TXS 0506+056 was first discovered as a radio source in 1983.[15] It was identified as an active galaxy in the 1990s, and a possible blazar in the early 2000s.[16] By 2009 it was regarded as a confirmed blazar and catalogued as a BL Lac object.[17] Gamma rays from TXS 0506+056 were detected by the EGRET and Fermi Gamma-ray Space Telescope missions.[16][18][19]
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.[6][22] It is also very bright in radio waves, in the top 1% of sources.[6] Given its distance, this makes TXS 0506+056 one of the most intrinsically powerful BL Lac objects known, particularly in high-energy gamma rays.[6][22]
On September 22, 2017, the IceCube Neutrino Observatory detected a high energy muon neutrino, dubbed IceCube-170922A.[7] The neutrino carried an energy of ~290 tera–electronvolts (TeV); for comparison, the Large Hadron Collider can generate a maximum energy of 13 TeV.[23] 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.[7]
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.[7][6] It was subsequently observed at other wavelengths of light across the electromagnetic spectrum, including radio, infrared, optical, X-rays and gamma-rays.[7][24] The detection of both neutrinos and light from the same object was an early example of multi-messenger astronomy.[11]
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.[22] An independent analysis found no gamma-ray flare during this earlier period of neutrino emission, but supported its association with the blazar.[6] The neutrinos emitted by TXS 0506+056 are six orders of magnitude higher in energy than those from any previously-identified astrophysical neutrino source.[6]
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,[25] though no cosmic rays from TXS 0506+056 have been directly observed.[11] 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.[7] The pion then decayed into a lepton and the neutrino. The neutrino interacts only weakly with matter, so it escaped the blazar.[7] 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.[7]
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.[26]
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.[27]
^ abcdefghPadovani, P.; Giommi, P.; Resconi, E.; Glauch, T.; Arsioli, B.; Sahakyan, N.; Huber, M. (2018). "Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as the first cosmic neutrino source". Monthly Notices of the Royal Astronomical Society. 480 (1): 192. arXiv:1807.04461. Bibcode:2018MNRAS.480..192P. doi:10.1093/mnras/sty1852.
^ abcdefghAartsen; et al. (The IceCube Collaboration, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S., INTEGRAL, Kanata, Kiso, Kapteyn, Liverpool Telescope, Subaru, Swift/NuSTAR, VERITAS, VLA/17B-403 teams) (12 July 2018). "Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A". Science. 361 (6398): eaat1378. arXiv:1807.08816. Bibcode:2018Sci...361.1378I. doi:10.1126/science.aat1378. PMID30002226. S2CID204803450.
^Douglas, James N; Bash, Frank N; Bozyan, F. Arakel; Torrence, Geoffrey W; Wolfe, Chip (1996). "The Texas Survey of Radio Sources Covering -35.5 degrees < declination < 71.5 degrees at 365 MHz". The Astronomical Journal. 111: 1945. Bibcode:1996AJ....111.1945D. doi:10.1086/117932.
^Lawrence, C. R; Bennett, C. L; Garcia-Barreto, J. A; Greenfield, P. E; Burke, B. F (1983). "5 GHz observations of sources in the Arecibo 611 MHz survey". The Astrophysical Journal Supplement Series. 51: 67. Bibcode:1983ApJS...51...67L. doi:10.1086/190840.
^Abdo, A. A; Ackermann, M; Ajello, M; Allafort, A; Antolini, E; Atwood, W. B; Axelsson, M; Baldini, L; Ballet, J; Barbiellini, G; Bastieri, D; Baughman, B. M; Bechtol, K; Bellazzini, R; Berenji, B; Blandford, R. D; Bloom, E. D; Bogart, J. R; Bonamente, E; Borgland, A. W; Bouvier, A; Bregeon, J; Brez, A; Brigida, M; Bruel, P; Buehler, R; Burnett, T. H; Buson, S; Caliandro, G. A; et al. (2010). "The First Catalog of Active Galactic Nuclei Detected by The Fermi Large Area Telescope". The Astrophysical Journal. 715 (1): 429–457. arXiv:1002.0150. Bibcode:2010ApJ...715..429A. doi:10.1088/0004-637X/715/1/429. S2CID119295892.
^Lister, M. L; Aller, M. F; Aller, H. D; Homan, D. C; Kellermann, K. I; Kovalev, Y. Y; Pushkarev, A. B; Richards, J. L; Ros, E; Savolainen, T (2013). "Mojave. X. Parsec-Scale Jet Orientation Variations and Superluminal Motion in Active Galactic Nuclei". The Astronomical Journal. 146 (5): 120. arXiv:1308.2713. Bibcode:2013AJ....146..120L. doi:10.1088/0004-6256/146/5/120. S2CID119270093.
^Richards, Joseph L; Max-Moerbeck, Walter; Pavlidou, Vasiliki; King, Oliver G; Pearson, Timothy J; Readhead, Anthony C. S; Reeves, Rodrigo; Shepherd, Martin C; Stevenson, Matthew A; Weintraub, Lawrence C; Fuhrmann, Lars; Angelakis, Emmanouil; Anton Zensus, J; Healey, Stephen E; Romani, Roger W; Shaw, Michael S; Grainge, Keith; Birkinshaw, Mark; Lancaster, Katy; Worrall, Diana M; Taylor, Gregory B; Cotter, Garret; Bustos, Ricardo (2011). "Blazars in the Fermi era: the OVRO 40m Telescope monitoring program". The Astrophysical Journal Supplement Series. 194 (2): 29. arXiv:1011.3111. Bibcode:2011ApJS..194...29R. doi:10.1088/0067-0049/194/2/29. S2CID50872974.
^De Angelis, Alessandro; Pimenta, Mario (2018). Introduction to particle and astroparticle physics (multimessenger astronomy and its particle physics foundations). Springer. doi:10.1007/978-3-319-78181-5. ISBN978-3-319-78181-5.