The Ultraviolet Continua of Blazars: A Reconsideration of IUE Archives

Abstract

The IUE archive data on blazars are reconsidered. They refer to 47 objects, 24 radio-strong BL Lac objects, 15 radio-weak BL Lac objects, and eight optically violently variable quasars, increasing substantially the sample examined by Ghisellini et al. (1986). For each object a dereddened spectral slope is evaluated starting from the extraction of Kinney et al. (1991) and Edelson et al. (1992), and deriving the extinction from the hydrogen column density obtained through radio data. The 1200-3000 Å average spectral index is found to be α ≃ 1, with radio-strong BL Lac objects being steeper (α_ν ≃ 1.2) than radio-weak ones (α_ν ≃ 0.7). The behavior of blazars in the UV does not depart much from that of PG quasars for which the average spectral index is α_ν ≃ ≃ 0.8. Comparing with recent results of Falomo et al. (1993a) on the infrared-optical continuum of blazars we find no indication of a steepening in the 8 × 10¹³ -2 × 10¹⁵ Hz band, while the extrapolation to soft X-rays indicates at most a modest steepening.