GRB 140206A: the most distant polarized Gamma-Ray Burst

Accepted. Received; in original form The nature of the prompt γ-ray emission of Gamma-Ray Bursts (GRBs) is still far from being completely elucidated. The measure of linear polarization is a powerful tool that can be used to put further constraints on the content and magnetization of the GRB relativistic outflows, as well as on the radiation processes at work. To date only a handful of polarization measurements are available for the prompt emission of GRBs. Here we present the analysis of the prompt emission of GRB 140206A, obtained with INTEGRAL/IBIS, Swift/BAT, and Fermi/GBM. Us-ing INTEGRAL/IBIS as a Compton polarimeter we were able to constrain the linear polarization level of the second peak of this GRB as being larger than 28 % at 90 % c.l. We also present the GRB afterglow optical spectroscopy obtained at the Telesco-pio Nazionale Galileo (TNG), which allowed us the measure the distance of this GRB, z=2.739. This distance value together with the polarization measure obtained with IBIS, allowed us to derive the deepest and most reliable limit to date (ξ <1×10−16) on the possibility of Lorentz Invariance Violation, measured through the vacuum bire-fringence effect on a cosmological source