Fermi Gamma-ray Space Telescope

Novel Search for TeV-Initiated Pair Cascades in the Intergalactic Medium

Wenlei Chen
(Manel Errando, James H. Buckley, Francesc Ferrer)


The observed magnetic fields in galaxies and galaxy clusters are believed to result from dynamo amplification of weak magnetic field seeds whose origin remains a long-standing open question. Beams of TeV gamma-rays from blazar jets can be used to infer the intensity, coherence length, and helicity of the intergalactic magnetic field. Intergalactic magnetic fields deflect the electron-positron pairs produced by TeV gamma-rays from blazars, resulting in broadened beams of secondary GeV gamma-rays known as pair halos. Such pair-cascades develop along the projected direction of the blazar jet, which is known from imaging radio observations. We searched for GeV pair halos in Fermi-LAT data around 12 high-synchrotron-peaked BL Lacs with well-determined jet orientation from VLBA radio observations. Our study exploits the expected asymmetry of blazar pair halos and uses advanced simulations of the pair cascades to improve the sensitivity of previous studies and increase the signal to noise. Although we find no significant detection, a 2-sigma hint for an extended pair halo along the direction of the jet appears in the stacked LAT data in the 30-300 GeV energy range, corresponding to an intergalactic magnetic field with strength of about 1E-15 Gauss. This magnetic field value is consistent with similar hints from independent studies using LAT data. We will present the results of our analysis and discuss the limitations of pair-halo searches due to astrophysical uncertainties. Finally, assuming that the apparent convergence on B ~ 1E-15 Gauss is not coincidental, we will outline a clear path towards a positive detection of blazar pair halos with future spaceborne and ground-based gamma-ray observatories.