Photometrically Triggered Keck Spectroscopy of Fermi BL Lac Objects

TL;DR

This study used photometrically triggered Keck spectroscopy to determine redshifts of Fermi blazars, including previously unobserved and challenging BL Lac objects, by capturing spectra during low jet emission states.

Contribution

The paper introduces a novel photometric monitoring strategy to trigger spectroscopy during low jet activity, enabling redshift measurements of elusive blazar objects.

Findings

Redshift of J0622+3326 confirmed as z=1.062

Redshifts for seven BL Lac objects were constrained

Spectroscopy during low jet states improves host galaxy detection

Abstract

We report on Keck spectra of ten Fermi blazars. J0622+3326, previously unobserved, is shown to be a flat-spectrum radio quasar at redshift z=1.062. The others are known BL Lac-type objects that have resisted previous attempts to secure redshifts. Using a photometric monitoring campaign with the 0.76 m Katzman Automatic Imaging Telescope at Lick Observatory, we identified epochs when the relativistic jet emission was fainter than usual, thus triggering the Keck spectroscopy. This strategy gives improved sensitivity to stars and ionized gas in the host galaxy, thereby providing improved redshift constraints for seven of these sources.