SWPS Scientist: Clio Sleater

clio I am a PhD student in the physics department, where I study with Prof. Steve Boggs at U.C. Berkeley’s Space Sciences Laboratory. My research is in the field of high energy astrophysics, the study of X-rays and gamma-rays emitted from astrophysical objects. The focus of my group is to develop novel instruments to detect gamma-rays from space. Currently, we are working on the Compton Spectrometer and Imager (COSI), a balloon-borne gamma-ray telescope. COSI is designed to measure polarization from compact objects such as neutron stars, black holes, and gamma-ray bursts; map the 511 keV positron annihilation line from the Galactic plane and bulge; and image lines of radioactive decay to learn more about stellar nucleosynthesis.

COSI is sensitive to photons in the energy range of 0.2-5 MeV, known as soft or medium energy gamma-rays (depending on who you ask). This energy range is referred to as the MeV gap and is the least astrophysically explored range across the electromagnetic spectrum. Due to high instrumental and atmospheric backgrounds, low interaction cross-sections, and the inherent difficulty of imaging at these energies, the sensitivity of MeV telescopes is currently much worse than the sensitivity of telescopes in neighboring energy ranges. Though many challenges come with observing MeV gamma-rays, this energy range is scientifically rich: we can learn a lot about signatures of stellar nucleosynthesis, positron annihilation, and emission from the most extreme environments.

MeV gamma-rays primarily interact with matter via a process called Compton scattering. To detect these gamma-rays, COSI utilizes 12 high purity Germanium detectors. When incoming gamma-rays Compton scatter in the COSI detectors, we use our understanding of Compton scattering to determine where in the sky the gamma-rays came from, as well as to measure the energy and polarization of the incoming gamma-ray. Because gamma-rays are absorbed in the atmosphere, gamma-ray telescopes need to be in space to study astrophysical objects. COSI is carried into space by a scientific balloon filled with helium, where it floats in the uppermost regions of the atmosphere. Balloons are much cheaper than satellites, so they are a great platform to develop new instruments.

I spent the first few years of my PhD working on building and calibrating the COSI instrument. My roles on the project have included testing and installing our shielding and cooling subsystems, writing the monitoring and commanding software, and any random tasks that need to get done. In the early summer of 2014, we built up COSI in our lab at Berkeley for the first time and tested it to ensure that it was working as expected. We then immediately took it apart and shipped the components to the NASA ballooning facility in Palestine, Texas. There, we spent two months rebuilding the instrument, this time including key components that NASA provided such as telemetry, batteries and solar panels. Once we had confirmed that our detectors were not adversely affected by noise from the NASA electronics, we concluded that COSI was ready to go to space!

Because mishaps can happen during balloon launches and flights, it’s best to launch in unpopulated areas. NASA has a couple of launch sites throughout the globe, including Palestine, Texas; Fort Sumner, New Mexico; McMurdo Station in Antarctica; and Wanaka, New Zealand. When launching from Palestine and Fort Sumner, the flights can only last about a day or two. When launching from McMurdo or Wanaka, however, the winds push the balloon over much less populated areas (Antarctica and the Pacific Ocean, respectively); thus, the balloon flight can last up to 56 days (the world record), or in theory even longer.

In November of 2014, we traveled to McMurdo Station in Antarctica to launch COSI. We worked day and night to get our instrument ready for launch by the first week of December. Once we were launch ready, we had to wait for the weather to be good enough to launch — basically, we need very low wind. COSI launched on December 28, 2014. Unfortunately, the balloon developed a leak during the second day of flight and the mission was terminated shortly thereafter. Another graduate student in my group went to recover the instrument on the Antarctic ice shelf, and it returned to Berkeley in April 2015 in good working order.

In February of 2016, we left for another balloon launch, this time from Wanaka, New Zealand. Again, we spent the first month of the trip preparing the payload for launch. This time, we had to wait 6 weeks for the weather to be good enough to launch! While waiting, we explored the beautiful area around Wanaka and some of us (myself included) got really into knitting. COSI finally launched on May 17, 2016. This time, the balloon remained healthy, and we had a 46 day flight! The payload eventually came down in Peru. Once again, we recovered the instrument and it’s now back in Berkeley.

During our 2016 flight, we got great exposure of the Galactic center and detected one gamma-ray burst. We also detected three compact objects including the Crab nebula (my area of study). Since then, I’ve been working on analyzing the data. It’s been quite a change going from working in the lab to sitting at a computer all day long, but the variety of the work I’ve done with COSI has made for an interesting and rewarding grad school experience!

To learn more about COSI and our adventures in Antarctica and New Zealand, check out cosi.ssl.berkeley.edu.