The two main issues of the thesis work are the study of multi-frequency spectral flux distributions and the analysis of flux and spectral variability of blazars. To this regard, the far-UV band (1000-3000 A) covered by the satellite International Ultraviolet Explorer (IUE, see Boggess et al. 1978 for details about the spacecraft and scientific instrumentation) has in AGN the important advantage, with respect to the optical and near-IR, of a lesser contamination by the stellar light of the host galaxy. This enables a reliable determination of the nonthermal continuum.
The thesis is divided as follows: Chapter 1 is devoted to the first part of the research program, consisting of the study of the near-IR, optical, UV spectral properties of blazars. In Section 1.1 are reconsidered the IUE archives, with
the aim of determining the average UV spectral properties of a rather large sample of blazars. In Section 1.2 are presented the near-IR, optical, UV spectra of BL Lacs based on simultaneous observations. One of these sources is a
1-ray emitter, so its overall radio-to-1-ray energy distribution is constructed and discussed in Section 1.4. The results of multi-frequency monitoring campaigns on the three blazars PKS 2155-304 (Section 2.1 ), OJ 287 (Section 2.2) and 3C 279 (Section 2.3) will be presented in Chapter 2. Particular enphasis will be put on the UV light curves, which represent my direct contribution to the wide international collaboration engaged for these multi-wavelength observing projects. The correlation of light curves at different wavelengths is also discussed in the case of PKS 2155-304 (§ 2.1.2). In Appendix A are reported the fit parameters for the 201 IUE spectra of PKS 2155-304. Appendices B and C contain the IUE extracted spectra of OJ 287 and 3C 279 respectively.
The work presented in this thesis is partially published or in progress. Part of the material included in Chapter 1 is presented in Pian & Treves (1993); Pian, Falomo, Scarpa & Treves (1994a) and Pian, Falomo, Ghisellini, Maraschi, Sambruna, Scarpa & Treves (1994c; Sections 1.1, 1.2 and 1.4 respectively). In Chapter 2 are collected the results presented in the papers by Urry et al. (1993), Edelson et al. (1994a), Pian et al. (1994b) and Maraschi et al. (1994b ), corresponding respectively to Sections 2.1.1, 2.1.2, 2.2, 2.3.
