arXiv subjects

Aleksei S. Nikonov

Publications and source records attributed to Aleksei S. Nikonov.

3 recordsLinked to original sources

arXiv2025

Imaging a ring-like structure and the extended jet of M87 at 86 GHz

The galaxy M87 is one of the prime targets for high resolution radio imaging to investigate the supermassive black hole, accretion flow, and relativistic jet. However, it remains challenging to observe them jointly. In 2018, GMVA+ALMA observations at 86 GHz enabled the simultaneous reconstruction of a ring structure and the extended jet emission. In order to analyze the ring and jet of M87, conventional 'CLEAN' algorithms were mainly employed alongside the RML method 'SMILI' in previous work. To test the robustness of the reconstructed structures of M87 GMVA+ALMA observations at 86 GHz, we estimate the ring diameter, width, and the extended jet emission with the possible central spine by two different novel imaging algorithms: 'resolve' and 'DoG-HiT'. Overall, reconstructions are consistent with the 'CLEAN' and 'SMILI' images. The ring structure of M87 is resolved at a higher resolution and the posterior distribution of M87 ring features is explored. The 'resolve' images show that the ring diameter is 60.9 +- 2.2 muas and its width is 16.0 +- 0.9 muas. The ring diameter and the ring width measured from the 'DoG-HiT' image are 61.0 muas and 20.6 muas, respectively. The ring diameter is therefore in agreement with the estimation in the previous work (64 +4 -8 muas). Two bright spots in the ring are reconstructed by four independent imaging methods. Therefore, the substructure in the ring most likely results from the data. A consistent limb-brightened jet structure is reconstructed by 'resolve' and 'DoG-HiT', albeit with a less pronounced central spine. Modern data-driven imaging methods confirm the ring and jet structure in M87, and complement traditional VLBI methods with novel perspectives on evaluating the significance of the recovered features. They confirm the result of the previous report.

astro-ph.HE↗

arXiv2024

Kilogauss magnetic field and jet dynamics in the quasar NRAO 530

The advancement of the Event Horizon Telescope has enabled the study of relativistic jets in active galactic nuclei down to sub-parsec linear scales even at high redshift. Quasi-simultaneous multifrequency observations provide insights into the physical conditions in compact regions and allow testing accretion theories. Initially we aimed at measuring the magnetic field strength close to the central supermassive black hole in NRAO 530 (1730-130) by studying frequency-dependent opacity of the jet matter, Faraday rotation and the spectral index in the mm-radio bands. NRAO 530 was observed quasi-simultaneously at 15, 22, 43, 86, and 227 GHz at four different very long baseline interferometer (VLBI) networks. By the means of imaging and model-fitting, we aligned the images, taken at different frequencies. We explored opacity along the jet and distribution of the linearly polarized emission in it. Our findings reveal that the jet of NRAO 530 at 86 and 227 GHz is transparent down to its origin, with 70 mJy emission detected at 227 GHz potentially originating from the accretion disk. The magnetic field strength near the black hole, estimated at $5r_\mathrm{g}$, is $3\times10^3-3\times10^4$ G (depending on the central black hole mass). These values represent some of the highest magnetic field strengths reported for active galaxies. We also report the first ever VLBI measurement of the Faraday rotation at 43-227 GHz, which reveals rotation measure values as high as -48000 rad/m2 consistent with higher particle density and stronger magnetic fields at the jet's outset. The complex shape of the jet in NRAO 530 is in line with the expected behavior of a precessing jet, with a period estimated to be around $6\pm4$~years.

astro-ph.GA↗

arXiv2024

Bayesian self-calibration and imaging in Very Long Baseline Interferometry

Self-calibration methods with the CLEAN algorithm have been widely employed in Very Long Baseline Interferometry (VLBI) data processing in order to correct antenna-based amplitude and phase corruptions present in the data. However, human interaction during the conventional CLEAN self-calibration process can impose a strong effective prior, which in turn may produce artifacts within the final image and hinder the reproducibility of final results. In this work, we aim to demonstrate a combined self-calibration and imaging method for VLBI data in a Bayesian inference framework. The method corrects for amplitude and phase gains for each antenna and polarization mode by inferring the temporal correlation of the gain solutions. We use Stokes I data of M87 taken with the Very Long Baseline Array (VLBA) at 43GHz, pre-calibrated using the rPICARD CASA-based pipeline. For antenna-based gain calibration and imaging, we use the Bayesian imaging software resolve. To estimate gain and image uncertainties, we use a Variational Inference method. We obtain a high-resolution M87 Stokes I image at 43GHz in conjunction with antenna-based gain solutions using our Bayesian self-calibration and imaging method. The core with counter-jet structure is better resolved, and extended jet emission is better described compared to the CLEAN reconstruction. Furthermore, uncertainty estimation of the image and antenna-based gains allows us to quantify the reliability of the result. Our Bayesian self-calibration and imaging method is able to reconstruct robust and reproducible Stokes I images and gain solutions with uncertainty estimation by taking into account the uncertainty information in the data.

astro-ph.IM↗