NONTHERMAL PAIR MODELS OF THE X-RAY SPECTRAL INDEX VARIABILITY IN NGC 4151

Reports of variability in the X-ray spectral index of AGN have recently become commonplace. In particular the results of an extensive monitoring campaign with EXOSAT and Ginga of the well-studied Seyfert 1.5 galaxy, NGC 4151, have shown that the X-ray photon index varies between approximately 1.35-1.7 and is correlated with the 2-10 keV flux. In this paper it is shown that the flux-index correlation in NGC 4151 can be understood in terms of currently popular 'pair models' of the X-ray emission in AGN which involve inverse Compton scattering of UV photons on relativistic electrons that are monoenergetically injected into the source region. The flux-index correlation can be reproduced by fluctuations in the Lorentz factor of the injected electrons, provided that the total number injected per unit time remains roughly constant and that the mean Lorentz factor is less than approximately 700. Such small Lorentz factors are also required to reproduce the high-energy cut-off in NGC 4151 at MeV energies. A peculiar acceleration mechanism for the electrons may thus be required. The data also constrain the electron compactness to lie in the range approximately 1-50 and the ratio of seed photon compactness to electron compactness to lie between approximately 0.05-1.0. The relevance of the results to other AGN is also discussed.