A compact detector technology applicable to all types of
cross-disciplinary scientific investigations has found a home
on a new CubeSat mission designed to find the electromagnetic
counterparts of events that generate gravitational waves.
NASA scientist Georgia de Nolfo and her collaborator,
astrophysicist Jeremy Perkins, recently received funding from
the agency’s Astrophysics Research and Analysis Program to
develop a CubeSat mission called BurstCube. This mission, which
will carry the compact sensor technology that de Nolfo
developed, will detect and localize gamma-ray bursts caused by
the collapse of massive stars and mergers of orbiting neutron stars. It also will detect solar flares
and other high-energy transients once it’s deployed into
low-Earth orbit in the early 2020s.
The cataclysmic deaths of massive stars and mergers of neutron stars are of
special interest to scientists because they produce
gravitational waves—literally, ripples in the fabric of
space-time that radiate out in all directions, much like what
happens when a stone is thrown into a pond.
Since the Laser Interferometer Gravitational Wave Observatory,
or LIGO, confirmed their existence a couple years ago, LIGO and
the European Virgo detectors have detected other events,
including the first-ever detection of gravitational waves from the merger of two
neutron stars announced in October 2017.
Less than two seconds after LIGO detected the waves washing
over Earth’s space-time, NASA’s Fermi Gamma-ray Space Telescope
detected a weak burst of high-energy light—the first burst to
be unambiguously connected to a gravitational-wave source.
These detections have opened a new window on the universe,
giving scientists a more complete view of these events that
complements knowledge obtained through traditional
observational techniques, which rely on detecting
electromagnetic radiation—light—in all its forms.
Perkins and de Nolfo, both scientists at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, see BurstCube as a
companion to Fermi in this search for gravitational-wave
sources. Though not as capable as the much larger Gamma-ray
Burst Monitor, or GBM, on Fermi, BurstCube will increase
coverage of the sky. Fermi-GBM observes the entire sky not
blocked by the Earth. “But what happens if an event occurs and
Fermi is on the other side of Earth, which is blocking its
view,” Perkins said. “Fermi won’t see the burst.”
BurstCube, which is expected to launch around the time
additional ground-based LIGO-type observatories begin
operations, will assist in detecting these fleeting,
hard-to-capture high-energy photons and help determine where
they originated. In addition to quickly reporting their
locations to the ground so that other telescopes can find the
event in other wavelengths and home in on its host galaxy,
BurstCube’s other job is to study the sources themselves.
BurstCube will use the same detector technology as Fermi’s GBM;
however, with important differences.
Under the concept de Nolfo has advanced through Goddard’s
Internal Research and Development program funding, the team
will position four blocks of cesium-iodide crystals, operating
as scintillators, in different orientations within the
spacecraft. When an incoming gamma ray strikes one of the
crystals, it will absorb the energy and luminesce, converting
that energy into optical light.
Four arrays of silicon photomultipliers and their associated
read-out devices each sit behind the four crystals. The
photomultipliers convert the light into an electrical pulse and
then amplify this signal by creating an avalanche of electrons.
This multiplying effect makes the detector far more sensitive
to this faint and fleeting gamma rays.
Unlike the photomultipliers on Fermi’s GBM, which are bulky and
resemble old-fashioned television tubes, de Nolfo’s devices are
made of silicon, a semiconductor material. “Compared with more
conventional photomultiplier tubes, silicon photomultipliers
significantly reduce mass, volume, power and cost,” Perkins
said. “The combination of the crystals and new readout devices
makes it possible to consider a compact, low-power instrument
that is readily deployable on a CubeSat platform.”
In another success for Goddard technology, the BurstCube team
also has baselined the Dellingr 6U CubeSat bus that a small
team of center scientists and engineers developed to show that
CubeSat platforms could be more reliable and capable of
gathering highly robust scientific data.
“This is high-demand technology,” de Nolfo said. “There are