title: A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source.
abstract: Most ultraluminous X-ray sources have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3 × 10(39) ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3 kiloelectronvolts) and a characteristic downturn in their spectra above about 5 kiloelectronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes or as emission from stellar-mass black holes accreting above their Eddington limit, analogous to some Galactic black holes at peak luminosity. Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole. Here we report that the X-ray source P13 in the galaxy NGC 7793 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10(40) ergs per second can be explained by supercritical accretion onto massive stellar-mass black holes.
authors.affiliation: Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France.
authors.forename: C
authors.initials: C
authors.lastname: Motch
authors.affiliation: Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France.
authors.forename: M W
authors.initials: MW
authors.lastname: Pakull
authors.affiliation: International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
authors.forename: R
authors.initials: R
authors.lastname: Soria
authors.affiliation: 1] Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France [2] Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain [3] Universidad de La Laguna, Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain [4] Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, Iowa 52242, USA.
authors.forename: F
authors.initials: F
authors.lastname: Grisé
authors.affiliation: 1] Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Octava Region, Chile [2] Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
authors.forename: G
authors.initials: G
authors.lastname: Pietrzyński