A New Likely Redback Millisecond Pulsar Binary with a Massive Neutron Star: 4FGL J2333.1--5527

Abstract: We present the discovery of a likely new redback millisecond pulsar binary associated with the \emph{Fermi} $\gamma$-ray source 4FGL J2333.1--5527. Using optical photometric and spectroscopic observations from the SOAR telescope, we identify a low-mass, main sequence-like companion in a 6.9-hr, highly inclined orbit around a suspected massive neutron star primary. Archival XMM-Newton X-ray observations show this system has a hard power-law spectrum $\Gamma = 1.6\pm0.3$ and $L_X \sim 5 \times 10^{31}$ erg s$^{-1}$, consistent with redback millisecond pulsar binaries. Our data suggest that for secondary masses typical of redbacks, the mass of the neutron star is likely well in excess of $\sim1.4\,M_{\odot}$, but future timing of the radio pulsar is necessary to bolster this tentative conclusion. This work shows that a bevy of nearby compact binaries still await discovery, and that unusually massive neutron stars continue to be common in redbacks.