Established through multiwavelength observations that blazars are AGN characterized by relativistic jets oriented along the line of sight. Identified the parent population as radio galaxies, and established that, once the (strong) effect of relativistic beaming is properly taken into account, the numbers and luminosities are as expected. Carried out the first intensive multiwavelength monitoring to probe the structure of the unresolved inner relativistic jets (which are orders of magnitude too small to be imaged but can be mapped via their temporal variations). Found the first correlations between X-ray and gamma-ray variations in light curves from blazars, which constrain the energetics of the relativistic electrons in the jet as well as their acceleration and radiation mechanisms.

Key papers:

PKS~2155-304: Relativistically Beamed Synchrotron Radiation from a BL Lac Object   C. M. Urry and R. F. Mushotzky, 1982, ApJ, 253, 38-46

Multiwavelength Monitoring of the BL Lacertae Object PKS~2155-304. I. The IUE Campaign   C. M. Urry, L. Maraschi, R. Edelson, A. Koratkar, J. Krolik, G. Madejski, E. Pian, G. Pike,G. Reichert, A. Treves, W. Wamsteker, R. Bohlin, J. Bregman, W. Brinkmann, L. Chiappetti, T. Courvoisier, A. V. Filippenko, H. Fink, I. M. George, Y. Kondo, P. G. Martin, H. R. Miller, P. O’Brien, J. M. Shull, M. Sitko, A. E. Szymkowiak, G. Tagliaferri, S. Wagner, and R. Warwick, 1993, ApJ, 411, 614-631

The 1993 Multiwavelength Campaign on 3C~279. I. The Radio to Gamma-Ray Energy Distribution in the Low State  L. Maraschi, P. Grandi, C. M. Urry, A. E. Wehrle, G. Madejski, H. H. Fink, G. Ghisellini, R. C. Hartman, A. Koratkar, E. Pian, H.-C. Thomas, A. Treves, M. F. Aller, H. D. Aller, C. D. Bailyn, T. J. Balonek, H. Bock, W. Collmar, I. S. Glass, S. J. Litchfield, I. M. McHardy, R. M’endez, J. Pesce, H. P. Reuter, E. I. Robson, H. Steppe, J. A. Stevens, H. Ter\asranta, and S. J. Wagner, 1994, ApJ (Letters), 435, L91-95

Multiwavelength Observations of a Dramatic High Energy Flare in the Blazar 3C~279   A. E. Wehrle, E. Pian, C. M. Urry, L. Maraschi, G. Ghisellini, R. C. Hartman, G. M. Madejski, F. Makino, A. P. Marscher, I. M. McHardy, S. J. Wagner, J. R. Webb, G. S. Aldering, M. F. Aller, H. D. Aller, D. E. Backman, T. J. Balonek, J. Bonnell, J. Caplinger, A. Celotti, W. Collmar, J. Dalton, A. Drucker, R. Falomo, C. E. Fichtel, W. Freudling, W. K. Gear, N. Gonzalez-Perez, P. Hall, H. Inoue, W. N. Johnson, D. Kazanas, M. R. Kidger, R. I. Kollgaard, Y. Kondo, J. Kurfess, A. J. Lawson, Y. C. Lin, B. McCollum, K. McNaron-Brown, A. D. Nair, S. Penton, J. E. Pesce, M. Pohl, C. M. Raiteri, M. Renda, E. I. Robson, R. M. Sambruna, A. F. Schirmer, M. Sikora, A. Sillanpaa, P. S. Smith, J. A. Stevens, J. Stocke, L. O. Takalo, H. Terasranta, D. J. Thompson, R. Thompson, M. Tornikoski, G. Tosti, P. Turcotte, A. Treves, S. C. Unwin, E. Valtaoja, M. Villata, W. Xu, and A. Zook, et al., 1998, ApJ, 497, 178-187

Multiwavelength Monitoring of the BL Lacertae Object PKS~2155-304 in May 1994. III. Probing the Inner Jet through Multiwavelength Correlations  C. M. Urry, A. Treves, L. Maraschi, H. Marshall, T. Kii, G. Madejski, S. Penton, J. E. Pesce, E. Pian, A. Celotti, R. Fujimoto, F. Makino, C. Otani, R. M. Sambruna, K. Sasaki, J. M. Shull, P. Smith, T. Takahashi, and M. Tashiro, 1997, ApJ, 486, 799-809

Complex Spectral Variability of Mrk~421  T. Takahashi, J.~Kataoka, G.~Madejski, J.~Mattox, C. M. Urry, S.~Wagner, F.~Aharonian, M.~Catanese, L.~Chiappetti, P.~Coppi, B.~Degrange, R.~Edelson, G.~Fossati, H.~Kubo, H.~Krawczynski, F.~Makino, A.~Markowitz, H.~Marshall, L.~Maraschi, F.~Piron, R.~Remillard, M.~Tashiro, H.~Terasranta, and T.~Weekes, and [19 authors], 2000, ApJ Letters, 542, L105-L110

BL Lac Objects and Blazars: Past, Present, and Future   C. M. Urry, 1999, in BL Lac Phenomenon, ed. L. Takalo and A. Sillanpaa (San Francisco: ASP), pp. 3-19