HUT Observations of the Far-Ultraviolet Spectrum of MKN 421

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Session 20 -- Astro 2
Display presentation, Tuesday, June 13, 1995, 9:20am - 6:30pm

[20.02] HUT Observations of the Far-Ultraviolet Spectrum of MKN 421

J.W. Kruk, A.F. Davidsen, G.A. Kriss (JHU)

We observed the BL-Lac object Markarian 421 with the Hopkins Ultraviolet Telescope on two occasions during the Astro-2 space shuttle mission in March 1995. The spectra cover the range from the Galactic Lyman edge at 912 \AA\ to 1850 \AA, with a resolution of 2-4 \AA. The individual spectra have typical signal-to-noise ratios of 10-18 per \AA, and the combined spectrum has a S/N of 14-25 per \AA. There was no sign of changes in incident flux during either observation, but the measured flux increased by 10\% over the 61 hours separating the two pointings. The mean flux was $1.0~\times~10^{-13}$ ergs cm$^{-2}$ s$^{-1}$ \AA$^{-1}$ at 1400 \AA. This is nearly double the historical average flux for MKN 421, and only 30\% below the highest flux recorded. The spectrum itself is well-represented by a single power law over our entire wavelength range, plus absorption by gas in the galactic ISM. The spectral index observed was $\alpha \approx 1.1$ (for $f_\nu~\propto~\nu^{-\alpha}$), and was the same for both pointings. This is somewhat larger than the average value for $\alpha$, and differs from past behavior in that values of $\alpha$ larger than 0.9 - 1.0 have tended to be correlated with lower than average flux.

This research is supported by NASA contract NAS 5-27000 to the Johns Hopkins University.

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