Increased X-ray activity for the black hole binary GX 339-4 detected with NuSTAR and Swift/XRT

ATel #14352; J. Garcia (Caltech), J. Tomsick (UC Berkeley), F. Harrison (Caltech), Riley Connors (Caltech), Guglielmo Mastroserio (Caltech)
on 26 Jan 2021; 18:16 UT
Credential Certification: Javier Garcia (javier@caltech.edu)

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The black hole binary GX 339-4 is an X-ray transient system typically displaying outbursts every 2-3 years. On 2021-01-04, radio observations with the MeerKAT radio telescope detected the source at 264 +/- 25 uJy (Atel #14336). We requested a Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al. 2013, ApJ, 770, 103) Target of Opportunity observation and obtained a ~28 ks observation on 2021-01-23.

The source was detected by NuSTAR at ~15 cts/s with both Focal Plane Modules (FPMA/B), with strong variability of ~50%. The source spectrum is well above the background in the entire 3-79 keV energy band. A simple power-law continuum including galactic absorption with the TBabs model (using a fixed hydrogen column of 6.e21 cm^-2) yields a photon index of 1.52, and the absorbed flux predicted by this model is ~3.3e-10 erg/cm^2/s (2-10 keV). A ratio plot reveals clear signatures of X-ray reflection off the accretion disk (Fe K emission and K-edge, and Compton hump). The Fe K line centroid is at 6.41 keV, visibly narrow, and with an equivalent width of ~25 eV.

Swift/XRT has begun monitoring the source and has obtained observations on 2021-01-23 and 2021-01-25. The first observation was performed in photon counting mode, leading to a high level of photon pile-up, but part of the second observation used windowed timing mode, and the spectrum is consistent with a power-law with a photon index of 1.53+/-0.11. The flux is ~4.3e-10 erg/cm^2/s (2-10 keV), indicating a small increase between the NuSTAR observation on 01-23 and the Swift observation on 01-25.

These findings are consistent with the rise of a new outburst in the hard state for this source.

Additional NuSTAR DDT observations will be requested in order to monitor the evolution of this source. We thank the NuSTAR and Swift Operations Teams for the prompt scheduling of these observations.