BY DR EMILY BALDWIN
Posted: 5 May, 2009
New details of high energy particles detected by NASA’s Fermi Gamma-ray Space Telescope were revealed yesterday at the American Physical Society meeting held in Denver.
Since its launch last June Fermi has discovered a new class of pulsars, probed gamma-ray bursts and watched flaring jets in galaxies billions of light years away. “Fermi’s Large Area Telescope is a state-of-the-art gamma-ray detector, but it’s also a terrific tool for investigating the high-energy electrons in cosmic rays,” says Alexander Moiseev of NASA’s Goddard Space Flight Center, who presented the findings.
The Large Area Telescope (LAT) on Fermi detects gamma-rays by tracking the electrons and positrons they produce after striking layers of tungsten. This ability also makes the LAT an excellent tool for exploring high-energy cosmic rays. Image: NASA/Goddard Space Flight Center Conceptual Image Lab.
Astronomers believe that the highest-energy cosmic rays - electrons, positrons, and atomic nuclei moving at nearly the speed of light - arise from exotic places within our Galaxy, such as the wreckage of exploded stars. Concentrating on the 4.5 million high-energy particles that struck Fermi’s Large Area Telescope (LAT) between 4 August 2008 and 31 January 2009, the team found evidence that both supplements and refutes other recent findings.
Last autumn, the balloon-borne experiment ATIC captured evidence for a dramatic spike in the number of cosmic rays at energies around 500 GeV. “Fermi would have seen this sharp feature if it was really there, but it didn’t,” says Luca Latronico, a team member at the National Institute of Nuclear Physics (INFN) in Italy. “With the LAT’s superior resolution and more than 100 times the number of electrons collected by balloon-borne experiments, we are seeing these cosmic rays with unprecedented accuracy.”
But compared to the number of cosmic rays at lower energies, more particles striking the LAT had energies greater than 100 billion electron volts (100 GeV) than expected based on previous experiments and models. The observation has implications similar to complementary measurements from other space- and ground-based telescopes, such as the PAMELA satellite and HESS array. These results suggest the presence of a nearby object that is beaming cosmic rays our way. “If these particles were emitted far away, they’d have lost a lot of their energy by the time they reached us,” explained Luca Baldini, another Fermi collaborator at INFN.
Since its launch last June Fermi has discovered a new class of pulsars, probed gamma-ray bursts and watched flaring jets in galaxies billions of light years away.
Unlike gamma rays, which travel from their sources in straight lines, cosmic rays can ricochet off gas atoms or become redirected by magnetic fields, randomizing the particle paths and making it difficult to tell where they originated. If a nearby source is sending electrons and positrons towards the Earth, the most likely culprit is a pulsar, the fast-spinning leftover of an exploded star. Or, somewhat more exotically, the particles could arise from the annihilation of hypothetical particles that make up so-called dark matter. This mysterious substance neither produces nor impedes light and reveals itself only by its gravitational effects.
According to Latronico, Fermi’s next step is to look for changes in the cosmic-ray electron flux in different parts of the sky. “If there is a nearby source, that search will help us unravel where to begin looking for it,” he says.