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We describe a technique for measuring the polarization of hard X-rays from a celestial source based on the angular distribution of that portion of the flux which is scattered off the top of the Earth's atmosphere. The scattering cross section depends not only on the scatter angle itself, but on the orientation of the scatter angle with respect to the incident polarization vector. Consequently, the distribution of the observed albedo flux will depend on the direction and the polarization properties (i.e., the level of polarization and polarization angle) of the source. Since the albedo component can represent a relatively large fraction (up to 40\%) of the direct source flux, there will generally be sufficient signal for making such a measurement. The sensitivity of this approach is therefore dictated by the effective area and the ability of a detector system to 'image' the albedo flux. Polarization sensitivities down to a level of a few percent for a moderately sized gamma-ray burst can be achieved with a relatively simple design.
The $4\pi$ coverage of the BATSE detectors on the Compton Gamma-Ray Observatory provides an opportunity to measure both the direct and the albedo flux from a given transient event (either a gamma-ray burst or solar flare). Although the BATSE design (with its large field-of-view for each detector) is not optimized for albedo polarimetry, we have nonetheless investigated the feasibility of this technique using BATSE data. Here we shall report on our progress to date.
