June 18, 2012 | 15
At a lecture I went to some years ago, astrophysicist Trevor Weekes compared garden-variety elementary particles to mosquitoes. They are plentiful and easy to find—indeed, they find you. But ultra-high-energy gamma rays, he said, are like elephants. They are fairly rare, but among the greatest of creatures. They often roam in spectacular habitats. Their sheer heft tests the limits of the laws of nature.
I naturally wanted to invite an article for Sci Am about these charismatic megaparticles, but for years I struggled with what the article would say. Although they may be the most powerful electromagnetic radiation known to science—photons with an energy of around a teraelectron-volts (TeV), the kinetic energy of a mosquito concentrated into a single quantum—once you use up all the superlatives in your thesaurus, what was there to say, really? At the time I saw Weekes speak, astronomers had found a grand total of about a dozen celestial sources of TeV gamma rays, and they were the usual suspects: giant black holes and suchlike. Teragammas had revealed nothing about the ecology of the universe which astronomers didn’t already know. They were like animals in a zoo rather than out in the wild: fun to look at before you move onto the baby penguins.
This has all changed in the past couple of years. Observatories have catalogued 136 TeV sources, which is enough to start doing systematic astronomy rather than freak-show physics. They have turned up some striking results, questioning conventional wisdom about pulsars and shedding some light on dark matter.
Blazars, giant black holes that just so happen to be oriented that we are looking down the barrel of the jets they spray out (see picture above), are the largest single category of TeV gamma source outside our galaxy. They are pretty extreme to begin with, but some go all out. They blaze with the intensity of a thousand Milky Way galaxies and can vary in brightness by a factor of five within an hour—a puzzlingly rapid time, too fast even for light to cross from one side of the black hole to the other. “They’re some of the wildest animals in the whole astronomical zoo,” says astrophysicist Chuck Dermer. “The luminosities are just incredible.”
Superlatives aside, last year Christoph Pfrommer, Philip Chang, and Avery Broderick proposed that TeV gammas from blazars play an unappreciated role in heating up intergalactic gas. The injection of thermal energy would prevent the gas from settling into galaxies—especially into small galaxies, whose gravitational fields are too weak to overcome the tendency to dissipate. This may solve one of the most perplexing puzzles in modern cosmology: the fact that dark matter should nucleate lots of miniature galaxies, yet doesn’t seem to do so.
The blazars listed in the TeV catalog are only a small fraction of the ones out there. To our instruments, all the others blur together, forming a diffuse glow spread over the entire sky. In the 1990s, the Compton satellite measured this gamma-ray background up to an energy of 0.1 TeV. Yet when Compton’s successor, the Fermi satellite, went to take a look, the background glow looked so different that it was as if astronomers were seeing it for the first time. The earlier observatory appears to have been miscalibrated at the highest energies.
The upshot is that blazars are not the only things bathing our sky in a diffuse glow of high-energy gammas. Dermer says they account for only about a sixth of the background. The rest must come from pulsars, collisions of cosmic rays produced by supernovae, and maybe the decay or annihilation of dark-matter particles. “We still cannot explain the intensity of the isotropic flux,” says physicist Steve Ritz, one of the leaders of the Fermi project. Astrophysicists gathered to discuss this mystery during a special session of the American Astronomical Society meeting in Anchorage last week.
Pulsars are another example of how recent measurements have forced theorists back to the drawing board. By rights, these hyperdense neutron stars should be denuded of very-high-energy gammas. Although the stars might well produce such gammas near their surface, the surrounding magnetosphere should snuff them out, while gammas produced at higher altitudes should be comparatively wimpy. “A lot of people discouraged us from looking at pulsed emissions from pulsars,” recalls gamma-ray astronomer Nepomuk Otte.
So when the MAGIC observatory saw hints of high-energy pulses from the pulsar at the heart of the Crab Nebula, Otte says few paid any attention. But he and his colleagues kept at it and, last year, Fermi and the VERITAS observatory confirmed photons with up to 0.4 TeV. “This has changed the picture that we have of how gamma rays are produced in the Crab pulsar,” Otte says. A new idea is that streams of electrons and positrons are carrying energy into the outer magnetosphere and converting into gammas there. Astrophysicists had known that neutron stars were complicated, but not this complicated.
The biggest wildcards in teragamma astrophysics are so-called dark accelerators. These are TeV gamma sources that astronomers have yet to see any other way; they do not seem to correspond to any star, nebula, or other discernible object. They are tantalizingly marked “UNID” in the database. They might turn out to be known systems such as pulsar nebulae, but there’s always the hope they are dark matter or some other never-before-seen species. “There’s a lot of speculation about them,” Otte says.
To know for sure what’s going on, astronomers need even more than 136 TeV sources. A thousand would be more like it. So they are now planning the next generation of observatory with telescopes scattered over a square kilometer of land. Like the animals of Madagascar, gammas have broken out of their zoo and returned to the wild—with emphasis on the word “wild.”