An Artificial Intelligence based approach for constraining the redshift of blazars using $\gamma$--ray observation

In this paper, we discuss an artificial intelligence based approach to constrain the redshift of blazars using combined $\gamma$--ray observations from the \emph{Fermi} Large Area Telescope (LAT) and ground based atmospheric Cherenkov telescopes (ACTs) in GeV and \emph{sub} TeV energy regimes respectively. The spectral measurements in GeV and TeV energy bands show a redshift dependent spectral break in the $\gamma$--ray spectra of blazars. We use this observational feature of blazars to constrain their redshift. The observed spectral information of blazars with known redshifts reported in the \emph{Fermi} catalogs (3FGL and 1FHL) and TeV catalog are used to train an Artificial Neural Network (ANN) based algorithm. The training of the ANN methodology is optimized using \emph{Levenberg - Marquardt} algorithm with $\gamma$--ray spectral indices and redshifts of 35 well observed blazars as input and output parameters respectively. After training, we use only observed spectral indices in GeV and sub TeV regimes for 10 blazars as inputs to predict their redshifts. The comparison of predicted redshifts by the ANN with the known redshift suggests that both the values are consistent within $\sim$ 18$\%$ uncertainty. The method proposed in the present work would be helpful in future for constraining or predicting the redshifts of the blazars using only observational $\gamma$--ray spectral informations obtained from the \emph{Fermi}-LAT and current generation IACTs as well as from the next generation Cherenkov Telescope Array (CTA) with improved source statistics.

K K Singh, V K Dhar, P J Meintjes · 2019