AGNs as Particle Accelerators

Yesterday Neronov et al. posted their paper:
Ultra-High Energy Cosmic Ray production in the polar cap regions of black hole magnetospheres.

It is becoming increasingly clear that the Ultra-High Energy Cosmic Rays (UHCR) are produced nearby in cosmological terms, i.e. at distances smaller or equal to the Greisen-Zatsepin-Kuzmin (GZK) radius of around a hundred Megaparsecs (Mpc).

Now we have detectors, big enough to detect these particles , Pierre Auger Observatory (PAO), and we thus need a good theory of the accelerators themselves. These are the biggest telescopes ever used by Mankind to observe the immediate neighborhood of our Universe. The better the knowledge of these natural-artificial hybrids, the better we will know our Universe.

The best model of AGNs is that they are Black Holes (BH) in the center of galaxies energizing their part of the universe to these very high energies we measure in the UHCRs at the PAO in Argentina.

These authors do a good job in clarifying the physics involved in these extraordinary accelerators.

They write:

"Simple order of magnitude estimates show that, in principle, the mechanism of particle acceleration in the vicinity of supermassive BHs can result in production of the ultra-high-energy cosmic rays (UHECR) (see e.g. [18]). At the same time, the existence of the Greisen-Zatsepin-Kuzmin (GZK) cutoff [19,20] in the spectrum of UHECR, found by the HiRes experiment [21] and confirmed recently by Pierre Auger Observatory [22], points to the astrophysical origin of the primary cosmic ray particles. In this case most of the cosmic rays with energies above the cut-off energy E =10²⁰ eV should come from nearby sources located at the distance D < 100 Mpc. Moreover, analysis of the combined HiRes, AGASA, SUGAR and Yakutsk data reveals anisotropy of arrival directions of the highest energy events, which could be related to the local Universe large scale structure [23,24]. Recently Pierre Auger Observatory has found a correlation of the arrival directions of the highest energy events with the sky positions of the nearby AGN [25]."

They find that the rotation axis of the BH has to be aligned within a few degrees with its magnetic field. This field has to be 10⁵ G, with a BH mass of 10⁸M_⊙, M_⊙ is the Sun's mass.

All this is exciting. I remember sitting in an auditorium at the University of California at Santa Barbara, when I was a student there, listening to a lecture by Prof. Chandrasekhar about stars. Somebody asked him if he believed BHs would be observed, he was one of the world experts on the subject, and he said he did not think so. He gave some reason about measuring the metric around the hole well enough to determine it was a BH, which he thought was difficult.

Now we are almost certain that to understand the UHECRs that we do observe, the best possible candidates are huge BHs in AGNs.

Everything is different and then so similar. I remember the late Mexican physicist Carlos Graeff Fernández, an expert on Gravity, that calculated the movement of particles in the magnetic field of the Earth for the then new science of Cosmic Ray Physics. Prof. Manuel Sandoval Vallarta directed him in his endeavors, since he was an active member of the then small cosmic ray community in the world. Together with Professor Compton he proved that cosmic rays were mainly protons.

Now we have to become experts on Applied Black Hole Physics, Prof Chandrasekhar would find that odd, to better understand our huge accelerator up in the sky.

Also interesting is the fact that Jerzy Plebanski that worked in Mexico from the early sixties until his death in 2005 was one of the first to study the motion of particles in General Relativity. These studies will come handy now that we need to know how charged particles move near BHs.

Every thing is similar and yet different. Now the problems seem harder but the tools are better.

Some directions of research are:

AGN - Correlations with UHECR
AGN - Source discrimination for UHECR
AGN - Autocorrelations