Many active galaxies harbor powerful relativistic jets, however, the detailedmechanisms of their formation and acceleration remain poorly understood. Toinvestigate the area of jet acceleration and collimation with the highestavailable angular resolution, we study the innermost region of the bipolar jetin the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC1052. We combined observations of NGC 1052 taken with VLBA, GMVA, and EHT overone week in the spring of 2017. For the first time, NGC 1052 was detected withthe EHT, providing a size of the central region in-between both jet bases of250 RS (Schwarzschild radii) perpendicular to the jet axes. This size estimatesupports previous studies of the jets expansion profile which suggest twobreaks of the profile at around 300 RS and 10000 RS distances to the core.Furthermore, we estimated the magnetic field to be 1.25 Gauss at a distance of22 {\mu}as from the central engine by fitting a synchrotron-self absorptionspectrum to the innermost emission feature, which shows a spectral turn-over atabout 130 GHz. Assuming a purely poloidal magnetic field, this implies an upperlimit on the magnetic field strength at the event horizon of 26000 Gauss, whichis consistent with previous measurements. The complex, low-brightness,double-sided jet structure in NGC 1052 makes it a challenge to detect thesource at millimeter (mm) wavelengths. However, our first EHT observations havedemonstrated that detection is possible up to at least 230 GHz. This studyoffers a glimpse through the dense surrounding torus and into the innermostcentral region, where the jets are formed. This has enabled us to finallyresolve this region and provide improved constraints on its expansion andmagnetic field strength.
Aims. Quasar strong gravitational lenses are important tools for puttingconstraints on the dark matter distribution, dark energy contribution, and theHubble-Lemaitre parameter. We aim to present a new supervised machinelearning-based method to identify these lenses in large astrometric surveys.The Gaia Focused Product Release (FPR) GravLens catalogue is designed for theidentification of multiply imaged quasars, as it provides astrometry andphotometry of all sources in the field of 4.7 million quasars. Methods. Our newapproach for automatically identifying four-image lens configurations in largecatalogues is based on the eXtreme Gradient Boosting classification algorithm.To train this supervised algorithm, we performed realistic simulations oflenses with four images that account for the statistical distribution of themorphology of the deflecting halos as measured in the EAGLE simulation. Weidentified the parameters discriminant for the classification and performed twodifferent trainings, namely, with and without distance information. Results.The performances of this method on the simulated data are quite good, with atrue positive rate and a true negative rate of about 99.99% and 99.84%,respectively. Our validation of the method on a small set of known quasarlenses demonstrates its efficiency, with 75% of known lenses being correctlyidentified. We applied our algorithm (both trainings) to more than 0.9 millionquadruplets selected from the Gaia FPR GravLens catalogue. We derived a list of1127 candidates with at least one score larger than 0.75, where each candidatehas two scores -- one from the model trained with distance information and onefrom the model trained without distance information -- and including 201 verygood candidates with both high scores.
The 2017 observing campaign of the Event Horizon Telescope (EHT) deliveredthe first very long baseline interferometry (VLBI) images at the observingfrequency of 230 GHz, leading to a number of unique studies on black holes andrelativistic jets from active galactic nuclei (AGN). In total, eighteen sourceswere observed: the main science targets, Sgr A* and M87 along with variouscalibrators. We investigated the morphology of the sixteen AGN in the EHT 2017data set, focusing on the properties of the VLBI cores: size, flux density, andbrightness temperature. We studied their dependence on the observing frequencyin order to compare it with the Blandford-K\"onigl (BK) jet model. We modeledthe source structure of seven AGN in the EHT 2017 data set using linearlypolarized circular Gaussian components and collected results for the other nineAGN from dedicated EHT publications, complemented by lower frequency data inthe 2-86 GHz range. Then, we studied the dependences of the VLBI core fluxdensity, size, and brightness temperature on the frequency measured in the AGNhost frame. We compared the observations with the BK jet model and estimatedthe magnetic field strength dependence on the distance from the central blackhole. Our results indicate a deviation from the standard BK model, particularlyin the decrease of the brightness temperature with the observing frequency.Either bulk acceleration of the jet material, energy transfer from the magneticfield to the particles, or both are required to explain the observations.