16 October, 2017
The Atacama Large Millimeter/submillimeter Array (ALMA) joined one of the biggest Target of Opportunity observing campaign ever launched worldwide. After the LIGO-Virgo observatory network detected gravitational waves passing through the Earth and positioned the source on 17 August 2017, many telescopes around the globe identified for the first time the electromagnetic counterpart to a gravitational wave source.
For the first time, astronomers have observed both gravitational waves and electromagnetic radiation (light) from the same event, thanks to a global collaborative effort and the quick reactions of observatories around the world. On 17 August 2017, the NSF‘s Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, working with the Virgo Interferometer in Italy, detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817. About two seconds later, two space observatories, NASA’s Fermi Gamma-Ray Space Telescope and ESA’s INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), detected a short gamma-ray burst (GRB) from the same area of the sky.
The LIGO–Virgo observatory network positioned the source within a vast region of the southern sky, the size of several hundred full Moons and containing millions of stars. As night fell in Chile, many telescopes peered at this patch of sky, searching for new sources. The Swope 1-metre telescope was the first to announce a new point of light. It appeared very close to NGC 4993, a lenticular galaxy in the constellation of Hydra, and VISTA observations pinpointed this source at infrared wavelengths almost at the same time. As night marched west across the globe, the Hawaiian island telescopes Pan-STARRS and Subaru also picked it up and watched it evolve rapidly.
“There are rare occasions when a scientist has the chance to witness a new era at its beginning,” said Elena Pian, an astronomer with INAF, Italy, and lead author of one of the Nature papers. “This is one such time!”
The astronomical community launched one of the biggest ever target of opportunity observing campaigns, and about 70 observatories around the world also observed the event.
The almost simultaneous detections of both gravitational waves and gamma-rays from GW170817 raised hopes that this object was indeed a long-sought kilonova and observations have revealed properties remarkably close to theoretical predictions. Kilonovas were suggested more than 30 years ago, but this marks the first confirmed observation.
Although observations with ALMA did not reveal any detection, it’s contribution helped to confirm that the “merger took place in empty space rather than inside a dense cloud of dust and gas, which is exactly what we expected if this event is similar to those that produce short gamma-ray bursts,” said astronomer Kate Alexander from Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts (USA) and author of a paper appearing in the ApJL. “ALMA, VLA, and Chandra (X-ray) observations show that the merger launched a jet of particles moving at nearly the speed of light, but not pointed directly at Earth. With ALMA, we also detected a supermassive black hole in the center of the galaxy where the merger occurred, which is glowing faintly as it slowly consumes gas.”
ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of South Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.