Intergalactic Magnetic Fields and Gamma Ray Observations of Extreme TeV
  Blazars
  The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly
probed through its effect on electromagnetic cascades initiated by a source of
TeV gamma-rays, such as active galactic nuclei (AGN). AGN that are sufficiently
luminous at TeV energies, "extreme TeV blazars" can produce detectable levels
of secondary radiation from inverse Compton (IC) scattering of the electrons in
the cascade, provided that the IGMF is not too large. We review recent work in
the literature which utilizes this idea to derive constraints on the IGMF for
three TeV-detected blazars-1ES 0229+200, 1ES 1218+304, and RGB J0710+591, and
we also investigate four other hard-spectrum TeV blazars in the same framework.
Through a recently developed detailed 3D particle-tracking Monte Carlo code,
incorporating all major effects of QED and cosmological expansion, we research
effects of major uncertainties such as the spectral properties of the source,
uncertainty in the UV - far IR extragalactic background light (EBL),
undersampled Very High Energy (VHE; energy > 100 GeV) coverage, past history of
gamma-ray emission, source vs. observer geometry, and jet AGN Doppler factor.
The implications of these effects on the recently reported lower limits of the
IGMF are thoroughly examined to conclude that presently available data are
compatible with a zero IGMF hypothesis.
