SIGMA observation of a steep spectral shape in NGC 4151 above 35 keV

Details SIGMA observation of a steep spectral shape in NGC 4151 above 35 keV SIGMA observation of a steep spectral shape in NGC 4151 above 35 keV

Mar 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992a%26a...256l..38j&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 256, no. 2, p. L38-L40.

Astronomy and Astrophysics

Astronomy

62

Active Galactic Nuclei, Astronomical Spectroscopy, Seyfert Galaxies, Spaceborne Astronomy, Background Radiation, Diffuse Radiation

Scientific paper

The SIGMA telescope on board the GRANAT satellite accumulated 82 hours of observation on the Seyfert galaxy NGC 4151 during 1990. The average photon spectrum obtained by combining all data is unusually steep (alpha about 3) and difficult to reconcile both with a previous detection of a few MeV gamma-ray excess in 1977 and a single power law mean galaxy spectrum of alpha about 1.5 in the 2 - 300 keV band, unless strong spectral variability is present above 50-100 keV. Further, its extrapolation to tens of MeV energies in order to meet the SAS-2 and COS-B upper limits does not require a break or rollover at high energies. On the contrary, comparison of SIGMA data with the observed range of soft X-ray spectra indicates the need for a break at E about 50 keV. Thus our result, which is similar to a previous spectral measurement of the HEAO-A4 instrument in June 1978, unambiguously establishes the existence, at least occasionally, of a steep hard X-ray spectrum in NGC 4151. Consideration of this result in the light of previous high energy measurements underlines the complex and variable nature of the NGC 4151 spectrum.

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