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J.F. Kartje, A. Konigl, C.-Y. Hwang, and S. Bowyer
We carried out a spectroscopic observation of the BL Lacertae object Mrk 421 with the Extreme Ultraviolet Explorer satellite over a 11-day period in late April/early May 1995 (~242 ksec useful time). During this period the source underwent a flare that was detected also in X-rays and TeV gamma-rays. The best continuous coverage of the flare was obtained by EUVE, which resolved the smooth rise and fall of the flux, measuring the variability of as much as a factor of ~1.5 over a span of ~2 days. The detected spectrum extended from ~65-100 A and could be fit with a power law of energy spectral index alpha_EUV ~= 3.5 +/- 0.8 for the measured Galactic hydrogen column density. The EUV spectrum is much steeper than the mean 1.5 - 5.0 keV X-ray spectrum, alpha_X = 1.63 +/- 0.02, measure simultaneously by the ASCA satellite. Furthermore, a simple power-law fit to the observed fluxes at 85 A and 1.5 keV (excluding the data from the first 3 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 A. 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 ~5E+21 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 v_i ~= 0.05c to f_v ~= 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 clouds' physical parameters and chemical composition.
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