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Faint X-ray sources in the core of the globular cluster M28

Author

Listed:
  • R. Danner

    (Mathematics and Astronomy 105-24, California Institute of Technology
    Max-Planck-Institute für extraterrestrische Physik)

  • S. R. Kulkarni

    (Mathematics and Astronomy 105-24, California Institute of Technology)

  • Y. Saito

    (Institute of Space and Astronautical Science, 3-1-1 Yoshinodai)

  • N. Kawai

    (The Institute of Physical and Chemical Research, 2-1 Hirosawa Wako)

Abstract

Globular clusters, the most ancient stellar groups in our galaxy, are known to contain bright X-ray sources, faint X-ray sources and millisecond pulsars. The bright X-ray sources are neutron stars accreting matter from a companion star1, and the millisecond pulsars are believed to be descendants of these sources2. But the origin of the faint X-ray sources remains unclear. Here we report satellite-based X-ray observations of the globular cluster M28 which reveal two faint X-ray sources; an extended source slightly offset from the centre of the cluster, and a point source. The point source pulsates with the same period as a well-known3,4 3-millisecond pulsar in M28. The nature of the extended source is more puzzling, however, and its spatial and spectral properties permit a range of plausible models. We argue that this source is either a collection of low-luminosity accreting neutron-star binaries or a synchrotron nebula powered by a recent outburst of energy from an unknown source. Sensitive optical and X-ray observations should be able to distinguish between these two possibilities.

Suggested Citation

  • R. Danner & S. R. Kulkarni & Y. Saito & N. Kawai, 1997. "Faint X-ray sources in the core of the globular cluster M28," Nature, Nature, vol. 388(6644), pages 751-753, August.
    • Handle: RePEc:nat:nature:v:388:y:1997:i:6644:d:10.1038_41962
    DOI: 10.1038/41962
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