Rapid optical and X-ray timing observations of GX 339−4: multicomponent optical variability in the low/hard state

Abstract

A rapid timing analysis of VLT/ULTRACAM and RXTE observations of the black\nhole binary GX 339-4 in its 2007 low/hard state is presented. The optical light\ncurves in the r', g' and u' filters show slow (~20 s) quasi-periodic\nvariability. Upon this is superposed fast flaring activity on times approaching\nthe best time resolution probed (~50 ms) and with maximum strengths of more\nthan twice the local mean. Power spectral analysis over ~0.004-10 Hz is\npresented, and shows that although the average optical variability amplitude is\nlower than that in X-rays, the peak variability power emerges at a higher\nFourier frequency in the optical. Energetically, we measure a large optical vs.\nX-ray flux ratio, higher than that seen when the source was fully\njet-dominated. Such a large ratio cannot be easily explained with a disc alone.\nThe optical:X-ray cross-spectrum shows a markedly different behaviour above and\nbelow ~0.2 Hz. The peak of the coherence function above this threshold is\nassociated with a short optical time lag, also seen as the dominant feature in\nthe time-domain cross-correlation at ~150 ms. The rms energy spectrum of these\nfast variations is best described by distinct physical components over the\noptical and X-ray regimes, and also suggests a maximal disc fraction of 20% at\n~5000 A. If the constant time delay is due to propagation of fluctuations to\n(or within) the jet, this is the clearest optical evidence to date of the\nlocation of this component. The low-frequency QPO is seen in the optical but\nnot in X-rays. Evidence of reprocessing emerges at the lowest Fourier\nfrequencies, with optical lags at ~10 s and strong coherence in the blue u'\nfilter. Simultaneous optical spectroscopy also shows the Bowen fluorescence\nblend, though its emission location is unclear. But canonical disc reprocessing\ncannot dominate the optical power easily, nor explain the fast variability.\n(abridged)\n