  Energy dependent phase lags reveal crucial information about the causal
relation between various spectral components and about the nature of the
accretion geometry around the compact objects. The time-lag and the fractional
root mean square (rms) spectra of GRS 1915+105 in its heartbeat oscillation
class/$\rho$ state show peculiar behaviour at the fundamental and harmonic
frequencies where the lags at the fundamental show a turn around at $\sim$ 10
keV while the lags at the harmonic do not show any turn around at least till
$\sim$ 20 keV. The magnitude of lags are of the order of few seconds and hence
cannot be attributed to the light travel time effects or Comptonization delays.
The continuum X-ray spectra can roughly be described by a disk blackbody and a
hard X-ray power-law component and from phase resolved spectroscopy it has been
shown that the inner disk radius varies during the oscillation. Here, we
propose that there is a delayed response of the inner disk radius (DROID) to
the accretion rate such that $r_{in}(t)\propto \dot{m}^\beta (t-\tau_d)$. The
fluctuating accretion rate drives the oscillations of the inner radius after a
time delay $\tau_d$ while the power-law component responds immediately. We show
that in such a scenario a pure sinusoidal oscillation of the accretion rate can
explain not only the shape and magnitude of energy dependent rms and time-lag
spectra at the fundamental but also the next harmonic with just four free
parameters.
