Simultaneous Observations of BL Lac Objects with the IUE and HEAO-2

We propose to obtain simultaneous observations of four BL Lac objects in the ultraviolet and X-ray (HEAO-2) regions to study the "instantaneous" spectral energy distribution and their variations. The results already obtained for MK 501 and the preliminary results for the three remaining objects obtained in 1979 point out the importance of the simultaneity of observation as the X-ray emission is variable in at least three of the proposed objects. Our objective is to investigate the energy generation mechanism in the BL Lac objects, which is quite probably akin to, if not the same as, that in quasars.