2 research outputs found
Polarimetric Properties of Event Horizon Telescope Targets from ALMA
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 > 103.3–105.5 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) × 105 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) × 105 rad m−2 at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source. We also report the first unambiguous measurement of RM toward the M87 nucleus at millimeter wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year timescale, spanning the range from −1.2 to 0.3 × 105 rad m−2 at 3 mm and −4.1 to 1.5 × 105 rad m−2 at 1.3 mm. Given this time variability, we argue that, unlike the case of Sgr A*, the RM in M87 does not provide an accurate estimate of the mass accretion rate onto the black hole. We put forward a two-component model, comprised of a variable compact region and a static extended region, that can simultaneously explain the polarimetric properties observed by both the EHT (on horizon scales) and ALMA (which observes the combined emission from both components). These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA
First M87 Event Horizon Telescope Results. VII. Polarization of the Ring
In 2017 April, the Event Horizon Telescope (EHT) observed the
near-horizon region around the supermassive black hole at the core of
the M87 galaxy. These 1.3 mm wavelength observations revealed a compact
asymmetric ring-like source morphology. This structure originates from
synchrotron emission produced by relativistic plasma located in the
immediate vicinity of the black hole. Here we present the corresponding
linear-polarimetric EHT images of the center of M87. We find that only a
part of the ring is significantly polarized. The resolved fractional
linear polarization has a maximum located in the southwest part of the
ring, where it rises to the level of similar to 15%. The polarization
position angles are arranged in a nearly azimuthal pattern. We perform
quantitative measurements of relevant polarimetric properties of the
compact emission and find evidence for the temporal evolution of the
polarized source structure over one week of EHT observations. The
details of the polarimetric data reduction and calibration methodology
are provided. We carry out the data analysis using multiple independent
imaging and modeling techniques, each of which is validated against a
suite of synthetic data sets. The gross polarimetric structure and its
apparent evolution with time are insensitive to the method used to
reconstruct the image. These polarimetric images carry information about
the structure of the magnetic fields responsible for the synchrotron
emission. Their physical interpretation is discussed in an accompanying
publication