  PolarLight is a space-borne X-ray polarimeter that measures the X-ray
polarization via electron tracking in an ionization chamber. It is a collimated
instrument and thus suffers from the background on the whole detector plane.
The majority of background events are induced by high energy charged particles
and show ionization morphologies distinct from those produced by X-rays of
interest. Comparing on-source and off-source observations, we find that the two
datasets display different distributions on image properties. The boundaries
between the source and background distributions are obtained and can be used
for background discrimination. Such a means can remove over 70% of the
background events measured with PolarLight. This approaches the theoretical
upper limit of the background fraction that is removable and justifies its
effectiveness. For observations with the Crab nebula, the background
contamination decreases from 25% to 8% after discrimination, indicative of a
polarimetric sensitivity of around 0.2 Crab for PolarLight. This work also
provides insights into future X-ray polarimetric telescopes.
