High-energy neutrino from flaring blazar

[BjarneModerator Super Nova]

Den blot 3. neutrinokilde er blevet identificeret som en blazar ved navn TXS 0506+056. En blazar er et supermassivt sort hul i centret af en galakse, som udsender en relativistisk jet i retning af Jorden. Man har længe haft mistanke om, at disse objekter under aktive perioder udsender neutrinoer plus gammastråler med meget høje energier. Det er nu blevet bekræftet, at retningen af en neutrino detekteret med IceCube detektoren falder sammen med en blazar med rødforskydninge z = 0.34.

Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

The IceCube Collaboration

Neutrino emission from a flaring blazar

Neutrinos interact only very weakly with matter, but giant detectors have succeeded in detecting small numbers of astrophysical neutrinos. Aside from a diffuse background, only two individual sources have been identified: the Sun and a nearby supernova in 1987. A multiteam collaboration detected a high-energy neutrino event whose arrival direction was consistent with a known blazar—a type of quasar with a relativistic jet oriented directly along our line of sight. The blazar, TXS 0506+056, was found to be undergoing a gamma-ray flare, prompting an extensive multiwavelength campaign. Motivated by this discovery, the IceCube collaboration examined lower-energy neutrinos detected over the previous several years, finding an excess emission at the location of the blazar. Thus, blazars are a source of astrophysical neutrinos.

INTRODUCTION

Neutrinos are tracers of cosmic-ray acceleration: electrically neutral and traveling at nearly the speed of light, they can escape the densest environments and may be traced back to their source of origin. High-energy neutrinos are expected to be produced in blazars: intense extragalactic radio, optical, x-ray, and, in some cases, γ-ray sources characterized by relativistic jets of plasma pointing close to our line of sight. Blazars are among the most powerful objects in the Universe and are widely speculated to be sources of high-energy cosmic rays. These cosmic rays generate high-energy neutrinos and γ-rays, which are produced when the cosmic rays accelerated in the jet interact with nearby gas or photons. On 22 September 2017, the cubic-kilometer IceCube Neutrino Observatory detected a ~290-TeV neutrino from a direction consistent with the flaring γ-ray blazar TXS 0506+056. We report the details of this observation and the results of a multiwavelength follow-up campaign.

RATIONALE

Multimessenger astronomy aims for globally coordinated observations of cosmic rays, neutrinos, gravitational waves, and electromagnetic radiation across a broad range of wavelengths. The combination is expected to yield crucial information on the mechanisms energizing the most powerful astrophysical sources. That the production of neutrinos is accompanied by electromagnetic radiation from the source favors the chances of a multiwavelength identification. In particular, a measured association of high-energy neutrinos with a flaring source of γ-rays would elucidate the mechanisms and conditions for acceleration of the highest-energy cosmic rays. The discovery of an extraterrestrial diffuse flux of high-energy neutrinos, announced by IceCube in 2013, has characteristic properties that hint at contributions from extragalactic sources, although the individual sources remain as yet unidentified. Continuously monitoring the entire sky for astrophysical neutrinos, IceCube provides real-time triggers for observatories around the world measuring γ-rays, x-rays, optical, radio, and gravitational waves, allowing for the potential identification of even rapidly fading sources.

RESULTS

A high-energy neutrino-induced muon track was detected on 22 September 2017, automatically generating an alert that was distributed worldwide within 1 min of detection and prompted follow-up searches by telescopes over a broad range of wavelengths. On 28 September 2017, the Fermi Large Area Telescope Collaboration reported that the direction of the neutrino was coincident with a cataloged γ-ray source, 0.1° from the neutrino direction. The source, a blazar known as TXS 0506+056 at a measured redshift of 0.34, was in a flaring state at the time with enhanced γ-ray activity in the GeV range. Follow-up observations by imaging atmospheric Cherenkov telescopes, notably the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes, revealed periods where the detected γ-ray flux from the blazar reached energies up to 400 GeV. Measurements of the source have also been completed at x-ray, optical, and radio wavelengths. We have investigated models associating neutrino and γ-ray production and find that correlation of the neutrino with the flare of TXS 0506+056 is statistically significant at the level of 3 standard deviations (sigma). On the basis of the redshift of TXS 0506+056, we derive constraints for the muon-neutrino luminosity for this source and find them to be similar to the luminosity observed in γ-rays.

CONCLUSION

The energies of the γ-rays and the neutrino indicate that blazar jets may accelerate cosmic rays to at least several PeV. The observed association of a high-energy neutrino with a blazar during a period of enhanced γ-ray emission suggests that blazars may indeed be one of the long-sought sources of very-high-energy cosmic rays, and hence responsible for a sizable fraction of the cosmic neutrino flux observed by IceCube.

Abstract

Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera–electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray–emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.

