'Neutrino factories' could hold the solution to the cosmic ray mystery

Jan 27, 2020

By Robert Lea

High-energy particles that bombard the Earth from the depths of space could originate from blazars, new research suggests.

An artist's illustration of a blazar accelerating neutrinos and cosmic rays to tremendous speeds. The supermassive black hole at the center of the accretion disk sends a narrow high-energy jet of matter into space, perpendicular to the disk (Image credit: DESY, Science Communication Lab)

New research has revealed that high-energy neutrinos and cosmic rays that bombard the Earth from deep space originate in blazars  —  actively galactic nuclei (AGN) that lurk at the center of galaxies and are powered by supermassive black holes.

Researchers know cosmic rays are charged particles from deep space that continuously strike Earth with energies as great as 1020 electron volts  —  a million times more energetic than the energies generated at the Large Hadron Collider(LHC). What could launch these particles with so much force that they travel billions of light-years has, however, remained a mystery.

This is because cosmic rays consist of electrically charged particles, meaning as they journey billions of light-years from their source to Earth, they are repeatedly deflected by the magnetic fields of galaxies, making their sources impossible to spot.

Some of the processes and events that launch cosmic rays also blast out astrophysical neutrinos, and these 'ghost-like' particles could be used as 'messengers' to solve this puzzle, a team of astrophysicists believes.

"Astrophysical neutrinos are produced exclusively in processes involving cosmic ray acceleration," team member and Julius-Maximilians-Universität (JMU) Würzburg astrophysics professor Sara Buson said in a statement.(opens in new tab)

Neutrinos are particles with no charge and very little mass that interact with matter so weakly that they pass through galaxies, planets, and even the human body almost without trace. Because they have no charge, neutrinos don't experience the same deflections as cosmic rays do, meaning their sources can be pinpointed more accurately.

In 2017, a neutrino signal was detected that could be traced back to the blazar TXS 0506+056. As a result, Buson suggested that blazars  —  which emit more radiation than the entire stellar population of the galaxies around them  — are responsible for blasting out high-energy neutrinos.

blazar 2.jpeg
This image taken by NASA's Wide-field Infrared Survey Explorer (WISE) shows a blazar - a voracious supermassive black hole inside a galaxy with a jet that happens to be pointed right toward Earth. (Image credit: NASA/JPL-Caltech/Kavli)

In 2021, she and her team set about solidifying this connection with a multi-messenger astronomy project, one that mixes 'traditional' astronomy with neutrino observations. These new results were achieved using data from the IceCube Neutrino Observatory  —  the most sensitive neutrino detector ever created  —  located deep beneath the ice of the south pole in Antarctica.

The team used this data to confirm that the location of blazars corresponded with the direction of astrophysical neutrinos often enough that this association couldn't be put down to chance alone, providing the first solid evidence of the connection between astrophysical neutrinos and blazars.

"After rolling the dice several times, we discovered that the random association can only exceed that of the real data once in a million trials," team member and scientist from the University of Geneva's astronomy department, Andrea Tramacere, said. "This is strong evidence that our associations are correct."

And because these neutrinos are created in sites where cosmic rays are accelerated and launched, this indicates that blazars are responsible for accelerating cosmic rays too. This could be a result of how the supermassive black hole at the heart of a blazar 'chews up' matter like gas and dust that surrounds them before it is 'fed'  —  or accreted  —  to their surface.

Rotating black holes which drag the very fabric of spacetime along with them , an effect called frame-dragging or Lense-Thirring precession ,  ensures matter around them can't stay still, which makes the acceleration of particles easier.

"The accretion process and the rotation of the black hole lead to the formation of relativistic jets, where particles are accelerated and emit radiation up to energies of a thousand billion [times higher than] that of visible light," Tramacere explained. "The discovery of the connection between these objects and the cosmic rays may be the 'Rosetta stone' of high-energy astrophysics."

According to Tramacere, the next step for this research is to investigate the difference between the types of blazars that emit neutrinos and those that don't.

"This will help us to understand the extent to which the environment and the accelerator 'talk' to each other," the University of Geneva scientist said. "We will then be able to rule out some models, improve the predictive power of others and, finally, add more pieces to the eternal puzzle of cosmic ray acceleration."

