NuSTAR Observation of GX 339-4 in a Bright Hard State

ATel #16425; O. K. Adegoke (Caltech), G. Mastroserio (University of Milan), J. A. Garcia (NASA/GSFC & Caltech), J. A. Tomsick (UC Berkeley, SSL), R. M. T. Connors (Villanova), F. Harrison (Caltech)
on 30 Jan 2024; 22:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Oluwashina Adegoke (oadegoke@caltech.edu)

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GX 339-4 is a black hole X-ray binary that undergoes an outburst every 1-2 years on average. For the past several weeks, it has showed a steady increase in X-ray activity since the onset of the current outburst (ATel #16260, #16302). Over this time, the radio brightness of GX 339-4 has also been observed to steadily increase based on data from MeerKAT (ATel #16421). Here, we report results from a recent NuSTAR observation of the source, carried out on 2024 January 19 for a duration of ~19 ks.

An absorbed (N_H fixed to 6e21 cm^-2) cutoff-powerlaw fit to its 3-79 keV spectra reveals strong relativistic reflection features including the iron K alpha line at ~6.4keV and the Compton hump that peaks around 25keV. The spectrum can be approximated by a model including a Comptonized-disk continuum, relativistic and distant reflection. The powerlaw photon index is 1.71+/-0.01 and the diskbb temperature kT_in is 0.29^{+0.01}_{-0.02} keV. The unabsorbed 3-79 keV flux is ~9 E-9 erg/cm2/s (about 400 mCrab). The power density computed from the timing analysis of the NuSTAR light curve shows a strong QPO at 0.46+/-0.01 Hz and the presence of broadband noise at lower frequencies. The total rms noise level in the 0.01-50 Hz band is 30%. The source appears to be in the bright hard state during this observation.

Since the 19th of January when this observation was made, the 2-20 keV MAXI light curve of the source has more than doubled and it has been confirmed to have transitioned from the hard state a few days after the NuSTAR observation (ATel #16424). Further multi-wavelength observations are encouraged.