RX J0123.4-7321 -- the story continues: major circumstellar disk loss and recovery

RX J0123.4-7321 is a well-established Be star X-ray binary system (BeXRB) in the Small Magellanic Cloud (SMC). Like many such systems the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. Previous work has shown that the optical and X-ray were characterised by regular outbursts at the proposed binary period of 119 d. However around February 2008 the optical behaviour changed substantially, with the previously regular optical outbursts ending. Reported here are new optical (OGLE) and X-ray (Swift) observations covering the period after 2008 which suggest an almost total circumstellar disc loss followed by a gradual recovery. This indicates the probable transition of a Be star to a B star, and back again. However, at the time of the most recent OGLE data (March 2020) the characteristic periodic outbursts had yet to return to their early state, indicating that the disk still had some re-building yet to complete.