Swift XRT observations find GX 339-4 in the soft state in Jan 2020

ATel #13447; J. A. Paice (Univ. Southampton, IUCAA), T. Belloni (INAF-OAB), S. Motta (Univ. Oxford), P. Gandhi (Univ. Southampton), A. Rao, R. Misra (IUCAA), D. M. Russell (NYU Abu Dhabi), Phil Charles (Univ. Southampton)
on 5 Feb 2020; 10:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

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We report on a Swift/XRT observation of the outbursting X-ray binary GX 339-4, determining its spectral state and brightness after coming out of sun constraint.

GX 339-4 is a black hole X-ray transient that entered outburst in late 2018. It was first detected by NuSTAR (ATel #12322), with later observations carried out by Swift (ATel #12413, #12690) and INTEGRAL (ATel #12420), finding it to be bright in both X-rays and optical. The source did not transition, but neither did it reach quiescence; in July/August 2019, Swift observations found the source to be rebrightening in both X-ray and Optical (ATel #13024). This renewed activity was observed by Swift (ATel #13113) and Astrosat (ATel #13264), and continued until the source went into sun constraint towards the end of 2019.

During the period of sun constraint, the all-sky monitors Swift/BAT and MAXI showed the source to rapidly rise to a brightness not seen since 2014. Then, around the 13th December, they detected a rapid drop in hard X-ray flux, while the soft X-rays continued to rise until around the 28th before they too began to decline. This was indicative of a transition from the hard to the soft state. However, without direct observation, no further details could be accurately assessed until the source came out of sun constraint.

Therefore, a Swift/XRT observation was requested to be carried out soon after the source became visible once more. It is this observation that we report on here.

The observation (ObsID: 00032490017) was carried out on 2020 January 25 for a duration of 534s in Window Timing (WT) mode. Swift XRT (XRT; Burrows et al. 2005 SSRv 120 165) data products were built using the automated pipeline (Evans et al. 2009 MNRAS 397 1177). The 1-10 keV data was found to be well fit with an absorbed disk black body (phabs(diskbb); reduced chi^2 = 1.29 for 479 d.o.f). nH = 0.53 [+/- 0.01] e22 cm^-2, with a disk temperature of 0.83 +/- 0.01 keV. The source was found to have an absorbed flux of 6.59 (+0.05,-0.07) e-9 erg/s/cm2 for the 1-10 keV band. All errors quoted are at 90% confidence. The spectrum includes two peaks at 2 and 2.4 keV that are not fit by the model.

The fractional RMS value of the resultant light curve was found to be less than 5% at 3 sigma accuracy (in the frequency range 1/2 - 1/128 Hz and in the energy range 0.2 - 10 keV). Due to this, the spectrum being well-fit by a disk black body model, and poorly fit by a simple absorbed power law, we therefore conclude that the source did indeed transition, and is currently in the soft state.

The source is still active, and is brighter than the last observations from before it went under sun constraint; Using the Swift Build XRT Products tool, Obs ID 00032898238 (observed 2019 October 29) shows the source to be well fit to a power law with Photon Index 1.38 (+0.15, -0.14) and an unabsorbed flux of 1.44 (+0.10, -0.09) e-9 erg/s/cm2, a factor of 6 less than this new observation. However, Swift/BAT and MAXI light curves clearly show that it is no longer at peak brightness and is sharply declining.

Despite being a very active source with roughly one outburst every couple of years, MAXI light curves show that this is the first time that the source has transitioned since 2014. Therefore, there currently exists a valuable opportunity to study its decline as the source visibility improves over the coming weeks and months for ground and space telescopes. To this end, Swift/XRT will continue to monitor the source roughly once a week, coincident with MeerKAT, and multiwavelength observations are very welcome.

We would like to thank the Swift Team for their scheduling of these observations.