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Discovery of a red supergiant counterpart to RX J004722.4-252051, a ULX in NGC 253

Heida, M.; Torres, M.A.P.; Jonker, P.G.; Servillat, M.; Repetto, S.; Roberts, T.P.; Walton, D.J.; Moon, D.-S.; Harrison, F.A.

Discovery of a red supergiant counterpart to RX J004722.4-252051, a ULX in NGC 253 Thumbnail


M. Heida

M.A.P. Torres

P.G. Jonker

M. Servillat

S. Repetto

D.J. Walton

D.-S. Moon

F.A. Harrison


We present two epochs of near-infrared spectroscopy of the candidate red supergiant counterpart to RX J004722.4–252051, a ULX in NGC 253. We measure radial velocities of the object and its approximate spectral type by cross-correlating our spectra with those of known red supergiants. Our VLT/X-shooter spectrum is best matched by that of early M-type supergiants, confirming the red supergiant nature of the candidate counterpart. The radial velocity of the spectrum, taken on 2014 August 23, is 417 ± 4 km s−1. This is consistent with the radial velocity measured in our spectrum taken with Magellan/MMIRS on 2013 June 28, of 410 ± 70 km s−1, although the large error on the latter implies that a radial velocity shift expected for a black hole of tens of M⊙ can easily be hidden. Using nebular emission lines we find that the radial velocity due to the rotation of NGC 253 is 351 ± 4 km s−1 at the position of the ULX. Thus the radial velocity of the counterpart confirms that the source is located in NGC 253, but also shows an offset with respect to the local bulk motion of the galaxy of 66 ± 6 km s−1. We argue that the most likely origin for this displacement lies either in a SN kick, requiring a system containing a ≳ 50 M⊙ black hole, and/or in orbital radial velocity variations in the ULX binary system, requiring a ≳ 100 M⊙ black hole. We therefore conclude that RX J004722.4–252051 is a strong candidate for a ULX containing a massive stellar black hole.


Heida, M., Torres, M., Jonker, P., Servillat, M., Repetto, S., Roberts, T., …Harrison, F. (2015). Discovery of a red supergiant counterpart to RX J004722.4-252051, a ULX in NGC 253. Monthly Notices of the Royal Astronomical Society, 453(4), 3510-3518.

Journal Article Type Article
Acceptance Date Aug 8, 2015
Publication Date Nov 11, 2015
Deposit Date Sep 16, 2015
Publicly Available Date Sep 22, 2015
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 453
Issue 4
Pages 3510-3518
Keywords Supergiants, Infrared: stars, X-rays: binaries, X-rays: individual: RX J004722.4-252051.
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Published Journal Article (3.1 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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