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
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
"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
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