# TXS 0506+056

TXS 0506+056 is a very high energy blazar – a quasar with a relativistic jet pointing directly towards Earth – of BL Lac-type. With a redshift of 0.3365 ± 0.0010, it has a luminosity distance of about 1.75 gigaparsecs (5.7 billion light-years). Its approximate location on the sky is off the left shoulder of the constellation Orion. Discovered as a radio source in 1983, the blazar has since been observed across the entire electromagnetic spectrum.
TXS 0506+056 is the first known source of high energy astrophysical neutrinos, identified following the IceCube-170922A neutrino event in an early example of multi-messenger astronomy. The only astronomical sources previously observed by neutrino detectors were the Sun and supernova 1987A, which were detected decades earlier at much lower neutrino energies.

## Observational history

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 (standard abbreviation TXS) 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 Gamma-ray 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 (TeV); 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 (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.
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.
In 2020, a study using MASTER global telescope network found that TXS 0506+056 was in an 'off' state in the optical spectrum 1 minute after the alert for IceCube-170922A event and switched back on 2 hours later. This would indicate that the blazar was in a state of neutrino efficiency.