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.
This is a preview of subscription content, access via your institution
Access options
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Buy this article
- Purchase on Springer Link
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
References
Lewin, W. H. G., van Paradijs, J. & van den Heuvel, E. (eds X-ray Binaries(Cambridge Univ. Press, (1995)).
Alpar, M. A., Cheng, A. F., Ruderman, M. A. & Shaham, J. Anew class of radio pulsars. 300, 728–730 (1982).
Lyne, A. G.et al. The discovery of a millisecond pulsar in the globular cluster M28. Nature 328, 399–401 (1987).
Saito, Y.et al. Detection of magnetospheric X-ray pulsations from the millisecond pulsar B1821−24. Astrophys. J. 477, L37–L40 (1997).
Trumper, J. ROSAT. Phys. Scripta T 7, 209–215 (1984).
Johnston, H. M., Verbunt, F. & Hasinger, G. ROSAT PSPC observations of globular clusters. Astron. Astrophys. 289, 763–774 (1994).
Danner, R., Kulkarni, S. R. & Thorsett, S. E. ROSAT observations of six millisecond pulsars. Astrophys. J. 436, L153 –L156 (1994).
Tanaka, Y., Inoue, H. & Holt, S. S. The X-ray astronomy satellite ASCA. Publ. Astron. Soc. Jpn 46, L37–L41 (1994).
Hasinger, G.et al. Adeep X-ray survey in the Lockman hole and the soft X-ray log N − log S. Astron. Astrophys. 275, 1–5 (1993).
Almaini, O.et al. Adeep ROSAT survey. 12. The X-ray-spectra of faint ROSAT sources. Mon. Not. R. Astron. Soc. 282, 295–303 (1996).
Rees, R. F. & Cudworth, K. M. Anew look at the globular cluster M28 Astron. J. 102, 152–158 (1991).
Krockenberger, M. & Grindlay, J. E. Discovery of diffuse X-ray emission in 47-tucanae. Astrophys. J. 451, 200–209 (1995).
Kulkarni, S. R., Goss, W. M., Wolszczan, A. & Middleditch, J. Deep radio synthesis images of globular clusters. Astrophys. J. 363, L5–L8 (1990).
Pacholczyk, A. G. Radio AstrophysicsCh. 7 (Freeman, San Francisco, (1970)).
Foster, R. S., Backer, D. C., Taylor, J. H. & Goss, W. M. Period derivative of the millisecond pulsar in the globular cluster M28. Astrophys. J. 326, L13–L15 (1988).
Verbunt, F., van Paradijs, J. & Elson, R. X-ray sources in globular clusters. Mon. Not. R. Astron. Soc. 210, 899–914 (1984).
Hasinger, G., Johnston, H. M. & Verbunt, F. Discovery of multiple X-ray sources in 47-Tucanae. Astron. Astrophys. 288, 466–471 (1994).
Grindlay, J. E.et al. Spectroscopic identification of probable cataclysmic variables in the globular cluster NGC-6397. Astrophys. J. 455, L47–L50 (1995).
Asai, K.et al. ASCA observations of soft X-ray transients in quiescence—X1608-52 and Cen X-4. Publ. Astron. Soc. Jpn 48, 257–263 (1996).
David, L. P., Harnden, F. R., Kearns, K. E. & Zombeck, M. V. in The ROSAT High Resolution Imager Calibration Report 1–52 (SAO, Cambridge, MA, (1996)).
Djorgovski, S. & Meylan, G. in Structure and Dynamics of Globular Clusters(eds Djorgovski, S. G. & Meylan, G.) 325–336 (Astron. Soc. Pacif., San Francisco, (1993)).
Trager, S. C., Djorgovski, S. & King, I. R. in Structure and Dynamics of Globular Clusters(eds Djorgovski, S. G. & Meylan, G.) 347–355 (Astron. Soc. Pacif., San Francisco, (1993)).
Cognard, I.et al. High precision timing observations of the millisecond pulsar PSR B1821−24 at Nancy. Astron. Astrophys. 311, 179–188 (1996).
Buccheri, R. & De Jaeger, O. C. in Timing Neutron Stars(eds ögelman, H. & van den Heuvel, E. P. J.) 95–111 (Kluwer, Dordrecht, (1989)).
Acknowledgements
We thank J. Grindlay and F. Verbunt for discussions. R.M.D. and S.R.K. thank NASA for support. This work was partially supported by the Scientific Research Fund of the Japanese Ministry of Education, Science, and Culture.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Danner, R., Kulkarni, S., Saito, Y. et al. Faint X-ray sources in the core of the globular cluster M28. Nature 388, 751–753 (1997). https://doi.org/10.1038/41962
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1038/41962
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.
