Detailed observations of NGC 4151 with IUE - IV. Absorption line spectrum and variability

Abstract

A detailed analysis is presented of the ultraviolet (λλ 1150–3200 Å) absorption spectrum of the NGC 4151 Seyfert nucleus. The IUE data base consisted of high-dispersion ($$\Delta\lambda\sim0.2$$Å) spectra at 5 epochs, and 137 low-dispersion ($$\Delta\lambda\sim4-8$$Å) spectra at 31 epochs from 1978 February to 1980 May, together with further low-dispersion data in 1980–81 with NGC 4151 in a very faint quiescent state. Over 30 UV absorption lines are identified, including resonance and excited fine-structure lines, and absorptions from higher metastable levels. There is a strong unidentified blend at λ 1364/1370 Å. Correlation analyses are presented for 16 strong features in the mean low-dispersion spectra for 31 epochs. This coverage includes a large flare in 1979 May. For the highly ionized species (notably Nv), equivalent widths (W_λ) are correlated with the non-thermal continuum flux. Several low-ion absorptions are perfectly correlated in W_λ amongst themselves; in general they show moderately strong anti-correlations with continuum flux. All absorption tines appear to be weak when the nucleus is very faint. Velocity ranges covered by absorptions are typically −1100 to +100 km s⁻¹ relative to the narrow emission lines, and show no obvious dependence on line excitation. Between faint and bright phases the lines of highly ionized species become slightly less negative in velocity but do not change dramatically in overall profile. Observed line widths at high dispersion, together with a detailed study of SiIV doublet ratios, yield an approximately constant effective b value of ∼500 km s⁻¹, that is, a constant spread in velocity of the ‘assemblage of clouds’ in the absorption region. Thus the absorption-line variations are due largely to changes in the column density (N) of the species absorbing, rather than to changes in the velocity-spread (b value) of the clouds. Our observations suggest a model for the BLR absorption region consisting of large numbers of clouds that are almost optically thin – at variance with the Ferland & Mushotzky models. A lower limit on the total H column density is obtained from those Si and C lines observable in the new UV spectra: N(H)≳10²¹cm⁻², consistent with observed X-ray absorptions. The population of CIII and SiIIIns np³P metastable absorbers is considerable: the present data imply N(CIII,³P)/N(CIV, g) exceeds 10⁻², too high for recombination-dominated population. This result together with observed recombination times and other considerations, points to electron densities in the range 10^8.5 to 10¹⁰ cm⁻³. Considering the likely space densities, the observed range of emission velocities covered, and the lack of any clear time lags in the absorbers' response to continuum flares, the most likely site of absorption is in a shell in the outermost parts of the BLR. Such a shell would have a thickness much less than 1 per cent of its distance from the centre; the mass of the absorption region would be roughly of order 1 M_⊙; and the mass loss rate from the NGC 4151 nucleus (if the loss were episodic rather than continuous) would only be about 10⁻²M_⊙ yr⁻¹.