Study of Optical Emission-Line Variability in Gamma-Ray Blazars

Since 2016 we have been observing optical spectra of quasars and radio galaxies from the VLBA-BU-Blazar sample of 38 gamma-ray bright active galactic nuclei using the 4.3 m Lowell Discovery Telescope (LDT, Flagstaff, AZ). The spectra are obtained in both quiescent and active states, with sampling of spectra increased during active states. Substantial variability of emission lines is detected in the quasar 1156+295 during the gamma-ray/optical outburst from late 2017 to early 2018. The spectra of the quasar show that Mg II λ2798 and Fe II emission lines vary contemporaneously with the continuum. The optical continuum is highly polarized, indicating that it is beamed synchrotron radiation arising from the relativistic jet. The negligible time delay between increases in emission line flux and the continuum during the outburst implies that the broad-line emitting clouds must be located near the jet, beyond the canonical broad-line region. This allows us to conclude that these clouds are a likely source of seed photons that are Compton up-scattered to gamma-ray energies by high-energy electrons in the jet. Using spectra for several other gamma-ray quasars obtained during quiescent and active states, we investigate whether the behavior observed in 1156+295 is common for gamma-ray quasars during gamma-ray/optical events. This research has been supported in part by National Science Foundation grant AST-1615796 and NASA Fermi Guest Investigator grants 80NSSC20K1504, 80NSSC20K1565, and 80NSSC20K1567.