The gamma-ray flux from TXS 0506+056 is highly variable, by at least a factor of a thousand, but on average it is in the top 4% of brightest gamma-ray sources on the sky. It is also very bright in radio waves, in the top 1% of sources. Given its distance, this makes TXS 0506+056 one of the most intrinsically powerful BL Lac objects known, particularly in high-energy gamma rays.
A search of this region in the sky, 1.33 degrees across, yielded only one likely source: TXS 0506+056, a previously-known blazar, which was found to be in a flaring state of high gamma ray emission. It was subsequently observed at other wavelengths of light across the electromagnetic spectrum, including radio, infrared, optical, X-rays and gamma-rays. The detection of both neutrinos and light from the same object was an early example of multi-messenger astronomy.
A search of archived neutrino data from IceCube found evidence for an earlier flare of lower-energy neutrinos in 2014-2015 (a form of precovery), which supports identification of the blazar as a source of neutrinos. An independent analysis found no gamma-ray flare during this earlier period of neutrino emission, but supported its association with the blazar. The neutrinos emitted by TXS 0506+056 are six orders of magnitude higher in energy than those from any previously-identified astrophysical neutrino source.
The observations of high energy neutrinos and gamma-rays from this source imply that it is also a source of cosmic rays, because all three should be produced by the same physical processes, though no cosmic rays from TXS 0506+056 have been directly observed. In the blazar, a charged pion was produced by the interaction of a high-energy proton or nucleus (i.e. a cosmic ray) with the radiation field or with matter. The pion then decayed into a lepton and the neutrino. The neutrino interacts only weakly with matter, so it escaped the blazar. Upon reaching Earth, the neutrino interacted with the Antarctic ice to produce a muon, which was observed by the Cherenkov radiation it generated as it moved through the IceCube detector.
^"A Multimessenger Picture of the Flaring Blazar TXS 0506+056: implications for High-Energy Neutrino Emission and Cosmic Ray Acceleration". arXiv:1807.04537. Bibcode:2018arXiv180704537K. given its redshift z = 0.3365 (Paiano et al. 2018) and a consensus cosmology, the luminosity distance of TXS 0506+056 is dL ≈ 1750Mpc.
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