See: https://www.space.com/neutrino-factories-blazars-cosmic-rays?utm_campaign=58E4DE65-C57F-4CD3-9A5A-609994E2C5A9

A partial offering from the original study:

"Beginning a Journey Across the Universe: The Discovery of Extragalactic Neutrino Factories"

from Astrophysical Journal Letters

Sara Buson1, Andrea Tramacere2, Leonard Pfeiffer1, Lenz Oswald1, Raniere de Menezes1, Alessandra Azzollini1, and Marco Ajello3

Published 2022 July 14 • © 2022. The Author(s). Published by the American Astronomical Society.
The Astrophysical Journal Letters, Volume 933, Number 2Citation Sara Buson et al 2022 ApJL 933 L43
Sara Buson1, Andrea Tramacere2, Leonard Pfeiffer1, Lenz Oswald1, Raniere de Menezes1, Alessandra Azzollini1, and Marco Ajello3

Published 2022 July 14 • © 2022. The Author(s). Published by the American Astronomical Society.
The Astrophysical Journal Letters, Volume 933, Number 2Citation Sara Buson et al 2022 ApJL 933 L43

Neutrinos are the most elusive particles in the universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long-standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them. Among the candidate sources of neutrinos, there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed toward Earth. Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work, we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at an unprecedented level of confidence, i.e., a chance probability of 6 × 10−7. Our statistical analysis provides the observational evidence that blazars are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators.

Cosmic rays are charged particles of energies up to 1020 eV, far higher than the most powerful human-attained particle accelerator, i.e., the Large Hadron Collider (LHC). The nature and origin of these particles arriving from deep outer space remain elusive and represent a foremost challenge for the astroparticle and astrophysics fields. Cosmic rays' birthplaces generate other particles, neutrinos and γ rays among them. Unlike γ rays, astrophysical neutrinos are solely created in processes involving cosmic-ray acceleration, making them unique smoking-gun signatures of a cosmic-ray source (Mészáros 2017). In 2013, the IceCube Collaboration reported the discovery of a diffuse flux of astrophysical neutrinos in the ≳100 TeV to 10 PeV energy range (IceCube Collaboration 2013; Aartsen et al. 2016). The origin of this diffuse flux is probably extragalactic but still has to be ascertained.

Among the candidate sources of high-energy neutrinos, there are blazars, 4 a class of extragalactic sources powered by supermassive black holes harbored at the center of their host galaxies (Hillas 1984; Winter 2013; Padovani et al. 2015; Palladino et al. 2019). Blazars efficiently convert the gravitational energy of accreting gas into kinetic energy of highly relativistic jets, pointed toward Earth (Padovani et al. 2017). In 2017, the potential association (Aartsen et al. 2018a, 2018b) of the γ-ray-bright blazar TXS 0506 + 056 with putative neutrino emission (chance probability at the ≳10−4 level) has put forward γ-ray blazars as promising neutrino point sources, hence cosmic-ray accelerators (Padovani et al. 2018; Gao et al. 2019; Oikonomou et al. 2021; Keivani et al. 2018; Murase et al. 2018). Further efforts extensively pursued the search for a link between high-energy neutrinos and blazars leading to a large debate, with claimed associations (chance probability ≳10−3) between blazars and high-energy neutrinos (Padovani et al. 2016; Kadler et al. 2016; Plavin et al. 2021; Hovatta et al. 2021; Abbasi et al. 2021), as well as contrasting findings (Aartsen et al. 2017a; Yuan et al. 2020). Previous studies were hampered by employing a sample of blazars selected according to the objects' electromagnetic properties in a preferential energy band. Besides, most searches rely on the assumptions of a correlation between the γ-ray/neutrino emission (Hooper et al. 2019; Giommi et al. 2020; Oikonomou et al. 2019; Garrappa et al. 2019; Franckowiak et al. 2020), often implying that the majority of the observed γ rays originate from the same emission region as neutrinos. As shown by several theoretical studies (e.g., Murase et al. 2016; Reimer et al. 2019; Aartsen et al. 2017a) and observational constraints (Aartsen et al. 2017a; Yuan et al. 2020), however, a bright GeV γ-ray-emitting blazar can unlikely be at the same time an efficient (cospatial) producer of high-energy neutrinos.

