Accurate positions for the ultraluminous X-ray sources NGC 7319-X4 and NGC 5474-X1 and limiting magnitudes for their optical counterparts

Authors

  • M. Heida,

    Corresponding author
    1. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
    2. Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, the Netherlands
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  • P. G. Jonker,

    1. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
    2. Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, the Netherlands
    3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
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  • M. A. P. Torres,

    1. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
    2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
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  • S. Mineo

    1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
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E-mail: m.heida@astro.ru.nl

ABSTRACT

In this paper, we report accurate Chandra positions for two ultraluminous X-ray sources (ULXs): NGC 7319-X4 at Right Ascension (RA) = 339bsl00064029 17(2), Declination (Dec.) = 33bsl00064974 76(2) and NGC 5474-X1 at RA = 211bsl00064248 59(3), Dec. = 53bsl00064635 84(3). We perform bore-sight corrections on the Chandra X-ray satellite observations of these sources to get these accurate positions of the X-ray sources and match these positions with archival optical data from the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope. We do not find the optical counterparts; the limiting absolute magnitudes of the observations in the WFPC2 standard magnitude system are B=−7.9, V=−8.7 and I=−9.3 for NGC 7319-X4 and U=−6.4 for NGC 5474-X1. We report on the X-ray spectral properties and find evidence for X-ray variability in NGC 5474-X1. Finally, we briefly discuss several options for the nature of these ULXs.

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