Instrument performance and simulation verification of the POLAR detector

Abstract

POLAR is a new satellite-born detector aiming to measure the polarization of an unprecedented number of Gamma-Ray Bursts in the 50-500 keV energy range. The instrument, launched on-board the Tiangong-2 Chinese Space lab on the 15th of September 2016, is designed to measure the polarization of the hard X-ray flux by measuring the distribution of the azimuthal scattering angles of the incoming photons. A detailed understanding of the polarimeter and specifically of the systematic effects induced by the instrument's non-uniformity are required for this purpose. In order to study the instrument's response to polarization, POLAR underwent a beam test at the European Synchrotron Radiation Facility in France. In this paper both the * Corresponding author beam test and the instrument performance will be described. This is followed by an overview of the Monte Carlo simulation tools developed for the instrument. Finally a comparison of the measured and simulated instrument performance will be provided and the instrument response to polarization will be presented.

Polarimetry with POLAR

“…The value of µ 100 is an instrument dependent variable which furthermore depends on the energy of the incoming photons and the incoming angle of the flux with respect to the instrument. In the case of POLAR this value is acquired using simulations verified using dedicated on ground calibration tests [24] and is of the order of 30 − 40% depending on the photon energy. A last parameter often used in polarization to indicate the statistical significance of a measurement is the Minimal Detectable Polarization (MDP) [26].…”

“…For example the potential induction of a fake polarization signal by instrument systematics needs to be studied in detail first as discussed earlier in this paper. For this purpose POLAR underwent detailed calibration tests prior to launch, the results of which are presented in [24] and show that the on ground performance of the instrument is well understood and can be accurately reproduced using Monte Carlo simulations. Furthermore the performance of the instrument in-orbit and potential differences with those measured on ground…”

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

“…This, together with its high sensitivity, makes it capable of almost continuously monitoring point sources such as PSRs. More details of the POLAR detectors construction can be reffered to [14,15], while the descriptions of the instrument on-ground and in-orbit performance and simulation software the can be referred to [16] and [17].…”

In-orbit instrument performance study and calibration for POLAR polarization measurements

“…These in-orbit calibration parameters include pedestal and noise levels of the FEE, the nonlinearity function of the gain in the electronics, the ADC threshold of each channel, the crosstalk matrix of each module and the gain of each channel. All these calibration parameters are not only necessary in the analysis pipeline to reconstruct the deposited energy of each bar and reduce the systematic effects from the instrument, but also work as the input parameters for the Monte Carlo simulations [19] that simulate the digitization process of the PMTs and FEEs and handle the event response, which is another important procedure of the polarization analysis by comparing the modulation curves between the measured data and the simulated data. Therefore, the methods to calculate all these calibration parameters using in-orbit data and their typical values are firstly discussed and presented.…”