GX 339-4 is still in the soft state

ATel #7434; F. Bernardini, D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 23 Apr 2015; 13:10 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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In October 2014 the black hole transient GX 339-4 entered a new outburst (ATel #6649). It then left the low-hard state and made a transition to the soft state (ATel #6960), which it entered in January 2015 and stayed for more than a month (ATel #7009). The source was reported (ATel #7201) to leave the soft state, likely entering the intermediate state, on March 3, 2015 (MJD 57090.5), according to the hardness ratio between Swift/BAT (15-50 keV) and MAXI/GSC (2-10 keV). Here, we report optical monitoring observations of GX 339-4 with the Las Cumbres Observatory Global Telescope Network (LCOGT), and an update on the X-ray evolution from Swift/BAT and MAXI/GSC.

We inspected the updated light curves of Swift/BAT and MAXI/GSC, covering up to April 18, 2015 (MJD 57130.5). The hardness ratio between Swift/BAT (15-50 keV) and MAXI/GSC (2-10 keV) suggests that the source first moved toward an intermediate state (MJD 57082.5) and then came back to the soft state, where it still lies at the moment with its 2-10 keV flux slowly decreasing. A link is provided below to the hardness intensity diagram and optical light curves.

We observed GX 339-4 with the 1-m network LCOGT telescopes starting on 14 March, 2015 (MJD 57095) in V, R and i'-band filters (exposures were 100 - 200 sec). The optical light curve shows variations on day-to-week timescales with magnitudes in the range V = 15.8 - 16.5, R = 15.3 - 15.9, i' = 15.2 - 15.9 over the last 5 weeks. The V - i' colour is ~ 0.4 - 0.5 mag on most dates and the i'-band fractional rms variability (time resolution ~200 sec), when measured is <5% (the amplitude is < 0.05 mJy, not de-reddened). Both V - i' colour and rms variability are typical of the soft state and not the hard state (see e.g. ATel #2573).

We also compared the 3-9 keV MAXI flux (derived using WebPIMMS by assuming a blackbody temperature of 1 keV and a hydrogen column density of 5*10e+21 cm^-2) to our V-band flux density (de-reddened using Av = 3.7; both nH and Av were taken from Coriat et al. 2009, MNRAS, 400, 123) on several dates. The position of the data on the X-ray/optical correlation in Figure 2 (right panel) of Coriat et al. (2009) cluster around Fx ~ 1e-8 erg/s/cm^2, F_nu_V ~ 40 mJy, which is consistent with the soft state correlation curve, and brighter than the previous (2002 to 2007) outbursts. The peak X-ray (3-9 keV MAXI) flux of 1.5e-8 erg/s/cm^2 on March 16, 2015 (MJD 57097) slightly exceeds that of all the 2002-2003, 2007 and 2010 outbursts. This is likely to be the brightest outburst (measured in this energy range) since before 2002. We conclude that the source is still currently in the soft state. Assuming an exponential decay can describe this outburst, the fit to the current MAXI light curve suggests the source should reach the soft to hard transition flux several months (~ 5) from now. This work makes use of observations from the LCOGT network.

Faulkes/LCOGT monitoring page of GX 339-4