On the continuum shape of broad absorption line quasars

The continuum shape of 26 broad absorption line (BAL) QSOs is shown to be virtually identical to that of non-BAL QSOs when the spectra of the former are dereddened. A procedure for dereddening of BAL QSO spectra is introduced, based on measurement of the flux in three bands and determination of the colour excess in comparison with an unreddened spectrum. This method allows us to derive the type and amount of extinction. We show that reddening in high-ionization BAL QSOs is comparatively small and the range of extinction laws varies from Milky Way, LMC to SMC type. On the other hand, low-ionization BAL QSOs seem to experience reddening which can be attributed to SMC type extinction alone. Comparison of the colour indices of composite spectra from Francis et al. (1991) and Zheng et al. (1997) with those derived from individual BAL QSO spectra shows that the Francis et al. (1991) composite might be a more suitable representative of an unreddened spectrum. In general, we conclude that the spectral energy distribution of normal QSOs can be used as an incident continuum shape for BAL QSOs.