Detection of Low Frequency QPO in GX 339-4 with NuSTAR

ATel #14484; Guglielmo Mastroserio (Caltech), Javier Garcia (Caltech), John Tomsick (UC Berkeley), Fiona Harrison (Caltech), Riley Connors (Caltech)
on 25 Mar 2021; 23:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Guglielmo Mastroserio (gullik@caltech.edu)

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The black hole binary GX 339-4 was detected in outburst starting around 2021-01-04 (ATel #14336). The outburst has been followed by different facilities (ATel #14455 #14440 #14419 #14415 #14400 #14384 #14367 #14354 #14352 #14351). The Neil Gehrels Swift Observatory monitoring, with almost daily cadence, is suggesting that the source is still in the hard state. The Swift/BAT count rate has been fairly constant in the past few weeks suggesting that the source is still in the hard state. There is no indication of state transition neither in the Swift/BAT nor in the Monitor of All-sky X-ray Image (MAXI) light curves.

The Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al. 2013, ApJ, 770, 103) observed the source 4 times, on 2021-01-23 (ATel #14352), 2021-02-05, 2021-02-20 and 2021-03-08. During the first two Epochs no significant signatures of Quasi Periodic Oscillations (QPOs) were detected in the power spectrum. However, the last two Epochs show a clear detection of a Low Frequency QPO, first observed at ~0.13 Hz with 10% rms on Epoch 3, and then at a higher frequency (~0.24 Hz, 10% rms) on Epoch 4. Strong signatures of reflection from the accretion disk are clearly observed in all Epochs, with similar characteristics (Fe K in emission, Fe K-edge, and Compton hump). The power-law continuum is fitted with Gamma~1.6-1.7, consistent with the source being in the hard state.

Additional NuSTAR DDT observations will be requested in order to monitor the evolution of this source. We thank the NuSTAR Operation Team for the prompt scheduling of these observations.

NuSTAR power density spectra