ST ScI Preprint #1353

NGC 3516: The Long Term Variability of the AGN Emission-Line and Absorption-Line Regions

NGC 3516 displays the strongest intrinsic, blue-shifted ultraviolet (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 years of IUE data. The broad (FWHM ~10,000 km s^-1) high ionization emission lines (HILs) of Ly 1215, CIV 1549, NV 1240, and HeII 1640 showed significant variation (a factor of ~2), while the low ionization lines (LILs) of MgII 2798 and FeII showed no significant variation (less than 7%) within the signal-to-noise of our data. No significant variability was detected in the narrow (FWHM ~1,000 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, blue-ward of line center, a picture generally consistent with a kinematically radial outflowing BLR. Interestingly, the profile shapes of both Ly and CIV are identical to one another, 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 CIV and MgII are indistinguishable. Moreover, although the MgII emission-line flux varies on longer timescales (~years) the profile shape of MgII 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.  Ly 1215, NV 1240, SiIV 1397, and CIV 1549), but we do not observe the higher velocity CIV 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. HST/GHRS spectra obtained of the CIV emission-line region prior to our observing campaign (Crenshaw, Maran & Mushotsky 1998), indicate that the narrow CIV absorption-line doublets are completely saturated. After degrading the HST/GHRS spectra to the resolution of HST/FOS 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.

Status:

Appeared in: The Astrophysical Journal

Affiliations:

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
² Department of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, Scotland, UK
³ Department of Physics Western Michigan University Kalamazoo, MI 49008
⁴ Department of Physics, Astronomy University of Leicester University Road Leicester, LE1 7RH, England, UK
⁵ Nuffield Radio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield, Cheshire, SK11 9DL, England, UK
⁶ Affiliated to the Astrophysics Division of ESA