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Astron. Astrophys. 330, 823-832 (1998)


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ROSAT evidence for AGN and superwind activity
in NGC 6240 and NGC 2782

Hartmut Schulz 1, 2, Stefanie Komossa 3, Thomas W. Berghöfer 3, 4 and Berto Boer 1

1 Astronomisches Institut der Ruhr-Universität, D-44780 Bochum, Germany
2 Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA
3 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
4 Space Sciences Laboratory, Berkeley, CA 94720, USA

Received 28 May 1997 / Accepted 20 October 1997

Abstract

We present ROSAT observations of the starburst galaxy NGC 2782 (HRI plus a weak PSPC frame) and the ultraluminous infrared galaxy NGC 6240 (PSPC). The (0.1-2.4) keV spectra of both objects appear similar. However, due to better sampling spectral modeling is only warranted in case of NGC 6240 for which both a single thermal Raymond-Smith model ([FORMULA] keV) or a hybrid model consisting of 80% power-law with the canonical photon index -1.9 plus 20% Raymond-Smith contribution ([FORMULA] keV) lead to good fits. However, the single thermal model turns out to be unlikely because it yields a luminosity of [FORMULA] erg s-1, which is hard to reach in a starburst superwind-scenario. The hybrid model leads to a more moderate luminosity of [FORMULA] erg s-1, of which [FORMULA] erg s-1 can be attributed to shocked superwind gas. We link the remaining [FORMULA] erg s-1 powerlaw to an AGN component because the alternative of inverse-compton scattering of the FIR radiation leads to a too low flux when estimated with available data. The result appears to be consistent with preliminarily announced ASCA observations.

For NGC 2782 we find [FORMULA] (0.1-2.4 keV) = [FORMULA] erg s-1 (within a factor of four) which can be explained by emission from a shocked superwind region with a high preshock density in agreement with earlier optical evidence for an outflowing supershell.

Key words: galaxies: active – galaxies: interactions – galaxies: starburst – galaxies: individual: NGC 2782, NGC 6240 – x-rays: galaxies

Send offprint requests to: H. Schulz

© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998
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