Second neutron star in globular cluster M4 .
Abstract
We show that the optical counterpart of the brightest X-ray source C-X 1 in M4 is a ∼ 20th magnitude star, located in the color-magnitude diagram on (or very close to) the main sequence of the cluster, and exhibiting sinusoidal variations of the flux. We find the X-ray flux to be also periodically variable, with X-ray and optical minima coinciding. Stability of the optical light curve, lack of UV-excess, and unrealistic mean density resulting from period-density relation for semidetached systems speak against the original identification of CX 1 as a cataclysmic variable. We argue that the X-ray active component of this system is a neutron star, probably a millisecond pulsar.
- Publication:
-
Memorie della Societa Astronomica Italiana
- Pub Date:
- 2013
- Bibcode:
- 2013MmSAI..84..598R
- Keywords:
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- binaries:close;
- globular clusters: individual (M4);
- X-rays: stars