Impact of the ISM magnetic field on GRB afterglow polarization

ABSTRACT Linear polarization has been measured in several gamma-ray burst (GRB) afterglows. After a few days, arises from the forward shock emission that depends on post-shock magnetic field. The latter can originate both compression of existing fields, here interstellar medium (ISM) field, and shock-generated instabilities. For short GRBs, previous modelling arising considered random field fully or partially confined to plane. However, ISM likely consists ordered components. Here we study impact more realistic having We present our semi-analytical model compute curves for different configurations. find presence an component, even significantly weaker than one, distinct signatures could be detectable. In component not observer plane, show (i) inside jet, angle θp either remains constant during all afterglow phase exhibits variations smaller 90° swing expected solely; (ii) off-axis observer, evolves $\theta _\mathrm{ p}^{\max }$, before jet break its opposite after break. also upper limit GRB 170817 requires plane is compatible with as large half one.