PoGO : The polarised gamma-ray observer
Introduction
The Polarised Gamma-ray Observer (PoGO) [1] is a balloon-borne instrument designed to measure the polarisation of soft gamma-rays over the energy range 30–. The polarisation is derived from the asymmetry in the distribution of azimuthal scattering angles for gamma-rays which Compton scatter within the instrument [2]. This technique is adapted from hard X-ray balloon [3] and satellite [4] spectrometers. During a 6– flight, PoGO will be able to detect at least 10% polarisation from sources and open a new observational window on gamma-ray emission and transportation mechanisms within a variety of astrophysical sources, such as Active Galactic Nuclei, pulsars and X-ray binary systems. The instrument is proposed by an international collaboration with participants from France (Ecole Polytechnique), Japan (Hiroshima University, Institute for Space and Astronautical Science, Tokyo Institute of Technology, Yamagata University), Sweden (The Royal Institute of Technology and Stockholm University) and the United States (NASA Goddard Space Flight Centre, Princeton University, Stanford Linear Accelerator Centre).
Polarisation measurements, from radio to ultraviolet wavelengths, have been used to extract information on radiation processes and on the structure and geometry of a range of astrophysical sources. A few examples are studies of plasma and magnetic phenomena on the Sun, magnetic fields in the interstellar medium and accretion onto magnetic compact stars.
While polarimetric observations have been very productive at longer wavelengths, X-ray and gamma-ray polarimetry is still in its infancy. In addition to the Sun there has only been two detections of polarisation at these energies. The first was made with an instrument on-board the OSO-8 satellite [5] in 1976. The Crab Nebula was viewed at energies of 2.6 and and Bragg diffraction was used to measure the photon polarisation. Recently, the detection of high linear polarisation, (80±20)%, in a gamma ray burst observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has been reported [6]. If confirmed, this observation will have far reaching implications for models of gamma-ray bursts.
Many of the X-ray and gamma-ray sources observed on the sky are expected not only to be polarised but also to exhibit time and energy variations in polarisation properties. These dependencies can be used for diagnostics and tests of source models which are outside the reach of present instruments. PoGO will provide the capability to make a series of breakthrough observations in key areas of high energy astrophysics.
Section snippets
Scientific aims
Prime targets for gamma-ray polarimetry are sources associated with black holes in active galactic nuclei and in galactic X-ray binaries, and magnetic neutron stars in the form of pulsars and X-ray binaries. Most of the astronomical high energy sources appear point-like to the present gamma-ray instruments. Studies of source geometry and structure must therefore be performed by indirect means. At longer wavelengths, the analysis of polarisation properties has proven a powerful technique to
Description of the PoGO instrument
A hexagonal close-packed array of 397 scintillator-based well-type phoswich detectors lies at the heart of PoGO. A single well unit is shown in Fig. 2. The well is built up from a stack of different scintillator materials. The base of the well is made from a short section of bismuth germanate (BGO). This is coupled to a solid rod of fast scintillator upon which a hollow tube (‘well’) of slow scintillator is mounted. The entire assembly is read-out by a single photomultiplier tube coupled to the
Mission plans
A balloon will carry PoGO to its operating altitude of about . Due to the restricted field of view, PoGO must be accurately pointed toward the astrophysical object it is imaging and this pointing be maintained during the flight. The first test flights (tentatively scheduled for 2007) will focus on measurements of northern-latitude (quasi-) steady sources such as the Crab, Cyg X-1 or Her X-1. In a single balloon flight, it is expected to be able to measure at least a 10%
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Cited by (9)
Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation
2009, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentMeasuring energy dependent polarization in soft γ-rays using Compton scattering in PoGOLite
2007, Astroparticle PhysicsBeam test of a prototype phoswich detector assembly for the PoGOLite astronomical soft gamma-ray polarimeter
2007, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentSimulation of a soft-gamma-ray polarimeter on board a microsatellite
2023, Nuclear Science and Techniques