Swift XRT observes GX 339-4 in the hard state in Jan 2018

ATel #11208; J. A. Paice, P. Gandhi (Univ. Southampton)
on 20 Jan 2018; 20:48 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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GX 339-4 is a black hole X-ray transient that entered outburst in September 2017 (ATel #10797). During early outburst, observations were carried out by Swift (ATel #10798), ATCA (ATel #10808), SALT (ATel #10820, #10864), NICER (ATel #10824), and NuSTAR (ATel #10825), among others. These observations found the source to be visible in X-rays, optical and radio bands, and also detected both sub-second and large-scale flaring activity.

On 1st November 2017, GX 339-4 became Sun constrained with Swift, and could not be observed until 20th January 2018. We have now observed the source again with Swift/XRT.

The observation (ObsID: 00032898161) was carried out on 2018-01-20T17:26:57 for a duration of 76s in Window Timing (WT) mode and 925s in Photon Counting (PC) mode. Swift XRT (XRT; Burrows et al. 2005 SSRv 120 165) data products built using the automated pipeline (Evans et al. 2009 MNRAS 397 1177) show that GX 339-4 has declined in flux compared to the previous observations taken late last year. Using a fit with an absorbed power law, the source is found to have an absorbed flux of 3.5 (+/-0.3)e-10 erg/s/cm2 for the 0.3-10 keV band, with photon index (gamma)=1.36(+/-0.16) and N(H)=5.1(+/-1.3)e21/cm2, for the PC mode (w-statistic=368/411). The results for the WT mode spectrum are consistent within the uncertainties, with Gamma=1.47(+/-0.23).

The source is still active, though the flux is lower by a factor of ~2-3 as compared to the peak fluxes seen by Swift/XRT in Oct/Nov 2017. The current photon index implies that the source is in the hard state, and the lower flux suggests that it may be in decline. Swift BAT and MAXI light curves are consistent with a declining trend.

This therefore may have been a failed outburst, not uncommon for this source, unless the soft state was very brief and missed during Sun constraint. There currently exists a valuable opportunity to study the 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 with a 5 day cadence over the coming month, and multiwavelength observations are welcome.

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