 

[BjarneModerator Super Nova]

Mange astronomiske navne for klasser af objekter afspejler ikke nødvendigvis disse objekters fysiske egenskaber. TXS 0506+056 betegnes skiftevis som en blazar eller et BL Lac objekt. Den variable stjerne BL Lac er nemlig slet ikke nogen stjerne, men den første prototype på en blazar. Man valgte at opfinde et passende navn for disse objekter uden relation til en “stjerne”. En blazar har et optisk spektrum uden emissions- eller absorptionslinjer, hvis man ser bort fra absorptionslinjer fra Mælkevejens interstellare stof. Det har derfor været meget vanskeligt at bestemme rødforskydninger for blazarerne.

Udsendelse af fotoner kan forekomme ved to fundamentalt forskellige processer: 1) Termisk emission fra f.eks. stjerneatmosfærer eller 2) ikke-termisk emission fra relativistiske elektroner i f.eks. Krabbetågen. Elektronerne er i det første tilfælde i termisk ligevægt, så deres hastigheder følger en Maxwell-fordeling (Gauss-fordeling). Elektronerne i et plasma med magnetfelter og chokbølger vil accelereres mod højere og højere energier ved gentagne passager frem og tilbage gennem chokfronten (Fermi-acceleration). Resultatet er en potenslovsfordeling af elektronernes energi, sådan at antallet falder med voksende energi. Disse relativistiske elektroner udsender fotoner ved to forskellige mekanismer: a) synkrotonstråling (ved acceleration i magnetfeltet) som f.eks. i Krabbetågen og b) sammenstød mellem elektroner og synkrotonfotoner, som sparkes op mod meget højere energier (omvendt Compton-spredning).

Disse mekanismer finder sted i et plasma (atomkerner og elektroner), som udgør en tynd jet, som bevæger sig mod os med en relativistisk hastighed meget tæt på lysets hastighed. Lyskilder, som bevæger sig nær lysets hastighed, er udsat for en relativistisk aberration, som er kendt fra Jordens bevægelse omkring solen. Aberrationen flytter Solens tilsyneladende retning 20.6 buesekunder i Jordens bevægelsesretning. Det lyder ikke af så meget, men det er tilstrækkeligt til at få støvpartikler til at spirallere ind i Solen. Ved den relativistiske jets høje hastighed bliver al udsendt stråling begrænset til en snæver lyskegle omkring jet-retningen. Fotonerne er desuden udsat for en kraftig blåforskydning. Vi observerer en blazar lige ned i jetten. Dette betyder, at den sammentrykkede og blåforskudte synkrotonstråling fuldstændig overstråler galaksen og det sorte huls omgivelser. Synkrotonstrålingen er uden emissionslinier. Eventuelle emissionslinjer kommer fra det sorte huls omgivelser. Der kræves et meget højt signal-støj-forhold for at måle disse linjer og dermed rødforskydningen. Objektets magnitude er ca. 15.

[BjarneModerator Super Nova]

Resultatet af elektroners acceleration via gentagne passager af magnetiskr chokbølger i en jet, som bevæger sig mod Jorden med en hastighed nær lysets, er som ovenfor beskrevet et ikke-termisk fotonspektrum med to meget brede toppe: a) Synkrotronstråling med et toppunkt nær det synlige vindue ved ca. 1 eV (eV = elektronvolt) og b) omvendt comptonspredning af de relativistiske elektroner på synkrotonstrålingen med et toppunkt i gammastråleområdet nær 1 GeV. Fotonspektret har et dybt minimum i røntgenområdet omkring ca. 10 keV (kiloelektronvolt). Disse mekanismer forklarer den elektromagnetiske stråling (fotoner) fra en blazar under et udbrud (flare), men den kan ikke forklare den målte neutrinos høje energi. Denne kræver acceleration af atomkerner bestående af nukleoner (protoner og neutroner) til meget høje energier, hvor en nukleon har energier af størrelsesorden 10 PeV (PeV = peta elektronvolt = 10¹⁵ eV).

Vi kan derfor umiddelbart slutte, at en blazar er i stand til at accelererer atomkerner til ekstremt høje energier. Sådanne atomkerner betegnes normalt for kosmiske stråler. Nukleoner med energier omkring 10 PeV vekselvirker med UV (ultraviolette) fotoner, idet der dannes ladede og neutrale pioner. En ladet pion henfalder (via en muon) til en elektron eller en positron og tre neutrinoer, som bevæger sig uforstyrret til Jorden. En neutral pion henfalder til to fotoner med samme energi som neutrinoerne. Måling af sådanne højenergetiske fotoner vil bekræfte, om der er en direkte relation mellem emissionen af fotoner og neutrinoer. Synkrotonstråling i røntgenområdet fra de sekundære elektroner og positroner overstiger imidlertid den målte røntgenstråling, hvorfor en realistisk model må baseres på acceleration af både elektroner og atomkerner (kosmiske stråler).

[BjarneModerator Super Nova]

Den officielle hjemmeside for “The IceCube Lab at the Amundsen-Scott South Pole Station” fortæller mere om opdagelsen med mange illustrationer:
https://news.wisc.edu/cosmic-rays/

Since cosmic rays were discovered in 1912, scientists have sought the origins of these mysterious particles. In September 2017, a flash of blue light in the ice deep beneath the South Pole set researchers on a path to resolving this century-old riddle.

By Terry Devitt

[Forfatter]

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