Statistical Properties of Gamma-Ray Burst Polarization

Abstract

The emission mechanism and the origin and structure of magnetic fields in gamma-ray burst (GRB) jets are among the most important open questions concerning the nature of the central engine of GRBs. In spite of extensive observational efforts, these questions remain to be answered and are difficult or even impossible to infer with the spectral and lightcurve information currently collected. Polarization measurements will lead to unambiguous answers to several of these questions. Recent developments in X-ray and y-ray polarimetry techniques have demonstrated a significant increase in sensitivity enabling several new mission concepts, e.g. POET (Polarimeters for Energetic Transients), providing wide field of view and broadband polarimetry measurements. If launched, missions of this kind would finally provide definitive measurements of GRB polarizations. We perform Monte Carlo simulations to derive the distribution of GRB polarizations in three emission models; the synchrotron model with a globally ordered magnetic field (SO model), the synchrotron model with a locally random magnetic field (SR model), and the Compton drag model (CD model). The results show that POET, or other polarimeters with similar capabilities, can constrain the GRB emission models by using the statistical properties of GRB polarizations. In particular, the ratio of the number of GRBs for which the polarization degrees can be measured to the number of GRBs that are detected (N m /N d) and the distributions of the polarization degrees (IT) can be used as the criteria. If N m /N d > 30% and n is clustered between 0.2 and 0.7, the SO model will be favored. If instead N m /N d < 15%, then the SR or CD model will be favored. If several events with n > 0.8 are observed, then the CD model will be favored.

The Compton Spectrometer and Imager (COSI)

“…Different theoretical models predict different levels of polarization [10,11], and thus polarimetry can shed light on magnetic field ordering and the orientation of an accretion disk or jet. COSI takes advantage of the polarization dependence of Compton-scattering in high sensitivity to gain insight into these extreme environments.…”

A high-energy Compton polarimeter for the POET SMEX mission

“…Fig. 1 shows the expected distribution of polarization levels that would be measured in the 60 -500 keV energy band, as a function of the peak energy (Ep) parameter of the Band spectrum 9 , for three different models: synchrotron emission from an ordered magnetic field (SO); synchrotron emission from a random magnetic field (SR), and inverse Compton emission from a random field ("Compton drag" model; CD) 3 Energy (keV) 1000 3 magnetic field model. Based on these calculations we estimate that, in order to discriminate between the models at better than the 3σ level, it will be necessary to observe at least ~35 GRBs with sufficient sensitivity to measure 30% polarization.…”

“…These measurements will provide insight into the structure of the relativistic jets believed to power GRB prompt emission 3 . The POET payload will consist of two wide field-of-view, non-imaging instruments: a Low Energy Polarimeter (LEP) operating in the X-ray band (2 -15 keV) via the photoelectric effect 4 , and a High Energy Polarimeter (HEP) operating in the hard X-ray to soft gamma-ray band (50 -500 keV) via Compton scattering.…”

Space-borne Hard X-ray Compton Polarimeter POLAR Data Center at PSI

“…It is equipped with an array of 1600 scintillator bars allowing for precise measurements of the polarization of hard X-rays in the energy range from 50 keV to 500 keV. It has a large field of view of about 1/3 of the sky and a sensitive area of about 80 cm 2 . Its minimal detectable polarization (MDP) for stronger Gamma-Ray Burst (GRB) events is lower than 10%.…”