Soft Lag and Its Implications for X‐Ray Time Variations in GRS 1915+105

The X-ray time variation of GRS 1915+105 is analyzed, focusing on the phase-lag properties in class θ. The energy and frequency dependence of the soft lag are derived. This energy dependence can be interpreted as Comptonization of high-energy photons by relatively soft electrons with temperatures of less than ~2 keV. A Monte Carlo simulation of the Comptonization of power-law photons by soft electrons is presented. This model can explain the energy spectrum, the energy dependence, and the frequency dependence of the soft lag. The size of the Compton cloud is estimated to be ~2 × 109 cm by comparing the simulation with the data. The time evolution in class θ, including the dip, is discussed.