Astrophysics At Very High Energies

by Felix Aharonian, Lars Bergström, and Charles Dermer
Springer, 2013, 361 pages
reviewed by T. Nelson

High energy astrophysics is the study of gamma rays created under the most extreme conditions known in the universe: immense gravitational fields, intense heat, crushing magnetic fields, and colossal shock waves. They interact with the atmosphere to produce a variety of radioactive and non-radioactive particles that can tell us about conditions that existed almost at the beginning of the cosmos.

One of the biggest unsolved questions in astrophysics is what the nature of dark matter could be. The experts in this book give their perspectives on this and related topics in three lengthy chapters.

Felix Aharonian

Cosmic gamma rays below 100 GeV are observable only from satellites, which have modest sensitivity (about 10⁻¹² erg cm⁻² / second). Particles and γ-rays more energetic than that (up to 100 TeV) are studied with ground-based Imaging Atmospheric Cherenkov Telescopes or IACTs, which measure the visible light, called Cherenkov radiation, that's produced when a particle exceeds the speed of light in a medium. These detectors, which are basically giant mirrors, pick up showers of photons—tiny flashes of light lasting only a few nanoseconds. At least two detectors 100 meters apart are needed to triangulate on the source and direction of the gamma rays. The number of photons allows calculation of the energy.

The gamma rays come from the most violent objects in the sky: black holes, pulsar-driven nebulae (called plerions), pulsar winds, pulsar microquasars, neutron stars, X-ray binaries, and supernova remnants. These bodies act as particle accelerators via “non-thermal” processes, i.e. nuclear reactions.

Mixed with gamma rays are a variety of charged particles, which while not exactly a nuisance are less informative than gamma rays because they are deflected by magnetic fields during transit, which makes it challenging to determine their origin. There's also a high diffuse thermal background produced by absorption of gamma rays by the interstellar medium, which happens by two important processes: inverse Compton scattering and photon-photon pair production. Absorption features in the spectra of Extragalactic Background Light (EBL) also give information about the formation of galaxies billions of years ago.

These processes are familiar to anyone who works with radiation: bremsstrahlung, or ‘braking radiation’, produced by deceleration of a charged particle; photomeson reactions, in which a proton reacts with a gamma-ray to produce a neutral π meson; photon-photon pair production, where two gamma rays interact to produce an electron and a positron, and so on. If bremsstrahlung is due to circular motion in a magnetic field, it is called synchrotron radiation.

Gamma rays are absorbed and re-emitted by various processes at rates that differ depending on the strength of the magnetic field, the accelerator processes in the star, the magnetic field, and the shock waves, often quite strong, in the atmosphere around it.

Untangling all these different processes is a tremendous challenge: the only information we have is the energy spectrum, so deduction plays a strong role. For instance, protons above 1PeV would be needed to get neutrinos in the 10–100 TeV range, which is the optimal energy for detection by IceCube or KM3NET neutrino detectors, but both of those detectors have low sensitivity, so we might miss them; instead we would have to look for synchrotron X-rays caused by secondary π^± meson decay.

The authors are honest about the challenges in analyzing the results:

Remarkably, a tendency of softening [lower energy] of the energy spectrum of gamma-rays with distance from the position of the pulsar has been found in the gamma-ray image of HESS J1825-137 [a pulsar wind nebula near the center of the galaxy]. This is indeed a strong argument in favour of the inverse Compton origin of TeV emission of this source. On the other hand, the TeV gamma-ray luminosity of this source is about 10% of the pulsar spin-down luminosity . . . . This makes quite difficult the interpretation of gamma-ray emission by the current spin-down power of the pulsar. [p.89]

(The name HESS in the pulsar wind nebula HESS J1825-137 refers to the High Energy Stereoscopic System, a cosmic ray observatory in Namibia.) All this logic, which often requires sophisticated computer modeling, can become quite complex, and the reader needs to take copious notes to organize it.

Lars Bergström

In the second chapter, Lars Bergström focuses on dark matter. We're now certain, he says, that neutrinos are not the main form of dark matter, but only a small fraction of it. And so he gives us just enough quantum field theory to make us hate it, and then goes on to a brief introduction to SUSY or supersymmetry.

Supersymmetry, says Bergström, would be a beautiful theory if it were not ‘broken’ (by which he means it has a broken symmetry). Breaking must have occurred since no super­sym­met­ric particle has ever been detected and unbroken symmetry would mean they all have the same mass, which they probably don't.

And so he theorizes about what spectral features would be seen and what properties they could have if the various particles existed. For dark matter that means mainly light neutralinos. Neutralinos are linear combinations of neutral gauge bosons called gauginos and neutral higgsinos, so there are four (or now maybe five) states: if they are small, they're gaugino-like; if they're big, they're higgsino-like. There are also squarks, gluinos, axions, axinos, Q-balls, and WIMPS (the last being a generic term meaning weakly-interacting massive particles, not part of any specific theory, so forget that one).

You would be forgiven for never having heard of any of these particles: as I mentioned, not one has ever actually been detected in any experiment on Earth or in any cosmic rays. But the theory, known as the MSSM (minimal supersymmetric extension of the Standard Model), says that light neutralinos have to be lighter than 7 TeV to stay within the upper limit calculated from the WMAP data. So some, er, “fine-tuning” is necessary and we can make a wild guess estimate of < 600 GeV, or about 638 times as heavy as a neutron (there is still ample speculation about the mass in the literature).

One question laymen always ask is whether dark matter could be black holes. Bergström says no: black holes come in three sizes: small (2–20 solar masses); medium (a few million solar masses) such as Sgr A* in the center of the Milky Way; and big (a few billion solar masses), as in active galactic nuclei (AGN). Even added together, they are way too small. The galactic ‘halos’, i.e., stars and dust, around these AGN black holes are a thousand times heavier than the AGNs. Anyway, black holes are not really black, so they're not “dark.” (And of course they aren't holes, either.)

Bergström says the MSSM has over a hundred free parameters. Results from the Linear Hadron Collider have ruled out wide ranges of masses for the supersymmetric particles, so a pessimist might say the walls are closing in on MSSM. An optimist would say they're zeroing in on the possibilities. And Bergström is most definitely an optimist.

Charles Dermer

The third chapter focuses on categorizing various objects. The author admits that it was mostly out of date when it was incorporated into this book, but it's still a great way to become familiar with objects including some of the 1900 pulsars that have been identified through radio surveys, such as the Crab Nebula, Vega pulsar, and Cygnus X-3. Some of the theory presented in the first chapter is also repeated, but from a different angle and a bit more math.

Nice color images of gamma-ray sky maps, color graphs, and a good index, but since publication better γ-ray data have come in, so read it for the background theory more than for the astronomy.

aug 13, 2024