Gammas from Heaven

Not long ago, physicists seeking to understand the cataclysmic events at the birth of the universe had to rely on massive, earthbound experiments in which beams of charged particles, steered by powerful magnetic fields, traveled in circles for miles before smashing into each other. Now, an increasing number of these particle physicists have turned to the skies, teaming with astronomers to launch spacecraft that can capture gamma rays from astrophysical processes with energies far greater than anything that can be generated in the most powerful atom smashers on Earth.

Carrying thousands to billions of times as much energy as visible-light photons, gamma rays “are telling us about the most energetic processes in the universe,” says David Thompson of NASA’s Goddard Space Flight Center in Greenbelt, Md. But detecting gamma radiation is no easy feat. Scientists have built a variety of ground-based detectors that capture the secondary radiation created when gamma rays crash into gas molecules in Earth’s atmosphere, but only a detector above the atmosphere can capture gamma rays directly.

Gamma-ray astronomy got a big boost in 1991 with the launch of the NASA’s now-defunct Compton Gamma Ray Observatory (CGRO). That push continued with missions such as the European Space Agency’s INTEGRAL satellite and NASA’s Swift spacecraft. But the agency’s GLAST (Gamma-ray Large Area Space Telescope) mission, set for launch next spring, will give scientists a view of the gamma-ray sky at higher energies and with sharper resolution and greater sensitivity than any previous craft has provided.

GLAST may shed light on dark matter, primordial black holes, and other cosmic oddities near and dear to the hearts of physicists and cosmologists. The Earth-orbiting craft will detect gamma rays with energies up to 300 gigaelectronvolts (GeV), far beyond the 20 GeV energies that previous instruments in space have reached. “That’s a huge discovery window,” says GLAST team member Thompson.

Recording rays

Every 3 hours, GLAST’s Large Area Telescope (LAT) will scan the entire sky, hunting for sources of gamma rays with energies from 30 million eV (MeV) to 300 GeV. Another suite of 14 separate detectors, the GLAST Burst Monitor (GBM), will cover a vast range of lower energies, from 8,000 eV up to 90 MeV. (Visible-light photons have energies of about 1 eV.) GLAST’s goal is reveal the origins of the mysterious and sporadic cosmic flashes known as gamma-ray bursts.

All in all, researchers will have a spacecraft capable of recording gamma-ray radiation over an energy range spanning seven orders of magnitude.

LAT is the modern-day version of the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument, which flew more than a decade ago on CGRO. Because gamma rays are so energetic, they can’t be focused or contained using lenses and mirrors as visible light can. EGRET’s detectors, relying on a technique originally developed for particle accelerators, were sensitive to energies up to about 20 GeV. EGRET recorded a total of 271 gamma-ray–emitting objects.

LAT uses a more sophisticated version of the same technology, designed for the abandoned Superconducting Supercollider project. It is expected to record thousands of sources. It has 16 tower-shaped gamma-ray detectors, each consisting of thin tungsten foils interleaved with silicon strips, giving a total collecting area of about 35 square meters. An incoming gamma ray that collides with a tungsten atom converts into an electron and its antiparticle, the positron. The silicon strips record the paths of the electron and the positron, from which the arrival direction of the gamma ray can be deduced. The total area of the silicon strips is more than 70 sq m, similar to the area of that in CERN’s spanking new Large Hadron Collider, expected to begin operation in Geneva early next year.

The electron and positron then pass into blocks of cesium iodide, which scintillates as they absorb the energy of each particle. The intensity of the flashes reveals the energy of the electron and positron, and therefore that of their parent gamma ray.

Although gamma-ray images from LAT will be fuzzy compared with the arrestingly sharp visible-light photos that the Hubble Space Telescope produces, they will nevertheless localize the brightest sources to within an area about one five-hundredth the diameter of the full moon. Astronomers expect the images to be the first to reveal structure in what previously appeared as featureless point sources on the gamma-ray sky.

Gammas in the galaxy …

At gamma-ray energies, the Milky Way forms a brilliant swath across the sky. Much of this high-energy emission has its origin in supernova remnants, expanding shells of gas created when a blast wave from an exploded star plows into surrounding space, sweeping up material along the way. Intense magnetic fields entrained in these shells of gas can, in theory, boost protons and other charged particles to nearly the speed of light, giving them energies 100 times higher than the most powerful ground-based accelerators can achieve. When these energized protons smash into atoms and molecules in surrounding space, they can spark gamma rays. LAT is expected to detect such sources in unprecedented numbers.

