Reverse and forward shock afterglow emission from steep jets viewed off-axis
Abdikamalov E. Beniamini P.
1 May 2025Oxford University Press
Monthly Notices of the Royal Astronomical Society
2025#539Issue 32707 - 2717 pp.
We study the morphology of gamma-ray burst (GRB) afterglows viewed off-axis using a simplified analytical model. We consider steep jets, which are expected to be the most common type. These jets, characterized by steep lateral gradients in energy and Lorentz factor, produce highly beamed emission. The observed signal is dominated by their minimum visible angle at any given time. Consequently, the afterglow morphology depends on when this angle begins to decrease, revealing the inner regions of the jet. Depending on whether this decrease occurs before, at, or after the reverse shock crosses the ejecta, three distinct classes of light curves emerge. In the first scenario, the de-beamed emission can produce a rapidly rising signal even prior to the reverse shock crossing. This is expected in GRBs with long duration, low energy, dense circumburst media, or combinations thereof. In some cases, the ejecta shell can be considered as effectively thick in the inner regions and effectively thin in the outer regions. For forward shocks, the temporal slopes in both regimes are identical, which makes it hard to detect the transition. Reverse shocks, however, have distinct temporal slopes, allowing potential detection of the transition in light curves if their emission surpasses that of the forward shock. The characteristic synchrotron frequency of de-beamed emission evolves independently of jet structure for forward shocks but depends on the lateral energy and Lorentz factor gradients for the reverse shock, with slower evolution for steep energy and shallow Lorentz factor gradients.
gamma-ray burst: general , gamma-rays: general , radiation mechanisms: general
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Department of Physics, Nazarbayev University, 53 Kabanbay Batyr Ave, Astana, 010000, Kazakhstan
Energetic Cosmos Laboratory, Nazarbayev University, 53 Kabanbay Batyr Ave, Astana, 010000, Kazakhstan
Department of Natural Sciences, The Open University of Israel, PO Box 808, Raanana, 4353701, Israel
Astrophysics Research Center of the Open university (ARCO), The Open University of Israel, PO Box 808, Raanana, 4353701, Israel
Department of Physics, The George Washington University, 725 21st Street NW, Washington, 20052, DC, United States
Department of Physics
Energetic Cosmos Laboratory
Department of Natural Sciences
Astrophysics Research Center of the Open university (ARCO)
Department of Physics
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