New outburst of the black-hole low-mass X-ray binary GX 339-4 detected with XB-NEWS and Swift/XRT

ATel #16260; Kevin Alabarta (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), David M. Russell (NYU Abu Dhabi), Sara Motta and M. Cristina Baglio (INAF-OAB), Payaswini Saikia, D. M. Bramich and Sandeep Rout (NYU Abu Dhabi), and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 28 Sep 2023; 06:37 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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GX 339-4 is a black-hole low-mass X-ray binary undergoing outbursts with recurrence times of approximately 1-2 years. During the last few months, GX 339-4 has shown low-level X-ray and optical activity, with fluxes slightly higher than those normally observed in quiescence (ATel #16096). Here we report on recent optical and X-ray observations that indicate that GX 339-4 has started to deviate significantly from the previously reported low-level activity and has entered an outburst phase.

The optical observations were performed with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network and light curves can be seen in the figure linked below. Since August 15 (MJD 60171.58), the optical flux (with magnitudes V ~ 19.1, R ~ 18.3 and i' ~ 18.2) has been brightening at a rate of ~0.03 mag/day (i' and R bands) and 0.04 mag/day (V band). On September 16 (MJD 60203.43), XB-NEWS measured optical magnitudes of GX 339-4 in the V, R and i' bands of 18.59 +/- 0.01, 17.98 +/- 0.01 and 17.66 +/- 0.01, respectively. These magnitudes are significantly brighter than those observed in the last months. Since then, GX 339-4 has brightened further to V = 17.64 +/- 0.01, R = 17.23 +/- 0.01 and i' = 17.12 +/- 0.01 on September 24, 2023 (MJD 60211.49).

The increase in flux since September 16 is higher at shorter wavelengths, ΔV = 0.95 +/- 0.01, ΔR = 0.75 +/- 0.01 and Δi' = 0.54 +/- 0.01, suggesting an important contribution of the accretion disc to the brightening of the source and the beginning of a new outburst.

GX 339-4 has also been monitored with the Swift/XRT on a regular basis. The observed X-ray count rate evolution is similar to that in the optical (see the light curve figure). After reaching a local minimum around August 10 (MJD 60166) the X-ray count rates have increased by a factor of 15, reaching 0.24 counts/s on September 23 (MJD 60210).

Multi-wavelength observations are encouraged to study the rise of the outburst.

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).

This material is based upon work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).

A link to the light curve is below.

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