In-flight Low Energy X-ray Calibration of POLAR Detector on TianGong2

Abstract

POLAR is a γ-ray burst polarimeter on board the TianGong2 space lab, which was launched into a near earth orbit in 2016 September 15. It is designed to measure the linear polarization of hard X-ray in the 50–500keV energy range. The detector has 25 modules, and each module has 64 plastic scintillator bars. With a total number of 1600 plastic scintillator bars, POLAR has a very large effective detection area and field of view. During its on-orbit operation, POLAR has detected multiple small flares. Their hard X-ray photon energies are usually less than 50keV, and can not be analyzed directly by using the on-orbit and ground-based high-energy calibration results. By combining the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) observation of the flare SOL2016112907 with the Monte Carlo simulation, this paper calibrates the response of POLAR in the energy band lower than 50keV. We find that the energy threshold (∼ 10keV) and conversion factor obtained by calibration are relatively stable, but they are quite different from the values obtained via calibration in the high-energy range, and that the differences for the different scintillator bars have no evident regularity.