EXO 0748-676: ATLAS data demonstrates the optical rise from quiescence
ATel #16648; L. Rhodes (Oxford), J. H. Gillanders (Oxford), A. Knight (Durham), S. Srivastav (Oxford)
on 11 Jun 2024; 15:27 UT
Credential Certification: Lauren Rhodes (lauren.rhodes@physics.ox.ac.uk)
Subjects: Optical, Binary, Neutron Star
Here we report optical observations 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 queried the ATLAS forced photometry server (https://fallingstar-data.com/forcedphot/) at the location of EXO 0748-676, to search for any optical activity across the last 4 months (since MJD 60350). The ATLAS system observed the field with a 1-3 day cadence across this 4 month period. The resulting light curve can be found at https://fallingstar-data.com/forcedphot/queue/1280775. On MJD 60415 (April 16th), the light curve sharply rises to a constant sequence of detections at ~18 (AB mag), with significant variability. Our detections are consistent with those from ATel #16646. The rise in the optical light curve predates the first X-ray detection by about two months.