In this work, we overcome the limitations of the previous searches by employing the largest available neutrino data set optimized for searches of point-like sources and a homogeneous clean sample of the blazar population. The paper is organized as follows: Section 2 lays out the working hypothesis, Section 3 presents the neutrino data and Section 4 the blazar sample, Section 5describes the statistical analysis and results, and Sections 6 and 7 present the discussion and conclusions.

Blazar theoretical models predict an emerging neutrino spectrum to be hard in the IceCube energy band, with an emission that follows a power law with index ≲−2 and peaks at ≳1 PeV energies (e.g., Mannheim 1993; Stecker 2013; Dermer et al. 2014; Murase et al. 2014; Petropoulou et al. 2015; Padovani et al. 2015). The bulk of the blazars' neutrino emission should reside at energies ≳1 PeV. Besides invoking theoretical models in support of this hypothesis, it is demanded by observational constraints, such as the IceCube collaboration stacking limit on γ-ray blazars (Aartsen et al. 2017a), which already excludes a substantial (<27%) contribution from this population in the ∼10 TeV/100 TeV energy range for an emerging neutrino soft spectrum (∝ E−2.5). The limit relaxes to 40% and 80% when assuming a hard spectrum, e.g., a power-law spectrum ∝ E−2.0, compatible with the IceCube diffuse flux measured above ∼200 TeV (Aartsen et al. 2016). Similar conclusions are drawn by independent, complementary studies (e.g., Yuan et al. 2020).

Motivated by these primers, one may foresee a correlation between blazars and astrophysical neutrinos, especially those of the highest observable energies (≳100 TeV). The IceCube Observatory is sensitive to different astrophysical neutrino energy ranges in the southern and northern celestial hemispheres. Given its location at the geographic south pole, Earth's opacity hampers the detection of the highest-energy astrophysical neutrinos from the northern hemisphere; for ≳100 TeV neutrino energies, the effect starts to be important at δ ∼ 30°. Therefore, the data collected for the northern sky are best capable of probing the TeV/sub-PeV range while the southern data are most sensitive to astrophysical neutrino fluxes in the PeV−EeV range (Abbasi et al. 2009, see also next Section 3). Because this work aims to test the hypothesis of blazars as high-energy neutrino emitters, we focus our search on the southern hemisphere first, which provides the most promising discovery ground. A forthcoming publication will address the expansion of this investigation to neutrinos observed at the lower energies (≲100 TeV).

blazar locations.jpeg
All-sky map in equatorial coordinates (J2000) of the IceCube neutrino local p-value logarithms denoted as L. Locations of PeVatron blazars associated with neutrino hotspots are pointed out by black squares. For visualization clarity, the label of 5BZCat objects is limited to reporting the unique numerical coordinate part. Unassociated hotspots are highlighted by green squares. The location of TXS 0506+056 is shown for reference (green circle). Squares are not to scale and serve the only purpose of highlighting the blazars' locations. The Galactic plane and Galactic center are shown for reference as a green line and star, respectively.

This work proves that at least part of the blazar population originates high-energy neutrinos and, hence, is capable of accelerating cosmic rays. A neutrino with observed energy E must be produced at redshift z with rest-frame energy Eν RF = (1 + z)E. If the neutrinos are produced by acceleration processes within the blazar jet, the relation between the rest-frame and observed energies is Eν RF = (1 + z)E/D, where
$D=\tfrac{1}{{\rm{\Gamma }}(1-{\beta }_{{\rm{\Gamma }}}\cos (\theta ))},$
is the beaming factor defined by the bulk Lorentz factor Γ, and θ is the viewing angle of the jet. Typical beaming factors for blazars are of the order of D ∼ 10 and viewing angles are of the order of ≲ few degrees. The production of neutrinos of energies Eν unavoidably requires hadrons to be accelerated to energies ∼20 × Eν /Z, Z being the atomic number (Halzen 2013). For the observed (minimum astrophysical) neutrino energies, i.e., between ∼100 TeV to ∼10 PeV energies, and assuming the acceleration of protons, the sample of PeVatron blazars diagnoses in situ acceleration of hadrons with energies above the PeV range.

