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 the Gamma Ray Observatory (GRO)/OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June and ASCA in 1993 May. The spectra are well modelled 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. When reflection is taken into account, the intrinsic X-ray energy spectral index is alpha similar to 0.8 and the plasma temperature is similar to 60 keV for both observations, conditions which imply an optical depth of similar to 1. The X-ray spectral index is within the range, alpha similar to 0.95+/-0.15, observed from other Seyfert Is. Also, the OSSE spectra of those and other observations of NGC 4151 are statistically indistinguishable from the average OSSE spectrum of radio-quiet Seyfert Is. Thus, NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/gamma-ray spectrum typical for Seyfert Is, and the main property distinguishing it from other Seyfert Is is a large absorbing column of similar to 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. Also, the Compton-reflection component in the Ginga/OSSE spectrum is a few times too small to account for the strength of the broad/redshifted line reported elsewhere to be found in this and other ASCA spectra of NGC 4151. 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 that variability in gamma-rays is 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 disc 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(+/-) pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is non-thermal.