  High-energy gamma-ray emission from the Galactic plane above ~100 MeV is
composed of three main contributions: diffuse emission from cosmic ray
interactions in the interstellar medium, emission from extended sources, such
as supernova remnants and pulsar wind nebulae, and emission from isolated
compact source populations. The diffuse emission and emission from the extended
sources provide the dominant contribution to the flux almost everywhere in the
inner Galaxy, preventing the detection of isolated compact sources. In spite of
this difficulty, compact sources in the Galactic plane can be singled out based
on the variability properties of their gamma-ray emission. Our aim is to find
sources in the Fermi data that show long-term variability. We performed a
systematic study of the emission variability from the Galactic plane, by
constructing the variability maps. We find that emission from several
directions along the Galactic plane is significantly variable on a time scale
of months. These directions include, in addition to known variable Galactic
sources and background blazars, the Galactic ridge region at positive Galactic
longitudes and several regions containing young pulsars. We argue that
variability on the time scale of months may be common to pulsars, originating
from the inner parts of pulsar wind nebulae, similarly to what is observed in
the Crab pulsar.
