On Aug. 17, Colleen Wilson-Hodge had already hit the gym and was headed to a training class at work when a message popped up on her phone. The Gamma-ray Burst Monitor (GBM) on NASA’s Fermi Gamma-ray Space Telescope had detected a gamma-ray burst or “GRB” — a brief flash of high-energy light.
“Nothing unusual,” recalls Wilson-Hodge, a NASA astrophysicist and principal investigator for the GBM instrument. GBM had caught the burst in real time, a process the team calls “triggering.” Its onboard flight computer automatically located and classified the event.
Andreas von Kienlin, the scientist on duty at the Max Planck Institute for Extraterrestrial Physics in Germany, dubbed the burst GRB 170817A and reported it to astronomers around the world, just as he had done for hundreds of other events like this.
As it turned out, GRB 170817A was no run of the mill gamma-ray burst. GBM had the given the world its first glimpse of light from the same source as gravitational waves — ripples in space and time.
“WAKE UP,” urged the subject line of an email that arrived only minutes later. It came from NASA astrophysicist Tyson Littenberg, a member of the LIGO Scientific Collaboration, a group of scientists focused on exploring the distant universe with twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in Washington and Louisiana and the Virgo detector in Europe.
LIGO scientists had picked up a gravitational wave signal from merging neutron stars — the smallest, densest stars in the universe. For decades, scientists have suspected that a smashup of two neutron stars might churn out gravitational waves and spew so-called “short” gamma-ray bursts — they were right.
“When we built GBM and launched it on Fermi in 2008, we designed it to detect gamma-ray bursts well,” said Wilson-Hodge. “Back then, it was only slated to fly for five years. Today, GBM is at the forefront of an entirely new type of science, ushering in this new era of multi-messenger astronomy.”
Gamma-ray bursts are the most powerful explosions in the cosmos. Since beginning operations, GBM has triggered on over 2,000 gamma-ray bursts. With 14 detectors pointed in different directions, GBM sees the entire sky not blocked by Earth. It’s sensitive to X-rays and gamma rays with energies between 8,000 and 40 million electron volts (eV). For comparison, the energy of visible light ranges between about 2 and 3 eV.
“GBM’s wide energy range and large field of view makes it a key instrument for detecting electromagnetic counterparts to gravitational waves,” said Adam Goldstein, a GBM team member at the Universities Space Research Association’s Science and Technology Institute in Huntsville, Alabama, and lead author of an article on the burst published in the Astrophysical Journal Letters.