The Far-Ultraviolet Spectrum of NGC 4151 as Observed with the Hopkins Ultraviolet Telescope on Astro-2

Details The Far-Ultraviolet Spectrum of NGC 4151 as Observed with the Hopkins Ultraviolet Telescope on Astro-2 The Far-Ultraviolet Spectrum of NGC 4151 as Observed with the Hopkins Ultraviolet Telescope on Astro-2

Mar 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...186.1102k&link_type=abstract

American Astronomical Society, 186th AAS Meeting, #11.02; Bulletin of the American Astronomical Society, Vol. 27, p.827

Astronomy and Astrophysics

Astronomy

Scientific paper

We observed NGC 4151 on six separate occasions at intervals of one to three days using the Hopkins Ultraviolet Telescope during the flight of Astro-2 aboard the space shuttle Endeavour in March 1995. The far-ultraviolet spectra cover the spectral range from the interstellar cutoff at 912 Angstroms to 1840 Angstroms with a spectral resolution of 2--4 Angstroms. Integrations of 300--1300 s yielded spectra with mean signal-to-noise ratios of 12--25 per Angstroms; the mean spectrum has a S/N of ~ 30. Slight variations are apparent among the six recorded spectra, particularly in the absorption line strengths, but comparison of the mean spectrum to that obtained during the flight of Astro-1 in December 1990 shows profound differences. The continuum, 5.0 times 10(-13) erg cm(-2) s(-1) Angstroms(-1) at 1450 Angstroms, is five times brighter, nearly the brightest UV flux ever observed for NGC 4151. The far-UV transparency near the Lyman limit has changed from optically thick to optically thin. We infer a neutral hydrogen column of ~ 3 times 10(17) cm(-2) broadened by a Doppler parameter of ~ 250 km s(-1) . All high-ionization absorption lines have strengthened considerably --- S VI lambda lambda 933,945, C III lambda lambda 977, O VI lambda lambda 1032,1038, N V lambda lambda 1239,1243, Si IV lambda lambda 1394,1403, and C IV lambda lambda 1548,1551. The Si IV doublet is observed at its optically thin limit. The high velocity of the absorbing neutral hydrogen, the change in transparency from Astro-1 to Astro-2, and the high ionization lines suggest a high temperature (few times 10(6) K), photoionized absorbing medium, possibly related to the ``warm" absorbers seen with moderate resolution X-ray telescopes. This work was supported by NASA contract NAS 5-27000 to the Johns Hopkins University.

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