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I've read that with short hard gamma ray bursts (shGRBs) associated with the coalescence of NS-NS and NS-BH binaries are expected to be beamed along the axis of the orbital angular momentum (i.e. perpendicular to the orbital plane) because the "baryon loading is lowest there". Here, NS = neutron star, BH = (stellar mass) black hole.

  1. What is meant by baryon loading?

  2. Why is it lowest along the direction of the orbital angular momentum?

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up vote 6 down vote accepted

The baryon load is either the number or the density of baryons. It is the amount of baryons the burst must either clear out or carry along.

It is low at the poles and high in the orbital plain because there is nothing but empty space at the poles, whereas in the orbital plane there can be an accretion disk as well as the two possibly disrupted orbiting bodies and their remnants, which can be a very significant amount of matter, and thus smother (or obscure) the burst in these directions.

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So in short, it's a metric for the total number of particles that are "in the way". – Brandon Enright Apr 9 '13 at 22:57

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