3C 279: ALMA detection of polarization drop and EVPA swing

ATel #17018; Seiji Kameno, Elizabeth Artur, Yoshiharu Asaki, Luciano Cerrigone, Paulo Cortes, Andres Guzman, Kevin Harrington, Ruediger Kneissl, Evgenia Koumpia, Cristian Lopez, Gabriel Marinello, Sergio Martin, Hugo Messias, Jorge Morgado, Priscilla Nowajewski, Andres Perez, Kurt Plarre, Matias Radiszcz, Tsuyoshi Sawada, Yiqing Song, Ignacio Toledo, Celia Verdugo, Baltasar Vila Vilaro (Joint ALMA Observatory), Gianni Cataldi, Hiroshi Nagai, Kouichiro Nakanishi (NAOJ), Edward Fomalont (NRAO), Rosita Paradino (INAF)
on 6 Feb 2025; 14:00 UT
Credential Certification: Seiji Kameno (seiji.kameno@alma.cl)

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The Atacama Large Millimeter/submillimeter Array (ALMA) is monitoring full-Stokes parameters of extragalactic radio sources at 40.0, 97.5, 154.9, 233.0, and 343.4 GHz. The telescope has detected an event of polarization drop coupled with swing of the electric vector position angle (EVPA) of the blazar J1256-0547 (3C 279) at the coordinate of RA=12:56:11.166573 DEC=-05.47.21.52515 in J2000.0. While the typical polarization degrees (median values in the recent decade) of 3C 279 are 5.4% (40 GHz), 7.5% (97.5 GHz), 7.7% (154.9 GHz), 9.1% (233 GHz), and 8.6% (343.4 GHz), they dropped down to the minimum of 0.6% - 1.1% around 2024-10-27. The EVPA of 3º - 40º before the minimum significantly changed to -70º - -60º after the minimum. At high frequency (233 GHz and 343.4 GHz), the EVPA continuously increased through the minima as 10º -> 45º -> 90º (-90º) -> -70º. EVPA at lower frequency than 154.9 GHz showed a different transition, decreased through the minima as 10º -> 0º -> -40º -> -60º. Stokes I, polarization degree, and EVPA (10-day median with uncertainty) are tabulated in https://www.alma.cl/~skameno/AMAPOLA/3C279Dec2024.txt. Similar EVPA jump was identified between 2020-03-18 and 2021-01-21 over missing data due to the COVID-19 pandemic. This time, EVPA was continuously traced.

Plots of Stokes I, polarized flux, and EVPA