Fermi Gamma-ray Space Telescope

Optical Variability in the Candidate Transitional Millisecond Pulsar 3FGL J0427.9-6704

Mark Kennedy
(R. P. Breton, C. J. Clark, V. S. Dhillon, T. R. Marsh, D. Mata Sánchez, J. Stringer, G. Voisin)


The gamma-ray source 3FGL J0427.9-6704 has been associated with the nearby LMXB 1SXPS J042749.2-670434, making it unique among the known LMXBs due to the presence of a gamma-ray eclipse. The optical light curve of the source suggests the binary is accreting, and the system has been proposed to belong in the transitional millisecond pulsar (tMSP) class. We have recently obtained a high time resolution multi-band optical light curve of the source. There is a deep optical eclipse present in the light curve, alongside flickering which is most likely coming from the accretion disc. There is evidence for significant heating of the secondary star, and modeling of the light curve constrains the inclination to be 84$pm$3degree, in line with the required inclination to produce a gamma-ray eclipse. We are unable to tightly constrain the primary mass (whose nature is still unclear), and do not find any evidence for a bi-modality in the systems optical flux (whose presence would have been the smoking gun for a tMSP classification). Finally, we find weak evidence for a ~21 min quasi-periodic oscillation in the light curve, we which attribute to a transient structure in the accretion disc.