A SEARCH FOR GAMMA-RAY BURST POLARIZATION USING THE IBIS COMPTON MODE DATA Mark H. Finger 1 , Chryssa Kouveliotou 2 , Lorenzo Natalucci 3 , Pietro Ubertini 3 , Angela Bazzano 3 , Memmo Federici 3 , Antony J. Bird 4 , and Anthony J. Dean 4 1 USRA, NSSTC, 320 Sparkman Dr., Huntsville, AL, 35805, U.S.A. 2 NASA/MSFC, NSSTC, 320 Sparkman Dr., Huntsville, AL, 35805, U.S.A. 3 IASF-Roma INAF, via del Fosso del Cavaliere 100, 00133 Roma, Italy 4 School of Physics & Astronomy, University of Southampton, SO17 1BJ, U.K. ABSTRACT The IBIS Compton mode data, which consists of events coincident between the ISGRI and PICsIT detector planes, is sensitive to gamma-ray polarization, due to the polarization dependence of the azimuthal distribution of Compton scatters. We present an analysis of the Comp- ton mode data obtained during GRB041219a, a bright gamma-ray burst which occurred within the IBIS fully- coded field of view. We discuss the impact of data losses and accidental coincidence events during the burst. We place a preliminary upper limit of 65% on any linear po- larization which persists throughout the burst. 1. COMPTON POLARIMETRY WITH IBIS The IBIS Compton mode – events coincident between IS- GRI and PICsIT, has polarimetry capabilities. The rate of events coincident between a given pixel in ISGRI and a given pixel (or multiple pixels) in PICsIT, depends on the Figure 1. The fractional amplitude (%) of the azimuthal modulation of the Compton cross section for 100% linear polarization. polarization of the incident gamma-rays due to the po- larization dependence of the Compton scattering cross- section. For an extensive review of Compton polarimetry see Lei, Dean & Hills [1]. For linearly polarized photons the Klein-Nishina cross section is dσ dΩ = r 2 0 2 E 2 1 E 2 0  E 1 E 0 + E 0 E 1 - sin 2 θ(1 + cos2φ)  (1) where r 0 is the classical electron radius, E 0 is the inci- dent photon energy, E 1 the scattered photon energy, θ the photon scatter angle, and φ the azimuth of the scat- ter about the incident photon direction as measured from the incident electric polarization vector. The incident and scattered photons energies are related kinematically by 1 E 1 = 1 E 0 + (1 - cos θ) m e c 2 (2) where m e is the electron mass. Figure 1 shows the frac- tional amplitude of the sinusoidal modulation of the az- imuthal scatter distribution given in equation 1. Figure 2. Azimuth angle distribution from a simulation of IBIS for 100% polarized 300 keV photons at normal incidence. The solid curve is a sinusoidal fit.