Gamma-Ray Polarimetry of the Crab Pulsar Observed by POLAR

Abstract

The X/𝛾 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 which took data from September 2016 to April 2017. 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 fitted values and corresponding 1𝜎 deviations for the averaged phase interval: (PD=14 +15 −10 %, PA=108 +33 −54 • ), for Peak 1: (PD=17 +18 −12 %, PA=174 +39 −36 • ) and for Peak 2: (PD=16 +16 −11 %, PA=78 +39 −30 •). Further more, the 3𝜎 upper limits on the polarization degree are for the averaged phase interval (55%), Peak 1 (66%) and Peak 2 (57%). Finally, to illustrate the capabilities of this method in the future, we simulated two years 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% and 10% respectively.

Performance of the X-Calibur Hard X-Ray Polarimetry Mission during its 2018/19 Long-Duration Balloon Flight

“…6 −9.3 ) % for off-pulse emission, in the 20-160 keV band [3]. The gamma-ray burst polarimeter PO-LAR found a similar polarization fraction of 14 +15 −10 % for phase-averaged emission in the 50-500 keV energy range, but unlike PoGO+ excluded the nebula from the measurement [4]. The Soft Gamma-ray Detector (SGD) on board the Hitomi satellite found a phase-integrated polarization fraction of (22.1 ± 10.6) %, in agreement with the PoGO+ result [5].…”

Response of the first POLAR-2 prototype to polarized beams

“…Finally, energy-resolved polarization studies were performed, however, no significant dependency on the energy was found [17]. Apart from studying GRBs, the data from POLAR was also used to perform spectral polarimetric measurements of the Crab pulsar [18]. The measurement results from POLAR agree with the majority of the GRB emission models as these predict low polarization levels.…”

“…published results, the large statistical errors did not allow to fully probe the emission mechanism [18]. For these purposes the POLAR-2 project was initiated in 2018.…”