Using multiple observational arguments, recent work has shown that cosmological gamma-ray bursts (GRBs) are typically viewed at angles within, or close to the cores of their relativistic jets. One of those arguments relied on the lack of tens-of-days-long periods of very shallow evolution that would be seen in the afterglow light curves of GRBs viewed at large angles. Motivated by these results, we consider that GRBs efficiently produce γ -rays only within a narrow region around the core. We show that, on these near-core lines of sight, structured jets naturally produce shallow phases in the X-ray afterglow of GRBs. These plateaus would be seen by a large fraction of observers and would last between 102-105 s. They naturally reproduce the observed distributions of time-scales and luminosities as well as the intercorrelations between plateau duration, plateau luminosity, and prompt γ -ray energy. An advantage of this interpretation is that it involves no late-time energy injection which would be both challenging from the point of view of the central engine and, as we show here, less natural given the observed correlations between plateau and prompt properties.
Bibliographical noteFunding Information:
We thank Jonathan Granot, Ehud Nakar, Pawan Kumar, and Maria Dainotti for helpful discussions. P. Beniamini’s research was funded in part by the Gordon and Betty Moore Foundation through grant GBMF5076. FD, RM, and RD acknowledge material and financial support from the Centre National d’Études Spatiales.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
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