Optical polarization of AT2019wey
ATel #14000; F. Bouzelou(Univ. of Crete, Greece), S. Kiehlmann (IA FORTH & Univ. of Crete, Greece), L. Markopoulioti (Univ. of Crete, Greece)
on 8 Sep 2020; 19:05 UT
Credential Certification: Dmitriy Blinov (blinov@physics.uoc.gr)
Subjects: Optical, AGN, Blazar, Transient
On Dec 7, 2019 the ATLAS group detected the optical transient AT2019wey, i.e. ATLAS19bcxp. Gaia identified another transient event at the same coordinates on Feb 12, 2020. An X-ray transient event (SRGA J043520.9+552226, i.e. SRGE J043523.3+552234) discovered by SRG/ART-XC on Mar 18, 2020 and confirmed by SRG/eROSITA was identified as a likely counterpart to AT2019wey (ATel #13571). Based on the X-ray observations and optical follow-up with the Sayan Observatory's AZT-33IK 1.6-m telescope this sources was claimed to be an BL Lac type object in an active state ATel #13576. ATel #13932 reported an increase of the X-ray flux density from Apr to Aug 2020 and suggested an accreating binary as the origin of observations presented in ATel #13921. Further X-ray observations were reported in ATel #13948 (MAXI/GSC), ATel #13957 (NuSTAR), and ATel #13976 (AstroSat). JVLA identified an unresolved source in C band at the location of AT2019wey on May 27, 2020 (ATel #13984) with a flat radio spectrum. A later measurement on Aug 02, 2020 indicated a steepening of the radio spectrum.
On 6 September 2020, we measured the R-band polarization of AT2019wey by using RoboPol, a four-channel polarimeter at the Skinakas 1.3m telescope in Crete, Greece. Since the line of sight has rather high extinction A_R = 2.2 mag (Schlafly et al. 2011) we have also measured two stars in the field of the transient located within 1 arcmin and 3 arcmin from it at: RA=04h35m25.5s DEC=+55d23m31s and RA=04h35m04.5s DEC=+55d21m28s. These stars are located in the distance range of 3.8-7.6 kpc and 5.8-10.8 kpc according to (Bailer-Jones et al. 2018). Therefore, they are likely tracing most of the interstellar medium on the line of sight according to the reddening profile (Green et al. 2019). The measured polarization of the two stars is well consistent within uncertainties. Moreover, averaged fractional polarization and polarization position angle of the two stars P=2.4+-0.3%, PA=-28.2+-3.4 deg are very close to polarization parameters of AT2019wey P=3.1+-0.3%, PA=-23.94+-3.2 deg. Thereby, we conclude that the intrinsic polarization of the transient is <0.1%. Given the tentative classification of the transient as a BL Lac, this value appears to be unexpectedly low (see e.g. Angelakis et al. 2016).
