Discovery of Compton reflection in NGC 4151

Pawel Magdziarz¹, Andrzej A. Zdziarski²

¹Astronomical Observatory, Jagiellonian University, Cracow, Poland
²Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland

We study the gamma-ray observation of NGC 4151 by GRO/OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June. The spectrum is 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. When reflection is taken into account, the intrinsic X-ray energy spectral index is alpha ~ 0.8 and the plasma temperature is ~ 60 keV. The X-ray spectral index is within the range, alpha 0.95 ± 0.15, observed from other Seyfert 1s. Also, the OSSE spectrum of NGC 4151 is statistically undistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s. Thus, NGC 4151 observed in 1991 June 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 ~10²³ cm^-2.