EUV Spectroscopy of BL Lac Objects

Recent observations of BL Lac objects have shown evidence for the presence of a deep and broad absorption feature at soft X-ray energies. The best data exist for PKS 2155-304, where this feature is believed to be associated with a Ly-alpha transition of hydrogenic oxygen (O VIII) that is broadened by a high-velocity outflow. If this interpretation is correct, then the spectral region sampled by the shortest wavelength bandpass of the EUVE spectrometer should exhibit a ``forest'' of potentially observable features associated with the 2s--2p transitions of L-shell species of iron (Fe XVIII -- XXIV). The distribution of equivalent width among these features as well as the observed line profiles are quite sensitive to the density and velocity structure of the outflow and may thus be of great diagnostic value. We have proposed Extreme Ultraviolet Explorer (EUVE) observations of two BL Lacs, PKS 2155-304 and Mkn 421, both of which have been definitely detected by the ROSAT Wide Field Camera. Our simulations show that the spectra should be observable at least out to wavelengths of order 100 Angstroms.