Multifrequency Observations of Z-Ray-Bright BL Lac Objects

Ten BL Lacertae objects discovered in the HEAO1 all-sky survey are brighter than 1 Uhuru flux unit (1 UFU ~= 2.4 x 10^-11 ergs cm^-2 s^-1), bright enough to yield useful spectral and variability information from the Ginga hard-X-ray satellite (2-30 keV) and the ROSAT extreme-ultraviolet/soft-X-ray satellite. These X-ray-selected BL Lacs have their peak emission in the far-ultraviolet or soft X-ray regimes, in contrast to the more traditional radio-selected BL Lacs. The ultraviolet -through- X-ray spectra of BL Lac objects, and any attendant variability, have important implications for the energetics and structure of BL Lacs. Furthermore, broad-band spectroscopy is a sensitive probe of the underlying energygeneration mechanism in BL Lacs, which may be related to that in other AGN. We propose to make coordinated, simultaneous observations using the IUE satellite in conjunction with ROSAT, Ginga, Voyager 2 (a far-ultraviolet spectrometer), and groundbased radio, submillimeter, infrared, and optical telescopes, in order to obtain the best possible broad-band spectra of BL Lac objects. These will be used to constrain models for the emitting region, as we have demonstrated during previous years of our ongoing IUE program. All ten targets will be easily detected in the ROSAT all-sky survey; furthermore, we have already been awarded time for longer ROSAT pointed observations of four of these BL Lacs, for which we therefore propose to repeat the multifrequency experiment. The proposed combination of instruments covers ten decades in frequency with unprecedented sensitivity and energy resolution. Understanding AGN is one of the primary questions in astrophysics; IUE has the potential to make important advances toward answering this question.