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ATel #14493; Ongoing state transition of the BH XRB GX 339-4 confirmed in the optical by XB-NEWS
on 30 Mar 2021; 16:24 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole X-ray binary (BH XRB) GX 339-4 entered an outburst in January 2021 (ATel #14351, #14367, #14352, #14354). For two months the source exhibited a slow brightening, observed at all reported wavelengths, remaining in a hard X-ray state (ATel #14384, #14400, Atel#14419, #14440, #14455, #14484). After this stable period, a low frequency quasi periodic oscillation was recently detected with NuSTAR (ATel#14484), first at ~0.13 Hz (2021-02-20, MJD 59265) and then at a higher frequency (~0.24 Hz, on 2021-03-08, MJD 59281). Recent NICER observations showed a strongly significant increase of the soft X-ray flux, together with a dramatic evolution of the spectral properties of the source, suggesting a possible transition to the soft state could be imminent (ATel #14490).

As part of an ongoing monitoring of ~50 low-mass X-ray binaries (LMXBs), GX 339-4 is regularly observed with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (e.g. ATel #14419). 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).

Our optical monitoring showed a slow optical brightening since our monitoring resumed after Sun constraint, from V = 16.74+/- 0.01, R = 15.90+/- 0.01, i' = 15.73+/- 0.01 on MJD 59235-6 (2021 Jan 21-22; ATel #14419), to a maximum flux of V=15.44+/- 0.01, R= 14.73+/- 0.01, i'=14.51+/-0.01 on MJD 59281 (2021 March 08). After this epoch however, the optical flux started to decline slowly until MJD 59290, when the magnitudes were: V=15.68+/- 0.01, R=15.06+/- 0.01, i'=14.77+/-0.01. During a 9-day gap in our monitoring, the flux dramatically dropped in all the optical filters, reaching the following magnitudes on MJD 59299 (2021 March 26): V=16.57+/-0.01, R=16.03+/-0.01, i'=15.88+/-0.02. In particular, the magnitude drop is 0.89+/-0.01, 0.97+/-0.01, 1.11+/-0.01 in the V, R, i' bands, respectively. This shows that the flux drop is more significant at lower frequencies, implying that the jet component is fading, as in previous outbursts during the hard to hard-intermediate state transition (e.g. Homan et al. 2005; Cadolle Bel et al. 2011; Buxton et al. 2012). Therefore, our optical monitoring supports the possible transition towards the soft state that has been suggested for GX 339-4 from an analysis of the X-ray spectral properties evolution (ATel #14490).

The latest LCO observations show the following magnitudes: V=16.59+/-0.01, R=16.10+/-0.01, i'=15.99+/-0.01 (MJD 59303; 2021 March 30), thus confirming the optical fading.

We encourage multi-wavelength observations over the coming days/weeks during the state transition. We will continue to monitor the source with LCO / Faulkes Telescopes.

The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO / Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Light curve of GX 339-4