The RoboPol optical polarization survey of gamma-ray-loud blazars
We present first results from RoboPol, a novel-design optical polarimeter operating at the Skinakas Observatory in Crete. The data, taken during the 2013 May–June commissioning of the instrument, constitute a single-epoch linear polarization survey of a sample of gamma-ray-loud blazars, defined according to unbiased and objective selection criteria, easily reproducible in simulations, as well as a comparison sample of, otherwise similar, gamma-ray-quiet blazars. As such, the results of this survey are appropriate for both phenomenological population studies and for tests of theoretical population models. We have measured polarization fractions as low as 0.015 down to R-mag of 17 and as low as 0.035 down to 18 mag. The hypothesis that the polarization fractions of gamma-ray-loud and gamma-ray-quiet blazars are drawn from the same distribution is rejected at the 3σ level. We therefore conclude that gamma-ray-loud and gamma-ray-quiet sources have different optical polarization properties. This is the first time this statistical difference is demonstrated in optical wavelengths. The polarization fraction distributions of both samples are well described by exponential distributions with averages of ⟨p⟩=6.4^(+0.9)_(−0.8)×10^(−2) for gamma-ray-loud blazars, and ⟨p⟩=3.2^(+2.0)_(−1.1)×10^(−2) for gamma-ray-quiet blazars. The most probable value for the difference of the means is 3.4^(+1.5)_(−2.0)×10^(−2). The distribution of polarization angles is statistically consistent with being uniform.
Additional Information© 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2014 May 3. Received 2014 April 16; in original form 2013 November 15. First published online June 16, 2014. We thank the referee, Dr Beverley Wills, for a constructive review that improved this paper. VP and EA thank Dr F. Mantovani, the internal MPIfR referee, for useful comments on this paper. We are grateful to A. Kougentakis, G. Paterakis, and A. Steiakaki, the technical team of the Skinakas Observatory, who tirelessly worked above and beyond their nominal duties to ensure the timely commissioning of RoboPol and the smooth and uninterrupted running of the RoboPol programme. The University of Crete group is acknowledging support by the 'RoboPol' project, which is implemented under the 'ARISTEIA' Action of the 'Operational Programme Education and Lifelong Learning' and is co-funded by the European Social Fund (ESF) and Greek National Resources. The NCU group is acknowledging support from the Polish National Science Centre (PNSC), grant number 2011/01/B/ST9/04618. This research is supported in part by NASA grants NNX11A043G and NSF grant AST-1109911. VP is acknowledging support by the European Commission Seventh Framework Programme (FP7) through the Marie Curie Career Integration Grant PCIG10-GA-2011-304001 'JetPop'. KT is acknowledging support by FP7 through Marie Curie Career Integration Grant PCIG-GA-2011-293531 'SFOnset'. VP, EA, KT, and JAZ would like to acknowledge partial support from the EU FP7 Grant PIRSES-GA-2012-31578 'EuroCal'. IM is supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. MB acknowledges support from the International Fulbright Science and Technology Award. TH was supported in part by the Academy of Finland project number 267324. The RoboPol Collaboration acknowledges observations support from the Skinakas Observatory, operated jointly by the University of Crete and the Foundation for Research and Technology – Hellas. Support from MPIfR, PNSC, the Caltech Optical Observatories, and IUCAA for the design and construction of the RoboPol polarimeter is also acknowledged.
Published - MNRAS-2014-Pavlidou-1693-705.pdf
Submitted - 1311.3304v2.pdf
Supplemental Material - suppl_data.zip