Optical, X-ray and infrared observations of GX 339-4 during the hard-to-soft state transition observed with LCO and REM

ATel #16460; Kevin Alabarta (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), David M. Russell, D. M. Bramich, Payaswini Saikia and Sandeep Rout (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU) P. Casella (INAF-OAR), S. Campana, S. Covino, P. D'Avanzo (INAF-OAB),  P. Goldoni (APC/IRFU), N. Masetti (INAF-OAS) and T. Munoz-Darias (IAC)
on 20 Feb 2024; 16:40 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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The latest outburst of GX 339-4 was first detected on September 16, 2023 (MJD 60203.43) with the Las Cumbres Observatory (LCO) network (ATel #16260). After that date, observations with Swift/XRT (ATel #16260), MAXI/GSC (ATel #16302), and MeerKAT (ATel #16421) revealed a brightening trend in X-rays and radio frequencies. After the Sun constraint, the system continued its brightening at radio (ATel #16421) and X-rays (ATel #16424, #16425), starting a hard-to-soft transition on January 25, 2024 (MJD 60334) as detected with MAXI/GSC (ATel #16424) and MeerKAT (ATel #16457).

Here, we report on new optical observations acquired with LCO. Since the last reported LCO observations (ATel #16260), GX 339-4 brightened until the Sun constraint on October 25, 2023 (MJD 60242.00), with magnitudes R=15.92 +/- 0.01 and i'=15.69 +/- 0.07, at an average rate of ~0.05 mag/day in both bands. The monitoring resumed on January 23, 2024 (MJD 60332.74) with magnitudes g'=16.62 +/- 0.01, r'=15.88 +/- 0.01 and i'=15.41 +/- 0.01. On January 24 (MJD 60333.74), the optical brightness decreased to g'=16.91 +/- 0.02, r'=16.17 +/- 0.01 and i'=15.69 +/- 0.01. The next observations (February 7; MJD 60347.34), displayed magnitudes of V=16.79 +/- 0.01, R=16.25 +/- 0.01 and i'=16.14 +/- 0.01. Since then, the source has faded more slowly in optical until February 18 (MJD 60358.09), with magnitudes V = 17.15 +/- 0.02, R = 16.65 +/- 0.01 and i' = 16.46 +/- 0.01.

GX 339-4 has also been monitored daily in X-ray with MAXI and Swift/BAT (bottom panels in the linked figure). Since the drop in the optical, the MAXI flux increased to ~1.1 c/s/cm2. In contrast, the Swift/BAT flux dropped from ~0.09 c/s/cm2 on January 25 (MJD 60334) to ~0.007 c/s/cm2 on February 1 (MJD 60341, red dotted line), indicating a hard-to-soft transition. After that, GX 339-4 remained approximately constant at that level until February 11 (MJD 60351), when the Swift/BAT flux increased (green dotted line). This suggests that the system may be slightly hardening again. After February 13 (MJD 60353), the source is not significantly detected with Swift/BAT.

This outburst has also been monitored with the Robotic Eye Mount (REM) 60cm telescope in La Silla (Chile) in the NIR (H-band, 1.64 micron). The monitoring started on January 27, 2024 (MJD 60336.36; H=14.00 +/- 0.03 mag) with a ~1 day cadence, ~3 days after the optical drop was detected with LCO. Therefore, it is not possible to state whether the drop is also observed in the NIR, as expected if the compact jet is quenched at the hard-to-soft transition.

Strong variability of up to 1 mag/day is detected in the NIR. The last observation on February 19 (MJD 60359.34) reports H=14.23 +/- 0.06 mag (on MJD 60358.34, it was H=13.22 +/- 0.03 mag). Considering the hardening of the source during that period and the fact that the source is still detected at radio frequencies (ATel #16457), the flickering observed in the NIR with REM is likely to be produced by the compact jet.

Multi-wavelength observations are encouraged to study the evolution of the outburst. We will continue to observe the system with LCO and REM.

The LCO observations of GX 339-4 are 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).

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