XMM–Newton observations of ultraluminous X–ray sources in nearby galaxies

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF–CNR) – Sezione di Bologna (Formerly Istituto TeSRE – CNR.) , via Gobetti 101, 40129, Bologna, Italy
² The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA

Corresponding author: L. Foschini, foschini@bo.iasf.cnr.it

Received: 6 May 2002
Accepted: 24 June 2002

Abstract

An XMM–Newton study of ultraluminous X–ray sources (ULX) has been performed in a sample of 10 nearby Seyfert galaxies. Eighteen ULX have been found with positional uncertainty of about . The large collecting area of XMM–Newton makes the statistics sufficient to perform spectral fitting with simple models in 8 cases. The main results of the present minisurvey strengthen the theory that the ULX could be accreting black holes in hard or soft state. In some cases, the contribution of the ULX to the overall X–ray flux appears to be dominant with respect to that of the active nucleus. In addition, 6 ULX present probable counterparts at other wavelengths (optical/infrared, radio). A multiwavelength observing strategy is required to better assess the nature of these sources.