Fermi opens new window on high-energy universe: gamma-ray telescope detects bursts and pulsars.

VANCOUVER -- Curtain up! Light the lights! In its first four months monitoring the heavens from orbit, NASA's Fermi Gamma-raySpaceTelescope has unveiled the activity of celestial objects that emit powerful gamma rays--photons that pack 20 million to more than 300 billion times the energy of visible light. The orbiting observatory features the first detectors in space capable of recording the most energetic of these photons.

For now, Fermi's flurry of initial findings--which include discoveries about gamma-ray bursts as well as a possible new class of pulsars, the rapidly spinning corpses of exploded stars--poses new puzzles. But ultimately the findings will offer new insight into the origin of these powerful emissions and the activity of pulsars, among the most enigmatic objects in the cosmos, says Stanford University's Peter Michelson, the principal investigator of Fermi's Large Area Telescope, or LAT, the device that records high-energy emissions.

Michelson and his Stanford colleague Aurelien Bouvier presented their results December 8 in Vancouver at the Texas Symposium on Relativistic Astrophysics. At press time, Fermi researchers were scheduled to report similar findings January 7 in Long Beach, Calif., at a meeting of the American Astronomical Society.

Bouvier's report focused on gamma-ray bursts, the ephemeral flashes of light that can signal the most powerful explosions in the universe since the Big Bang. Long-duration bursts--those lasting more than a second or two--may be the birth cries of black holes, created as jets of material zoom out of collapsing stars. Short bursts may signal the coalescence of two elderly neutron stars that have been orbiting each other for billions of years.

In his talk, Bouvier announced that the LAT had recorded the highest-energy emissions ever detected from gamma-ray bursts. Among a trio of such detections, the telescope did not record any energetic radiation until well after Fermi's other instrument, the Gamma-ray Burst Monitor, detected the low-energy components of the same bursts.

The time delay between the onset of high- and low-energy emissions--which amounted to five seconds in a burst discovered September 19--suggests that the high-energy gamma rays from bursts might be produced at different places or by different particles than the lower-energy radiation, Bouvier says. Gamma-ray bursts are believed to be generated when clusters of charged particles in a jet racing out of a collapsed star collide with each other.

Within a jet, it may be easier--and quicker--for electrons, which are relatively lightweight, to rev up to high speeds and crash into each other, producing the early, lower-energy part of these bursts, he says. And it's possible that protons, which are heavier and thus take longer to accelerate, contribute to the higher-energy component some time later.

Another, more intriguing--and much more speculative--idea may explain at least part of the delay, Bouvier adds.

The highest-energy photons--13 giga-electronvolts--from the September 19 event arrived a full 15.5 seconds later than the lowest-energy emissions. Moreover, the burst originated in an extremely remote galaxy, 12.2 billion light-years from Earth. Many theories of quantum gravity predict that spacetime on its tiniest scale isn't continuous but is as malleable and variable as sea foam. Because of this foaminess, not all photons would travel at the same speed. Even though photons have no mass, those with higher energies--which translate into higher gravitational potentials, according to Einstein's E=[mc.sup.2]--would travel slightly slower through space and arrive slightly later than lower-energy photons. The effect would be tiny, but over a journey of 12.2 billion light-years, it might be detectable.

Two of the three bursts detected by the LAT--the September event and one recorded on August 25--belong to a class of bursts that last for over a second. But on October 24, the telescope detected a first--extremely high-energy emissions from a short gamma-ray burst, which lasted for only a few tenths of a second.

The Fermi data support the idea that although long and short bursts have different origins, "the sources of the outflow in both cases share many similarities and are probably sharing the same physical mechanism," comments Ehud Nakar of Caltech in Pasadena.

Michelson reported that the LAT has now recorded 14 previously unknown pulsars in our galaxy. These pulsars have been found to emit only gamma rays, not radio waves, as most of the 1,800 known pulsars do. If Fermi continues to find gamma-ray-only pulsars at such a high rate, it could indicate that the galaxy has as many gamma-ray pulsars as radio pulsars, says Caltech's Shri Kulkarni.

A theory developed in 1995 by Roger Romani of Stanford and a collaborator may explain why Fermi so quickly found the gamma-ray-emitting pulsars. In that theory, all pulsars emit both radio waves and gamma rays, The region where each type of radiation is generated is determined by the pulsar's strong magnetic field. Gamma rays are emitted from a higher-altitude region and fan out over a wider area than the lower-altitude, narrow beams of radio waves. So gamma rays from pulsars may be more likely than radio waves to sweep over Earth's vicinity and be noticed. The new pulsars were not detected before because their gamma-ray energies were too high to be detected by previous spacecraft.

With the new Fermi data, "it's very exciting that we may finally be uncovering the mechanism of these powerful cosmic accelerators" says Romani.

Fermi's grand view

This map shows the location in the Milky Way of the 15 radiosilent, gamma-ray-only pulsars detected by the Fermi Gamma-ray Space Telescope, 14 of which had never been observed before. Pulsars detected by Fermi's predecessor, CGRO, and those previously discovered through radio emissions are shown as well. In the brief time the Fermi telescope has been in orbit, it has also recorded three gamma-ray burst events.

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Birth of a burst

Jets zoom out from a massive, collapsing star in this illustration. Collisions between parcels of high-speed charged particles within jets are thought to generate long-duration gamma-ray bursts. The Fermi telescope found that the highest-energy gamma-ray emissions from three bursts--two long-duration, one short--lag behind the lower-energy emissions, a finding still awaiting explanation.

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Back Story | AT 40-PLUS, PULSARS STILL PACK SURPRISES

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* 1967 Jocelyn Bell and Antony Hewish (shown) discover the first pulsar by recording rapid pulses of radio waves emitted by a celestial object.

* 1968 Discovery of a pulsar that emits bursts of radio waves 30 times a second at the center of the Crab nebula (shown), the remnant of a supernova witnessed by the Chinese in 1054.

1974 Joseph Taylor and Russell Hulse discover the first pulsar in a binary system, and show that the pulsar is slowing at exactly the rate predicted by Einstein's theory of general relativity.

1982 First pulsar found to spin with a rotation period of just 1.6 milliseconds, about 20 times faster than previous finds.

* 2008 First gamma-ray-only pulsars found with the Fermi Gamma-ray Space Telescope (illustration shown).