Abstract
The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from the Esrange Space Center in Sweden to an altitude of approximately 40 km. Several flights have been conducted, resulting in two independent measurements of the Crab polarization and one of Cygnus X-1. Here, a review of the PoGO mission is presented, including a description of the payload and the flight campaigns, and a discussion of some of the scientific results obtained to date.
Highlights
PoGO+ is a balloon-borne hard X-ray Compton polarimetry telescope operating in the energy range ∼20–180 keV
Unlike satellite-based measurements, scientific ballooning missions allow modifications and upgrades to the payload design to be implemented based on flight data and experience, increasing the chance of success in a re-flight
PoGO+ has successfully conducted polarimetric observations of the Crab system and Cygnus X-1 in the energy range ∼20–180 keV, based on experience from the flight of the PoGOLite “Pathfinder.” Results presented for the Crab [14] are the first in this energy range
Summary
PoGO+ is a balloon-borne hard X-ray Compton polarimetry telescope operating in the energy range ∼20–180 keV. Using a two-dimensional position-sensitive device inside a free-moving gimbal, which automatically locates and follows the sun, this tracker can provide the instrument’s azimuth based on its location and the position of the sun These sensors provide absolute information for instrument pointing. The camera identifies the pixel coordinates of the guide star and matches these with the expected coordinates, calculated in real time based on the instrument position, altitude, and time of day, correcting for the field rotation of the star field as time goes by In this tracking mode, motor feedbacks are generated based on the difference between the expected and the observed pixel coordinates of the relevant star.
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