  We present a unifying scenario to address the physical origin of the
diversity of X-ray lightcurves within the rho variability class of the
microquasar GRS 1915+105. This 'heartbeat' state is characterized by a bright
flare that recurs every ~50-100 seconds, but the profile and duration of the
flares varies significantly from observation to observation. Based on a
comprehensive, phase-resolved study of heartbeats in the RXTE archive, we
demonstrate that very different X-ray lightcurves 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 lightcurve
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 lightcurves, 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.