The telescope is also likely to pin down the origin of a significant portion of the spectrum of cosmic rays—charged particles that bombard Earth’s upper atmosphere. The highest energy cosmic rays move too fast to be confined to any single galaxy, but their lower-energy cousins are confined by individual galaxies’ magnetic fields. Any such particles that strike Earth must therefore have arisen within the Milky Way, and scientists have had strong hints for years that these lower-energy cosmic rays also have their origin in particle acceleration by supernova remnants.

The same fast protons that create gamma rays when they strike atoms will produce a host of other particles, including neutral pions. The neutral pions in turn would decay into gamma rays with energies of around 67 MeV.

Several other gamma-ray telescopes, including EGRE7, have already detected 67-MeV gamma rays coming from the center of the Milky Way. But these craft lacked the spatial resolution and sensitivity to match the radiation with specific supernova remnants. LAT can determine exactly where in the Milky Way the gamma rays are coming from, and so can check whether supernova remnants are indeed the source.

… And from afar

Looking well beyond the Milky Way, GLAST will examine a subset of active galactic nuclei—galaxies that have supermassive black holes at their hearts, fed by swirling disks of gas. In a process that’s still not well understood, jets of gas shoot out of these galactic cores at right angles to the disk, generating a stream of emissions from radio waves to visible light and on up to gamma rays.

Active galactic nuclei whose jets happen to point directly at Earth are especially conspicuous in the sky, and only they produce detectable gamma-ray emission. Known as blazars, they’re prime targets for GLAST. “With gamma rays [from blazars], we’re looking right down the barrel of the gun of an active galactic nuclei,” says Thompson.

Even more puzzling are gamma-ray bursts. Lasting from a few thousandths of a second to several minutes, these cosmic flashes are among the most energetic explosions in the universe. The longer bursts, lasting more than about 2 seconds, are associated with the collapse of massive stars into neutron stars or black holes, while the short-duration ones may be the swan song of two elderly neutron stars about to coalesce. Short or long, a single burst unleashes more energy than the sun will put out during its entire 10-billion-year lifetime, notes GLAST scientist Neil Gehrels of the Goddard Space Flight Center.

The suite of detectors in GBM, GLAST’s dedicated burst experiment, looks at the entire sky at once except for a small region blocked by Earth. GBM is expected to find 200 bursts a year, double the average number found by NASA’s Swift satellite. Once the GBM locates a burst, it alerts LAT to observe that portion of the sky.

Because Swift has an onboard ultraviolet and visible-light telescope that can home in on bursts, the craft will still be better than GLAST at pinning down precise burst locations. But with its two instruments working in tandem, GLAST will be able to examine bursts up to much higher energies. GLAST will “tell us about the high-energy emission of gamma-ray bursts, way above 1 MeV,” notes theorist Andrew MacFadyen of New York University. The 1990s’ EGRET experiment revealed a puzzle, he notes: Some bursts emit a large fraction of their energy at energies above 1 GeV and may even emit longer at those high energies than they do at lower energies. If GLAST bears this out, “it would be a significant constraint on both the models and emission mechanism” of bursts, MacFadyen adds.

It’s all relative

GLAST even has a shot at testing a fundamental principle of Einstein’s theory of relativity. That theory stipulates that all photons travel through the vacuum of space at exactly the same speed—2.99 x 10⁸ meters/second—regardless of their energy.

But Einstein’s theory describes gravity only as it operates over large distances. In some proposed models of quantum gravity, which attempt to marry Einstein’s theory of gravity to the physics of the subatomic realm, higher-energy photons may travel more slowly than lower-energy ones. In such theories, the vacuum seethes with the constant annihilation and creation of subatomic particles, which in turn create tiny fluctuations in the fabric of space-time.

Those fluctuations would be sensed more acutely by higher-energy gamma-rays because these photons have shorter wavelengths. As a result, high-energy gamma rays would travel a tad more slowly than their lower-energy counterparts.

Because GLAST is sensitive to an extraordinarily broad range of gamma-ray energies, it could look for this effect by examining a galaxy or quasar, billions of light years away, that emits gamma rays over a similarly broad range. Differences in gamma-ray travel speed would mean that a pattern of emission at lower energies would arrive slightly earlier than the corresponding pattern at higher energies.

