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Optical Counterparts of the Nearest Ultraluminous X-Ray Sources

Gladstone, J.C.; Copperwheat, C.; Heinke, C.O.; Roberts, T.P.; Cartwright, T.F.; Levan, A.J.; Goad, M.R.

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J.C. Gladstone

C. Copperwheat

C.O. Heinke

T.F. Cartwright

A.J. Levan

M.R. Goad


We present a photometric survey of the optical counterparts of ultraluminous X-ray sources (ULXs) observed with the Hubble Space Telescope (HST) in nearby (5 Mpc) galaxies. Of the 33 ULXs with HST and Chandra data, 9 have no visible counterpart, placing limits on their MV of ~ –4 to –9, enabling us to rule out O-type companions in 4 cases. The refined positions of two ULXs place them in the nucleus of their host galaxy. They are removed from our sample. Of the 22 remaining ULXs, 13 have one possible optical counterpart, while multiple are visible within the error regions of other ULXs. By calculating the number of chance coincidences, we estimate that 13 ± 5 are the true counterparts. We attempt to constrain the nature of the companions by fitting the spectral energy distribution and MV to obtain candidate spectral types. We can rule out O-type companions in 20 cases, while we find that one ULX (NGC 253 ULX2) excludes all OB-type companions. Fitting with X-ray irradiated models provides constraints on the donor star mass and radius. For seven ULXs, we are able to impose inclination-dependent upper and/or lower limits on the black holes' mass, if the extinction to the assumed companion star is not larger than the Galactic column. These are NGC 55 ULX1, NGC 253 ULX1, NGC 253 ULX2, NGC 253 XMM6, Ho IX X-1, IC342 X-1, and NGC 5204 X-1. This suggests that 10 ULXs do not have O companions, while none of the 18 fitted rule out B-type companions.


Gladstone, J., Copperwheat, C., Heinke, C., Roberts, T., Cartwright, T., Levan, A., & Goad, M. (2013). Optical Counterparts of the Nearest Ultraluminous X-Ray Sources. Astrophysical Journal Supplement, 206(2), Article 14.

Journal Article Type Article
Publication Date Jun 1, 2013
Deposit Date Sep 11, 2013
Publicly Available Date Feb 12, 2014
Journal Astrophysical Journal Supplement
Print ISSN 0067-0049
Electronic ISSN 1538-4365
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 206
Issue 2
Article Number 14
Keywords Accretion, Accretion disks, Binaries, Black hole physic, X-rays.


Published Journal Article (1.5 Mb)

Copyright Statement
© 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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