The extreme ultraviolet spectrum of the bl lacertae object markarian 421

We carried out a spectroscopic observation of the BL Lacertae object Mrk 421 with the Extreme Ultraviolet Explorer (EUVE) over an 11 day period in 1995 late April to early May (∼242 ks useful time). During this period, the source underwent a flare that was detected also in X-rays and TeV γ-rays. The best continuous coverage of the flare was obtained by EUVE, which resolved the smooth rise and fall of the flux, measuring a variability of as much as a factor of ∼1.5 over a span of ∼2 days. The detected spectrum extended from γ65 to 100 Å and could be fitted with a power law of energy spectral index α_EUV ≈ 3.5 ± 0.8 for the measured Galactic hydrogen column density. The EUV spectrum is much steeper than the mean 1.5-7.5 keV X-ray spectrum, α_x = 1.63 ± 0.02, measured simultaneously by the Advanced Satellite for Cosmology and Astrophysics. Furthermore, a simple power-law fit to the observed fluxes at 85 Å and 1.5 keV (excluding the data from the first 4 days, the time of maximum variability) significantly overestimates the flux at the shortest detected EUV wavelengths. These two findings imply that strong absorption is occurring between ∼65 and ∼75 Å. Such absorption is quite similar to that detected previously in our observation of the BL Lacertae object PKS 2155-304. We demonstrate that this absorption can be attributed to a superposition of Doppler-smeared absorption lines originating in high-velocity, QSO-type nuclear clouds of total column density ∼5 × 10²¹ cm^-2 that are ionized by the beamed continuum of the associated relativistic jet. We identify the lines as mostly L- and M-shell transitions of Mg and Ne. The data suggest that the velocity range spanned by the clouds is relatively small (from ν_i ≈ 0.05c to ν_f ≈ 0.1c). We find that such a range is consistent with a scenario in which the clouds are initially accelerated to v_i by a magnetized outflow from a nuclear accretion disk, with radiation pressure further accelerating them to v_f after they enter the beamed emission cone of the jet. We also compute the expected cloud absorption lines in the UV and soft X-ray regimes and use these results to constrain the physical parameters of the clouds and their chemical composition.