Hard X-Ray Spectral Feature Detected in the Region of MCG 8-11-11

Details Hard X-Ray Spectral Feature Detected in the Region of MCG 8-11-11 Hard X-Ray Spectral Feature Detected in the Region of MCG 8-11-11

Feb 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...475l..89p&link_type=abstract

Astrophysical Journal Letters v.475, p.L89

Astronomy and Astrophysics

Astronomy

2

Galaxies: Individual Alphanumeric: Mcg 8-11-11, Galaxies: Seyfert, Radiation Mechanisms: Nonthermal, X-Rays: Galaxies, X-Rays: General

Scientific paper

During a balloon flight of the High Energy Astronomical Telescope on 1995 July 29, a region of the sky of 2.dg5 width around the Seyfert galaxy MCG 8-11-11 was studied over the photon energy range 20--300 keV. An excess at the 9 sigma level---well described by a power-law photon spectrum with a slope of alpha = 2.1 +/- 1.2, plus a Gaussian line centered at 112 +/- 6 keV having a flux of (9.0 +/- 3.4) × 10-4 photons cm-2 s-1 and a FWHM of 32 +/- 6 keV---was detected in the energy range 20--160 keV. A contemporary detection of a strong gamma -ray flux in the veto system of the telescope suggests that the observed spectral feature may be the result of double-Compton backscattering of a 511 keV line source incident on an external cloud. If associated with the Seyfert galaxy MCG 8-11-11, the present measurement would provide the first evidence for electron-positron pair annihilation flux emission from an active galactic nucleus. An observation of the Crab Nebula as an in-flight calibration source of the telescope is also decribed.

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