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An optical counterpart to the anomalous X-ray pulsar 4U0142+61

Nature volume 408, pages 689–692 (2000)Cite this article

Abstract

The energy source of the anomalous X-ray pulsars1 (AXPs) is not understood, hence their designation as anomalous. Unlike binary X-ray pulsars, no companions are seen, so the energy cannot be supplied by accretion of matter from a companion star. The loss of rotational energy, which powers radio pulsars, is insufficient to power AXPs. Two models are generally considered: accretion from a large disk left over from the birth process2,3, or decay of a very strong magnetic field (1015 G) associated with a ‘magnetar’4. The lack of counterparts at other wavelengths has hampered progress in our understanding of these objects. Here we report deep optical observations of the field around 4U0142+61, which is the brightest AXP in X-rays. The source has no associated supernova remnant, which, together with its spin-down timescale of ∼105 yr (ref. 5), suggests that it may be relatively old. We find an object with peculiar optical colours at the position of the X-ray source, and argue that it is the optical counterpart. The optical emission is too faint to admit the presence of a large accretion disk, but may be consistent with magnetospheric emission from a magnetar.

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Figure 1: Keck images around the X-ray position of 4U0142+61.
Figure 2: Colour–magnitude and colour–colour diagrams for stars near the position of 4U0142+61.
Figure 3: Energy distribution for 4U0142+61.

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Acknowledgements

We thank D. Kaplan for undertaking the observations at the 60-inch telescope, and acknowledge helpful discussions with B. Paul and F. Verbunt. Model atmospheres for hot white dwarfs were kindly provided by J. Heise. The observations were obtained at the W. M. Keck Observatory on Mauna Kea, Hawaii, which is operated by the California Association for Research in Astronomy. This research made use of the SIMBAD data base. M.H.v.K. is supported by the Royal Netherlands Academy of Science (KNAW). S.R.K.'s research is supported, in part, by grants from the National Science Foundation and the National Aeronautics and Space Administration.

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Authors and Affiliations

  1. Astronomical Institute, Utrecht University, PO Box 80000, Utrecht, 3508 TA, The Netherlands

    F. Hulleman & M. H. van Kerkwijk

  2. Palomar Observatory 105-24, California Institute of Technology, Pasadena, 91125, California, USA

    S. R. Kulkarni

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  1. F. Hulleman
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  3. S. R. Kulkarni
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Correspondence to M. H. van Kerkwijk.

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Hulleman, F., van Kerkwijk, M. & Kulkarni, S. An optical counterpart to the anomalous X-ray pulsar 4U0142+61. Nature 408, 689–692 (2000). https://doi.org/10.1038/35047024

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