ν/νsub(V)), we find best fit values of α = 0.5 +- 0.1, Wsub(Lyα) = 90 +- 20 A for the sample. Evidence is presented for a steepening of the mean optical spectrum towards shorter wavelengths. (author)