In-flight energy calibration of the space-borne Compton polarimeter POLAR

TL;DR

This paper details the in-flight energy calibration method for the POLAR space-borne detector, crucial for accurate polarization measurements of X-rays from gamma-ray bursts and solar flares, using radioactive sources and Monte Carlo simulations.

Contribution

It introduces a novel in-flight calibration approach for POLAR, combining Monte Carlo simulations and laboratory tests to ensure precise energy measurements during space operations.

Findings

Calibration method verified by laboratory tests.

Monte Carlo simulations support calibration accuracy.

In-flight data confirms calibration effectiveness.

Abstract

POLAR is a compact wide-field space-borne detector for precise measurements of the linear polarisation of hard X-rays emitted by transient sources in the energy range from 50 keV to 500 keV. It consists of a 40$\times$40 array of plastic scintillator bars used as a detection material. The bars are grouped in 25 detector modules. The energy range sensitivity of POLAR is optimized to match with the prompt emission photons from the gamma-ray bursts (GRBs). Polarization measurements of the prompt emission would probe source geometries, emission mechanisms and magnetic structures in GRB jets. The instrument can also detect hard X-rays from solar flares and be used for precise measurement of their polarisation. POLAR was launched into a low Earth orbit on-board the Chinese space-lab TG-2 on September 15th, 2016. To achieve high accuracies in polarisation measurements it is essential to assure both before and during the flight a precise energy calibration. Such calibrations are performed with four low activity $^{22}$Na radioactive sources placed inside the instrument. Energy conversion factors are related to Compton edge positions from the collinear annihilation photons from the sources. This paper presents main principles of the in-flight calibration, describes studies of the method based on Monte Carlo simulations and its laboratory verification and finally provides some observation results based on the in-flight data analysis.