Gamma-Ray Burst Polarimetry with the POLAR and POLAR-2 missions

TL;DR

Gamma-Ray Burst prompt emission polarization is a powerful diagnostic for constraining emission mechanisms and jet geometry, but early polarimeters yielded limited significance. This paper reviews POLAR’s six-month mission results, which show low time-integrated polarization with hints of polarization-angle evolution in time-resolved analyses, highlighting statistical limitations for energy-resolved measurements. It then details the design, calibration, and space-qualification plan for POLAR-2, an upgraded, modular Compton polarimeter that dramatically enhances sensitivity and enables robust time- and energy-resolved polarimetry for a larger GRB sample. The authors outline the expected scientific impact, including multi-messenger opportunities and rapid follow-up capabilities, and report POLAR-2’s current status toward a 2027 launch to the China Space Station. Overall, the POLAR-POLAR-2 program represents a substantial advancement in GRB polarimetry, with significant implications for models of prompt emission, magnetic fields, and jet structure.

Abstract

Gamma-Ray Bursts are among the most powerful and violent events in the Universe. Despite over half a century of observations of these transient sources, many open questions remain about their nature and the physical emission mechanisms at play. Polarization measurements of the GRB prompt gamma-ray emission have long been theorized to be able to answer most of these questions. Early polarization measurements did not allow to draw clear conclusions because of limited significance. With the aim of better characterizing the polarization of these prompt emissions, a compact Gamma-Ray polarimeter called POLAR has been sent to space as part of the Tiangong-2 Chinese space lab for 6 months of operations starting September 2016. The instrument detected 55 GRBs as well as several pulsars. Time-integrated polarization analysis of the 14 brightest detected GRBs has shown that the prompt emission is lowly polarized or fully unpolarized. However, time-resolved analysis depicted strong hints of an evolving polarization angle within single pulses, washing out the polarization degree in time-integrated analyses. Energy-resolved polarization analysis has shown no constraining results due to limited statistics. Hence, a more sensitive $γ$-ray polarimeter is required to perform detailed energy and time-resolved polarization analysis of the prompt gamma-ray emission of GRBs. Based on the success of the POLAR mission, a larger-scale instrument, approved for launch to the China Space Station (CSS) in 2027, is currently being developed by a Swiss, Chinese, Polish, and German collaboration. Thanks to its large sensitivity in the 20-800~keV range, POLAR-2 will produce polarization measurements of at least 50 GRBs per year with a precision equal to or higher than the best results published by POLAR, allowing for good quality time and energy resolved analysis.

Figures (11)

• Figure 1: Left: POLAR's CAD design and exploded view of a polarimeter module (taken from Estella_thesis, with permission). Right: POLAR scintillator targets wrapped with 3M Vikuti Enhanced Specular Reflector films.
• Figure 2: Left: Epeak or Ec [keV] (depending on spectral model: Band function Band93 or cutoff powerlaw) as a function of the burst fluence in the 10-1000 keV band and $T_{90}$ time interval for the 38 GRBs detected by POLAR with an off-axis angle $<90^\circ$. The markers' color indicates whether the burst was analyzed for polarization. Right: Posterior distributions of the PD [%] for the 14 GRBs for which polarization analysis has been performed in the POLAR catalog paper (taken from POLAR_catalog, with permission).
• Figure 3: Left: Exploded CAD model of a POLAR-2 polarimeter module (taken from NDA_thesis, with permission). Right: Exploded CAD model of the POLAR-2 instrument (taken from NDA_thesis, with permission).
• Figure 4: Left: Map of the expected radiation dose per polarimeter module normalized by the lowest value (taken from SiPM_irradiation_POLAR-2, with permission). It should be noted that the former module configuration was used here; they are now disposed in a 10$\times$10 array. Right: Temperature dependence of the fit parameters of the sensor dark current annealing-induced exponential recovery for Hamamatsu's S13360 SiPMs with 25, 50, and 75 $\mu$m microcell sizes (taken from Annealing, with permission).
• Figure 5: Definition of the X-Y-Z frame with respect to system. The Z axis is defined as the vertical direction (along the scintillator length), and the X and Y directions are defined in the horizontal plane along the scintillators. The readout system is placed in the aluminum frame. Accelerometers are placed on the flange of the module (green) and near the dampers (red) in order to monitor the resonance spectrum during and between the different tests (taken from NDA_thesis, with permission).