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

TL;DR

This study documents the significant loss and subsequent gradual recovery of the circumstellar disk in the Be star X-ray binary RX J0123.4-7321, highlighting the dynamic nature of its stellar environment over more than a decade.

Contribution

It provides new long-term optical and X-ray observations showing the disk loss and re-building process in a Be star system, illustrating a transition between Be and B star states.

Findings

Almost total circumstellar disk loss after 2008

Gradual disk re-building observed up to 2020

Periodic outbursts had not yet resumed by 2020

Abstract

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.