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The Question

(Submitted June 14, 1999)

Please explain the phenomena associated with the luminosity of GRB990510 that has been explored recently. Thank you.

The Answer

GRB 990510's optical afterglow was well-placed to be viewed continuously for many hours during its first day after the burst. As a result, we have an unprecedented multi-color, continuous light curve for it.

The light curve shows that the light does not follow a straight power law decay, but falls off more rapidly than would be extrapolated from the early observations.

This is what would be predicted if the burst were not isotropic but was, rather, in the form of a jet. As the jet slows down it spreads out, and so it no longer shines as brightly in the direction of the jet as it would otherwise, letting some emission go off to the side.

The afterglow from this GRB was also polarized by 1.6%, which also suggests a jet.

If the emission from GRBs is in the form of jets, then that means that there is less energy in the burst than a straightforward calculation from observations would suggest.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

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