Fig. A.1
Geometry of the hotspot emission. 
is the normal to the surface of
the hotspot and 
is the normal of the surface
,
which forms an angle θn with
. θflow/ 2
(see text) is equal to θn when the momentum flux
integrated in azimuthal angle between 0 and θn equals 68% of the momentum
flux integrated over the total hemisphere (between 0 and
).
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