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X-ray brightening of GX 339-4 in late September 2017

ATel #10798; P. Gandhi, D. Altamirano, D. M. Russell, C. Knigge, M. Middleton, A. Veledina, A. Beri, J. Paice
on 29 Sep 2017; 10:38 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 10808, 10820, 10825

Following the report of optical brightening of the Galactic black hole X-ray transient GX 339-4 (ATel #10797), we requested Swift follow-up of the source.

The X-Ray Telescope (XRT; Burrows et al. 2005 SSRv 120 165) data products from the automated pipeline (Evans et al. 2009 MNRAS 397 1177) show the source to be clearly detected. A crude fit with an absorbed power-law returns a flux of 7(+/-1)e-11 erg/s/cm2 for the 0.3-10 keV band, for a photon index (Gamma)=1.6(+0.4,-0.3) and an N(H)=7(+4,-3)e21/cm2.

GX 339-4 is the prototypical black hole X-ray transient, having undergone outbursts every ~2-3 years for the past ~15 years. However, there has been an extended period of inactivity (based on Swift/BAT) over the past ~year and more. This may be suggestive of a longer disc build-up time leading to a bright outburst, but this remains to be verified.

There is no strong rise in BAT and MAXI based upon the most recent publicly available data. This, combined with the optical rise, suggests that the source has been caught at an early stage. Studying these rising phases is typically hampered by the fast outburst rise times. Multi-wavelength observations are strongly encouraged. XRT will continue to monitor the source with a ~2-3 day cadence over the coming ~2 weeks.

We thank the Swift team and Brad Cenko for quickly scheduling these observations.