Soft γ-ray emission from the region of MCG8 –11 –11

Abstract

The galaxy MCG8 –11 –11 inside the 0.093 square degrees error-box of the X-ray source 2A0551 + 466 (ref. 1), was recently found to be a Type 1 Seyfert galaxy². A 40-arcs error-box from the A3 experiment on HEAO1 has confined the X-ray emission to the nucleus or bar of this 14-m_v galaxy³. The redshift is z = 0.0205 (ref. 2), corresponding to a distance of 123 Mpc (H₀ = 50 km s⁻¹ Mpc⁻¹). The 2–10 keV luminosity ranges from 5 × 10⁴³erg s⁻¹ to 1.2 × 10⁴⁴erg s⁻¹ (refs 1,3). Hard X-ray emission above 20 keV from the region of MCG8 –11 ‐11 has been recently reported⁴, and the A2 experiment on HEAO 1 has provided for the first time the emission spectrum of this object up to 30 keV (ref. 5). During a balloon flight on 30 September 1979 from Palestine, Texas of the Milan/Southampton (MISO) low-energy γ-ray telescope, an excess at the 3.9σ level in the flux above 90 keV was detected from a region of the sky containing MCG8 –11 –11. The emission spectrum, evaluated in the energy range 0.02–19 MeV, has similar spectral characteristics to those observed with the same telescope and associated with the Seyfert galaxy NGC4151 (ref. 6). Both spectra show evidence for a high-energy cutoff close to 3 MeV and this feature could be interpreted on the basis of the ‘Penrose–Compton scattering’ process⁷. A large amount of the observed low-energy γ-ray diffuse background⁸ could be produced by a few per cent of the X-ray emitting Seyfert galaxies having a γ-ray luminosity comparable with that observed from the region of NGC4151 or MCG8 –11 –11.