Swift sees the outburst brightening of GX 339-4 in the hard state

ATel #12413; J. A. Paice (Southampton, IUCAA), P. Gandhi (Southampton), M. Pahari (Southampton)
on 21 Jan 2019; 13:48 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 its most recent outburst in November 2018. The outburst was discovered by MeerKAT (ATel #12287), and then later confirmed by NuSTAR (ATel #12322). These observations found the source to be visible in both Radio and X-rays. Due to a sun constraint that has only just lifted, this outburst has yet to be studied in any great detail.

A new Swift observation (ObsID: 00032898182) was carried out on 2019-01-21T01:40:46 for a duration of 970s in Window Timing (WT) mode with the X-Ray Telescope (XRT; Burrows et al. 2005 SSRv 120 165). Using the standard XRT pipeline products (Evans et al. 2009 MNRAS 397 1177) and a similar fit to the NuSTAR report in ATel #12322 with an absorbed power law (TBabs*pow) with a fixed hydrogen column density of N_H = 5.e21 cm^-2, the source is found to have a flux of ~5.3e-10 erg/cm^2/s, an increase of more than a factor of 20 in flux since the NuSTAR observation taken on 2018 December 21. The photon index was found to be ~1.54 (+/- 0.02), in agreement with said previous observation. The XRT fit was carried out over 1-10 keV and the goodness-of-fit chi^2=302 with 283 d.o.f.

Swift XRT lightcurves show significant variability in the source, finding multiple flares which frequently increase the base count rate (of ~10 ct/s) by a factor of three.

Swift UVOT (UVOT; Roming et al. 2005 SSRv 120 95) also observed in the V band, finding an AB mag of 16.17 +/- 0.1, as measured in a 5 arcsec radius aperture and subtracting a neighbouring source-free background region. This is a fairly typical optical flux seen at the bright end during previous source outbursts (e.g. Buxton et al. 2012 AJ 143 130).

The photon index, X-ray flux and X-ray variability all point towards GX 339-4 still being in the hard state. It is difficult to say whether or not the source is still brightening or is in decline; the Swift/BAT lightcurve (https://swift.gsfc.nasa.gov/results/transients/weak/GX339-4/) implies that the source may have plateaued, but this remains to be tested. Swift/XRT will continue to monitor this source with a 3-day cadence over the next two weeks to determine its current state, and multiwavelength observations are welcome. Unfortunately, the source still remains Sun-constrained for night-time ground observing.

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