Evolution of the truncated disc and inner hot-flow of GX 339–4

¹ School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
e-mail: pchainakun@g.sut.ac.th
² Department of Physics, Faculty of Science, Srinakharinwirot University, Bangkok 10110, Thailand
³ National Astronomical Research Institute of Thailand, Chiang Mai 50180, Thailand
⁴ H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK

Received: 3 August 2020
Accepted: 9 November 2020

Abstract

Aims. We study the changes in geometry of the truncated disc and the inner hot-flow of GX 339–4 by analysing the power spectral density (PSD) extracted from six XMM-Newton observations taken at the very end of an outburst.

Methods. We developed a theoretical model of the PSD of GX 339–4 in the 0.3–0.7 keV (thermal-reverberation-dominated) and 0.7–1.5 keV (disc-continuum-dominated) energy bands. The model assumes the standard accretion disc to be truncated at a specific radius, inside of which are two distinct hot-flow zones: one spectrally soft and the other spectrally hard. The effects of disc fluctuations and thermal reverberation are taken into account.

Results. This model successfully produces the traditional bumpy PSD profiles and provides good fits to the GX 339–4 data. The truncation radius is found to increase from r_trc ∼ 10 to 55r_g as the source luminosity decreases, confirming that the truncation radius can be characterized as a function of luminosity. Keeping in mind the large uncertainty in previous measurements of the truncation radius, our values are larger than some obtained from spectroscopic analysis, but smaller than those implied by reverberation lag analysis. Furthermore, the size of two inner hot-flow zones that are spectrally hard and spectrally soft also increases from ∼5 to 27r_g and from ∼3 to 26r_g, respectively, as the flux decreases. We find that the radial range of the inner hard zone is always larger than the range of the soft hot-flow zone, but by a comparatively small factor of ∼1.1–2.2