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We analyze X-ray light curves of the blazar Mrk 421 obtained from the SoftX-ray Imaging Telescope and the Large Area X-Ray Proportional Counterinstrument onboard the Indian space telescope $AstroSat$ and archivalobservations 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 negativebelow a characteristic "break-timescale". Galactic black hole X-ray binariesand Seyfert galaxies exhibit a similar characteristic timescale in their X-rayvariability that is proportional to their respective black hole mass. X-rays inthese objects are produced in the accretion disk or corona. Hence, such atimescale is believed to be linked to the properties of the accretion flow. Anyrelation observed between events in the accretion disk and those in the jet canbe used to characterize the disk-jet connection. However, evidence of such linkhave been scarce and indirect. Mrk 421 is a BL Lac object which has a prominentjet pointed towards us and a weak disk emission, and it is assumed that most ofits X-rays are generated in the jet. Hence, existence of the break in its X-rayPSD may indicate that changes in the accretion disk, which may be the source ofthe break timescale are translating into the jet, where the X-rays areproduced.

Possible Accretion Disk Origin of the Emission Variability of a Blazar Jet