The galaxy Centaurus A is one of the favorite extragalactic objects of X-ray and gamma-ray astronomers. The reason is, that it is rather close by at a distance of around 4 Mpc or about 12 million light years. And, the galaxy harbors a super massive black hole which accretes matter from its surrounding: This engine produces a large amount of radiation, especially in the X-ray and gamma-ray domain. The central part of Cen A is enshrouded by gas and dust, and thus the high-energy photons are at the same time the only way to peek into the center. In addition, Cen A displays jets originating in the central engine, which shoot matter far out into the surrounding galaxy, up to distances of 50,000 light years from the core. So, when we look at the heart of Cen A in the X-ray and gamma-rays, do we actually see emission coming from this highly energetic jets, or do we observe the matter which is heated during the accretion process?
The gamma-ray satellite Fermi has detected both, extended and core emission, at energies higher than 100 MeV from Cen A. INTEGRAL was now used in order to clarify what drives the emission process in the X-ray and soft gamma-ray range. Its imager IBIS and spectrograph SPI are especially well suited for this task. They are the only instruments today, which can track the spectrum of Cen A all the way from the keV into the MeV region.
The top panel shows the INTEGRAL/SPI map (40 - 1000 keV). The map is overlaid with contours of the radio emission of Cen A, which shows the extended structure arising from the jets. Although SPI indicates some extended emission in the north of the core, the significance of this blob is too low to claim a detection, and using this as an upper limit for the hard X-ray emission, the results agree with what has been seen at other wavelengths.
In the lower panel, the superb spectrum of Cen A is shown, as it appears in the different INTEGRAL instruments. Differences, for example between the two JEM-X spectra (black and red), indicate some flux and spectral variability. In addition, the gamma-ray data from Fermi/LAT are displayed. The whole spectrum appears to connect smoothly. The big gap in the MeV range remains, as since the decommissioning of the Compton Gamma-Ray Observatory (CGRO) in 2000 we do not have any instrument in space, which can detect photons here.
Careful modeling shows, that the hard X-ray data are consistent with an origin in the accretion disk of the super massive black hole, but also the jet might contribute here. Most likely, it is a mix of both components. INTEGRAL shows some aspects of Cen A, which are typical for jet emission, like the smooth spectrum all the way up to the MeV range and the possible connection to the gamma-rays. But other aspects point toward accretion processes, like the iron line at 6.4 keV and the rather persistent X-ray spectrum.
With its brightness and all the special features, Cen A will keep astronomers busy for some time to come. And the X-ray spectrum up to the MeV range taken by INTEGRAL will be the best available for many years.
Reference and more details:
V. Beckmann, P. Jean, P. Lubinski, S. Soldi and R. Terrier, A&A 531, A70 (2011)
