  We present a timing analysis of two Rossi X-ray Timing Explorer observations
of the microquasar GRS 1915+105 during the heartbeat state. The
phase-frequency-power maps show that the intermediate-frequency aperiodic X-ray
variability weakens as the source softens in the slow rise phase, and when the
quasi-periodic oscillation disappears in the rise phase of the pulse of the
double-peaked class its sub-harmonic is still present with a hard phase lag. In
the slow rise phase, the energy-frequency-power maps show that most of the
aperiodic variability is produced in the corona, and may also induce the
aperiodic variability observed at low energies from an accretion disk, which is
further supported by the soft phase lag especially in the
intermediate-frequency range (with a time delay up to 20 ms). In the rise phase
of the pulse, the low-frequency aperiodic variability is enhanced significantly
and there is a prominent hard lag (with a time delay up to 50 ms), indicating
that the variability is induced by extension of the disk toward small radii as
implied by the increase in flux and propagates into the corona. However, during
the hard pulse of the double-peaked class, the variability shows no significant
lag, which may be attributed to an optically thick corona. These timing results
are generally consistent with the spectral results presented by Neilsen et al.
(2011, 2012) which indicated that the slow rise phase corresponds to a local
Eddington limit and the rise phase of the pulse corresponds to a radiation
pressure instability in the disk.
