RADIATION PRESSURE AND MASS EJECTION IN ρ-LIKE STATES OF GRS 1915+105

We present a unifying scenario to address the physical origin of the diversity of X-ray light curves within the rho variability class of the microquasar GRS 1915+105. This heartbeat state is characterized by a bright flare that recurs every similar to 50-100 s, but the profile and duration of the flares vary significantly from observation to observation. Based on a comprehensive, phase-resolved study of heartbeats in the Rossi X-ray Timing Explorer archive, we demonstrate that very different X-ray light curves do not require origins in different accretion processes. Indeed, our detailed comparison of the phase-resolved spectra of a double-peaked oscillation and a single-peaked oscillation shows that different cycles can have basically similar X-ray spectral evolution. We argue that all heartbeat oscillations can be understood as the result of a combination of a thermal-viscous radiation pressure instability, a local Eddington limit in the disk, and a sudden, radiation-pressure-driven evaporation or ejection event in the inner accretion disk. This ejection appears to be a universal, fundamental part of the rho state, and is largely responsible for a hard X-ray pulse seen in the light curve of all cycles. We suggest that the detailed shape of oscillations in the mass accretion rate through the disk is responsible for the phenomenological differences between different rho-type light curves, and we discuss how future time-dependent simulations of disk instabilities may provide new insights into the role of radiation pressure in the accretion flow.