The nuclear jet and core of TXS 0506+056 could be gravitationally lensed

Abstract

Aims. TXS 0506+056 was the first active galactic nucleus to be identified as a convincing counterpart of an extremely high-energy neutrino, IceCube-170922A. Prior to this, IceCube data revealed evidence of enhanced neutrino activity in 2014–2015 from the direction of the same source. Renewed neutrino activity from TXS 0506+056 was confirmed by Baikal-GVD on April 18, 2021, and another IceCube detection on September 18, 2022. We revisited this blazar to study its evolution in light of the new high-energy data.Methods. We reanalyzed VLBA data observed at 8 GHz between 2010 and 2019, as well as 15 GHz data observed between 2019 and 2023. We combined these new data with 15 GHz data obtained earlier and performed a detailed study of the temporal evolution of the parsec-scale radio structure to search for a possible correlation with the neutrino events. We complemented our study of the morphology with an investigation of the radio, optical (Abastumani Observatory), and Fermi-LAT light curves.Results. The parsec-scale jet continues to reveal unexpected and puzzling properties that are not comparable with those of any known typical blazar jet. The total of all the jet component positions, obtained from this and earlier work, reveals a very wide-spread distribution in xy-coordinates. This could indicate an interaction of jet components with surrounding clouds, most likely material from the broad line region. However, a more detailed investigation reveals that the arrangement of jet component positions changes drastically with time. While the jet morphology resembles a crossing of two jets around 2010, a sudden ordering into a ring-like structure sets in around 2016 with the start of the major radio flare. In addition to this ring-like accumulation of jet components, we find arc-like gatherings of jet features in the jet before (2011–2016) and during the major radio peak, which occurred from 2016 to 2023. We discuss the possibility that the radio core flaring and the ring-like arrangement of jet components with time is explained by gravitational lensing.Conclusions. If we interpret the observed distribution of the jet components as being related to a potential neutrino generation mechanism, our calculations show that the neutrino production site could be either co-spatial with or within a distance of about 10^{18 cm of the core, which corresponds to the broad line region. If instead we take the data at face value and examine them in a more rigorous way, we find that strong gravitational lensing might alter the very long-baseline interferometry morphology of TXS 0506+056. As no intervening galaxy is found in the optical data, we assume the lens is an isolated non-accreting supermassive black hole. At an intermediate distance, a mass of 105 − 106 solar masses would be required. Assuming instead that TXS 0506+056 is a binary or dual system with a second non-emitting black hole acting as the lens, then gravitational lensing by a supermassive black hole of 1010 solar masses located some kiloparsecs away significantly changes the morphology of the jet of TXS 0506+056 and magnifies the jet and core flux density.}