A second neutron star in M4?

TL;DR

This paper identifies the optical counterpart of X-ray source CX 1 in M4 as a main sequence star with sinusoidal flux variations, suggesting the X-ray source is likely a neutron star, possibly a millisecond pulsar, rather than a cataclysmic variable.

Contribution

It provides evidence that CX 1 is a neutron star system in M4, challenging previous identification as a cataclysmic variable.

Findings

Optical counterpart is a main sequence star with sinusoidal flux variations.

X-ray and optical minima coincide, indicating a linked variability.

The system is unlikely to be a cataclysmic variable due to density and UV-excess considerations.

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).