GX 339-4 may have reached the peak of its rebrightening

ATel #13113; Anjali Rao (UoS), John A. Paice (UoS, IUCAA), David M. Russell (NYU Abu Dhabi), Poshak Gandhi (UoS), Sara Motta (Oxford), Fraser Lewis (Faulks Telescope Project & Astrophysics Research Institute, LJMU), R. Misra (IUCAA), David A. Buckley (South African Astronomical Observatory)
on 17 Sep 2019; 14:20 UT
Credential Certification: Anjali Rao (a.rao@soton.ac.uk)

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GX 339-4 is a Galactic black hole X-ray binary that exhibits frequent outbursts. Its latest outburst, beginning in December 2018 (ATel #12287, #12322), has been regularly observed by Niel Gehrels Swift Observatory (See Burrows et al. 2005) since emerging from sun constraint in January 2019.

The first Swift/XRT observation of this outburst took place on January 21, 2019, and the flux was found to be 5.3e-10 erg/s/cm2 (1-10 keV) (ATel #12413). The flux then declined to a minimum level of ~1.5e-11 ergs/s/cm2 (0.3-10 keV) in May, which continued until late July, when the rebrightening in X-rays began (ATel #13024). The two latest observations show the following fluxes:

OBS_ID DATE TIME FLUX (0.3-10 keV) ergs/s/cm2

00032898231 2019-09-09 05:14:35 4.5(+/-0.5)e-10

00032898232 2019-09-16 04:42:35 4.1(+/-0.4)e-10

The Faulkes Telescope has been regularly monitoring the source over the course of both the outburst and rebrightening. It reveals that the source continued to brighten in i' band to a magnitude of 15.480 +/-0.003 on September 16, 2019, significantly brighter than the value of 16.55 +/-0.02 reported on August 12, 2019 (ATel #13024). Click here for a link to full Faulkes Telescope lightcurves.

This is the first instance since the rebrightening began that the source has not shown a definite rise in flux. The current flux is also comparable to the peak observed in January, and it is possible that the source may have peaked in its rebrightening. We encourage follow-up observations, since multiwavelength studies of such rebrightenings remain few and far between. A multiwavelength observation later this week is being coordinated; please visit this SmartNet page for more information for scheduling and participating observatories.

For the above analysis, we used results from automated Swift/XRT products. All observations were obtained in the PC mode and can be fit with an absorbed power law model with photon index varying between ~1.2-2.0 and N_H ~(0.3-1.9) e22 /cm^2. We thank the Swift team for their scheduling of these observations.

The LCO observations are part of an on-going monitoring campaign of ~40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

X-ray/Optical monitoring of GX 339-4