Four Years of Extreme Ultraviolet Observations of Markarian 421. I. Spectral Analysis

We analyzed the ~950 ks of spectroscopic data accumulated by the Extreme Ultraviolet Explorer (EUVE) satellite between 1994 and 1997 for the BL Lacertae object Markarian 421. The EUV spectrum is well detected in the 70-110 Å (112-177 eV) range and can be fitted by a power-law model plus an absorption feature in the ~71-75 Å range. Previous studies of EUV absorption features in Mrk 421 and in the other EUV bright BL Lac object, PKS 2155-304, explain this absorption feature as a superposition of Doppler-smeared absorption lines (mainly L- and M-shell transitions of Mg and Ne) originating in high-velocity gas clouds ionized by the beamed continuum of the associated relativistic jet. We show that, for example, Fe IX L could also be a possibility consistent with the marginal detection of oxygen absorption lines in the X-ray range. However physical models are highly sensitive to the assumptions on the photoionizing continuum and the surrounding gas.