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The components of blazar jets that emit radiation span a factor of $10^{10}$ in scale. The spatial structure of these emitting regions depends on the observed energy. Photons emitted at different sites cross the lens plane at different distances from the mass-weighted center of the lens. Thus there are differences in magnification ratios and time delays between the images of lensed blazars observed at different energies. When the lens structure and redshift are known from optical observations, these constraints can elucidate the structure of the source at high energies. At these energies, current technology is inadequate to resolve these sources and the observed light curve is thus the sum of the images. Durations of $gamma$-ray flares are short compared with typical time delays; thus both the magnification ratio and the time delay can be measured for the delayed counterparts. These measurements are a basis for localizing the emitting region along the jet. To demonstrate the power of strong gravitational lensing, we build a toy model based on the best studied and the nearest relativistic jet M87.
The formation of merging binary black holes can occur through multiple astrophysical channels such as, e.g., isolated binary evolution and dynamical formation or, alternatively, have a primordial origin. Increasingly large gravitational-wave catalogs
Gravitational lensing is a potentially powerful tool for elucidating the origin of gamma-ray emission from distant sources. Cosmic lenses magnify the emission from distance sources and produce time delays between mirage images. Gravitationally-induce
When gravitational waves pass near massive astrophysical objects, they can be gravitationally lensed. The lensing can split them into multiple wave-fronts, magnify them, or imprint beating patterns on the waves. Here we focus on the multiple images p
Electrons can be accelerated to ultrarelativistic energies at strong (high-Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova. Collisionless shock waves also form in the flow of particles from the S
We review the importance of Centaurus A in high energy astrophysics as a nearby object with many of the properties expected of a major source of very high energy cosmic rays and gamma-rays. We examine observational techniques and the results so far o