Title: Case for Centaurus A as the main source of ultrahigh-energy cosmic rays
Authors: Silvia Mollerach and Esteban Roulet
First Author’s Institution: Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Consejo Nacional de Investigaciones Científicas y Te ́cnicas (CONICET), Avenida Bustillo 9500, R8402AGP, Bariloche, Argentina
Status: Accepted for publication in Physical Review D [closed access]
Head, shoulders, knees, and… ankles?
Cosmic rays pose a frustrating problem in both astronomy and particle physics. We detect tens to hundreds to millions of charged particles (mostly protons and atomic nuclei) raining down on Earth every second, but we have no idea where they come from. Unlike the other cosmic “messengers”: light, gravitational waves, and neutrinos, cosmic ray paths get bent by magnetic fields in our Solar System, the Milky Way, and beyond. Since cosmic ray paths aren’t linear, it’s almost impossible to track them back to where they came from.
One figure that you can always expect to see in a cosmic ray paper or talk is the cosmic ray spectrum – the flux (number of particles per area per unit time; kind of like a brightness) of cosmic rays distributed over all the energies at which we detect them (see Figure 1). Cosmic ray physicists have collectively agreed that this spectrum looks vaguely like a human leg, and have therefore named the observed features (the bumps and dips) after various parts of the human leg (however, some iterations of this figure show a “second knee” – which is not a feature of most human legs)! We understand that the cosmic rays at the very low energy part of the spectrum (way below the knee) come from the Sun.
The cosmic rays with energies between those produced by the Sun (solar cosmic rays; < 1010 eV) and those at the energies of the knee can be created and accelerated in systems within our own Milky Way (such as supernovae), but these quickly hit a particle acceleration energy limit (due to the maximum acceleration that occurs in Galactic sources by shocks or magnetic fields) at around the PeV (1015 eV) energies of the knee. This leads to the hypothesis that the highest energy cosmic rays we observe – called ultra-high energy cosmic rays (UHECRs) – are probably being accelerated in systems outside of our Galaxy, such as supermassive black hole (SMBH) jets. These jets are huge outflows of matter, originating near black holes extending well beyond the galaxies that host them.
Today’s paper argues that a single source: the closest galaxy hosting an active SMBH, Centaurus A (Cen A; see Figure 2), is the main factory producing and accelerating the UHECRs that we detect on Earth. They create a model where the only source of UHECRs is Cen A (as opposed to multiple different extragalactic sources) and compare this model to the observed cosmic ray spectrum.
Higher energies, heftier cosmic rays
We can both infer and measure that most UHECRs are not light particles, such as electrons, or even protons, because these light particles are quickly accelerated in loops (due to magnetic fields) that shoot them out of the Milky Way, so that we wouldn’t detect many of them on Earth. Heavier nuclei (we only worry about the atomic nucleus, because electrons are easily stripped off) from elements like Carbon, Nitrogen, and Oxygen (usually lumped together as CNO); or Iron have more “rigidity”, meaning it takes a lot more time to shoot them out of the Milky Way. In that time, they can be continuously accelerated to higher energies and trace back more accurately to their sources of origin, if they haven’t travelled too far.
The authors use this idea to argue that UHECRs must come from nearby extragalactic sources (e.g., the closest galaxies to us), or we wouldn’t observe any of the “heavy” nuclei in the UHECR spectrum. The further a cosmic ray travels, the more likely it is to get hit by stray light in the universe, like a cosmic microwave background (CMB) or extragalactic background light (EBL) photon, which breaks apart a heavy nucleus into lighter nuclei through a process called photodisintegration. They note that this argument is consistent with measurements from a cosmic ray experiment, the Pierre Auger Observatory, which sees a hot spot of the highest energy cosmic rays near Cen A.
To the simulations we go!
The authors try to model Cen A as the dominant UHECR source by starting with a simple setup that generates a population of cosmic rays coming from Cen A, with energies and compositions matching those observed by the Pierre Auger Observatory. They also simulate a very different scenario for comparison – consisting of an assortment of many cosmic-ray-producing sources, each having unique output energy spectra and compositions. They find that both model outputs fit the observed UHECR spectrum adequately, with a slight preference for the multi-source model. However, they note that some of the key physical characteristics, such as the simulated cosmic ray energies and composition (the types of nuclei in the spectrum), that are output from their model point to Cen A being a more likely candidate for the observed UHECR population.
Next, the authors add more realistic conditions to their simulations to refine their models. They add in the following components:
- A model of the Milky Way’s cosmic ray contribution: these are particles accelerated in supernova remnants and other Galactic systems up to the cosmic ray “knee” around PeV energies.
- Injecting more realistic compositions based on the observed abundances of elements in the Universe (taking into account photodisintegration).
- Adding in magnetic fields to see how particles are bent away from their original source trajectories.
- Giving Cen A a finite source lifetime: we know that particle acceleration in AGNs is not steady and goes through bursts. Additionally, we know that the source hasn’t existed forever. These considerations will add variation of the particles with time.
They find that these corrections allow for a much better fit to the data than before, especially considering that a single source is favoured by the physical characteristics mentioned above. Additionally, they find that for elements heavier than Hydrogen, which get deflected considerably less by magnetic fields, a hotspot around Cen A should arise (see Figure 3).
Further studies about the specific abundances of cosmic rays coming from the Cen A area will be important to distinguish if the Cen A model for UHECRs is truly favourable over the model consisting of a population of sources. Additionally, neutrino observatories like the IceCube neutrino observatory can search for the rare smoking gun neutrino signals that would be produced alongside cosmic rays. An excess of neutrinos at the Cen A location would confirm that the source can produce cosmic rays, and give us more information about both Cen A and the huge populations of cosmic rays we detect on Earth.
Edited by Catherine Slaughter
Featured image credit: Canva media library
