Low-level activity of the black-hole low-mass X-ray binary GX 339-4 with LCO and Swift/XRT

ATel #16096; Kevin Alabarta (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), David M. Russell (NYU Abu Dhabi), Sara Motta (INAF-Brera), M. Cristina Baglio, Payaswini Saikia, Dan Bramich (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 26 Jun 2023; 14:35 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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GX 339-4 is a transient low-mass black-hole X-ray binary that undergoes frequent outbursts, with recurrence times of approximately 1-2 years. Its latest outburst was first detected in X-rays 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). GX 339-4 was already decaying when monitoring stopped due to Sun angle constraints in October (optical) and November (X-rays) of 2022.

Here we report on the detection of low-level X-ray and optical activity from GX 339-4. X-ray and optical monitoring observations resumed in January of 2023; light curves are shown in the figure linked below. The X-ray observations were performed with the Swift/XRT. The first Swift observations in January showed a source count rate of ~1e-2 cts/s (0.3-10 keV), consistent with, but at the high end of, count rates seen in past quiescent periods. However, during the next couple of months, the count rate was observed to rise slowly, reaching a peak count rate of ~9e-2 cts/s around mid-May. Soon after, the count rate started to drop, to ~2e-2 cts/s on June 18. In the most recent Swift observation (June 25) the count rate had increased again, to ~0.05 cts/s. A combined X-ray spectrum from the two observations with the highest count rates (May 14 and 21) was fitted with an absorbed power law (Nh fixed to 6e21 cm^-2). We obtained a photon index of 1.67+/-0.17, consistent with a hard spectral state, and an unabsorbed 0.3-10 keV flux of (6.1+/-0.6)e-12 erg/s/cm^2. For a distance of 8 kpc the latter corresponds to a luminosity of ~4.6e34 erg/s.

The optical observations were performed with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network. Initial detections made on January 25, 2023 (MJD 59969.73) and January 27, 2023 in the i' (i' = 18.227 +/- 0.009) and V bands (V = 19.27 +/- 0.02), respectively, were close to, but slightly brighter than the quiescent level (V ~ 19.5, i' ~ 18.5). Similar to what was seen in X-rays, the optical evolution showed a slow-increasing trend for the next few months. The i' and V band magnitudes both increased by ~ 0.5, peaked around the same time as the X-rays, and started to drop slightly soon after. The latest magnitudes obtained with LCO on June 26, 2023 (MJD 60120.06) are V = 19.07 +/- 0.02 and i' = 18.09 +/- 0.02.

It is not clear whether GX 339-4 is returning to its normal quiescent level, or if the low-level activity will evolve into a proper outburst. We encourage multi-wavelength observations over the coming /weeks/months to follow the evolution of GX 339-4. We will continue to monitor the source with LCO and the weekly Swift observations will continue until at least mid-August.

The LCO observations of GX 339-4 were performed as part of an ongoing monitoring program of ~50 low-mass X-ray binaries (Lewis et al. 2008). 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). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

X-ray (top) and optical (bottom) light curves of GX 339-4