NuSTAR confirms a new outburst for the Black Hole Binary GX 339-4

ATel #12322; J. A. Garcia (Caltech), B. Grefenstette (Caltech), F. Harrison (Caltech), J. Tomsick (Berkeley), H. Miyasaka (Caltech)
on 22 Dec 2018; 00:52 UT
Credential Certification: Javier Garcia (javier@caltech.edu)

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We report on new X-ray observations of the black hole binary system GX 339-4. GX 339-4 is a prototypical black hole binary with recurring outbursts every 2-3 years. A significant increase in the radio luminosity has been recently reported (Tremou et al. 2018, Atel #12287) indicating a new outburst has started. While monitoring is currently not attainable with most X-ray instruments due to Sun constrain, observations are possible using the Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al. 2013, ApJ, 770, 103). Thus, with requested Directors Discretionary Time (DDT), we have triggered a single ~17.5 ks NuSTAR observation of GX 339-4. The observation was performed when the source was roughly 25 degrees away from the Sun. This reduces the overall astrometric accuracy of NuSTAR as the star tracker used to reconstruct the source on the sky was blinded by the Sun. We used a relatively large (100") extraction region centered on the apparent position of the source to account for the jitter in the source location. High level science products (lightcurves, spectra, and response files) are minimally affected when operating in this mode as long as the source is bright. The source is detected at ~6 counts per second by both Focal Plane Modules A and B (FPMA/FPMB), well above the background signal in the entire NuSTAR band (3-79 keV). We measure the source flux at ~2.4e-11 erg/cm^2/s (2-10 keV). The spectra were fitted with an absorbed power-law model (TBabs*pow) with a fixed hydrogen column density of N_H = 5.e21 cm^-2. We observe a hard power-law continuum with Gamma = 1.55 +/- 0.01. Clear signatures of X-ray reprocessing (reflection) are observed in the form of Fe K emission, Fe K edge, and possibly an excess above ~20 keV due to electron scattering in the reflection (Compton hump). By fitting the Fe K emission with a Gaussian profile with fixed width (sigma=0.01 keV), we measure the centroid energy at E = 6.39 +\- 0.06 keV, and an equivalent width of ~57 eV. The hard continuum, flux level, and presence of reflection features are consistent with the beginning of a new outburst, and show remarkable resemblance with the last (failed) outburst of this source observed in October 2017-January 2018 (Garcia et al. 2017, Atel #10825), as well as with previous NuSTAR and Suzaku measurements at similar luminosities (Wang-Ji et al. 2018, ApJ 855, 61, Tomsick et al. 2009, ApJL, 707, 87). Given that observations with other X-ray telescopes will not be possible until middle January 2019, additional NuSTAR DDT observations will be requested in order to monitor the evolution of this source until the Sun exclusion period is over. We thank the NuSTAR SOC team for the prompt scheduling of this observation.