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The Nature of the Bright ULX X-2 in NGC 3921: A Chandra Position and HST Candidate Counterpart

Jonker, P.G.; Heida, M.; Torres, M.A.P.; Miller-Jones, J.C.A.; Fabian, A.C.; Ratti, E.M.; Miniutti, G.; Walton, D.J.; Roberts, T.P.

The Nature of the Bright ULX X-2 in NGC 3921: A Chandra Position and HST Candidate Counterpart Thumbnail


P.G. Jonker

M. Heida

M.A.P. Torres

J.C.A. Miller-Jones

A.C. Fabian

E.M. Ratti

G. Miniutti

D.J. Walton


We report on Chandra observations of the bright ultraluminous X-ray (ULX) source in NGC 3921. Previous XMM-Newton observations reported in the literature show the presence of a bright ULX at a 0.5–10 keV luminosity of 2 × 1040 erg s−1. Our Chandra observation finds the source at a lower luminosity of ≈8 × 1039 erg s−1; furthermore, we provide a Chandra position of the ULX accurate to 0. 7 at 90% confidence. The X-ray variability makes it unlikely that the high luminosity is caused by several separate X-ray sources. In three epochs of archival Hubble Space Telescope observations, we find a candidate counterpart to the ULX. There is direct evidence for variability between the two epochs of WFPC2 F814W observations with the observation obtained in 2000 showing a brighter source. Furthermore, converting the 1994 F336W and 2000 F300W WFPC2 and the 2010 F336W WFC3 observations to the Johnson U-band filter assuming a spectral type of O7I, we find evidence for a brightening of the U-band light in 2000. Using the higher resolution WFC3 observations, we resolve the candidate counterpart into two sources of similar color. We discuss the nature of the ULX and the probable association with the optical counterpart(s). Finally, we investigate a potential new explanation for some (bright) ULXs as the decaying stages of flares caused by the tidal disruption of a star by a recoiled supermassive black hole. However, we find that there should be at most only one of such systems within z = 0.08.


Jonker, P., Heida, M., Torres, M., Miller-Jones, J., Fabian, A., Ratti, E., …Roberts, T. (2012). The Nature of the Bright ULX X-2 in NGC 3921: A Chandra Position and HST Candidate Counterpart. Astrophysical Journal, 758(1), Article 28.

Journal Article Type Article
Publication Date Oct 10, 2012
Deposit Date Mar 26, 2013
Publicly Available Date Feb 12, 2014
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 758
Issue 1
Article Number 28
Keywords Binaries, Close, X-rays.


Published Journal Article (991 Kb)

Copyright Statement
© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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