Scientist Looks for Specific Energy Emissions to Identify Sources of Cosmic Positrons

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)

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.”

While the technology is a few years away, Kierans said, they want to start development now in anticipation of results from other proposed mission concepts, such as the Compton Spectrometer and Imager (COSI) and the All-sky Medium Energy Gamma-ray Observatory Explorer (AMEGO-X), now under study. Once 511keV hot spots have been identified in the galaxy, Kierans’ instrument can perform more detailed observations.

In order to accomplish this, Kierans requires optics that can focus and image gamma rays.

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 done before. While Chandra collects photons in a similar way, and is very effective within the 1 to 10 keV band, it is unable to focus photons at higher energies.

Kierans said if the technology was used in a large telescope, it would amount to bringing unprecedented NuSTAR-like capabilities to the gamma-ray range. NuSTAR is the current record-holder for high-energy focusing capabilities; it is sensitive to X-rays from 3 to 79 keV.

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.

“Grazing incidence X-ray optics optimized for high energies could focus gamma-ray photons to a much smaller point on the telescope detectors than previous gamma-ray instruments operating at 511 keV,” Gurgew said.

This both increases the telescope’s sensitivity to gamma-ray light, and allows for much higher-resolution imaging of these sources, she said.

Gurgew is working on going beyond what X-ray missions such as Chandra or XMM-Newton have done in focusing photons. She anticipates extending the energy band for focusing optics up to several hundred keV, into the gamma ray band.

“So instead of just a single thin film of highly reflective materials, like the iridium Chandra uses, we use a more complex structure — a multilayer coating,” Gurgew said.

Such a coating boosts reflectivity, allowing for imaging at higher energies, Gurgew said. This technology will ideally be able to image anywhere from 2 to 200 keV, she said. However, Kierans’ project demands imaging at an even higher energy — so the goal is to build on this work to investigate a multilayer coating capable of imaging at 511 keV.

Looking Forward

The technology is still in the very early stages of development. Kierans said the next step involves proposing for an Astrophysics Research and Analysis Program (APRA) technology and development grant to process small samples of a few different multilayer coatings and test them in the lab.

For a 511 keV telescope, the optics would require hundreds of individual layers with a thickness measured in dozens of angstroms. Each layer could be only about 10 atoms thick. This is particularly challenging since the individual layers also need to be very smooth. Understanding how different combinations of materials interact with one another during multilayer processing is crucial.

Kierans said performing tests of small mirror substrates would be the first step in this development. It is still unclear whether the current state-of-the-art predictions of multilayer coating performance are valid at gamma-ray energies, and comparing the results of laboratory measurements with models will be a big step in our understanding of how we can extend this technology to the 511 keV goal, Kierans said.

If there is ever a desire to reach energy levels higher than 511 keV, Gurgew said they may run into issues while working at such a small scale. Essentially, the higher the energy, the thinner the layer becomes, and you run into a point where you can’t physically deposit thin or smooth enough layers to achieve that energy.

One other technological challenge is that a 511 keV focusing telescope needs to have a large focal length, at 50 to 100 meters, with really small grazing angles, between 0.01 and 0.04 degrees. This will require precision formation flying using two satellites: one for the mirror and one for the telescope detector. The 511 keV focusing telescope without a doubt has its challenges, said Gurgew, but if proven successful over time, could revolutionize gammaray astrophysics as we know it.

“Proposing to get something like this launched would be obviously a wonderful next step,” Kierans said. “But being realistic, expanding the energy range would be the next most feasible step in terms of advancing the technology in other science directions as well. Because if we can image a large band of energy, then others are going to be interested in using it for wider gamma-ray energies, not just 511 keV.”

Gurgew said demonstrating the optics capability at 511 keV is likely the best way to move forward with the project.

As far as the current guesses are for what they hope to find when searching for the source of positrons with this technology some people think the answer could be related to dark matter or something beyond the standard model of physics.

“Once we do understand the sources of positrons,” Kierans said, “the 511 keV emission can be used as a tool to better understand their galactic environments.”

Carolyn.A.Kierans@nasa.gov or 301-286-7628