Fermi Gamma-ray Space Telescope

Photometric Redshifts of 3FGL Fermi BL Lacs with Swift-UVOT and GROND

A. Kaur
D. H. Hartmann, M. Ajello, A. Rau, J. Greiner, P. Schady, N. Gehrels

Abstract:

BL Lacertae (BL Lac) objects, a sub population of blazars are the largest known population of gamma-ray sources. The absence of bright circumnuclear radiation fields and the abundance of > 10 GeV photons establish them as excellent probes for the Extragalactic Background Light (EBL). However, in order to achieve this, redshifts measurements for these sources are essential. We select BL Lac sample from 3FGL Fermi catalog and then utilize Swift-UVOT with GROND, the optical-infrared imager at the 2.2 m Max-Planck-ESO telescope, La Silla observatory, Chile to calculate the photometric redshifts for sources located at z ? 1.3 with accuracy of ?z/(1+z) ? 10 %. Simultaneous observations are conducted with these two instruments utilizing a total of 13 filters to produce Spectral Energy Distributions (SEDs) for these objects. The photo-z estimation is based on the dropout in the continuum flux that occurs in these SEDs due to the neutral hydrogen absorption bluewards of ? = 91.2 nm (Lyman Limit) along the line of sight. For a source at z ? 1.3, this dropout would correspond to a Lyman shift at ? ? 209.7 nm. Detection of this discontinuity in the Swift-UVOT-GROND SEDs allows us to estimate the redshifts of BL Lacs, which is a very challenging task spectroscopically, due to the lack of spectral features. This program allows us to increase substantially the sample size of high-z BL Lacs with measured redshifts.