NGC 3516: The Long-Term Variability of the Active Galactic Nucleus Emission-Line and Absorption-Line Regions

Details NGC 3516: The Long-Term Variability of the Active Galactic Nucleus Emission-Line and Absorption-Line Regions NGC 3516: The Long-Term Variability of the Active Galactic Nucleus Emission-Line and Absorption-Line Regions

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...524..707g&link_type=abstract

The Astrophysical Journal, Volume 524, Issue 2, pp. 707-731.

Astronomy and Astrophysics

Astronomy

14

Galaxies: Individual: Ngc Number: Ngc 3516, Galaxies: Seyfert, Ultraviolet: Galaxies

Scientific paper

NGC 3516 displays the strongest intrinsic blueshifted UV absorption lines of any Seyfert 1 galaxy. In this paper, we report on UV spectrophotometric monitoring data obtained over a period of 11 months using the Hubble Space Telescope Faint Object Spectrograph (HST/FOS). The observations were at ~2 Å resolution and covered the wavelength range 1150-3300 Å. During the (HST/FOS) observation period, the UV continuum showed a factor of 5 variation, spanning almost the entire range in continuum flux seen in over 15 yr of IUE data. The broad (FWHM~10,000 km s^-1) high-ionization emission lines (HILs) of Lyα λ1215, C IV λ1549, N V λ1240, and He II λ1640 showed significant variation (a factor of ~2), while the low-ionization lines (LILs) of Mg II λ2798 and Fe II showed no significant variation (less than 7%) within the signal-to-noise ratio of our data. No significant variability was detected in the narrow (FWHM~1000 km s^-1) emission line strengths over the course of our campaign. The profile shapes of the HILs also display significant variation, with the largest flux changes occurring in the core of the line, blueward of line center, a picture generally consistent with a kinematically radial outflowing broad-line region. Interestingly, the profile shapes of Lyα and C IV are identical, suggesting a common kinematic origin for these lines. This is supported by an absence of variation in their respective line ratios despite significant flux changes. Perhaps even more surprisingly, at high continuum levels the profile shapes of C IV and Mg II are indistinguishable. Moreover, although the Mg II emission-line flux varies on longer timescales (~years), the profile shape of the Mg II emission line appears invariant on timescales of a decade. Our spectra clearly show narrow (FWHM~200-500 km s^-1) intrinsic absorption lines of highly ionized species (e.g., Lyα λ1215, N V λ1240, Si IV λ1397, and C IV λ1549), but we do not observe the higher velocity C IV absorption-line component seen in the historic (pre-1989) IUE/SWP spectra. The absence of a detectable variation in the narrow absorption line strengths over the course of our observing campaign (11 months) suggests absorption in ions close to their peak in fractional abundance. We find no evidence for a change in the strength of the narrow absorption lines, suggesting that the absorbing gas remains stable on timescales of at least 20 months. The line widths and ionization state of the intrinsic absorbers are found to be consistent with a narrow-line region origin for the absorbing gas.

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