Broad-band γ-ray and X-ray spectra of NGC 4151 and their implications for physical processes and geometry.
Abstract
We study γ-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 α∼0.8 and the plasma temperature is ∼60 keV for both observations, conditions which imply an optical depth of ∼1. The X-ray spectral index is within the range, α∼=0.95±0.15, observed from other Seyfert 1s. Also, the OSSE spectra of those and other observations of NGC 4151 are statistically indistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s. Thus, NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/γ-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. We find no evidence for a strong, broad and redshifted Fe Kαline component in the ASCA spectrum of the 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 γ-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 the e± pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is non-thermal. We consider the implications of their observations for physical processes in NGC 4151. They study Comptonization in the X-ray/γ-ray source as well as the presence of e± pairs and non-thermal acceleration. From that, the authors obtain strong constraints on the parameters and geometry of the central region of NGC 4151.- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 1996
- DOI:
- 10.1093/mnras/283.1.193
- arXiv:
- arXiv:astro-ph/9607015
- Bibcode:
- 1996MNRAS.283..193Z
- Keywords:
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- Seyfert Galaxies: X-Ray Spectra;
- Seyfert Galaxies: Gamma-Ray Spectra;
- Seyfert Galaxies: Radiation Mechanisms;
- galaxies: individual: NGC4151 -- galaxies: Seyfert -- gamma-rays: observations -- gamma-rays: theory X-rays: galaxies;
- Astrophysics
- E-Print:
- 15 pages, 8 Postscript figures, MNRAS accepted