The object has been detected by numerous astronomical surveys, so has numerous valid source designations. The most commonly used, TXS 0506+056, comes from its inclusion in the Texas Survey of radio sources and its approximate equatorial coordinates in the B1950 equinox used by that survey. TXS 0506+056 was first discovered as a radio source in 1983. It was identified as an active galaxy in the 1990s, and a possible blazar in the early 2000s. By 2009 it was regarded as a confirmed blazar and catalogued as a BL Lac object. Gamma rays from TXS 0506+056 were detected by the EGRET and Fermi Space Telescope missions. Radio observations using very-long-baseline interferometry have shown apparent superluminal motion in the blazar's jet. TXS 0506+056 is one of the blazars regularly monitored by the OVRO 40 meter Telescope, so has an almost-continuous radio light curve recorded from 2008 onwards. 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.
Neutrino emission
On September 22, 2017, the IceCube Neutrino Observatory detected a high energy muon neutrino, dubbed IceCube-170922A. The neutrino carried an energy of ~290 tera–electronvolts ; for comparison, the Large Hadron Collider can generate a maximum energy of 13 TeV. Within one minute of the neutrino detection, IceCube sent an automated alert to astronomers around the world with coordinates to search for a possible source. 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, 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 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. Analysis of 16 very long baseline radio array 15-GHz observations between 2009 and 2018 of TXS 0506+056 revealed the presence of a curved jet or potentially a collision of two jets, which could explain the 2014-2015 neutrino generation at the time of a low gamma-ray flux and indicate that TXS 0506+056 might be an atypical blazar.
