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NuSTAR Observation of GX 339-4 in the early stages of its 2017 outburst

ATel #10825; J. A. Garcia (Caltech), F. Harrison (Caltech), J. Tomsick (Berkeley), K. Foster (Caltech), F. Fuerst (ESAC), D. Walton (Cambridge), Y. Xu (Caltech), J. Steiner (MIT), J. McClintock (SAO), J. Wilms (Remeis), T. Dauser (Remeis), T. Maccarone (TTU), T. Dincer (Yale), C. Bailyn (Yale), N. Sridhar (IISER-B), J. Wang (Fudan)
on 5 Oct 2017; 22:06 UT
Credential Certification: Javier Garcia (javier@caltech.edu)

Subjects: X-ray, Binary, Black Hole

GX 339-4 is a transient binary black hole system with recurrent outbursts every 2-3 years, with the last full outburst observed during 2014-2016. A new outburst has been reported after a brightening observed in the optical on September 2017 (ATel #10797), as well as detections in X-rays with Swift (ATel #10798), and in radio with ATCA (ATel # 10808). We have triggered the first of a series of ToO observations from our GO program with the Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al. 2013, ApJ, 770, 103).

NuSTAR observed GX 339-4 on 2017 October 2 03:40:06 UT with an exposure time of ~23.2 ks. Both Focal Point Modules A and B (FPMA/FPMB) detected the source at approximately 2 counts per second, and a flux of ~2.6 mCrab (~8.e-11 ergs/cm^2/s; 2-10 keV). The source counts consistently dominate the background counts over the entire band (3-79 keV). This is the lowest flux observed by NuSTAR for this source, including the latest observations during the decay of the 2015 outburst.

The spectra were fitted with a Comptonization model (nthComp) including Galactic absorption (TBabs) with a column density fixed at N_H=5.e21 cm^-2. A hard power-law continuum is detected with Gamma=1.62+/-0.01. No cutoff at high energies can be detected. The residuals show weak emission feature near 6.4 keV, consistent with Fe K reflection from the accretion disk. However, no Fe K-edge or Compton hump signatures can be detected. We fitted the emission with a Gaussian profile finding the centroid at E=6.43(+0.08,-0.06) keV, with a poorly constrained width of sigma=0.13+/-0.13 keV. We measured the equivalent width of the Fe K line to be EW=91(+2,-1) eV, which is remarkably consistent with a previous Suzaku measurement at a similar flux level (Tomsick et al. 2009, ApJL, 707, 87).

Additional NuSTAR observations will be triggered later in the outburst in order to track the evolution of the Fe K emission and other disk reflection features during the rise of the system in its hard state. We thank the NuSTAR team for the prompt scheduling of this observation.