Volume 17 • Issue 4 • Summer 2021
cuttingedge • goddard’s emerging technologies
Scientist Looks for Specific Energy Emissions to Identify Sources of Cosmic Positrons The universe contains far more than what can be seen with even the most advanced telescopes currently functioning. Even the Milky Way galaxy contains secrets scientists are still trying to pry out of the darkness. One of these mysteries lies in the center of the galaxy, where electrons and positrons collide and annihilate, transforming into gamma rays of a characteristic energy. Scientists discovered the radiation — called the positron annihilation line, measured at 511 kiloelectron volts (keV) — decades ago, but the sources of the positrons remain unknown. Goddard astrophysicist Dr. Carolyn Kierans is developing a telescope concept that would provide the necessary resolution to identify where the positrons – the antimatter counterpart of electrons – are coming from. “There are regions in the galaxy that we know should be emitting 511 keV because massive stars emit positrons as they evolve,” Kierans said. “If we point a high-angular-resolution telescope at one of these sources, we could confirm for the first time that we see a positron source.”
In this composite image of Cygnus OB2, X-rays from Chandra (red diffuse emission and blue point sources) are shown with optical data from the Isaac Newton Telescope (diffuse emission in light blue) and infrared data from the Spitzer Space Telescope (orange). (Image credit: X-ray: NASA/CXC/SAO/J. Drake et al;
H-alpha: Univ. of Hertfordshire/INT/IPHAS; Infrared: NASA/JPL-Caltech/Spitzer) While the technology is a few years away, Kierans said, they want to start done before. While Chandra collects photons in a development now in anticipation of results from similar way, and is very effective within the 1 to 10 other proposed mission concepts, such as the keV band, it is unable to focus photons at higher Compton Spectrometer and Imager (COSI) and the energies. All-sky Medium Energy Gamma-ray Observatory Explorer (AMEGO-X), now under study. Once 511Kierans said if the technology was used in a large keV hot spots have been identified in the galaxy, telescope, it would amount to bringing unprecKierans’ instrument can perform more detailed edented NuSTAR-like capabilities to the gamma-ray observations. range. NuSTAR is the current record-holder for
In order to accomplish this, Kierans requires optics that can focus and image gamma rays.
high-energy focusing capabilities; it is sensitive to X-rays from 3 to 79 keV.
Dr. Danielle Gurgew, a NASA postdoctoral fellow at Goddard, said the technology concept is exciting because it would provide an advantage for several realms in gamma-ray astrophysics. Both Gurgew and Kierans emphasized that imaging in the sense of focusing gamma rays has never been
Gurgew is working to develop grazing incident reflectors for so-called hard X-rays. Hard X-rays have energies greater than 10 keV, she said. At still higher energies, such as 511 keV, the radiation takes the form of gamma rays.
www.nasa.gov/gsfctechnology
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