In the realm of energy and astrophysics, a team of researchers from the Indian Institute of Technology Guwahati, led by T. Mondal, S. Chakraborty, L. Resmi, and D. Bose, has delved into the intriguing phenomenon of gamma-ray bursts (GRBs) and their afterglows. Their work, recently published in the journal Monthly Notices of the Royal Astronomical Society, aims to refine models that explain the detection of very high energy (VHE) photons from GRB afterglows, with a particular focus on the extreme event GRB 221009A.
Gamma-ray bursts are intense explosions that release a tremendous amount of energy in the form of gamma rays. The afterglows that follow these bursts emit radiation across various wavelengths, including very high energy photons in the GeV-TeV range. To understand these emissions, researchers have developed models that consider the structure of the jet and the environment around the burst. Traditional models, such as the top-hat jet model, assume a sharp edge to the jet, but these do not always align with observed data.
The team’s research introduces a Gaussian structured-jet model, which posits a smooth angular decline in the jet’s energy distribution. This model naturally accounts for early bright peaks in the afterglow for observers aligned with the jet’s axis and delayed, softer peaks for observers at higher inclinations. The gradual decline in the Gaussian model suppresses excessive lateral expansion, making it a compelling alternative to traditional models.
The researchers implemented this model to explain TeV afterglows from adiabatic forward shocks propagating in a wind-driven medium. They found that the timing and intensity of the TeV peak depend significantly on the jet’s geometry, kinetic energy, wind density, and microphysical parameters that scale the synchrotron self-Compton (SSC) component. Their simulations revealed that only about ten percent of TeV events exceed the sensitivity of the Cherenkov Telescope Array (CTA), a ground-based observatory designed to detect VHE photons. These detectable events typically arise from near core-aligned views with high kinetic energy and wind density, moderate initial Lorentz factor and downstream magnetic field, and a relatively large fraction of energy in nonthermal electrons.
Applying their model to the extreme event GRB 221009A, the researchers performed multi-band fits that included wind-modified dynamics, Klein-Nishina effects, and extragalactic background light (EBL) attenuation. They found that a mildly off-axis geometry could reproduce the observed X-ray and GeV-TeV light curves. This work highlights the importance of considering realistic jet structures and complex circumburst environments in understanding the afterglows of gamma-ray bursts, which could have implications for future observations and models in the field of high-energy astrophysics.
For the energy sector, understanding the mechanisms behind gamma-ray bursts and their afterglows can contribute to the development of advanced detection technologies and the exploration of new energy sources. The insights gained from this research could also inform the design of future space-based and ground-based observatories aimed at studying high-energy phenomena in the universe.
This article is based on research available at arXiv.