A second neutron star in M4?

Abstract

We show that the optical counterpart of the X-ray source CX 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).

11 pages, 4 figures, accepted by ApJ Letters

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• Black Holes in Short Period X-ray Binaries and the Transition to Radiatively Inefficient Accretion
• M4 Core Project with HST - III. Search for variable stars in the primary field
• Exotica in the Globular Cluster M4, Studied with Chandra, HST, and the VLA