Multi-messenger determination of the viewing angle θ_v and circum-merger medium density n in the case of GRB170817A. We present 1-σ confidence regions (solid line: median; dashed line: 68% confidence limits) obtained from the GW data assuming the source localization (red), the radio afterglow’s properties around its peak (black, see Eq. (1)) and very long baseline interferometry imaging measurements (blue). Green triangles show the upper limit on n deduced from the (as yet) undetected kilonova afterglow. The preferred region for θ_v and n is highlighted in purple. The text gives details andreferences.