The true nature of the progenitor to GRBs remains elusive ; one characteristic that would constrain our understanding of the GRB mechanism considerably is gamma-ray polarimetry measurements of the initial burst flux .
We present a method that interprets the prompt GRB flux as it Compton scatters off the Earth ’ s atmosphere , based on detailed modelling of both the Earth ’ s atmosphere and the orbiting detectors .
The BATSE mission aboard the CGRO monitored the whole sky in the 20 keV – 1 MeV energy band continuously from April 1991 until June 2000 .
We present the BATSE Albedo Polarimetry System ( BAPS ) , and show that GRB 930131 and GRB 960924 provide evidence of polarisation in their prompt flux that is consistent with degrees of polarisation of \Pi > 35 % and \Pi > 50 % respectively .
While the evidence of polarisation is strong , the method is unable to strongly constrain the degree of polarisation beyond a systematics based estimation .
Hence the implications on GRB theory are unclear , and further measurements essential .