COMPTEL observations of Centaurus A at MeV energies in the years 1991 to 1995

H. Steinle, W. Collmar, R. Diehl, G.G. Lichti, V. Schönfelder, A.W. Strong
and members of the international COMPTEL Collaboration

During the Compton Gamma-Ray Observatory (CGRO) observations made in the years 1991 to 1995, the region on the sky including the radio galaxy Centaurus A was in the wide field-of-view of COMPTEL in 15 pointings of various durations. The analysis of the data shows emission consistent with the position of Cen A and we detected this AGN in two emission states.

The spectra in the COMPTEL energy range 0.75 - 30 MeV are consistent with the Cen A spectra in the energy range 0.05 - 4 MeV from OSSE [1] and the 0.1 - 1 GeV data points from EGRET [2], both instruments also on board CGRO. The left figure shows the intermediate emission state spectrum measured simultaneously with all three instruments. This state was detected in only one (the first) observation. All subsequent observations showed Cen A to be in a low emission state. The spectral form differs between the two observed emission states. A comparison of the spectral energy distribution of the two states is shown in the right figure. The high energy break in the broken power-law fits shifts significantly between the two states and the high energy spectrum gets harder in the low emission state. From the spectra of the combined OSSE, COMPTEL and EGRET data, the gamma-ray luminosity in the energy interval 50 keV to 1 GeV can be derived for the two emission states. Assuming a distance of 3.5 Mpc (H0 = 50 km s-1 Mpc-1; z = 0.0006; q0 = 0.5) and an isotropically emitting source, we derive Lmg = 5 ×1042 erg s-1 and Llg = 3 ×1042 erg s-1 for the intermediate and low emission state, respectively.

Cen A is the nearest active galaxy and the viewing angle with respect to the jet axis is very large ( > 70). The contemporaneous OSSE, COMPTEL, and EGRET spectra stretch the covered energy range in this observations over almost five decades and the spectral shape and the emission detected above 1 MeV puts hard constraints on models for the gamma-ray emission of this AGN. The large viewing angle with respect to the jet axis requires scattering of the high energy photons - usually assumed to be produced by relativistic boosting - into this direction or alternatively the production of high energy photons which are emitted isotropically. Although models exist which, under certain conditions, can produce sufficient high energy emission visible from the side of the jet, they may not be able to fully account for the observations.

[1] Kinzer R.L., Johnson W.N., Dermer C.D., et al., 1995, ApJ 449, 105-118
[2] Nolan P.L., Bertsch D.L., Chiang J., et al., 1996, ApJ 459, 100-109


The full version of this paper has been published in Astronomy & Astrophysics 330, 97-107, 1998.

Also available in postscript .

(Optical picture: Anglo Australian Telescope)

More information on Centaurus A can be found in the dedicated Centaurus A pages.

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