PoGOLite - A High Sensitivity Balloon-Borne Soft Gamma-ray Polarimeter

TL;DR

PoGOLite is a balloon-borne gamma-ray polarimeter designed to detect polarization in the 25-80 keV range, enabling insights into high-energy astrophysical processes with high sensitivity and background suppression.

Contribution

This paper introduces PoGOLite, a novel balloon-borne instrument employing advanced detector technology and shielding to measure soft gamma-ray polarization with unprecedented sensitivity.

Findings

Capable of detecting 10% polarization from 200mCrab sources in 6 hours

Achieves high S/N ratio through specialized background reduction techniques

Will differentiate emission models for pulsars and identify components in Cygnus X-1

Abstract

We describe a new balloon-borne instrument (PoGOLite) capable of detecting 10% polarisation from 200mCrab point-like sources between 25 and 80keV in one 6 hour flight. Polarisation measurements in the soft gamma-ray band are expected to provide a powerful probe into high-energy emission mechanisms as well as the distribution of magnetic fields, radiation fields and interstellar matter. At present, only exploratory polarisation measurements have been carried out in the soft gamma-ray band. Reduction of the large background produced by cosmic-ray particles has been the biggest challenge. PoGOLite uses Compton scattering and photo-absorption in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence shield and a thick polyethylene neutron shield. The narrow FOV (1.25msr) obtained with well-type phoswich detector technology and the use of thick background shields enhance the detected S/N ratio. Event selections based on recorded phototube waveforms and Compton kinematics reduce the background to that expected for a 40-100mCrab source between 25 and 50keV. A 6 hour observation on the Crab will differentiate between the Polar Cap/Slot Gap, Outer Gap, and Caustic models with greater than 5 sigma; and also cleanly identify the Compton reflection component in the Cygnus X-1 hard state. The first flight is planned for 2010 and long-duration flights from Sweden to Northern Canada are foreseen thereafter.