March 5, 2004  


In a Virtual Sky, Astronomers Find Dark Matter

By Davide Castelvecchi

Making sense of the data coming from the Large Area Telescope (LAT), one of two main instruments of the GLAST mission, will take highly trained eyes and sophisticated software. Once the NASA probe is in orbit in 2007, astronomers will be able to hit the ground running thanks to three rounds of a simulation drill called Data Challenges, or DC. After six months of preparation, the first round started last December and ended successfully with a SLAC workshop in February.

An image from Data Challenge 1. This is a simulation of what the sky would look like if our eyes could see gamma rays. The darker, horizontal band is the Milky Way. Most of the visible point-like sources are fast-rotating pulsars (within the galaxy) or accreting black holes at the center of other galaxies. Image by Julie McEnery (NASA/GSFC)

"We took a major step forward in preparing for the launch of GLAST," said Stanfordís Peter Michelson, Principal Investigator of the SLAC-based LAT project. Steve Ritz of NASAís Goddard Space Flight Center led the overall DC effort. Ritz is both Project and Instrument Scientist for the GLAST mission.

An international team coordinated by Richard Dubois (SLD), a particle physicist on SLACís LAT team, wrote the simulation software, which ran on SLACís computing system. The software churned out data that was meant to look like it came from the LAT detector.

Meanwhile, LAT astronomer Seth Digel (GLAST) coordinated the data analysis team. American, French and Italian physicists wrote the software tools astronomers will need to analyze the data, whether it is simulated or real.

"This level of end-to-end simulation is almost unprecedented for a space astrophysics mission," said Michelson. On December 8, both the data and the analysis tools were presented to the GLAST community at a meeting on the Stanford campus.

The first DC round simulated most known sources of cosmic gamma rays, the highly energetic radiation GLAST is meant to observe. Each round of the drill will include more and more sources of gamma rays, requiring the equivalent of several days of SLACís full computing power.

Because LATís images will have tens of times the definition of those from previous gamma ray telescopes, astronomers will have to be prepared to witness entirely new phenomena. In this round of simulations, Ritz told the Ďtest-takersí to watch out for some surprises, and they readily realized that his model produced clues of dark matter at the center of our galaxy.

Dark matter has not yet been directly observed in reality, though physicists say it has to be six times more abundant than ordinary matter in order to explain the shape of some galaxies. The nature of dark matter is one of the mysteries that GLAST may help to solve.

The DC exercise has been an opportunity for particle physicists, astrophysicists, astronomers and software experts to learn about each otherís science ends and means. "The fun part was putting the team together to pull this off," Dubois said.

Gamma rays are some of the most energetic radiation found in nature, billions of times more energetic than visible light. They are emitted by the nuclear reactions that happen in such dramatic cosmic phenomena as black holes and supernovae, the exploding stars that can temporarily outshine entire galaxies. To observe the gamma-ray sky, astronomers need high-altitude balloons or orbiting satellites. Luckily for us, the earthís atmosphere is an effective shield against the highly destructive radiation in space.


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Tuesday March 02, 2004 by Emily Ball