NGC 3516: The long-term variability of the active galactic nucleus emission-line and absorption-line regions

NGC 3516 displays the strongest intrinsic blueshifted UV absorption lines o f any Seyfert 1 galaxy. In this paper, we report on UV spectrophotometric m onitoring data obtained over a period of 11 months using the Hubble Space T elescope Faint Object Spectrograph (HST/FOS). The observations were at simi lar to 2 Angstrom resolution and covered the wavelength range 1150-3300 Ang strom. During the (HST/FOS) observation period, the UV continuum showed a f actor of 5 variation, spanning almost the entire range in continuum flux se en in over 15 yr of IUE data. The broad (FWHM similar to 10,000 km s(-1)) h igh-ionization emission lines (HILs) of Ly alpha lambda 1215, C Iv lambda 1 549, N v lambda 1240, and He II lambda,1640 showed significant variation (a factor of similar to 2), while the low-ionization lines (LILs) of Mg II la mbda 2798 and Fe II showed no significant variation (less than 7%) within t he signal-to-noise ratio of our data. No significant variability was detect ed in the narrow (FWHM similar to 1000 km s(-1)) emission line strengths ov er 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 prof ile shapes of Ly alpha and C Iv are identical, suggesting a common kinemati c origin for these lines. This is supported by an absence of variation in t heir respective line ratios despite significant flux changes. Perhaps even more surprisingly, at high continuum levels the profile shapes of C Iv and Mg n are indistinguishable. Moreover, although the Mg II emission-line flux varies on longer timescales (similar to years), the profile shape of the M g II emission line appears invariant on timescales of a decade. Our spectra clearly show narrow (FWHM similar to 200-500 km s(-1)) intrinsic absorptio n lines of highly ionized species (e.g., Ly alpha lambda 1215, N v lambda 1 240, Si Iv lambda 1397, and C Iv lambda 1549), but we do not observe the hi gher velocity C Iv absorption-line component seen in the historic (pre-1989 ) IUE/SWP spectra. The absence of a detectable variation in the narrow abso rption line strengths over the course of our observing campaign (11 months) suggests absorption in ions close to their peak in fractional abundance. W e find no evidence for a change in the strength of the narrow absorption li nes, suggesting that the absorbing gas remains stable on timescales of at l east 20 months. The line widths and ionization state of the intrinsic absor bers are found to be consistent with a narrow-line region origin for the ab sorbing gas.