POLAR-2 -- Latest Developments of the Next Generation GRB Polarimeter

TL;DR

POLAR-2 tackles the open challenge of GRB prompt-emission polarization by deploying a SiPM-based, 100-module polarimeter with an energy range of $30$-$800$ keV and a target of ≥ 200 GRBs per year. Beam-test validation at ESRF demonstrates higher light yield, reduced cross-talk, and an enhanced modulation factor, enabling time- and energy-resolved polarimetry across a large sample. Operating on the China Space Station with rapid alerts, POLAR-2 will advance multimessenger GRB studies and constrain models of jet structure and magnetic fields. A thorough calibration and production program ensures readiness by 2027, positioning POLAR-2 to deliver high-impact measurements that can discriminate among GRB emission mechanisms.

Abstract

Gamma-Ray Bursts (GRBs) are among the most energetic events in the Universe. Despite over 50 years of research and measurements their prompt emission remains poorly understood, with key questions surrounding the structure of relativistic jets, magnetic field configurations, and dominant radiation mechanisms. Polarization measurements are critical in resolving these uncertainties. The POLAR mission, operational in 2016-2017 on Tiangong-2, provided the most statistically significant GRB polarization data. Its results indicated low time-averaged polarization with hints of temporal evolution. However, POLAR's limited sensitivity, small effective area, and restricted energy range prevented more detailed time- and energy-resolved analyses in addition to a larger sample of GRB polarization measurements. POLAR-2 is designed to address these limitations by offering a fourfold increase in effective area (at least) and an extended energy range of 30-800 keV by utilizing Silicon Photomultipliers (SiPMs) and an updated module design, enabling the differentiation of competing GRB emission models. The instrument comprises of 100 polarimeter modules (each with 64 plastic scintillator bars), wherein the polarization angle is extracted through Compton Scattering of the gammas. The polarimeter module design was validated during an ESRF beam test campaign in 2023. The instrument was developed by a joint effort of Switzerland, China, Poland and Germany and is planned for launch in 2027. Currently, POLAR-2 is in its production phase with the first module targets being produced. We will provide an overview of the current status of the development.

Figures (5)

• Figure 1: Exploded view of the POLAR-2 payload
• Figure 2: a) POLAR-2 module exploded view. It consists of 64 scintillator bars, read out by four SiPM arrays connected to a front-end electronics mounted on a thermal mechanical support. Scintillators and FEE are inserted into a CFRP socket (figure taken with permissions from de_angelis_2023_thesis). b) Photo of the POLAR-2 front-end electronics (FEE).
• Figure 3: A selection of the results from the ESRF beam test campaign for a single POLAR-2 polarimeter module. The results show a good correspondence of the simulated detector response (blue) with respect to the measured data (red) (figures taken with permissions from Kole_2024).
• Figure 4: Expected POLAR-2 a) effective area for polarized events and b) modulation factor inferred from the ESRF beam test (figures taken with permissions from Kole_2024)
• Figure 5: a) Photo showing the size of the hole when drilled inside the scintillator compared to a standard scintillator bar. On the center hole is drilled in the tagged module. b) Activity of the "tagged" bar measured with a gamma detector after it has been placed wrapped and assembled inside module. c) Hit map of the Am-241 inside the "tagged" module, highlighting "tagged" bar.