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EXO 0748-676: ATLAS optical data shows possible return to quiescence

ATel #17004; Lauren Rhodes (TSI/McGill), Amy Knight (Durham)
on 30 Jan 2025; 18:36 UT
Credential Certification: Lauren Rhodes (Lauren.rhodes@mcgill.ca)

Subjects: Optical, Binary, Neutron Star

Here we report on a change in the optical properties of EXO 0748-676, obtained during normal survey operations with ATLAS. The neutron star X-ray binary EXO 0748-676 was detected by the Neil Gehrels Swift Burst Alert Telescope and X-ray Telescope on 10th June 2024 (D’Elia et al 2024, GCN #36653), signalling the beginning of an outburst. Subsequent observations by the Las Cumbres Observatory confirmed a return to outburst (Baglio et al 2024, ATel #16646). EXO 0748-676 has been in quiescence since the end of 2008 (Degenaar et al 2011, MNRAS, 412, 3, pp1409-1418).

ATLAS is a quadruple 0.5-m telescope system with two units in Hawaii, one in Chile and one in South Africa (see Tonry et al. 2018, PASP, 130:064505), routinely surveying the visible sky on a daily basis. We promptly process all data with our transient science server (Smith et al. 2020, PASP, 132:085002).

We have been tracking the outburst using the ATLAS forced photometry server at the location of EXO 0748-676, to search for any optical activity across the last ~12 months (since MJD 60350). The ATLAS system observed the field with a 1-3 day cadence across this ~year-long period. The resulting light curve can be found at https://fallingstar-data.com/forcedphot/queue/2162280.

For about seven months, the light curve has been relatively flat between 17.5 - 18.0 magnitudes. In the last month, the light curve has shown a substantial decay but we note has not reached its pre-outburst levels.