Identification of a 12-17 d time-scale in X-ray observations of GRS 1915+105

Measurement of the energy emitted from accreting astrophysical systems provides an observational constraint on the plasma processes that may be operating within the disc. Here we examine the continual time-variation over the past 6 yr of the total X-ray flux from the microquasar GRS 1915+105. The application of differencing and rescaling techniques to RXTE All-Sky Monitor (ASM) data shows that the small amplitude fluctuations scale up to 12-17 d. A 17-d time-scale in the X-ray fluctuations corresponds to half the measured binary orbital period of this system (33.5 ± 1.5 d). While this may be coincidental, it is possible that these two time-scales may be linked by, for example, a turbulent cascade in the accretion disc driven by a tidally induced two-armed spiral shock corotating with the binary system. Temporal scaling is found only in the ever-present small fluctuations, and not in the intermittent larger-amplitude fluctuations. This is consistent with the basic model for this source which consists of a steady, cold outer disc and an unstable inner disc.