Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610

TL;DR

This paper reports the discovery of a new redback millisecond pulsar candidate near a gamma-ray source, identified through optical and X-ray observations, with implications for future pulsar detection efforts.

Contribution

It introduces a newly identified redback millisecond pulsar candidate associated with 4FGL J0940.3-7610, based on optical, X-ray, and orbital analysis, suggesting a promising target for pulsar searches.

Findings

Optical and X-ray observations support the redback pulsar candidate nature.

Preliminary modeling indicates a low-mass neutron star with a more massive companion.

System properties suggest potential radio eclipses, explaining previous non-detections.

Abstract

We have discovered a new candidate redback millisecond pulsar binary near the center of the error ellipse of the bright unassociated Fermi-LAT $\gamma$-ray source 4FGL J0940.3-7610. The candidate counterpart is a variable optical source that also shows faint X-ray emission. Optical photometric and spectroscopic monitoring with the SOAR telescope indicates the companion is a low-mass star in a 6.5-hr orbit around an invisible primary, showing both ellipsoidal variations and irradiation and consistent with the properties of known redback millisecond pulsar binaries. Given the orbital parameters, preliminary modeling of the optical light curves suggests an edge-on inclination and a low-mass ($\sim 1.2$ - $1.4\,M_{\odot}$) neutron star, along with a secondary mass somewhat more massive than typical $\gtrsim 0.4\,M_{\odot}$. This combination of inclination and secondary properties could make radio eclipses more likely for this system, explaining its previous non-discovery in radio pulsation searches. Hence 4FGL J0940.3-7610 may be a strong candidate for a focused search for $\gamma$-ray pulsations to enable the future detection of a millisecond pulsar.