The ultra-compact broad emision line region in NGC 4151.

Abstract

New observations of the UV continuum and emission lines of a Seyfert I galaxy are presented in which the time resolution is adequate to describe the character of variability. Using the IUE satellite, the nucleus of NGC 4151 was observed with a mean interval of 3.4 d during 2 months. The flux of the continuum and of most emission lines underwent large variations, going through two large maxima and one deep minimum. The ratio of maximum to minimum flux was 3.1 for the continuum a 1455 a but only 1.24 at 5000 A. The UV continuum below 1900 a underwent small but significant spectral variations ({DELTA}α = 0.9). These variations took the form of a abrupt 'hardening' which preceded a major outburst by about 12 d. The degree of variability of a given line depends on the strength and the extension of its wings; the FWZI of the C IV λ 1549, Mg II λ2798 and C III] λ 1909 lines are 30 000, 20 000 and 3500 km s^-1^, respectively, while their fractional variations are 0.22, 0.12 and ~0. Apart from C III] λ1909 which did not vary, the emission line variations correlate extremely well with those of the continuum, if allowance is made for a small systematic delay. Within the uncertainty, the delay is the same for the C IV λ1549 and the Mg II λ2798 line: {DELTA} T = 4 +/- 3 d. The absence of a response of the lines on time-scales longer than ~10 d further implies that the transfer functions of the lines are very narrow, therefore suggesting that most of the gas is concentrated at small radial distances ~4 light-days from the ionizing source. The fractional variation of the C IV λ1549 wings (|V| > 3000 km s^-1^), 0.34, is much larger than that of the line core (|V| < 3000 km s^-1^), 0.12. The same pattern is present, though less pronounced, in the Mg II λ2798 line as well. Moreover, the response of the core of the C IV λ1549 line to the continuum variations is delayed by 3.2 +/- 3 d with respect to the response of its wings. This pattern strongly suggests that the volocity dispersion of the gas increases toward small radial distances from the centre. The simultaneity of the variations in the blue and in the red wing, of both C IV and Mg II, rules out the possibility that radial motions dominate the velocity field of the gas in the inner BLR of NGC 4151. The C IV/C III] and C IV/Mg II flux ratios, especially during an outburst, are much higher in NGC 4151 than in an average AGN. Hence, existing photoionization models are not appropriate to describe the inner BLR in NGC 4151 and their predictions concerning its size are irrelevant. In the wings, C IV/C III] > 50 and C IV/Mg II ~10 on the average, suggesting that the densities and the ionization parameter of the gas in the inner BLR are very large.