Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution

Abstract

3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique – global Very Long Baseline Interferometry (VLBI) at 1.3 mm (230 GHz) – to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variable γ-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array (ALMA), at an angular resolution of ∼20 μas (at a redshift of z = 0.536 this corresponds to ∼0.13 pc ∼ 1700 Schwarzschild radii with a black hole mass MBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation. We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across different imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI “core”. This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet. We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 c and ∼20 c (∼1.3 and ∼1.7 μas day−1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3 mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≲1010 K, a magnitude or more lower than typical values seen at ≥7 mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.

Polarimetric Properties of Event Horizon Telescope Targets from ALMA

“…The jet-like feature is oriented at approximately 40°, i.e., is roughly aligned with the EVPA in the core. Ultra-high-resolution images with the EHT reveal a jet component approximately along the same PA but on angular scales 10 5 times smaller (Kim et al 2020).…”

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

“…The recovery of D-terms for a highly polarized source like 3C 279 would require an extremely accurate source model in both total intensity and polarization. However, 3C 279ʼs complex structure in both total and polarized intensity add to the difficulty of imaging and calibrating the source (Kim et al 2020). Furthermore, interferometric-ALMA measurements taken contemporaneously to our EHT campaign found that 3C 279 may have non-negligible Stokes  (see Goddi et al 2021), which breaks the Stokes =  0 assumptions made in most of our calibration and imaging pipelines.…”

“…For full-array leakage calibration, we focus on two additional sources: J1924-2914 and NRAO 530 (calibrators for the second EHT primary target, Sgr A * ), which are compact and relatively weakly polarized. The main calibrator for M87 in total intensity, 3C 279 (Kim et al 2020;Paper IV), is bright and strongly polarized on longer baselines and is not used in this work. The properties and analysis of the calibrators are discussed in more detail in Appendices J and K.…”

“…Four targets in the EHT 2017 observations fit these criteria: M87, 3C 279, J1924-2914, and NRAO 530. Stokes  images of M87 and 3C 279 have been published (Papers I-VI; Kim et al 2020). Final Stokes  images for the Sgr A * calibrators NRAO 530 and J1924-2914 will be presented in upcoming publications (S. Issaoun et al 2021, in preparation;S.…”

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

“…With results from the 2017 EHT campaign for 3C 279 already published (Kim et al 2020) 2021, in preparation), and many more in the pipeline, we expect the context and impact of the multi-wavelength results to significantly broaden in the coming years. The precision EHT mm-VLBI images together with broadband SED information will be essential ingredients for developing a fuller understanding of the AGN phenomenon.…”