Phase-resolved gamma-ray spectroscopy of the Crab pulsar observed by POLAR

Abstract

The POLAR detector is a space based Gamma-Ray Burst (GRB) polarimeter sensitive in the 15-500 keV energy range. Apart from its main scientific goal as a Gamma-Ray Burst polarimeter it is also able to detect photons from pulsars in orbit. By using the six-months in-orbit observation data, significant pulsation from the PSR B0531+21 (Crab pulsar) was obtained. In this work, we present the precise timing analysis of the Crab pulsar, together with a phase-resolved spectroscopic study using a joint-fitting method adapted for wide field of view instruments like POLAR. By using single power law fitting over the pulsed phase, we obtained spectral indices ranging from 1.718 to 2.315, and confirmed the spectral evolution in a reverse S shape which is homogenous with results from other missions over broadband. We will also show, based on the POLAR in-orbit performance and Geant4 Monte-Carlo simulation, the inferred capabilities of POLAR-2, the proposed follow-up mission of POLAR on board the China Space Station (CSS), for pulsars studies.

The POLAR gamma-ray burst polarization catalog

“…Measurements of the pulsar are possible, albeit challenging, using time-resolved analysis. The Crab pulsar was previously used to calibrate the timing precision of POLAR (Li et al 2017) and its energy response (Li et al 2019). Analysis of the polarization is currently ongoing (Li et al 2020).…”

Development and science perspectives of the POLAR-2 instrument: a large scale GRB polarimeter

“…Time-resolved analysis showed a hint for continuous evolution of the PA with time that washes out the PD on time-integrated analysis for several GRBs [7], but the lack of statistics does not allow to exclude a discrete single change of the PA. Some non GRB related analyses were also performed using the POLAR data and are still ongoing, especially spectral and polarization analyses of the Crab pulsar as discussed in [8] and more recently in [9].…”

Polarization measurements of the Crab Pulsar with POLAR

“…At present, we have confirmed pulsations for the Crab pulsar and PSR B1509-58 by searching known pulsars from POLAR data, their pulse profiles are shown in Figure 1. The timing analysis is based on [3]. And the best periodical parameters are shown in Table 1.…”

“…Before proceeding with the polarization studies of the Crab pulsar using POLAR data, we decided to further check the accuracy of instrumental responses of all incoming directions by performing a phase-resolved spectroscopy analysis [3]. After extracting spectrum and joint-fitting to it, we obtain the spectral indices (single power-law) as a function of pulse phase of the Crab pulsar, which are shown in Figure 5.…”

“…And we verified that this method is adequate for the reconstruction of polarization, including Polarization Angle (PA) and Polarization Degree). Limited by the paper length, we skip the technical details about the de-rotation method, but it will be refered to [6].…”