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Astron. Astrophys. 339, 345-354 (1998)

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The ROSAT view of NGC 1365 and the luminous highly variable off-nuclear X-ray source NGC 1365-X1

Stefanie Komossa 1 and Hartmut Schulz 2

1 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
2 Astronomisches Institut, Ruhr-Universität, D-44780 Bochum, Germany

Received 27 May 1998 / Accepted 28 August 1998


We present an X-ray spectral and spatial analysis of the composite starburst/Seyfert galaxy NGC 1365. Excellent fits of the ROSAT PSPC spectrum are obtained by combining a soft thermal component with a hard powerlaw. The hard component may be either seen directly or can be explained by scattering of the AGN powerlaw at circumnuclear warm high-column-density gas. A compilation of the multi-wavelength properties of NGC 1365 and comparison with hard X-ray selected AGNs shows that the hard component of NGC 1365 is too faint compared to its broad Balmer line components challenging simple unified models. According to analytical estimates, supernova driven outflow can fully account for the X-ray luminosity in the Raymond-Smith component if the observed IR emission is mainly provided by the central starburst. We do not find obvious optical counterparts for three faint PSPC sources south of the nucleus. In particular, there is no coincidence with the two supernovae reported in NGC 1365.

With the ROSAT HRI data, we have precisely located the extraordinary southwest X-ray source NGC 1365-X1 which falls on one of the subordinate spiral arms. The source is found to be highly variable (a factor [FORMULA] 10) on the timescale of months. Intrinsic to NGC 1365, its huge luminosity makes it exceptional among stellar X-ray sources. At present, the most likely interpretation seems to be an ultra-powerful X-ray binary.

Key words: galaxies: active – galaxies: starburst – galaxies: individual: NGC 1365 – X-rays: galaxies

Send offprint requests to: Stefanie Komossa, (skomossa@xray.mpe.mpg.de)

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998