Abstract
Context. The neutrino event IceCube−170922A appears to originate from the BL Lac object TXS 0506+056. To understand the neutrino creation process and to localize the emission site, we studied the radio images of the jet at 15 GHz.Aims. Other BL Lac objects show properties similar to those of TXS 0506+056, such as multiwavelength variability or a curved jet. However, to date only TXS 0506+056 has been identified as neutrino emitter. The aim of this paper is to determine what makes the parsec-scale jet of TXS 0506+056 specific in this respect.Methods. We reanalyzed and remodeled 16 VLBA 15 GHz observations between 2009 and 2018. We thoroughly examined the jet kinematics and flux-density evolution of individual jet components during the time of enhanced neutrino activity between September 2014 and March 2015, and in particular before and after the neutrino event.Results. Our results suggest that the jet is very strongly curved and most likely observable under a special viewing angle of close to zero. We may observe the interaction between jet features that cross each other’s paths. We find subsequent flux-density flaring of six components passing the likely collision site. In addition, we find a strong indication for precession of the inner jet, and model a precession period of about 10 yr via the Lense-Thirring effect. We discuss an alternative scenario, which is the interpretation of observing the signature of two jets within TXS 0506+056, again hinting toward a collision of jetted material. We essentially suggest that the neutrino emission may result from the interaction of jetted material in combination with a special viewing angle and jet precession.Conclusions. We propose that the enhanced neutrino activity during the neutrino flare in 2014–2015 and the single EHE neutrino IceCube-170922A could have been generated by a cosmic collision within TXS 0506+056. Our findings seem capable of explaining the neutrino generation at the time of a low gamma-ray flux and also indicate that TXS 0506+056 might be an atypical blazar. It seems to be the first time that a potential collision of two jets on parsec scales has been reported and that the detection of a cosmic neutrino might be traced back to a cosmic jet-collision.
Highlights
It has been speculated for many years that cosmic neutrino production can be assigned to the most powerful accelerators in space, the jets of active galactic nuclei (AGN; e.g., Ginzburg & Syrovatskii 1963; Rachen & Biermann 1993)
We propose that the enhanced neutrino activity during the neutrino flare in 2014–2015 and the single extremely high-energy (EHE) neutrino IceCube-170922A could have been generated by a cosmic collision within TXS 0506+056
Results of the VLBA data analysis We tested several identification scenarios based on the available data and present the kinematic scenarios which seemed to be the most reliable and robust
Summary
It has been speculated for many years that cosmic neutrino production can be assigned to the most powerful accelerators in space, the jets of active galactic nuclei (AGN; e.g., Ginzburg & Syrovatskii 1963; Rachen & Biermann 1993). The resulting bootstrapped visibility data sets were fitted in difmap using the original bestfit model as the initial guess It was done 300 times for each epoch. Our error estimates capture the dependence between difmap model parameters, they do not account for a systematic error uncertainty due to self-calibration and core shuttle effect (Lisakov et al 2017) The former results from the true brightness distribution that is non-Gaussian. For TXS 0506+056 the brightest component of the jet can be identified and traced across the epochs We use this feature as the reference position and calculate all component motions and apparent speeds with regard to this core position. This second jet would require a different origin (core), not coinciding with the brightest feature of this jet The position of this second core (as indicated in Fig. 1b) cannot be determined unambiguously from the currently available data. We use the one brightest component as reference position for all the jet components and their apparent motions
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