  We analyze X-ray light curves of the blazar Mrk 421 obtained from the Soft
X-ray Imaging Telescope and the Large Area X-Ray Proportional Counter
instrument onboard the Indian space telescope $AstroSat$ and archival
observations from $Swift$. We show that the X-ray power spectral density (PSD)
is a piece-wise power-law with a break, i.e., the index becomes more negative
below a characteristic "break-timescale". Galactic black hole X-ray binaries
and Seyfert galaxies exhibit a similar characteristic timescale in their X-ray
variability that is proportional to their respective black hole mass. X-rays in
these objects are produced in the accretion disk or corona. Hence, such a
timescale is believed to be linked to the properties of the accretion flow. Any
relation observed between events in the accretion disk and those in the jet can
be used to characterize the disk-jet connection. However, evidence of such link
have been scarce and indirect. Mrk 421 is a BL Lac object which has a prominent
jet pointed towards us and a weak disk emission, and it is assumed that most of
its X-rays are generated in the jet. Hence, existence of the break in its X-ray
PSD may indicate that changes in the accretion disk, which may be the source of
the break timescale are translating into the jet, where the X-rays are
produced.
