Vol 21, No 9

Discrimination of background events in the PolarLight X-ray polarimeter

Jiahuan Zhu, Hong Li, Hua Feng, Jiahui Huang, Xiangyun Long, Qiong Wu, Weichun Jiang, Massimo Minuti, Saverio Citraro, Hikmat Nasimi, Dongxin Yang, Jiandong Yu, Ge Jin, Ming Zeng, Peng An, Luca Baldini, Ronaldo Bellazzini, Alessandro Brez, Luca Latronico, Carmelo Sgrò, Gloria Spandre, Michele Pinchera, Fabio Muleri, Paolo Soffitta, Enrico Costa


Abstract 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.


Keywords instrumentation: polarimeters — methods: data analysis — X-rays: general

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