Abstract
Optical identification of globular cluster X-ray sources is essential if their evolutionary state and relation to the galactic bulge sources is to be understood. The 10 high-luminosity (⩾1036 erg s−1) X-ray sources associated with globular clusters have very accurate (∼1 arc s) positions from the Einstein Observatory survey1,2. No globular cluster source has yet been optically identified, primarily because all of the bright X-ray sources are located close to the dynamical centre (implying total masses of ∼1.5M⊙ (ref. 2)), where crowding makes optical photometry and spectroscopy extremely difficult. The only northern cluster containing a bright X-ray source is M15, which has the extremely ultraviolet (U − B = −1.2) and variable star AC211 near the X-ray error circle3,4. The reported spectrum is that of an A star5,6, with no emission lines which are normally present in galactic low-mass X-ray binaries. We have now discovered7,8 strong He II 4,686-Å emission from the region of AC211, and weaker Balmer emission. We conclude that it is an intrinsically high-luminosity (Lx⩾1038erg s−1) X-ray source on the basis of the optical brightness of the star and the He II flux. As the observed Lx is only 6 × 1036 erg s−1, this implies that the system is at a high inclination and that only scattered X-rays from an extended region are observed. The mass transfer rate required to account for this is very high and places important constraints on the evolutionary state of the mass-losing star which, for its optical brightness, must either be a red giant or horizontal branch star. The location of AC211 questions the accuracy of the Einstein Observatory positions.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Hertz, P. & Grindlay, J. E. Astrophys. J. 275, 105–119 (1983).
Grindlay, J. E., Hertz, P., Steiner, J. E., Murray, S. S. & Lightman, A. P. Astrophys. J. 282, L13–L16 (1984).
Aurière, M., Le Fèvre, O. & Terzan, A. Astr. Astrophys. 138, 415–420 (1984).
Aurière, M., Cordoni, J. P. & Koch-Miramond, L. IAU Circ. No. 4101 (1985).
Grindlay, J. E. in Compact Galactic and Extragalactic X-ray Sources (eds Tanaka, Y. & Lewin, W. H. G.) 215 (Proc. Japan-U.S. Seminar, ISAS, Tokyo, 1985).
Grindlay, J. E. in Evolution of Galactic X-ray Binaries (eds Truemper, J., Lewin, W. H. G. & Brinkmann, W.) 25–41 (NATO ASI, Reidel, Dordrecht, 1986).
Charles, P. A., Jones, D. C., Naylor, T. & Boksenberg, A. IAU Circ. No. 4118 (1985).
Charles, P. A., Aurière, M., Ilovaisky, S. & Koch-Miramond, L. IAU Circ. No. 4146 (1985).
Hesser, J. E., Shawl, S. J. & Meyer, J. E. Publ. astr. Soc. Pacif. 98, 403–422 (1986).
Aurière, M., Maucherat, A., Cordoni, J.-P., Fort, B. & Picat, J. P. Astr. Astrophys. 158, 158–160 (1986).
Bradt, H. V. D. & McClintock, J. E. A. Rev. Astr. Astrophys. 21, 13–66 (1983).
van Paradijs, J. in Accretion Driven Stellar X-ray Sources (eds Lewin, W. H. G. & van den Heuvel, E. P. J.) 189–260 (Cambridge University Press, 1983).
White, N. E. & Mason, K. O. Space Sci. Rev. 40, 167–194 (1985).
Savonije, G. J. in Accretion Driven Stellar X-ray Sources (eds Lewin, W. H. G. & van den Heuvel, E. P. J.) 343–366 (Cambridge University Press, 1983).
Renzini, A. in Horizontal-Branch and UV-Bright Stars (ed. Philip, A. G. D.) 19–29 (Davis, New York, 1985).
McClintock, J. E., Canizares, C. R. & Backman, D. E. Astrophys. J. 223, L75–L78 (1978).
Ilovaisky, S. A. & Chevalier, C. IAU Circ. No. 4146 (1985).
Zinn, R. Astrophys. J. 193, 593–605 (1974).
Sweigart, A. V. in Horizontal-Branch and UV-Bright Stars (ed. Philip, A. G. D.) 3–18 (Davis, New York, 1985).
Battistini, P., Bregoli, G., Fusi Pecci, F., Lolli, M. & Epps Bingham, E. A. Astr. Astrophys. Suppl. 61, 487 (1985).
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Charles, P., Jones, D. & Naylor, T. An emission-line object in the core of M15. Nature 323, 417–419 (1986). https://doi.org/10.1038/323417a0
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1038/323417a0
- Springer Nature Limited