In an announcement last week, NASA selected the Compton Spectrometer and Imager (COSI) mission, which is led by UC Berkeley’s Space Sciences Laboratory (SSL), to advance toward a scheduled launch in 2025. The mission budget is estimated at $145 million, not including launch costs.
The telescope will map low energy, or "soft,” gamma ray emissions over the full sky during its two-year mission. Gamma rays are produced during the decay of unstable isotopes of atoms such as iron, aluminum and titanium, which are produced when massive stars explode. By mapping gamma ray emissions throughout the galaxy, COSI will be able to chart new and recent supernovae and help the understanding of how they manufacture and seed the galaxy with elements.
COSI "will revolutionize our understanding of the creation and destruction of matter in the galaxy and beyond,” said SSL research scientist John Tomsick , the mission’s principal investigator, in a video tweeted last week by Thomas Zurbuchen, associate administrator for the NASA’s Science Mission Directorate in Washington.
-For more than 60 years, NASA has provided opportunities for inventive, smaller-scale missions to fill knowledge gaps where we still seek answers,- Zurbuchen said, in announcing the award. -COSI will answer questions about the origin of the chemical elements in our own Milky Way galaxy, the very ingredients critical to the formation of Earth itself.-
One key objective of the mission is to understand why the Milky Way contains more positrons, which are the antimatter partners of electrons, than would be expected based on the known sources, primarily supernovae, in the galaxy. When positrons encounter electrons, the two annihilate one another and produce pairs of gamma rays of the same specific energy. Mapping this gamma ray wavelength will tell astronomers where the positrons are.
"There are more positrons out there than can be accounted for from supernovae alone, and the origin of the excess is unknown,” Tomsick said. "So, one thing we’re going to do is map in detail where the positrons are coming from to understand how they’re produced.”
Other possible sources of positrons include the supermassive black hole at the center of the galaxy, an unseen population of small black holes about the mass of our sun, or the annihilation of dark matter particles, which could theoretically produce positrons.
Mapping the galaxy in a new gamma ray bandThe new gamma ray imager is called a Compton telescope because it determines the location from which a gamma ray comes by how it scatters off electrons - Compton scattering - inside its germanium detectors. The detectors, based on concepts developed at Lawrence Berkeley National Laboratory, have 20 times better energy resolution than detectors in a similar soft gamma ray telescope (COMPTEL) that flew aboard the now defunct Compton Gamma Ray Observatory. Also, the area of the sky observed by COSI at any given time is more than four times larger than COMPTEL-s. In fact, COSI will observe the entire sky every day during the mission.
The telescope also can determine the polarization of the gamma rays it detects. Polarized light - like the reflected light from a swimming pool - is produced by various processes in space, including some gamma ray bursts. COSI will play an important role in understanding these transient bursts.
"With our large field of view, we have a 25% chance that any given gamma ray burst will be in our field of view,” Tomsick said. "We expect to see on the order of 200 gamma ray bursts during a two-year mission, and we’ll be able to measure polarization for about half of these. That tells us about the geometry of the source, where the gamma ray is coming from and how it is produced.”
COSI can also help astronomers understand neutron star mergers, which produce gravitational waves, as well as bursts of high energy radiation, including gamma rays.
Tomsick is an expert on astrophysical sources of X-rays and gamma rays. Nearly 20 years ago, he discovered the first X-ray jets produced by black holes as they gobble up the gas and dust around them.
He and the COSI team, including Steven Boggs , formerly at UC Berkeley and now at UC San Diego and deputy principal investigator of the COSI mission, spent decades developing their technology through flights on scientific balloons. In 2016, they flew a version of the gamma ray telescope aboard NASA’s super pressure balloon, which is designed for long flights and heavy lifts. A 46-day circumnavigation of the Southern Hemisphere in 2016 proved that the instruments work as planned: The team discovered, imaged and measured the polarization of a gamma ray burst and mapped the gamma ray emission in the galaxy that accompanies the annihilation of positrons and electrons.
"Ballooning has been essential for developing the COSI instrument, but for science, this is going to be so much better from the satellite than it was from the balloon,” Tomsick said.
With its focus on low-energy gamma rays, COSI will complement the orbiting Fermi satellite, which maps high energy gamma ray emissions in the universe.
"COSI will survey the whole sky in this new energy band,” he said. "We already know there’s a lot of interesting physics that leads to low energy gamma ray emission.”
Other key members of the COSI team at SSL include project scientist Andreas Zoglauer; project manager Cathy Chou; payload manager Alex Lowell; electrical engineer Brent Mochizuki; and graduate students Hadar Lazar, Jacqueline Beechert and Hannah Gulick. SSL will be working with the Naval Research Laboratory, NASA’s Goddard Space Flight Center, Northrop Grumman, Clemson University, Louisiana State University, Los Alamos National Laboratory and several international science partners.
COSI is the newest small mission explorer (SMEX) satellite in NASA’s Explorers Program, the agency’s oldest continuous program, which provides frequent, low-cost access to space using principal investigator-led space research relevant to astrophysics and heliophysics. COSI was one of 18 original telescope proposals, which in 2019 were whittled down to four that NASA funded for mission concept studies. So far, COSI is the only one to be cleared for the development phase.