Broad-band gamma-ray and X-ray spectra of NGC 4151 and their   implications for physical processes and geometry
  We study gamma-ray observations of NGC 4151 by GRO/OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June and with ASCA in 1993 May. The spectra are well modeled by thermal Comptonization and a dual neutral absorber. We also find, for the first time for NGC 4151, a Compton-reflection spectral component in the Ginga/OSSE data. The OSSE spectra of those and other observations of NGC 4151 are statistically undistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s. NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/gamma-ray spectrum typical for Seyfert 1s, and the main property distinguishing it from other Seyfert 1s is a large absorbing column of $\sim 10^{23}$ cm$^{-2}$. We find no evidence for a strong, broad and redshifted, Fe K$\alpha$ line component in the ASCA spectrum of 1993 May.   On the other hand, we confirm previous studies in that archival X-ray data do imply strong intrinsic X-ray variability and hardness of the intrinsic spectrum in low X-ray states. An observed softening of the intrinsic X-ray spectrum with the increasing flux implies variability in gamma-rays weaker than in X-rays, which agrees with the 100 keV flux changing only within a factor of 2 in archival OSSE and Granat/SIGMA observations.   The relative hardness of the intrinsic X-ray spectrum rules out the homogeneous hot corona/cold disk model for this source. Instead, the hot plasma has to subtend a small solid angle as seen from the source of UV radiation. If the hot plasma is purely thermal, it consists of electrons rather than $e^+e^-$ pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is nonthermal. 
A. A. Zdziarski, W. N. Johnson and P. Magdziarz