The RoboPol optical polarization survey of gamma-ray-loud blazars

Abstract

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 |$\langle p \rangle =6.4 ^{+0.9}_{-0.8}\times 10^{-2}$| for gamma-ray-loud blazars, and |$\langle p \rangle =3.2 ^{+2.0}_{-1.1}\times 10^{-2}$| for gamma-ray-quiet blazars. The most probable value for the difference of the means is |$3.4^{+1.5}_{-2.0}\times 10^{-2}$|⁠. The distribution of polarization angles is statistically consistent with being uniform.