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New outburst of GX 339-4 detected by Faulkes Telescope South

ATel #10797; David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Poshak Gandhi (Southampton)
on 29 Sep 2017; 10:30 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 10798, 10808, 10820, 10825, 10864

We have been monitoring the black hole X-ray binary GX 339-4 with the 2-m Faulkes Telescope South (at Siding Spring, Australia) since September 2007 (e.g. ATel #1586; #1962, #2459, #2547, #3191, #4162, Cadolle Bel et al. 2011). Since its last outburst in 2014 - 2016 (ATel #6649, #6960, #7009, #7201, #7434, #7649, #7962, #7977), the source has been in (or close to) quiescence. The system likely entered quiescence after 2016-07-10 (MJD 57579) when the magnitudes were V = 19.40 +- 0.12, R = 18.45 +- 0.05, i' = 18.32 +- 0.04, slightly above the quiescent level (these are Vega mags, except the SDSS i'-band which is SDSS mag which adopts the AB system). Almost all data since then are consistent with the quiescent level of V ~ 20, R ~ 19, i' ~ 18.7, with significant flickering on some dates (these magnitudes include the flux from a few close by stars, see Shahbaz et al. 2001, ATel #4162).

GX 339-4 has entered a new outburst in September 2017, with the following recent magnitudes:

2017-08-24 (MJD 57989.4): V = 20.36 +- 0.28, R = 18.94 +- 0.06, i' = 18.73 +- 0.04
2017-09-14 (MJD 58010.49): V = 17.18 +- 0.03, R = 16.54 +- 0.01, i' = 16.68 +- 0.01
2017-09-14 (MJD 58010.50): V = 17.35 +- 0.03, R = 16.63 +- 0.01, i' = 16.74 +- 0.01
2017-09-20 (MJD 58016.5): V = 18.04 +- 0.10, R = 17.46 +- 0.04, i' = 17.77 +- 0.07
2017-09-26 (MJD 58022.4): V = 17.42 +- 0.15, R = 16.51 +- 0.04, i' = 16.52 +- 0.02

The new outburst began between 2017-08-24 and 2017-09-14. The latest magnitudes on 2017-09-26 are ~ 2.2 to 2.6 mag brighter than the quiescent level. There appears to be high amplitude variability, with changes (both fading and brightening) of up to ~ 1 magnitude in a week. From the two observations in each filter taken on 2017-09-14, there is variability of ~ 0.1 to 0.2 mag in 20 minutes. This variability is typical of the hard state of GX 339-4, in which the optical fractional rms can be ~ 10 to 20 %.

The outbursts of GX 339-4 usually rise slowly, and this is the early stages of the 2017 outburst. No significant detections are apparent so far in the recent MAXI and Swift BAT light curves. Black hole X-ray binary outbursts are rarely monitored in their initial stages, near the onset of the outburst. We therefore encourage multiwavelength observations during these early stages of the new outburst of GX 339-4. We will continue monitoring the source at optical wavelengths.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory (LCO). A link to the updated light curves of GX 339-4 is included below.

GX 339-4 Faulkes Telescope light curve