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Optical monitoring of GX 339-4 suggests the source is approaching state transition

ATel #14419; Payaswini Saikia, Maria Cristina Baglio, David M. Russell, D. M. Bramich (NYU Abu Dhabi), Eleonora Caruso (NYU Shanghai), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 22 Feb 2021; 17:42 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, Ultra-Violet, Black Hole, Transient

Referred to by ATel #: 14440, 14455, 14484, 14490, 14493, 14494, 14504, 14507, 15598, 15615

The black hole low-mass X-ray binary GX 339-4 is a transient system that typically goes into outburst every 1-3 years. A recent outburst of the source was detected by the MeerKAT radio telescope (ATel #14336) in the first week of January 2021. The onset of an outburst was also confirmed with significant X-ray brightening observed by Swift/BAT (ATel #14351), NuSTAR (ATel #14352), INTEGRAL (ATel #14354), MAXI/GSC (#14367), NICER (ATel #14384) and AstroSat (ATel #14400).

We have been closely monitoring the source at optical wavelengths since 2007 with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, in particular the 2-m Faulkes Telescope South (e.g. ATel #1586, #2270, #7434, #10797, #13113, Cadolle Bel et al. 2011, A&A, 534, A119). When GX 339-4 came out of Sun constraint in the third week of January, we found significant enhancement of activity from the source at optical wavelengths, with magnitude i' = 15.73 on MJD 59235.7 (2021 Jan 21); and V = 16.74 and R = 15.90 on MJD 59236.7 (2021 Jan 22). This is more than 3 magnitudes brighter compared to the last observations of GX 339-4 with LCO which were obtained during quiescence (MJD 59131.4, 2020 October 9) when the optical magnitudes were i' = 18.85, V = 20.08 and R = 19.16. All errors are << 0.1 mag.

The most recent optical magnitudes obtained on MJD 59265.3 (Feb 20) show that the source is still brightening with current optical magnitudes i' = 14.60, V = 15.62 and R = 14.86. From studies of previous optical/infrared behaviour of GX 339-4 with SMARTS (Buxton et al. 2012, AJ, 143, 130) and our LCO monitoring, we find that since 2002 there have only been two outbursts as bright as V < 15.62 mag (in the years 2002 and 2010), both of which made the state transition around V ~ 14.8-15.0 mag. When the source made the transition in 2004, it was 0.3 mag fainter than now, while in 2014 it transitioned at a very similar brightness to the current magnitude. All other outbursts had peak mags of V >~ 15.8 and did not make the transition, remaining in the hard state. Hence our optical monitoring of the source during this outburst in comparison with its behaviour in the previous years, suggests that it is very likely to make the state transition soon.

The source was also observed at UV wavelengths with Swift/UVOT between Jan 23 and Feb 6, and here we report the following (AB) magnitudes: UVW2= 21.12 +- 0.15 (MJD 59237.9; Jan 23), UVW1= 19.48 +- 0.07 (MJD 59239.2; Jan 25), U= 18.10 +- 0.04 (MJD 59244.4; Jan 30), UVM2= 21.16 +- 0.19 (MJD 59246.5; Feb 1), U= 17.88 +- 0.04 and UVW1= 19.31 +- 0.06 (MJD 59248.4; Feb 3). This shows a brightening in the UV with respect to the last observation performed before the start of the current outburst, on MJD 59153.2 (2020 Oct 31), when an upper limit of UVW2>22.19 mag was measured.

The source is currently in a bright hard state, which is useful for studying compact jets (e.g. Gandhi et al. 2011, ApJ, 740, L13). Over the state transition we expect a UV fade (Yan & Yu 2012, MNRAS, 427, L11) then an optical-IR fade (e.g. Coriat et al. 2009, Cadolle Bel et al. 2011, Buxton et al. 2012) as the jet quenches, then possibly the launching of discrete radio jet ejections. We therefore encourage multi-wavelength observations over the coming days/weeks.

We will continue to monitor the source as part of an on-going monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. The analysis of the LCO data are performed with the real-time data analysis pipeline, the ''X-ray Binary New Early Warning System'' (XB-NEWS); see Russell et al. 2019 (AN, 340, 278), Pirbhoy et al. 2020 (ATel #13451) and Goodwin et al. 2020 (MNRAS, 498, 3429) for details.

Optical LCO light curves of GX 339-4