Joint spectral-timing modelling of the hard lags in GX 339-4: constraints on reflection models

TL;DR

This study tests a reflection model for hard X-ray lags in GX 339-4 but finds it incompatible with spectral data, suggesting propagation effects in the accretion flow are more likely responsible.

Contribution

The paper applies joint spectral-timing modelling to test reflection models for X-ray lags in a black hole binary, providing constraints on reflection's role in observed time delays.

Findings

Reflection model cannot fit both lag and spectral data simultaneously.

Large lags imply a highly flared disc inconsistent with Fe K extalpha ext{} emission.

Propagation effects in the accretion flow likely cause the observed lags.

Abstract

The X-ray variations of hard state black hole X-ray binaries above 2 keV show 'hard lags', in that the variations at harder energies follow variations at softer energies, with a time-lag \tau depending on frequency \nu approximately as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain this time delay, including fluctuations propagating through an accretion flow, spectral variations during coronal flares, Comptonisation in the extended hot corona or a jet, or time-delays due to large-scale reflection from the accretion disc. In principle these models can be used to predict the shape of the energy spectrum as well as the frequency-dependence of the time-lags, through the construction of energy-dependent response functions which map the emission as a function of time-delay in the system. Here we use this approach to test a simple reflection model for the frequency-dependent lags seen in the hard state of GX 339-4, by simultaneously fitting the model to the frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and 2009. Our model cannot simultaneously fit both the lag and spectral data, since the relatively large lags require an extremely flared disc which subtends a large solid angle to the continuum at large radii, in disagreement with the observed Fe K\alpha emission. Therefore, we consider it more likely that the lags > 2 keV are caused by propagation effects in the accretion flow, possibly related to the accretion disc fluctuations which have been observed previously.