Low Frequency Coherence Break in The Soft X-ray State of GRS 1915+105

We present results from the analysis of X-ray power density spectra and coherence when GRS 1915+105 is in soft states. We use three data sets that belong to $\mu$, $\phi$ and $\delta$ classes in Belloni et al (2000). We find that the power density spectra appear to be complex, with several features between 0.01 and 10 Hz. The coherence deviates from unity above a characteristic frequency. We discuss our results in different models. The corona size in the sphere-disk model implied by this break frequency is on the order of 10$^4$ GM/c$^2$, which is unphysical. Our results are more consistent with the prediction of the model of a planar corona sustained by magnetic flares, in which the characteristic frequency is associated with the longest time-scale of an individual flare, which is about eight seconds.