This analysis finds that 10 out of the 19 IceCube hotspots located in the southern sky likely originated from blazars. We observe a roughly even distribution of neutrino hotspots across the southern sky. This corroborates the hypothesis that the dominant origin of these neutrino sources is blazars, which are isotropically distributed in the sky. The fact that half of the astrophysical-likely hotspots are associated with blazars fosters the idea that these newly discovered PeVatron blazars may be the dominant population of steady neutrino emitters resolved by IceCube at observed energies E ≳ 100 TeV.

It is important to put the discovery of PeVatron blazars in the context of recent works. Our findings are consistent with previous limits on the contribution by γ-ray blazars to the diffuse high-energy neutrino flux observed by IceCube (Aartsen et al. 2017a, 2021; Yuan et al. 2020), being only a small fraction (≲30%) of the neutrino-emitter blazars detected also at GeV γ rays (see also Appendix C). This suggests that in the blazars' engine the neutrino emission is weakly related to the observed γ-ray emission. This implies different production sites for the bulk of the observed neutrinos and GeV γ rays in blazars (Murase et al. 2016; Reimer et al. 2019).

Our finding indicates a firm indirect detection of extragalactic cosmic-ray factories with in situ acceleration of cosmic rays to PeV energies and, possibly, up to the EeV regime (assuming the acceleration of protons). PeVatron blazars shed a new perspective on the properties of the cosmic-ray spectrum, as well as offer a promising probe to test fundamental particle-physics properties beyond the energy region accessible by LHC. The nondetection of individual ≳10 PeV likely astrophysical neutrinos over a decade of IceCube observations (IceCube Collaboration et al. 2021) may imply a physical intrinsic limit for PeVatron blazars, i.e., related to the maximum energy of the parent cosmic rays. Nonetheless, the lack of statistics above tens of PeV could be simply due to the sensitivity of IceCube that at those energies degrades rapidly. In the latter case, PeVatron blazars may accelerate hadrons to much higher energies, fostering the tantalizing prospect that the observed high-energy astrophysical neutrinos and UHECRs could be produced by the same population of cosmologically distributed sources (Waxman 2014; Murase et al. 2012). The forthcoming generation of new neutrino detectors such as IceCube-Gen2 (Aartsen et al. 2021), the Cubic Kilometre Neutrino Telescope (KM3NeT, Adrián-Martínez et al. 2016), the The Pacific Ocean Neutrino Experiment (P-ONE, Agostini et al. 2020), the Radio Neutrino Observatory in Greenland (Aguilar et al. 2021 RNO-G,) and the Giant Radio Array for Neutrino Detection (GRAND, Alvarez-Muniz et al. 2020) project has the potential of shedding light into this.

This work was supported by the European Research Council, ERC Starting grant MessMapp, S.B. Principal Investigator, under contract No. 949555. S.B. and A.T. are grateful for valuable conversation to M. Santander, K. Murase, M. Petropoulou, J. DeLaunay, G. Illuminati, D. Caprioli, D. Bastieri, R. D'Abrusco, M. Giroletti, A. Maselli, F. Massaro, S. I. Stathopoulos, and A. Kouchner. This work has made use of data from the Space Science Data Center (SSDC), a facility of the Italian Space Agency (ASI), and data provided by the IceCube Observatory.

See: https://iopscience.iop.org/article/10.3847/2041-8213/ac7d5b

The outcomes presented above are in agreement with the conclusions that gamma-ray-weak blazars may harbor efficient cosmic-ray accelerators able to produce ∼PeV (peta electron volt)* neutrinos, motivating to explore physical models with predictions in the X-ray and MeV spectral range.

* ∼PeV (peta electron volt) comparative energies:

Petaelectronvolt (PeV) is multiple (see prefix Peta) of the derived metric measurement unit of energy electronvolt.

1 PeV = 36 749 328 470 883 EhPeV>EhEh>PeVWhat is Eh
See: https://www.aqua-calc.com/what-is/energy/petaelectronvolt