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

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 40×40 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, arXiv:1710.08918v4 [physics.ins-det] 5 Jun 2018 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.

A fully automated data center for the space-borne hard X-ray Compton polarimeter POLAR developed at PSI

“…In this mode, the instrument collects data continuously apart from short passages through the South Atlantic Anomaly (SAA) where the normal mode is temporarily replaced by the SAA mode. More details on the POLAR instrument and its operations can be found in Refs., e.g., [1,2,3,4].…”

“…Afterward, data merging by the coincidence algorithm described in Ref. [1] is applied to the level-2 datasets. This step also enables extraction of the in-flight calibration events.…”

“…The 22 Na nuclei emit two collinear photons with energies of 511 keV. Selection of these two collinear photons during off-line analysis allows for proper calibration of the whole detector [1].…”

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

“…The POLAR flight model contains four 22 Na sources, the activity of each was approximately 100 Bq at the time of launch. The four sources are positioned in different corners of four different modules in the instrument and are used for the gain calibration of all 1600 channels [23]. The events of the two characteristic back-to-back emission 511 keV photons generated by the positrons released from the 22 Na sources can be selected out from the inorbit background events by applying some geometrical cuts in combination with the knowledge of the locations of the four sources in the 2D detection plane of the instrument.…”

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

“…POLAR [97][98][99] is a compact wide field polarimeter developed by an international collaboration of Switzerland, China and Poland. It was launched on 15 September 2016 to be hosted on-board the China Space Laboratory TG-2.…”