USRA scientists play role in detecting light from gravitational waves event
After the neutron stars merged, the remains of the jets that produced the gamma-ray burst continue expanding into space, as shown in this illustration. After nine days, the jet directed toward us had spread laterally enough that observers could detect its X-ray emission. Credit: NASA's Goddard Space Flight Center/CI Lab
Columbia, Maryland—October 16, 2017. A team of scientists from Universities Space Research Association (USRA) noticed on August 17, 2017, that missing data for the most recent transient event or “trigger” of the Gamma-ray Burst Monitor (GBM) on-board NASA’s Fermi satellite had halted the automatic processing of the data. There was no hint that anything extraordinary was happening.
The Gamma-ray Burst Monitor (GBM) on Fermi, picked up a brief pulse of gamma rays that seconds later was automatically reported to the astronomical community as a short GRB. NASA's Fermi Gamma-ray Space Telescope caught high-energy light from an explosion associated with the event.
And then NASA's Swift, Hubble and Chandra missions, along with dozens of ground-based observatories, captured the fading glow of the blast's expanding debris. Something eventful was happening!
The GBM trigger time and the initial estimate of the sky position of the GRB on the sky had been communicated automatically via the GRB Coordinates Network within 14 seconds of its detection. The missing step would confirm the trigger was from a Gamma-Ray Burst (GRB) and provide the final and best location on the sky for follow-up observations. USRA scientist Valerie Connaughton was about to alert the duty scientist, Andreas von Kienlin at Max Planck Institute in Germany, that this trigger required manual intervention to finish processing, but no big rush.
Six minutes after that came the subject line of an email screaming “WAKE UP!!!!” from her NASA MSFC colleague Tyson Littenberg, a scientist with the Laser Interferometer Gravitational-Wave Observatory (LIGO). This perfectly ordinary GRB “had a friend”. As Littenberg’s email disclosed, LIGO data pipelines found that the GBM trigger was close in time to a promising gravitational-wave signal in the LIGO-interferometer at Hanford, WA. The signal looked very much like what to expect from a pair of merging neutron stars, which had never before been seen.
And so started the excitement of the gravitational wave event.
With the coincidence in time between the GRB, called GRB 170817A, and the gravitational wave, called GW170817, LIGO scientists knew they were on to something exciting. An hour later, scientists reviewing data from ESA's (European Space Agency) INTEGRAL satellite also reported detecting the GRB.
No GRB is truly ordinary, each one announcing the death of at least one star and the birth of a black hole, but GRB 170817A, detected in coincidence with the gravitational wave signal, solves a decades-old mystery regarding the origin of so-called short GRBs. Long GRBs are firmly tied via supernova observations to the collapse of massive stars. The likely connection between short GRBs - those lasting less than 2 seconds - and the merger of two neutron stars was only circumstantial. The unambiguous link between GRB 170817A and GW170817 is a smoking gun.
Compact objects such as neutron stars are born during the collapse of stars a few times larger than our sun via a supernova explosion. The neutron stars can form rotating pairs that last a billion or more years before merging in a final spiral, emitting gravitational radiation according to Einstein’s general theory of relativity. Any evidence we see of this particular merger made a journey of 140 million light years to reach the Earth’s neighborhood. The 1.7 seconds separating the detection of the gravitational wave from the gamma-ray signal allows us to calculate differences in speed between gravity and light. “We can say they are the same to within about one part in one quadrillion, proving once again that Einstein was right and his detractors wrong.” says Eric Burns, a NASA Postdoctoral Program fellow involved in the analysis of GRB 170817A and GW170817.
Only members of the LIGO-Virgo electromagnetic follow-up group were privy to the coincidence between the GBM trigger and the signal at Hanford. Today, the LIGO and Virgo teams announce this first detection of a gravitational-wave signal from the merger of two neutron stars and the follow-up observers can loosen their tongues.
Realizing the importance of GRB 170817A, USRA scientist Adam Goldstein quickly produced the best possible GBM map to guide follow-up observers inside the secrecy bubble to the right spot on the sky.
Goldstein noted, “I was cautiously optimistic that the two detections were related, but when I was finally able to see the updated LIGO/Virgo sky map a few hours later, I knew we had observed something very special.” The LIGO/Virgo map tightened the patch of sky observers needed to scour for other signals from the merger. They were not disappointed. An optical transient shone in the nearby galaxy NGC 4993. At a distance of about 130 million light years from Earth, this matched the distance measured from the gravitational wave data. It is much closer than the typical GRB distance of a billion light years or more.
A “kilonova” detected in the optical and ultraviolet energy bands revealed yet another sign of the merger, arising from the expulsion of high-density radioactive material. A fading X-ray signal typically measured shortly after a GRB was not detected until days later, suggesting the GRB was not initially within our direct line-of-sight. This could also explain the relative weakness of the gamma-ray signal seen with GBM, an unusual observing perspective that can be experienced only in the nearby universe because everything appears dimmer when viewed other than down the barrel.
Building a picture of this unique event involved five space-based observatories, four run by NASA: Fermi, Swift, Hubble, and Chandra, and one by the European Space Agency: the INTErnational Gamma-Ray Astrophysics Laboratory or INTEGRAL, and many ground-based telescopes covering the electromagnetic spectrum in addition to the three gravitational-wave interferometers.
Fermi's GBM instrument is operated by NASA’s Marshall Space Flight Center in Huntsville, Alabama.
Dozens of scientific papers will appear in the peer-reviewed scientific literature in the days following this announcement, three of them involving USRA members of the Fermi GBM team, with Goldstein and Connaughton joined by William Cleveland the software architect who designed and maintains the GBM data pipeline and William Paciesas, a former Principal Investigator for the GBM experiment.