@article{ca93cbf3a0294ea7a87506d700608b43,
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 × 1039 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.",
author = "C. Motch and Pakull, {M. W.} and R. Soria and F. Gris{\'e} and G. Pietrzy{\'n}ski",
note = "Funding Information: Acknowledgements R.S. acknowledges an Australian Research Council Discovery Projects funding scheme (project number DP120102393). F.G. acknowledges support from CNES (CNRS/INSU/CNES contract no.92532) and partly from the Spanish Ministry of Science and Innovation (MICINN) under grant AYA 2010-18080. Support from the Ideas Plus programme of the Polish Ministry of Science is also acknowledged. We thank J. Orosz for providing us with the ELC code. We acknowledge the Swift team for executing our observing programme. This work is based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme IDs 084.D-0881 and 087.D-0602, and uses observations made with the NASA/ESA Hubble Space Telescope (HST) and obtained from the Hubble Legacy Archive,whichisacollaborationbetweenthe SpaceTelescopeScienceInstitute(STScI/ NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). The scientific results reported in this article are based in part on observations made by the Chandra X-ray Observatory and in part on observations obtained with XMM-Newton (OBSIDs 0693760101 and 0693760401), an ESA science mission with instruments and contributions directly funded by ESA member states and NASA. Publisher Copyright: {\textcopyright}2014 Macmillan Publishers Limited. All rights reserved.",
year = "2014",
month = oct,
day = "9",
doi = "10.1038/nature13730",
language = "English (US)",
volume = "514",
pages = "198--201",
journal = "Nature",
issn = "0028-0836",
publisher = "Nature Publishing Group",
number = "7521",
}