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Extremely High Optical Polarization Observed in the Blazar PKS1502+106

ATel #11047; Paul S. Smith (Steward Observatory, University of Arizona)
on 10 Dec 2017; 08:24 UT
Credential Certification: Paul S. Smith (psmith@as.arizona.edu)

Subjects: Optical, AGN, Blazar, Quasar

As part of a long-term program at Steward Observatory (SO) to optically monitor gamma-ray-bright blazars during the Fermi Gamma-ray Space Telescope mission, PKS1502+106 (z = 1.839) was observed from 12:17-12:35 on December 9, 2017 UTC. Using the 1.54-m Kuiper Telescope on Mt. Bigelow, Arizona and the SPOL spectropolarimeter, the optical linear polarization of the blazar was found to be 47.4% +/- 0.1% at a position angle of 85.2 +/- 0.1 degrees. This is among the highest levels of polarization ever observed for a blazar and surpasses the highest value previously measured in this program of 45.2% for PKS1506+106 during an optical outburst in February 2009. The spectropolarimetry reveals that both the polarization degree and position angle are constant with wavelength from 400-755 nm, except that a decrease in polarization to about 36% is observed at the redshifted position of the C IV emission line in the spectrum, consistent with the emission line being unpolarized. PKS1502+106 is also optically bright. Preliminary differential spectrophotometry with a field comparison star indicates that the blazar is at it brightest since monitoring began in 2009 at SO, with it being nearly 4 magnitudes brighter than the faintest photometric measurement obtained during the program. It is nearly 0.7 magnitudes brighter than in late September 2017, when the last measurements were made before the object was lost in the glare of the sun. Continued monitoring of this major outburst of PKS1502+106 as it climbs higher into the morning sky is encouraged. Data from the SO blazar monitoring program are public and can be accessed at http://james.as.arizona.edu/~psmith/Fermi .