Seeing the light of neutron star collisions

UNIVERSITY PARK, Pa. — When two neutron stars collided on Aug. 17, a widespread search for electromagnetic radiation from the event led to observations of light from the afterglow of the explosion, finally connecting a gravitational-wave-producing event with conventional astronomy using light, according to an international team of astronomers.

Previous gravitational-wave detections by LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo, a European observatory based in Pisa, Italy, were caused by collisions of two black holes. Black hole collisions generally are not expected to result in electromagnetic emissions and none were detected.

"A complete picture of compact object mergers, however, requires the detection of an electromagnetic counterpart," the researchers report online today (Oct. 16) in Science.

The Aug.17 detection of a gravitational wave from the collision of two neutron stars by gravitational wave observatories in the U.S. and Europe initiated a rapid cascade of observations by a variety of orbiting and ground-based telescopes in search of an electromagnetic counterpart.

Two seconds after detection of the gravitational wave, the Gamma Ray Burst monitor on NASA's Fermi spacecraft detected a short gamma ray burst in the area of the gravitational wave's origin.

While the Swift Gamma Ray Burst Explorer — a NASA satellite in low Earth orbit containing three instruments: the Burst Alert Telescope, the X-ray Telescope and the Ultraviolet/Optical Telescope — can view one-sixth of the sky at a time, it did not see the gamma ray burst because that portion of the sky was not then visible to Swift. Penn State is in charge of the Mission Operations Center for Swift. The satellite orbits the Earth every 96 minutes and can maneuver to observe a target in as little as 90 seconds.

Once the Swift team knew the appropriate area to search, it put the satellite's instruments into action. Swift is especially valuable in this type of event because it can reposition to a target very quickly. In this case, the telescope was retargeted approximately 16 minutes after being notified by LIGO/Virgo, and began to search for an electromagnetic counterpart.

Read more about Swift's involvement in detecting the neutron star collision here: https://www.eurekalert.org/pub_releases/2017-10/ps-stl101617.php

To speak with Penn State's Swift researchers, contact Joslyn Neiderer at jms1140@psu.edu.

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