1. Université de Toulouse, UPS, CESR, 9 Avenue du Colonel Roche, F-31028 Toulouse Cedex 9, France
2. CNRS, UMR5187, F-31028 Toulouse, France
3. Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
4. Present address: Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK.

Correspondence to: Sean A. Farrell^1,2,4Natalie A. Webb^1,2 Correspondence and requests for materials should be addressed to S.A.F. (Email: saf28@star.le.ac.uk) or N.A.W. (Email: natalie.webb@cesr.fr).

Abstract

Ultraluminous X-ray sources are extragalactic objects located outside the nucleus of the host galaxy with bolometric luminosities¹ exceeding 10³⁹ erg s^-1. These extreme luminosities—if the emission is isotropic and below the theoretical (Eddington) limit, where the radiation pressure is balanced by the gravitational pressure—imply the presence of an accreting black hole with a mass of 10²–10⁵ solar masses (). The existence of such intermediate-mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive. Here we report the detection of a variable X-ray source with a maximum 0.2–10 keV luminosity of up to 1.1  10⁴² erg s^-1 in the edge-on spiral galaxy ESO 243-49, with an implied conservative lower limit for the mass of the black hole of 500.

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