Using powerful x-rays produced by the National Synchrotron Light Source (NSLS) as a calibration tool, a team of researchers from NASA's Goddard Space Flight Center is developing an instrument to probe deep space phenomena such as gamma rays and black holes.
Counter clockwise, from back, researchers Syed Khalid (NSLS), C.J. Martoff (Temple University), Kevin Black (Rock Creek Scientific), Michael Dion (Temple University), and Joe Hill (NASA). Not pictured, Zachary Prieskorn (University of Iowa).
Some celestial objects, including the moon, the stars, and more exotic events like gamma ray bursts - one of the most powerful explosions in the universe - emit "light" in the form of x-rays. In order to learn more about this light and the sources that produced it, scientists measure four characteristics of an x-ray's photons: their number, direction, energy and frequency, and polarization.
"Because astronomical sources are so far away, we need to use these tools to determine what they are and how they behave," said Universities Space Research Association scientist Joanne "Joe" Hill, who works at NASA's Goddard Space Flight Center.
Scientists have figured out ways to measure the first three characteristics, but the fourth - polarization - remains tricky. The vibrating electric and magnetic fields that make up x-rays often point in different directions, known as polarization states. When the orientation of the fields is completely random, the degree of polarization is 0 percent. When the fields all point in the same direction, the poralization is 100 percent. Scientists have a difficult time determining the polarization of astronomical sources because their signals are too faint or largely unpolarized. But Hill's group is developing a detector able to do the job, and thus, provide researchers with a detailed picture of the insides of celestial objects.
The device, called a time projection chamber (TPC), is commonly used in accelerator physics experiments to track the subatomic debris produced in high-speed particle collisions. This gas-filled chamber also serves as a very sensitive and versatile method for detecting high-energy x-rays produced in space. When an x-ray collides with a gas atom in the TPC, it kicks out an electron. The electron travels through the gas, colliding with other atoms and creating a cloud of electrons along its path. The instrument will measure the electron cloud to determine the direction of the x-ray's electric field, which is the same as its polarization.
"Scientists have been developing various designs for astronomical x-ray polarimeters since the 1970s, but, except for one bright source - the Crab Nebula - they were unable to measure the polarization from the very small and faint fraction of celestial light that reaches us," Hill said. "With the TPC, we'll finally be able to understand the polarization of these photons, which can tell us more about what's happening around and inside the extreme environments produced by objects such as black holes and supernovae."
The group performed initial tests on the TPC at NSLS beamline X19A, where they used the highly polarized synchrotron light to calibrate the detector and gain a better understanding of how it will work in space. They hope to return to the Lab in the spring to perform more tests.
If all goes well, the TPC prototype will then be replicated for two upcoming NASA missions: the Gamma-ray Burst Polarimeter (GRBP) and the Gravity and Extreme Magnetism SMEX (GEMS). GRBP, scheduled to launch in December 2009, will be the first mission to definitively measure high-energy polarization from gamma-ray bursts, huge explosions from halfway across the universe. GRBP will build on information from NASA's Swift mission, a rapidly moving satellite that has been chasing gamma-ray bursts since November 2004. The GEMS mission, which aims to study the magnetic field surrounding black holes, dead stars, and other celestial objects, is still being reviewed within NASA. If it's given the green light, GEMS would launch around 2013.
The research team is comprised of scientists from NASA's Goddard Space Flight Center, the Universities Space Research Association, Rock Creek Scientific, the University of Iowa, and Temple University.
2008-975 INT/EXT | Media & Communications Office
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