Connection between optical and γ-ray variability in blazars
We use optical data from the Palomar Transient Factory (PTF) and the Catalina Real-Time Transient Survey (CRTS) to study the variability of γ-ray-detected and non-detected objects in a large population of active galactic nuclei selected from the Candidate Gamma-Ray Blazar Survey and Fermi Gamma-Ray Space Telescope catalogues. Our samples include 714 sources with PTF data and 1244 sources with CRTS data. We calculate the intrinsic modulation index to quantify the optical variability amplitude in these samples. We find the γ-ray-detected objects to be more variable than the non-detected ones. The flat spectrum radio quasars (FSRQs) are more variable than the BL Lac objects in our sample, but the significance of the difference depends on the sample used. When dividing the objects based on their synchrotron peak frequency, we find the low synchrotron peaked (LSP) objects to be significantly more variable than the high synchrotron peaked (HSP) ones, explaining the difference between the FSRQs and BL Lacs. This could be due to the LSPs being observed near their electron energy peak, while in the HSPs the emission is caused by lower energy electrons, which cool more slowly. We also find a significant correlation between the optical and γ-ray fluxes that is stronger in the HSP BL Lacs than in the FSRQs. The FSRQs in our sample are also more Compton dominated than the HSP BL Lacs. These findings are consistent with models where the γ-ray emission of HSP objects is produced by the synchrotron self-Compton mechanism, while the LSP objects need an additional external Compton component that increases the scatter in the flux–flux correlation.
Additional Information© 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2013 December 27. Received 2013 December 27; in original form 2013 September 16. First published online: January 30, 2014. We thank the referee, Tigran Arshakian, for useful comments that helped to improve the paper. TH thanks Elina Lindfors for useful discussions. TH was supported in part by the Jenny and Antti Wihuri foundation. VP acknowledges support by the European Comission Seventh Framework Programme (FP7) through the Marie Curie Career Integration Grant PCIG10-GA-2011-304001 'JetPop' and the EU FP7 Grant PIRSES-GA-2012-31578 'EuroCal'. The OVRO 40-m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The CSS survey is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the US National Science Foundation under grant AST-0909182. This paper is based on observations obtained with the Samuel Oschin Telescope as part of the Palomar Transient Factory project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford and the Weizmann Institute of Science. It is also partially based on observations obtained as part of the Intermediate Palomar Transient Factory project, a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Millwakee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, the Kavli Institute for the Physics and Mathematics of the Universe and the Inter-University Centre for Astronomy and Astrophysics. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.
Published - MNRAS-2014-Hovatta-690-702.pdf
Submitted - 1401.0538v1.pdf
Supplemental Material - CRTS_data_table.txt
Supplemental Material - PTF_data_table.txt