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Ongoing rotations of optical polarization plane in two blazars

ATel #7865; D. Blinov (Univ. of Crete/St. Petersburg Univ.), E. N. Einoder (Caltech), K. Kokolakis, I. Liodakis, E. Makrydopoulou, G. V. Panopoulou (Univ. of Crete) on behalf of the RoboPol collaboration
on 2 Aug 2015; 02:03 UT
Credential Certification: Dmitriy Blinov (blinov@physics.uoc.gr)

Subjects: Optical, Gamma Ray, AGN, Black Hole, Blazar

We perform optical polarimetric observations within the framework of the RoboPol program. Observations are carried out at the 1.3 m telescope of the Skinakas observatory in the R-band. Our preliminary analysis indicates that the polarization planes of two blazars RX J1809.3+2041 (RA=18h09m25.5s, DEC=+20d41m31s) and RX J1836.2+3136 (RA=18h36m21.2s, DEC=+31d36m27s) are currently rotating.
For RX J1809.3+2041, the rotation started around June, 21 2015. The amplitude of the rotation so far is ~200 degrees with average rotation rate ~5 deg/day. Average polarization degree during the rotation is around 3% and shows no clear trend. Preliminary analysis of the publicly available Fermi LAT data for the source 3FGL J1809.4+2040, which is positionally consistent with the blazar, shows no increase in the gamma-ray emission. Upper limit on the average gamma-ray photon flux (E>100MeV) during the rotation period is 0.51x10^-8 ph cm^-2 s^-1, which is lower than the average flux reported in 3FGL catalog (0.56+/-0.42)x10^-8 ph cm^-2 s^-1.
For RX J1836.2+3136, the rotation started around June, 26 2015 with average rate ~3.6 deg/day and current amplitude is ~128 deg. Average polarization degree varied between 2 and 7% during the rotation. Averaged gamma-ray photon flux (E>100MeV) during the rotation period <4.5x10^-8 ph cm^-2 s^-1, while the average flux reported in 3FGL catalog (2.0+/-0.6)x10^-8 ph cm^-2 s^-1.
Multifrequency observations of the blazars are encouraged.

The RoboPol program aims to understand the AGN physics through optical linear polarization monitoring of a large sample of gamma-ray loud blazars as well as a comparison sample of gamma-ray quiet ones. It utilizes a novel-design 4-channel optical polarimeter mounted on the 1.3-m Skinakas telescope in Crete. It is a collaboration between the University of Crete (Greece), Max-Planck-Institut fur Radioastronomie (Germany), California Institute of Technology (USA), Nicolaus Copernicus University (Poland) and the Inter-University Centre for Astronomy and Astrophysics (India).