Volume 275 - 4th Annual Conference on High Energy Astrophysics in Southern Africa (HEASA 2016) - Astropartical Physics and Cosmology (Chair: Iu Sushch)
Gravitational light bending prevents gamma-gamma absorption in gravitational lenses
H. Thiersen* and M. Böttcher
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Published on: June 23, 2017
Abstract
The magnification effect due to gravitational lensing enhances the chances of detecting moderate redshift ($z \sim 1$) sources in very-high energy (VHE; $E > 100$GeV) $\gamma$-rays by ground-based Atmospheric Cherenkov Telescope facilities. It has been shown in previous work that this prospect is not hampered by potential $\gamma$-$\gamma$ absorption effects by the intervening (lensing) galaxy, nor by any individual star within the intervening galaxy. In this paper, we expand this study to simulate the light bending effect of a realistic ensemble of stars. We first demonstrate that, for realistic parameters of the galaxy's star field, it is extremely unlikely (probability $\le 10^{-6}$) that the direct line of sight between the $\gamma$-ray source and the observer passes by any star in the field close enough to be subject to significant $\gamma$-$\gamma$ absorption. Our simulations then focus on the rare cases where $\gamma$-$\gamma$ absorption by (at least) one individual star might be non-negligible. We show that gravitational light bending will have the effect of avoiding the $\gamma$-$\gamma$ absorption spheres around massive stars in the intervening galaxy. This reinforces prospects of using VHE $\gamma$-ray observations of lensed blazars to probe the location of the $\gamma$-ray emission region in those blazars.
DOI: https://doi.org/10.22323/1.275.0015
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