For the first time, astronomers have observed both gravitational waves and light (electromagnetic radiation) from two neutron stars colliding, or merging if you want, thanks to an effort and the quick reactions of both ESO and others around the world.
The never-before-seen event was announced today at a press conference by representatives of organizations involved in an international collaboration.
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LIGO Observatory in the United States, and VIRGO in Italy, the new gravitational wave detector only operated simultaneously for a few weeks, almost immediately spots black hole merger, and detected gravitational waves passing the Earth.
All four previous gravitational wave detections have come from the mergers of black holes, which are events that don’t emit light.
But these gravitational waves, ripples in the fabric of space and time, created by the speed of very massive objects throughout the Universe. Such event is, for example, the merging of neutron stars, the incredibly dense, collapsed cores of high-mass stars left behind after supernovae.
Because neutron stars don’t swallow all the things they encounter, the gravitational waves were accompanied by photons. They spiraled around each other before colliding into one another, creating an extended afterglow, visible to telescopes on Earth.
The burst of rapidly expanding radioactive heavy chemical elements left the kilonova, moving as fast as one-fifth of the speed of light. The color of the kilonova shifted from very blue to very red over the next few days, a faster change than that seen in any other observed stellar explosion.
The LIGO–Virgo observatory found the source within the southern sky, An area the size of several hundred full Moons and containing millions of stars. The team sent out an alert to telescopes to allow them to perform observations of the area.
The Swope 1-metre telescope was the first to detect and announced a new point of light. It located very close to NGC 4993, a lenticular galaxy in the constellation of Hydra. VISTA observations pinpointed this source at infrared wavelengths almost at the same time.
“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!”
Within few hours of the detection, thousands of astronomers operating up to 70 ground-based and space-based telescopes were searching the sky — spotting the explosive leftovers of the merger. They continued to observe the event for weeks after the collision, learning more about how the chaotic object evolved.
Those two neutron stars that collide estimated at about 1.1 to 1.6 times the mass of the Sun, a relatively low mass, while black holes have been greater than 20 solar masses.
This means that those stars spent more time orbiting at a close distance before colliding. This made the detection of gravitational waves in nearly 100 seconds; while black hole mergers have produced detectable waves for only a fraction of a second.
Stefano Covino, a lead author of one of the Nature Astronomy papers, said: “The data we have so far are an amazingly close match to theory. It is a triumph for the theorists, a confirmation that the LIGO–VIRGO events are real, and an achievement for ESO to have gathered such an astonishing data set on the kilonova.”
“ESO’s great strength is that it has a wide range of telescopes and instruments to tackle big and complex astronomical projects and at short notice. We have entered a new era of multi-messenger astronomy!” concludes Andrew Levan, lead author of one of the papers.
“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the Universe,” said France A. Córdova, director of the National Science Foundation.
The beginning of “multi-messenger astronomy”
The light was the only tool astronomers used to study objects in space. Scientists could estimate distant objects by observing them in various wavelengths of light — from visible light to X-rays and infrared light, we can’t see.
But now, they can use both gravitational waves and light to study celestial events together. This new era is known as “multi-messenger astronomy.”
“This is a revolution in astronomy, of having thousands of astronomers focus on one source for weeks and having this collaboration unravel in seconds, in hours, then days, and weeks,” Vicky Kalogera, an astrophysicist at Northwestern University and one of the LIGO collaborators, tells The Verge. “For us, that’s the Holy Grail.”
