Calibration of Gamma-ray Burst Polarimeter POLAR

TL;DR

This paper details the calibration methodology and results for POLAR, a space-based gamma-ray burst polarimeter, enhancing the accuracy of polarization measurements crucial for understanding GRB mechanisms.

Contribution

It introduces a dedicated calibration approach for POLAR, combining laboratory sources and synchrotron X-ray beams, to improve measurement precision.

Findings

Calibration of energy response and conversion factors

Determination of threshold values and crosstalk contributions

Assessment of polarization modulation factors

Abstract

Gamma Ray Bursts (GRBs) are the strongest explosions in the universe which might be associated with creation of black holes. Magnetic field structure and burst dynamics may influence polarization of the emitted gamma-rays. Precise polarization detection can be an ultimate tool to unveil the true GRB mechanism. POLAR is a space-borne Compton scattering detector for precise measurements of the GRB polarization. It consists of a 40$\times$40 array of plastic scintillator bars read out by 25 multi-anode PMTs (MaPMTs). It is scheduled to be launched into space in 2016 onboard of the Chinese space laboratory TG2. We present a dedicated methodology for POLAR calibration and some calibration results based on the combined use of the laboratory radioactive sources and polarized X-ray beams from the European Synchrotron Radiation Facility. They include calibration of the energy response, computation of the energy conversion factor vs. high voltage as well as determination of the threshold values, crosstalk contributions and polarization modulation factors.