  Fast radio bursts (FRBs) are cosmic sources emitting millisecond-duration
radio bursts. Although several hundreds FRBs have been discovered, their
physical nature and central engine remain unclear. The variations of Faraday
rotation measure and dispersion measure, due to local environment, are crucial
clues to understanding their physical nature. The recent observations on the
rotation measure of FRB 20201124A show a significant variation on a day time
scale. Intriguingly, the oscillation of rotation measure supports that the
local contribution can change sign, which indicates the magnetic field reversal
along the line of sight. Here we present a physical model that explains
observed characteristics of FRB 20201124A and proposes that repeating signal
comes from a binary system containing a magnetar and a Be star with a decretion
disk. When the magnetar approaches the periastron, the propagation of radio
waves through the disk of the Be star naturally leads to the observed varying
rotation measure, depolarization, large scattering timescale, and Faraday
conversion. This study will prompt to search for FRB signals from Be/X-ray
binaries.
