Optical, X-ray, and $\gamma$-ray observations of the candidate transitional millisecond pulsar 4FGL J0427.8-6704

Abstract

We present an optical, X-ray, and $\gamma$-ray study of 1SXPS J042749.2-670434, an eclipsing X-ray binary which has an associated $\gamma$-ray counterpart, 4FGL J0427.8-6704. This association has led to the source being classified as a transitional millisecond pulsar (tMSP) in an accreting state. We analyse 10.5 years of Fermi LAT data, and detect a $\gamma$-ray eclipse at the same phase as optical and X-ray eclipses at the >5$\sigma$ level, a significant improvement on the 2.8$\sigma$level of the previous detection. The confirmation of this eclipse solidifies the association between the X-ray source and the $\gamma$-ray source, strengthening the tMSP classification. However, analysis of several optical data sets and an X-ray observation do not reveal a change in the source's median brightness over long timescales or a bi-modality on short timescales. Instead, the light curve is dominated by flickering which has a correlation time of 2.6 min alongside a potential quasi-periodic oscillation at $\sim$21 min. The mass of the primary and secondary star are constrained to be $M_1=1.43^{+0.33}_{-0.19}$ M$_{\odot}$ and $M_2=0.3^{+0.17}_{-0.12}$ M$_{\odot}$ through modelling of the optical light curve. While this is still consistent with a white dwarf primary, we favour the transitional millisecond pulsar in a low accretion state classification due to the significance of the $\gamma$-ray eclipse detection.