Gamma-ray polarimetry of the Crab pulsar observed by POLAR

Abstract

ABSTRACT The X/gamma-ray polarimetry of the Crab pulsar/nebula is believed to hold crucial information on their emission models. In the past, several missions have shown evidence of polarized emission from the Crab. The significance of these measurements remains, however, limited. New measurements are therefore required. POLAR is a wide Field of View Compton-scattering polarimeter (sensitive in 50–500 keV) onboard the Chinese spacelab Tiangong-2 that took data from 2016 September to 2017 April. Although not designed to perform polarization measurements of pulsars, we present here a novel method, which can be applied to POLAR as well as that of other wide Field of View polarimeters. The novel polarimetric joint-fitting method for the Crab pulsar observations with POLAR, allows us to obtain constraining measurements of the pulsar component. The best-fitting values and corresponding 1σ deviations for the averaged phase interval: (PD = $14\substack{+15 \\-10}$ per cent, PA = $108\substack{+33 \\-54} ^{\circ }$), for Peak 1: (PD = $17\substack{+18 \\-12}$ per cent, PA = $174\substack{+39 \\-36} ^{\circ }$) and for Peak 2: (PD = $16\substack{+16 \\-11}$ per cent, PA = $78\substack{+39 \\-30} ^{\circ }$). Furthermore, the 3σ upper limits on the polarization degree are for the averaged phase interval (55 per cent), Peak 1 (66 per cent) and Peak 2 (57 per cent). Finally, to illustrate the capabilities of this method in the future, we simulated 2 yr observation to the Crab pulsar with POLAR-2. The results show that POLAR-2 is able to confirm the emission to be polarized with 5σ and 4σ confidence level if the Crab pulsar is polarized at $20{{\,\rm per\,cent}}$ and $10{{\,\rm per\,cent}}$, respectively.