XMM-Newton observations of NGC 253: Resolving the emission components in the disk and nuclear area ^*

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany
² Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
³ Columbia Astrophys. Lab. and Dept. of Phys., Columbia Univ., 550 W. 120th St., New York, NY 10027, USA
⁴ NIS-2, Space and Remote Sensing Sciences, MS D436 Los Alamos National Lab., Los Alamos, NM 87545, USA
⁵ Mullard Space Science Lab., University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁶ Istituto TeSRE/CNR, Via Gobetti 101, 40129 Bologna, Italy
⁷ SOC, Villafranca, Apartado 50727, 28080 Madrid, Spain
⁸ DAPNIA/Service d'Astrophysique, Bât. 709, l'Orme des Merisiers, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
⁹ School of Physics & Astronomy, University Birmingham, Birmingham B15 2TT, UK
¹⁰ Department of Physics, Carnegie Mellon University, USA
¹¹ Department of Physics, University of California, Santa Barbara, CA 93106, USA

Corresponding author: W. Pietsch, wnp@mpe.mpg.de

Received: 2 October 2000
Accepted: 27 October 2000

Abstract

We describe the first XMM-Newton observations of the starburst galaxy NGC 253. As known from previous X-ray observations, NGC 253 shows a mixture of extended (disk and halo) and point-source emission. The high XMM-Newton throughput allows a detailed investigation of the spatial, spectral and variability properties of these components simultaneously. We characterize the brightest sources by their hardness ratios, detect a bright X-ray transient ~70´´SSW of the nucleus, and show the spectrum and light curve of the brightest point source (~30´´S of the nucleus, most likely a black-hole X-ray binary, BHXRB). The unresolved emission of two disk regions can be modeled by two thin thermal plasma components (temperatures of ~0.13 and 0.4 keV) plus residual harder emission, with the lower temperature component originating from above the disk. The nuclear spectrum can be modeled by a three temperature plasma (~0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus, as no non-thermal component, that would point at a significant contribution from an active nucleus (AGN), is needed. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 yr^-1. The unprecedented combination of RGS and EPIC also sheds new light on the emission of the complex nuclear region, the X-ray plume and the disk diffuse emission. In particular, EPIC images reveal that the limb-brightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured. The plume spectrum can again be modeled by a three temperature thermal plasma containing the two low temperature nuclear components (though less absorbed) plus an unabsorbed 0.15 keV component similar to the disk spectra. This points to new interpretations as to the make up of the starburst-driven outflow.