Correlation of time lag and photon index in GX 339-4

Abstract

Context. Black hole transients, as a class, exhibit during their outbursts a correlation between the time lag of hard photons with respect to softer ones and the photon index of the hard X-ray power law. The correlation is not very tight and therefore it is necessary to examine it source by source. Aims. The objective of the present work is to investigate in detail the correlation between the time lag and the photon index in GX 339-4, which is the best studied black hole transient. Methods. We have obtained RXTE energy spectra and light curves and have computed the photon index and the time lag of the 9-15 keV photons with respect to the 2-6 keV photons. The observations cover the first stages of the hard state, the pure hard state, and the hard-intermediate state.Results. We have found a tight correlation between time lag and photon index Γ in the hard and hard-intermediate states. At low Γ, the correlation is positive; it becomes negative at high Γ. By assuming that the hard X-ray power-law index Γ is produced by inverse Compton scattering of soft disk photons in the jet, we have reproduced the entire correlation by varying two parameters in the jet: the radius of the jet at its base R 0 and the Thomson optical depth along the jet τ . We have found that as the luminosity of the source increases, R 0 initially increases and then decreases. This behavior is expected in the context of the Cosmic Battery. Conclusions. Our jet model nicely explains the correlation with reasonable values of the parameters R 0 and τ . These parameters also correlate between themselves. As a further test of our model, we predict the break frequency in the radio spectrum as a function of the photon index during the rising part of an outburst.

Inclination Dependence Of The Time-Lag – Photon-Index Correlation In BHXRBs And Its Explanation With A Simple Jet Model

“…A nice correlation ( Fig. 1) has been found in the BHXRB GX 339-4 [9]. One can see that, as the source moves from the quiescent state to the hard state and then to the hard intermediate one (filled circles), Γ increases monotonically and the time-lag increases in the hard state and then flattens out.…”

“…One can see that, as the source moves from the quiescent state to the hard state and then to the hard intermediate one (filled circles), Γ increases monotonically and the time-lag increases in the hard state and then flattens out. We have explained this correlation with a simple jet model [9]. The model is the same, as the one we used before, to explain the energy spectra [6], the dependence of the time-lags on Fourier frequency [14], the correlation between the time-lag and Γ in Cyg X-1 (Kylafis et al 2008), and the correlation between time-lag and cut-off energy in GX 339-4 [15].…”

Spectro-temporal studies of rapid transition of the quasi periodic oscillations in the black hole source H1743-322

“…As discussed above most of the observational evidence related to the type-B QPO lead to a possible jet origin and a precessing jet in the inner region. This model also warrants a truncation in the disk, possibly arising due to the base of a jet, with the jet being the source of hard X-rays via comptonization (Liska et al 2018;Kylafis et al 2020;Kylafis & Reig 2018; Reig…”

Generation and Transport of Magnetic Flux in Accretion–Ejection Flows

“…C: The Bright Intermediate state. The system transitions from the Hard to the Soft High state, and the radius of the compact jet quickly diminishes in size at its base [56]. We associate this with a reduction of the size of the inner JED.…”

“…It takes a few months for the outburst to rise to its highest luminosity. During that time, a compact jet appears, whose radius at its base seems to increase with time as the system evolves from the quiescent to the high-hard state [56]. Quoting [5], "whenever a disk is capable of driving jets, these will carry away a fraction of the released accretion (gravitational) energy, and the disk luminosity will be quenched (it radiates only a small fraction of the accretion power)".…”