The active galaxy NGC 4151: Archetype or exception?

Abstract

The Seyfert galaxy NGC 4151 harbors in its nucleus the most intensively studied AGN (Active Galactic Nucleus). Among the brightest AGN (in apparent luminosity) it is the most widely variable and the variations of its ultraviolet and X-ray spectrum have been studied on time scales ranging from hours to decades. These observations have formed the basis of methods and models which have been found to generally apply to broad emission line AGN: the rich and complex relation between the X-ray and UV variations, the comptonization model of the X-ray spectrum from medium X-ray to $\gamma$ -rays, the reverberation mapping, the stratification in velocity and physical conditions of the gas in the broad line region, and a method to estimate the black hole mass from emission line variability. The large barred spiral which hosts this nucleus has been extensively studied especially in the central region. Inflow of gas along the $x_1$ and possibly also the $x_2$ orbits have been detected, but since the accretion disk is not in the galactic plane (as evidenced by the significant angle separating the radio axis and the rotation axis of the galaxy) the incoming gas seen on kpcs scale must, as it flows further inward, move out of the galactic plane, along trajectories which are entirely unknown. There is more to learn on NGC 4151. In fact, the best is yet to come. Three avenues of investigation appear particularly promising: 1) The variations in flux and spectral shape of the X-ray continuum and its relationship with the UV variations are the key to understanding the specifics of the Comptonization model. Progress on this point will come from repeated simultaneous observations of the UV spectrum and of the entire X-ray and $\gamma$ -ray spectrum. This will also give insights on the structure of the disk in the last stable orbits, the formation and structure of the corona and in the end, the process of energy production. Exciting results on these topics are expected in the near future from Chandra-AXAF, XMM and INTEGRAL. The Chandra and XMM (which have short energy range) main contributions will, however, be line diagnostics and for Chandra, imaging of the soft diffuse emission. 2) The search for the gas inflow which merges into and/or forms the torus could finally be successful. Several powerful approaches are possible: observing molecular lines in emission with millimeter arrays of increasing baseline and collecting area; using the nuclear radio structure as background source to observe free-free and atomic or molecular lines in absorption. 3) The observations of NGC 4151 during a state of deep minimum will provide a unique oportunity to observe the X-ray spectrum of a Seyfert 1 nucleus at epochs of very low accretion rate, to identify the nature of the narrow variable lines, to determine the stellar population of a currently active nucleus, and measure the mass of the black hole from the stellar lines. NGC 4151 at minimum states should be a target of opportunity for all space missions. In addition, observations on time scales of 10 years or more, especially following a deep minimum, will allow one to map emitting regions of size up to $\sim$ 1pc, thereby overlapping with the linear scale directly mapped with large radio telescopes.