1 research outputs found
"Orphan" afterglows in the Universal Structured Jet Model for gamma-ray bursts
The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Serendipitous
searches of afterglows, especially at radio wavelengths, have long been the
classic alternative. These survey data have been interpreted assuming a
uniformly emitting jet with sharp edges (``top-hat'' jet), in which case the
ratio of weakly relativistically beamed afterglows to GRBs scales with the jet
solid angle. In this paper, we consider, instead, a very wide outflow with a
luminosity that decreases across the emitting surface. In particular, we adopt
the universal structured jet (USJ) model, that is an alternative to the top-hat
model for the structure of the jet. However, the interpretation of the survey
data is very different: in the USJ model we only observe the emission within
the jet aperture and the observed ratio of prompt emission rate to afterglow
rate should solely depend on selection effects. We compute the number and rate
of afterglows expected in all-sky snapshot observations as a function of the
survey sensitivity. We find that the current (negative) results for OA searches
are in agreement with our expectations. In radio and X-ray bands this was
mainly due to the low sensitivity of the surveys, while in the optical band the
sky-coverage was not sufficient. In general we find that X-ray surveys are poor
tools for OA searches, if the jet is structured. On the other hand, the FIRST
radio survey and future instruments like the Allen Telescope Array (in the
radio band) and especially GAIA, Pan-Starrs and LSST (in the optical band) will
have chances to detect afterglows.Comment: 10 pages, 8 figures. MNRAS accepted. Moderate revision