科研之友
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摘要
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 similar to 2 Angstrom resolution and covered the wavelength range 1150-3300 Angstrom. 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 similar to 10,000 km s(-1)) high-ionization emission lines (HILs) of Ly alpha lambda 1215, C Iv lambda 1549, 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 lambda 2798 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 similar to 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 Ly alpha 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 n are indistinguishable. Moreover, although the Mg II emission-line flux varies on longer timescales (similar to years), the profile shape of the Mg II emission line appears invariant on timescales of a decade. Our spectra clearly show narrow (FWHM similar to 200-500 km s(-1)) intrinsic absorption lines of highly ionized species (e.g., Ly alpha lambda 1215, N v lambda 1240, Si Iv lambda 1397, and C Iv lambda 1549), 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.
- 出版日期1999-10-20
