Polarimetric Properties of Event Horizon Telescope Targets from ALMA

Abstract

We present the results from a full polarization study carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) during the first Very Long Baseline Interferometry (VLBI) campaign, which was conducted in 2017 April in the λ3 mm and λ1.3 mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A * , M87, and a dozen radio-loud active galactic nuclei (AGNs), in the two bands at several epochs in a time window of 10 days. We detect high linear polarization fractions (2%-15%) and large rotation measures (RM > 10 3.3 -10 5.5 rad m −2 ), confirming the trends of previous AGN studies at millimeter wavelengths. We find that blazars are more strongly polarized than other AGNs in the sample, while exhibiting (on average) order-of-magnitude lower RM values, consistent with the AGN viewing angle unification scheme. For Sgr A * we report a mean RM of (−4.2 ± 0.3) × 10 5 rad m −2 at 1.3 mm, consistent with measurements over the past decade and, for the first time, an RM of (-2.1 ± 0.1) × 10 5 rad m −2 at 3 mm, suggesting

First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon

“…The data also provide an upper limit on the net circular polarization fraction (Equation ( 7)) of the core of |v| net  0.3%, with a magnitude and sign that vary over the four observed epochs. Goddi et al (2021) also measured an EVPA that varies with wavelength across the ALMA band; the slope of EVPA with wavelength is consistent with EVPA ∝ λ 2 , as expected for Faraday rotation. The inferred rotation measure (Equation (5), for frequencies/wavelengths in ALMA Band 6) is also time variable and changes sign between 2017 April 5 and 11, with a maximum magnitude |RM| ; 1.5 × 10 5 rad m −2 .…”

“…These images reveal that a significant fraction of the ring emission is linearly polarized, as expected for synchrotron radiation. The EHT polarimetric measurements are consistent with unresolved observations of the radio core at the same frequency with the Submillimeter Array (SMA; Kuo et al 2014) and the Atacama Large Millimeter/submillimeter Array (ALMA; Goddi et al 2021). They also provide a detailed view of the polarized emission region on event-horizon scales near the black hole.…”

“…2. The image-integrated net circular polarization |v| net satisfies an upper limit from ALMA-only measurements reported in Goddi et al (2021): |v| net 0.8%. 3.…”

“…Unresolved observations measure the net (image-integrated) polarization fractions ( ) ( ) where the sums are over all pixels i in the resolved image. In addition to the signed circular polarization fraction v net , we also frequently consider the absolute value |v net |, as circular polarization measurements of the M87 * core at 230 GHz do not constrain its sign (Goddi et al 2021).…”

First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

“…where the sum is over the pixels indexed by i. ALMA measured |m| net = 2.7% on 2017 April 11 (Goddi et al 2021), but this measurement includes emission at large scales outside of the 120 μas field of view of the EHT images. We also consider the intensity-weighted average polarization fraction across the resolved EHT image:…”