Low-luminosity states of the black hole candidate GX 339-4. II. Timing analysis

Here we present timing analysis of a set of eight Rossi X-Ray, Timing Explo rer (RXTE) observations of the black hole candidate GX 339-4 that were take n during its hard/low state. On long timescales, the RXTE All Sky Monitor d ata reveal evidence of a 240 day periodicity, comparable to timescales expe cted from warped, precessing accretion disks. On short timescales all obser vations save one show evidence of a persistent f(QPO) approximate to 0.3 Hz quasi-periodic oscillation. The broadband (10(-3) to 10(2) Hz) power appea rs to be dominated by two independent processes that can be modeled as very broad Lorentzian fits with Q less than or similar to 1. The coherence func tion between soft and hard photon variability shows that if these are truly independent processes, then they are individually coherent, but they are i ncoherent with one another. This is evidenced by the fact that the coherenc e function between the hard and soft variability is near unity between 5 x 10(-3) to 10 Hz but shows evidence of a dip at f approximate to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the power spe ctral density. As in Cyg X-1, the coherence also drops dramatically at freq uencies greater than or similar to 10 Hz. Also as in Cyg X-1, the hard phot on variability is seen to lag the soft photon variability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time l ag appears to be positively correlated with the flux of GX 339-4. We discus s all of these observations in light of current theoretical models of both black hole spectra and temporal variability.