The SMC X-ray binary SXP4.78: a new Type II outburst and the identification and study of the optical counterpart

ABSTRACT SXP4.78 was originally discovered in 2000 as a pulsar in the Small Magellanic Cloud by the Rossi X-ray Timing Explorer, but it was not spatially located at that time. A new detection in 2018 with the Neil GehrelsSwift Observatory during a Type II outburst permitted its position to be accurately located and its optical counterpart to be identified. We report X-ray and optical monitoring covering epochs before and during the outburst. Using photometric data, we show the long-term variability of the Be disc where we present flux and colour changes associated with the disc growth and decay over a period of ∼6000 d. We show evidence of disc growth during the recent outburst through an increase in the H α equivalent width and photometric flux. Period analysis was performed using both optical photometric and spectroscopic data, but with no significant detection of an orbital period. A modest periodic signature of 2.65 d was detected from the Optical Gravitational Lensing Experiment (OGLE) I-band data, but we attribute that to the non-radial pulsations of the Be star. We also obtained a blue spectrum from the Southern African Large Telescope, which permits us to classify the spectral type as B0.5 IV–V.