Neutrinos are among the most abundant particles in the Universe. Tens of billions of naturally produced neutrinos pass through your fingertip every second, but because they interact weakly with matter, they are elusive and hard to detect. They are produced naturally through the decay of terrestrial radioactive material, as well as artificially in nuclear reactors. These interactions, known as beta decay, are also the driving force of nuclear fusion processes fuelling stars, thus producing a population of astrophysical neutrinos. The neutrinos escape the inner core of stars with typical energies up to a few tens of MeV, barely interacting with the outer layers of the star. Through the construction of dedicated neutrino observatories, such as the Homestake experiment which first detected neutrinos from the Sun, these elusive particles can be used to study regions that traditional photonic observations cannot probe (Fig. 1). Whereas solar observations in the radio to X-rays shows the outer layers of the Sun, the detection of solar neutrinos provides us with an ‘image’ of the solar core.
There also exists a more energetic population of astrophysical neutrinos. Recently, a handful of events were detected by the largest neutrino observatory so far constructed, called the IceCube. The observatory covers a kilometre-cubed volume in the dense Antarctic ice. Ice is a sufficiently dense material to increase the detection rates, and is therefore equipped to search for the highest-energy neutrinos. Their origin is however not yet known. The twenty-eight confirmed Tev-PeV neutrino events give as yet too little statistical information to trace their origin. There are however a few hints on where to look, as neutrino production constrains the conditions of the source environment.
The sources must be extremely powerful, and able to accelerate hadrons, such as protons or neutrons, to ultra-high energies. The hadrons interact with surrounding matter or radiation, which eventually leads to the emission of high-energy neutrinos. One of the known sources capable of generating such high energies are black holes. An active galactic nucleus (AGN) is a galaxy with a super-massive black hole at its centre, typically with masses of order 10^8 that of the Sun. The black hole core of the AGN outshines the rest of the galaxy, and in some cases powerful jets are emitted at relativistic speeds (Fig. 2).
The AGN population varies in both intensity of emission and morphology. A broad classification scheme of AGN is determined by the viewing angle of the jet. Radio galaxies are viewed face-on, as seen in Figure 2. Blazars are AGN with their jets pointed in our line of sight, and can be further sub-divided into the low-luminosity BL Lac population, and the high-luminosity flat spectrum radio quasar (FSRQ) population.
I am Idunn Jacobsen, a final year PhD-student, and my research here at MSSL aims to shed some light on this highest-energy neutrino population, exploit their energy content to look at the inner mechanisms of AGN, and that of the AGN jets. The detection of ultra-high-energy cosmic ray protons confirms the existence of an astrophysical origin of energetic protons. Neutrinos are a natural product of protons interacting with cosmic and galactic radiation fields, hence a high-energy neutrino population is guaranteed to exist. Few galactic populations are however known to be sufficiently energetic or numerous to be the source of the highest-energy emission, so an extragalactic origin is required. Particle physics provides us with the theory of particle interactions necessary to produce these neutrinos, and the core of AGN provides the necessary power to produce these energetic particles. Using models of neutrino production in astrophysical sources, the neutrino emission from a single source can be calculated. Convolving the neutrino emission of one model source with the total population of AGN in the Universe will then give us an estimate of the high-energy neutrino background which can be observed by observatories, such as the IceCube.
We have carried out this calculation, following two model approaches. Both models are motivated by the detection of cosmic rays by the Pierre Auger Observatory. The Koers & Tinyakov (KT, 2008) model uses the closest AGN to us, Centaurus A, as a laboratory for neutrino production. Assuming a correlation between the neutrino and cosmic ray emission, a neutrino energy spectrum for this source is computed. The model of Becker & Biermann (BB, 2009) takes advantage of optical depths of the interacting radiation fields with which the energetic protons interact to obtain a neutrino production rate.
We use X-ray surveys to map the evolution of the AGN population. X-ray observations are indicative of AGN emission, and the X-ray luminosity scales with the accretion power of the black hole. Hence, it is reasonable to expect that a powerful black hole core produces a higher rate of energetic particles. It can therefore be used to scale the neutrino emission, as we expect the high-energy neutrino population to be emitted with correspondingly higher X-ray luminosity. We find that the scaling model chosen greatly affects the resultant neutrino spectra.
Our calculations are summarised below. Figure 3 shows the resultant diffuse neutrino emission from the five AGN populations considered, calculated using the KT model prescription. Compared to the upper limit of detection set by IceCube, all populations are rejected as potential neutrino producing sources.
In Figure 4 we show the resultant neutrino spectra calculated using the BB model prescription. The two panels represents two different luminosity scaling models. Here we find that the BL Lac population falls below the detection limit for both models, however the BB1 scaling model also accepts all blazars as candidate sources. Our results reflect the importance of the luminosity scaling model chosen. Whereas the KT model calculations use Centaurus A as a model source, and a linear luminosity relation, the BB model derives its model source from the luminosity and optical depth of the target radiation field, and the relationship between the AGN jet and accretion disk luminosities. This has a power-law relation, and the model source has an X-ray luminosity a few orders of magnitude greater than that of the KT model.
The only source population which is accepted within the scope of any of these models is the BL Lac population. The BB1 model also found that all blazars are accepted neutrino sources. This is promising for future neutrino studies. A point-source detection of neutrinos requires emission beamed in our line of sight, which is provided by blazars. Thus, by combining photonic observations of possible neutrino sources, for example X-ray detected AGN populations, or gamma-ray loud blazars, we can therefore approach the neutrino production in the sources through the diffuse energy background, as well as investigate the emission of point sources. These methods are complimentary in the search for the physical mechanisms of AGN black holes, their jets, and the AGN structure. Because neutrino emission is tightly linked to processes producing energetic cosmic rays and gamma rays, a combination of these observations may therefore eventually give us a neutrino image of the hidden regions of AGN black holes and jets, effectively allowing us to look inside the AGN cores.