Thursday, April 23, 2015

Book Review: What Are Gamma Ray Bursts?

Joshua S. Bloom's What Are Gamma Ray Bursts is the second book in the Princeton University Press Frontiers in Physics Series. It shares the same concise and compact format as the earlier volume on the First Stars and Galaxies in the Universe, together with the same annoyingly small type face. It's also very reasonably priced.

Gamma Ray Bursts, first discovered as a side effect of a program to monitor the nuclear test ban treaty, are extraordinarily intense and very brief, with the duration of the gamma ray pulse being anything from less than a second to several seconds. During this time they are thousands of times brighter than a quasar and millions of times brighter than a supernova or a galaxy.

Bloom traces the history of our understanding of this phenomenon, and discusses the physics believed to be involved in the phenomena. There are still many uncertainties, but it is generally believed that there at least three different types of GRBs. The so-called soft gamma ray repeaters are the most least intense and most likely to be found nearby. They are believed to be neutron stars with exceptionally intense magnetic fields - magnetars - and their gamma rays are believed to be produced mostly from their rotational kinetic energy. A second type, producers of the briefest pulses, are thought to result from the mergers of two closely orbiting neutron stars. The most potent GRBs probably result from the spectacular death of a massive star, a so-called collapsar, with most of the mass of the star collapsing into a black hole while a small portion of the mass is expelled in ultra-relativistic polar jets.

These last events seem to have happened mainly in the past. The most distant GRBs happened when the universe was relatively young, and the rate of occurrence seems to have declined rather steeply in the last seven billion years or so. It's likely that there is a metallicity effect (metals being what astronomers call all the elements produced only in stars - everything except hydrogen and helium.)

The book has significant technical content, but much of the discussion is at a level readily appreciated by astronomy fans with only a bit of physics training. Overall, a very good book, suitable for many readers, from amateur fan to physicists and astronomers specializing in other areas.