Resolving the Inner Parsec of the Blazar J1924–2914 with the Event Horizon Telescope

Abstract

The blazar J1924–2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic center’s black hole Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with the unprecedented 20 μas resolution of the EHT. J1924–2914 is a very compact flat-spectrum radio source with strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the EHT (April 5–11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28), giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties from the subparsec to 100 pc scales. We combine the multifrequency images of J1924–2914 to study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise rotation of the jet projected position angle of about 90° between 2.3 and 230 GHz. Linearly polarized intensity images of J1924–2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal magnetic fields in the blazar compact core.

The Event Horizon Telescope Image of the Quasar NRAO 530

“…Measurements on baselines longer than 2.5 Gλ indicate a deviation from the Gaussian model, in agreement with the resolved source consisting of more than one component. Both EHT Sgr A * calibrators, NRAO 530 and J1924-2914 (Issaoun et al 2022) are fairly compact and bright sources at 230 GHz. During the week-long EHT observing run, their source structure and flux density did not show significant variations (see Figure 3).…”

“…The polarimetric leakage calibration follows procedures outlined in EHTC et al (2021), where D-terms of stations with an intra-site VLBI baseline (ALMA, APEX, SMA, and JCMT) were calculated through a multisource fitting procedure (Martí-Vidal et al 2021), and several estimates for D-terms of remaining stations were reported. Here we employ the fiducial D-terms given by Issaoun et al (2022) based on the analysis presented in EHTC et al (2021). Some imaging algorithms that we use ignore the leakage calibration and estimate D-terms independently, providing an additional consistency check of the calibration procedures and their impact on the resulting images (see Section 3).…”

“…Rather than carrying out an iterative selfcalibration procedure, DMC fits the complex gains at every station simultaneously with the full Stokes image structure. For NRAO 530 imaging, a field of view of 300 μas and a 30 × 30 pixel grid were used, the data were scan-averaged, and D-terms were fixed following Issaoun et al (2022). A comprehensive description of the DMC model and implementation can be found in Pesce (2021).…”

“…The averaging procedure involves aligning images to maximize their cross-correlation, re-gridding them to a common resolution and field of view, and taking a standard arithmetic mean on a pixel-by-pixel basis. It follows the practice of EHTC et al (2019dEHTC et al ( , 2022c, Kim et al (2020), andIssaoun et al (2022), and it is designed to make the resulting image-domain morphology more robust against methodspecific systematics.…”

The Kelvin-Helmholtz instability at the boundary of relativistic magnetized jets

“…They can be easily incorporated into global MHD simulations of jet launching and propagation, to identify KH-unstable surfaces [19,34,35]. On the observational side, claims have been made that the KHI is observed along Active Galactic Nuclei (AGN) jets, based on the geometry of the outflow [36,37]. Our formulae can place this claim on solid grounds, if estimates of the field strength and orientation and of the flow velocities are available.…”