Archival ATLAS observations of EXO 0748-676

ATel #16649; Albert Kong (NTHU)
on 11 Jun 2024; 16:12 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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Following the discovery of the new outburst of the neutron star X-ray binary EXO 0748-676 in both X-ray (GCN #36653) and optical (ATel #16646) bands, we searched for the ATLAS database to investigate the optical variability. Using the ATLAS forced photometry server (https://fallingstar-data.com/forcedphot/), we extracted the long-term optical light curve of EXO 0748-676. On 2024 April 17, it is clear from the ATLAS image that the optical counterpart was seen and the source varies between 17 and 19 mag since then. It is almost 2 months earlier than the X-ray detection by Swift (GCN #36653). Before that, the source is below the detection limit typically about 19 mag. It is worth noting that the quiescent optical magnitude of EXO 0748-676 is about R=22 mag (Hynes and Jones 2009).

We attempted to search for the 0.159d orbital period (Wolff et al. 2002; Hynes and Jones 2009) by using the Lomb-Scargle periodogram. We found several significant peaks between 0.14d and 0.19d. Although the 0.159d orbital period is not significant, it may be due to the sparse data. High cadence observations or longer term of monitoring are required to confirm the optical orbital period in this new outburst.

We acknowledge that Rhodes et al. (ATel #16648) also reported the ATLAS discovery while this ATel was in preparation.

ATLAS light curve of EXO 0748-676 from 2021 Dec to 2024 June