X-ray and radio observations reveal GX 339-4 has returned to the hard state

ATel #16621; T. D. Russell (INAF/IASF Palermo), F. Carotenuto (University of Oxford), J. C. A. Miller-Jones (ICRAR/Curtin), P. Atri (Astron), M. Del Santo (INAF/IASF Palermo), A. Marino (ICE/CSIC)
on 16 May 2024; 22:34 UT
Credential Certification: Thomas Russell (thomas.russell@inaf.it)

Tweet

The Galactic black hole X-ray binary GX 339-4 has been in outburst since late September 2023 (ATel #16260), undergoing a hard-to-soft X-ray state transition in January 2024 (Atels #16424, #16457, #16460). Recent X-ray observations taken with XRT on board the Neil Gehrels Swift Observatory (Swift) and radio observations with the Australia Telescope Compact Array (ATCA) show that the source has now returned to the hard state.

Swift-XRT has been monitoring GX 339-4 every few days over the past few weeks. These observations show a gradual hardening of the X-ray spectrum, as the source transitioned from the soft to the hard X-ray state. The most recent Swift-XRT observation taken on 2024-05-15 (MJD 60445) reveals an X-ray spectrum that is well described by a simple absorbed power law model. Using the X-Ray Spectral Fitting Package (XSPEC v12.14; Arnaud 1996) the 0.5-10 keV X-ray data are best fit with an equivalent hydrogen column, NH, of (4.0 +/- 0.04)E21 cm^-2 and photon index, Γ, of 1.75 +/- 0.06, corresponding to an absorbed 0.5-10 keV X-ray flux of (3.95 +/- 0.11)E-10 erg/s/cm^2. All errors are 1-sigma. The X-ray spectral shape of the latest Swift-XRT observation is consistent with a black hole X-ray binary within the hard X-ray state.

Regular ATCA radio observations taken over the same period show the radio emission from GX 339-4 progressively brighten, although the radio spectrum remained steep (with a spectral index, α, of around -0.7, where S_ν ∝ν^α). Most recently, ATCA observed GX 339-4 between 2024-05-14 20:12 UT and 2024-05-14 21:00 UT (MJD 60444.86 +/- 0.02). Data were recorded simultaneously at central frequencies of 5.5 and 9 GHz, with 2 GHz of bandwidth at each frequency. We used PKS B1934-638 for primary flux and bandpass calibration, and B1646-50 for secondary gain calibration. Data were flagged for radio frequency interference, calibrated, and imaged using standard procedures in the Common Astronomy Software Applications package (CASA v5.1; The CASA team et al. 2022). For imaging the target, we used a Briggs robustness parameter of 0 as the best compromise between sensitivity and resolution. Fitting for a point source in the image plane, we measured flux densities of 3.95 +/- 0.06 mJy at 5.5 GHz and 3.75 +/- 0.05 mJy at 9 GHz. The flux densities correspond to a radio spectral index, α, of -0.10 +/- 0.05, consistent with a compact jet launched during the hard X-ray state.

Further observations are planned. Multiwavelength observations are strongly encouraged.

We thank the Swift-XRT team for scheduling the X-ray observations, and Jamie Stevens and ATCA staff for scheduling the radio observations. We acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site.