Volume 531, July 2011
|Number of page(s)||7|
|Published online||15 June 2011|
The hard X-ray emission of Centaurus A⋆
François Arago Centre, APC, Université Paris Diderot, CNRS/IN2P3, CEA/DSM,
Observatoire de Paris,
13 rue Watt,
Paris Cedex 13,
2 Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
3 CNRS, IRAP, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
4 Centrum Astronomiczne im. M. Kopernika, Rabiańska 8, 87-100 Toruń, Poland
5 Laboratoire AIM – CNRS – CEA/DSM – Université Paris Diderot (UMR 7158), CEA Saclay, DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
Accepted: 21 April 2011
Context. The radio galaxy Cen A has been detected all the way up to the TeV energy range. This raises the question about the dominant emission mechanisms in the high-energy domain.
Aims. Spectral analysis allows us to put constraints on the possible emission processes. Here we study the hard X-ray emission, in order to distinguish between a thermal and a non-thermal inverse Compton process.
Methods. Using hard X-ray data provided by INTEGRAL, we determined the cut-off of the power-law spectrum in the hard X-ray domain (3−1000 keV). In addition, INTEGRAL data are used to study the spectral variability. The extended emission detected in the gamma-rays by Fermi/LAT is investigated using the data of the spectrometre SPI in the 40−1000 keV range.
Results. The hard X-ray spectrum of Cen A shows a significant cut-off at energies keV with an underlying power-law of photon index Γ = 1.73 ± 0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kTe = 206 ± 62 keV within the optically thin corona with Compton parameter . The reflection component is significant at the 1.9σ level with , and a reflection strength R > 0.3 can be excluded on a 3σ level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f3−30 keV = 1.2−9.2 × 10-10 erg cm-2 s-1, NH = 7−16 × 1022 cm-2, and Γ = 1.75−1.87. Extending the cut-off power-law or the Comptonisation model to the gamma-ray range shows that they cannot account for the high-energy emission. On the other hand, a broken or curved power-law model can also represent the data, therefore a non-thermal origin of the X-ray to GeV emission cannot be ruled out. The analysis of the SPI data provides no sign of significant emission from the radio lobes and gives a 3σ upper limit of f40−1000 keV ≲ 1.1 × 10-3 ph cm-2 s-1.
Conclusions. While gamma-rays, as detected by CGRO and Fermi, are caused by non-thermal (jet) processes, the main process in the hard X-ray emission of Cen A is still not unambiguously determined, since it is either dominated by thermal inverse Compton emission or by non-thermal emission from the base of the jet.
Key words: galaxies: active / galaxies: Seyfert / galaxies: individual: Cen A / X-rays: galaxies
© ESO, 2011
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.