Optical observations of the outburst of the black-hole low-mass X-ray binary GX 339-4 with LCO

ATel #15598; Kevin Alabarta, M. Cristina Baglio, David M. Russell, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Poshak Gandhi (Univ. of Southampton), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 8 Sep 2022; 09:27 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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GX 339-4 is a transient black-hole low-mass X-ray binary that undergoes periodic outbursts (with a recurrence time of approximately 1-2 years). A new outburst was detected on August 29, 2022 (MJD 59820), with MAXI/GSC (ATel #15577) and later confirmed with Swift/XRT (ATel #15578), MeerKAT (ATel #15580), NICER and NuSTAR (ATel #15585), and VLT/VISIR (ATel #15596).

We report on optical observations of GX 339-4, performed as part of an ongoing monitoring of ~50 low-mass X-ray binaries (LMXBs) with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (e.g. ATel #14419). LCO images are processed and reduced, and magnitudes are extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

After the end of its last outburst in January 2022, GX 339-4 has remained in quiescence and its optical magnitudes in the V, R and i' bands have been stable at the following average levels: V= 19.59 +/- 0.01, R = 18.89 +/- 0.06 and i' = 18.55 +/- 0.01 (note that there are a few faint stars that contaminate the aperture during quiescence; GX 339-4 may be fainter than these magnitude values). The start of the current outburst at optical wavebands likely happened between July 16, 2022 (MJD 59776) and July 29, 2022 (MJD 59789), at least 8 days earlier than the first reported Swift/XRT detection on MJD 59797 (ATel #15578); since then, GX 339-4 has continued brightening at a rate of 0.07 mag/day in V- and i' bands. The latest magnitudes obtained with XB-NEWS on September 5 (MJD 59827.1) are V = 16.68 +/- 0.01, R = 15.98 +/- 0.07 and i' = 15.58 +/- 0.01, suggesting that the system will continue to rise in the upcoming days, and the peak of the outburst has yet to be reached.

We encourage multi-wavelength observations over the coming days/weeks until the end of October, when the source will not be visible with LCO. We will continue to monitor the source with LCO.

We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical Lightcurve of GX 339-4