X-ray and optical reflaring in GX 339-4 in July/August 2019

ATel #13024; Anjali Rao (UoS), Poshak Gandhi (UoS), David M. Russell (NYU Abu Dhabi), Sara Motta (Oxford), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), John A. Paice (UoS, IUCAA)
on 13 Aug 2019; 22:00 UT
Credential Certification: Anjali Rao (a.rao@soton.ac.uk)

Tweet

GX 339-4 is a well known Galactic X-ray binary and a black hole candidate. The object has been exhibiting regular outbursts, with its latest outburst reported in December 2018 (ATel #12287, #12322). The brightness of the source has been in steady decline since February 2019. The last reported flux was 2.1 +/- 0.2 e-10 erg/cm^2/s in 0.5-10 keV on April 09, 2019 (ATel #12690).

We have examined a series of the Swift XRT (Burrows et al. 2005) observations of the source starting from April 02, 2019 and find that the source never reached quiescence. The lowest flux level was recorded on June 19, 2019 (ObsId: 00032898217) with an observed flux of 1.18(+0.24,-0.20)e-11 erg/cm^2/s in 0.3-10 keV, which is about two orders of magnitude brighter than its faintest X-ray flux seen in quiescence (See Corbel et al. 2013). Observations after June provide evidence of a fluctuating rise with the last observation obtained on August 12, 2019 03:14:36 UTC showing an observed flux of 5.1(+0.9,-0.8)e-11 erg/cm^2/s in 0.3-10 keV.

For the above analysis, we used results from automated Swift/XRT products (Evans et al. 2009). 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.

Using Faulkes Telescope monitoring, we also found a rise in i' band brightness of ~ 1.0 mag between June 11, 2019 and August 12, 2019. The last measured i' mag is 16.55 +/-0.02 on August 12, 2019 03:05:29 UTC.

It is unclear whether the observed rise in flux is a temporary re-flare on the path to decline from the peak of the last outburst, or whether it can be expected to continue to rise further. Multiwavelength monitoring is encouraged.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0811.2336) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). DMR acknowledges the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

X-ray/Optical monitoring of GX 339-4