High speed SALT photometry of GX 339-4 during its current outburst

ATel #15610; J. Brink, D. A. H. Buckley (South African Astronomical Observatory/University of Cape Town), P. Gandhi (University of Southampton)
on 11 Sep 2022; 08:46 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

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We report on optical photometry of the transient black-hole low-mass X-ray binary, GX 339-4, undertaken with the Southern African Large Telescope (SALT) during the current outburst, reported in ATels #15577, #15578, #15580, #15585, #15598. On 2022-09-06 we obtained 0.15 second cadence white light optical photometry with SALT using the Robert Stobie Spectrograph (RSS) in slot mode, covering the interval 19:20:24.594 to 19:49:59.817 UTC. The resulting light curve shows rapid optical flaring, characteristic of the hard state of the source (e.g. Gandhi et al. 2010, MNRAS 407, 2166). The source fractional r.m.s variability amplitude (Vaughan et al. 2003, MNRAS 345, 1271) measured from the light curve is 0.36. The strongest flares are found to be higher than 3 times the average flux at peak. A Lomb-Scargle power spectrum shows a possible QPO at 8.9 mHz, however this needs to be confirmed and red noise cannot be ruled out at this stage. Low frequency optical and near-infrared QPOs have previously been detected in GX 339-4, but were typically found at higher frequencies, closer to 0.1 Hz (e.g. Gandhi et al. 2010, Vincentelli et al. 2019, ApJ 887, L19). These observations were undertaken under the SALT transient follow-up program 2021-2-LSP-001).