Title:A model for the alternating phase lags associated with QPOs in X-ray binaries

Abstract: We present a theoretical model for the alternating phase lags associated with QPO fundamental and harmonic frequencies observed in some Galactic black-hole candidates. We assume that the accretion flow exhibits a transition from an outer cool, optically thick accretion disk to a hot, inner advection-dominated accretion flow (ADAF), and that the QPOs are related to small-scale oscillations of the accretion rate and the transition radius. We present an analytical estimate of the expected phase lags at the fundamental and first harmonic frequencies of the QPOs and perform detailed time-dependent Monte-Carlo simualtions of the radiation transport in the oscillating ADAF / cool disk system. We find that this model is well suited to reproduce alternating phase lags between the fundamental and the first harmonic. It also naturally explains the trend observed in GRS 1915+105 that, as the soft X-ray luminosity increases, the QPO frequency increases and the phase lag associated with the QPO fundamental frequency changes sign from positive to negative. The relation between the disk temperature and the QPO frequency observed in GRS 1915+105 is consistent with a secular instability modulating the disk evaporation at the transition radius.