In searching for such a difference, GLAST would be following up on recent findings from the ground-based MAGIC telescope in Spain’s Canary Islands. This telescope records visible-light emissions produced by fast-moving debris generated when atoms in Earth’s atmosphere are struck by gamma rays of even higher energy than those GLAST can detect. MAGIC recently examined two flares from the black hole at the center of the galaxy Markarian 501.

For one of the flares, recorded on July 9, 2005, an updated analysis reveals that gamma rays in the range of 1.2 teraelectronvolt (TeV)–10 TeV arrived 4 minutes later than those in the lower energy range of 0.25–0.6 TeV, Jordi Albert of the University of Würzburg in Germany and his colleagues recently reported online (http://xxx.lanl.gov/abs/0708.2889). The team can’t rule out the possibility that the black hole may simply have emitted the high-energy gamma rays later, however.

Into the darkness

GLAST may also shed light on dark matter, the invisible material that theorists say accounts for 80 percent of all the mass in the universe and keeps galaxies and galaxy clusters from flying apart. Dark matter is believed to be made of exotic particles unlike those such as electrons and protons that make up ordinary matter.

One such particle has its roots in a theory of elementary particle physics called supersymmetry. Elementary particles are either bosons, which can clump together, or fermions, which cannot. Supersymmetry posits that every particle has a more massive relative, called its superpartner, belonging to the opposite class. For instance, the proposed superpartner of the electron, a fermion, is called the selectron, and would be a boson.

Although superpartners have yet to be observed, theorists have suggested that they might be the building blocks of dark matter.

That’s where GLAST enters the picture. Among the supersymmetric candidates for dark matter, the least massive is the gravitino, superpartner to the graviton. The graviton, itself hypothetical, is proposed as the quantum particle relating to a gravitational field in the same way as photons relate to electromagnetic field. Gravitinos can decay into photons, including gamma rays, along with other particles that would last longer than the age of the universe. If dark matter consists of gravitinos, their decay would produce a diffuse gamma-ray glow that GLAST might detect, say theorists Alejandro Ibarra and David Tran of the Deutsches Elektronen-Synchrotron facility in Hamburg, Germany, in a paper recently posted online (http://xxx.lanl.gov/abs/0709.4593).

GLAST will also search for signs of another group of supersymmetric particles known as weakly interacting massive particles, or WIMPS, that have also been proposed as dark matter candidates. When two WIMPS collide, they annihilate, producing a shower of more-familiar particles, including gamma rays.

It will be tricky for LAT to distinguish gamma rays produced by decaying dark matter from radiation generated by supernovas, hot gas around black holes, and other conventional sources. But the search will be easier if, as theorists suggest, dark matter has a clumpy distribution, so that some parts of the sky will show more-intense gamma emission than others.

In addition, WIMP collisions would generate gamma rays with a specific energy, whereas black holes and supernovas produce gamma rays over a wide energy range, and a cacophony of other radiation besides.

“That’s the rosy scenario [but] we might not have a signal that’s strong enough to rule out” ordinary astrophysical explanations, cautions GLAST scientist Peter Michelson of Stanford University.

Even more speculatively, says Michelson, GLAST might also hunt for signs of evaporating black holes forged in the early universe. Black holes, contrary to popular notion, aren’t truly black. Quantum theory, Stephen Hawking showed in 1974, reveals that the boundary between the outside and inside of a black hole is slightly fuzzy. As result, black holes leak radiation into space. The smaller their mass, the more radiation they emit.

Supermassive black holes at the centers of galaxies or starsize ones in the Milky Way are far too massive to emit detectable amounts of Hawking radiation. But if black holes weighing only about a millionth the mass of the sun came into existence soon after the Big Bang, as some theorists have suggested, they would emit significant amounts of radiation, including gamma rays. As they emit energy, the black holes lose mass, which makes them emit even more intensely. This runaway process ends with an abrupt explosion. Some theorists have even suggested that some gamma-ray bursts represent the explosive evaporation of tiny black holes.

Finding a link between gravity and quantum mechanics would be monumental, but the greatest discoveries GLAST will make may be those that aren’t anticipated.

“Let us not forget about the unknown,” says Thompson. “Despite all our best efforts,” he notes, “over half the gamma-ray sources spotted by EGRET remain unidentified. They don’t seem to be pulsars, they don’t seem to be blazars. They’re mysteries to be solved.”

GLAST will find thousands of new sources. “We’ve only scratched the surface,” says Thompson, of what the gamma-ray sky may hold.