Colliding neutron stars
The ESA Integral satellite recently played a crucial role in the discovery of the flash of gamma rays linked to gravitational waves released by the collision of two neutron stars.
On August 17, a burst of gamma rays lit up in space for almost two seconds. It was immediately registered by Integral and NASA's Fermi satellite.
These short gamma-ray bursts are not uncommon: whole catches are approximately 20 each year. But this one was special: just a few seconds before the two satellites saw the explosion, a completely different instrument was unleashed on Earth.
One of the two detectors of the experiment of the Observatory of gravitational waves of the laser interferometer (LIGO), in the USA. UU., Registered the passage of gravitational waves: fluctuations in the fabric of space-time caused by powerful cosmic events.
"This is a pioneering discovery, revealing for the first time the gravitational waves and the highly energetic light released by the same cosmic source," says Erik Kuulkers, Integral project scientist at ESA.
Before this finding, gravitational waves had been confirmed four times: in all cases, they went back to pairs of black holes that fused together.
The two LIGO detectors had been the first in September 2015, followed by two others at the end of 2015 and the beginning of 2017. Recently, on August 14, the fourth observation of gravitational waves also involved the European instrument Virgo in Italy.
These detections won the Nobel Prize in physics earlier this month from the founding scientists of LIGO.
Gravitational waves are the only "messenger" expected when black holes collide. After these four measurements, scientists from all over the world began to search with terrestrial and space telescopes for possible luminous bursts linked to gravitational waves.
"We contributed to these previous searches with Integral, looking for the emission of gamma rays or X-rays and without finding any, as expected from the vast majority of theories," says Volodymyr Savchenko of the Center for Integrated Data Sciences in Geneva, Switzerland.
Integral gamma-ray observatory
This time, however, the story took a different turn.
Other cosmic clashes are suspected to release not only gravitational waves but also light across the electromagnetic spectrum. This can happen, for example, when the collision involves one or more neutron stars – like black holes, they are compact remnants of what were once massive stars.
Merging neutron stars have also been thought to be the long-sought sources of short gamma-ray bursts, though no observational proof had yet been found.
Until August.
“We realised that we were witnessing something historic when we saw the notification of Fermi’s and LIGO’s detections appear on our internal network almost at the same time, and soon after we saw the confirmation in the data from Integral's SPI instrument, too,” says Carlo Ferrigno, from the Integral Science Data Centre.
“Nothing like this had happened before: it was clearly the signature of a neutron star merger,” adds Volodymyr.
Gamma-ray buist after gravitational waves
Normally, an alert of only one of the three gravitational wave detectors would not arouse curiosity so suddenly, but the coincidence with the gamma-ray burst detected from space caused the LIGO / Virgo scientists to look again.
Later it appeared that both LIGO detectors had recorded the gravitational waves. Due to its lower sensitivity and different orientation, Virgo produced a smaller response, but the combination of the three sets of measurements was crucial in locating the source.
The data pointed to a patch of 28 squares in the sky, equivalent to a square that covers approximately 10 times the diameter of the full Moon on each side. The gravitational wave signal indicated that the source is only 130 million light years away.
Without further delay, a large number of terrestrial and space telescopes were turned over to this portion of the sky.
Host galaxy
Around half a day after the detections, scientists from several optical observatories, including the telescopes of the European Southern Observatory in Chile, discovered something new near the nucleus of the galaxy NGC 4993. Sitting at the distance indicated by LIGO / Virgo, it was fair what I would expect to see in visible light as the neutron stars merged.
With the position of the known source, a large number of observatories and other sensors continued to watch it for several days and, in some cases, weeks, looking for light and particles emitted as a result of the collision. Many are still watching it.
After the initial detection of the explosion, Integral observed him for five and a half days. No more gamma rays were detected, an important fact to understand how the neutron stars merged.
The extensive follow-up campaign revealed signals across the spectrum, first in the ultraviolet, visible and infrared bands, then in X-rays and, eventually, radio wavelengths.
"What we are witnessing is clearly a kilonova: the neutron-rich material released in the fusion is impacting its environment, forging a large number of heavy elements in the process," explains Carlo.
"This amazing discovery was possible thanks to the excellent collaboration of thousands of people working in different observatories and experiments around the world," says Erik.
"We are delighted that Integral can provide a crucial contribution to confirm the nature of such a rare phenomenon that scientists have been searching for decades."
"This is the closest short burst of gamma rays detected between those that we have measured the distance, and by far the faintest one, almost a million times less bright than the average," says Volodymyr.
"We believe that the unusual properties of this source indicate that the powerful jets that arise in the cosmic shock of the neutron stars do not point directly at us, as in most gamma-ray bursts detected."
With a high sensitivity to gamma rays and an almost complete sky coverage for short events, Integral is among the best astronomical facilities to monitor gamma-ray bursts.
When the LIGO / Virgo sensors begin their observations again, with improved sensitivity, by the end of 2018, it is crucial that as many gamma-ray satellites are active as possible to verify the detections of gravitational waves.
Meanwhile, ESA is working on the next generation of gravitational-wave experiments, taking the quest to space with LISA, the Laser Interferometer Space Antenna.
Planned for launch in 2034, LISA will be sensitive to gravitational waves of less frequency than those detected with terrestrial instruments. These are released by the collisions of even more exotic cosmic objects: supermassive black holes, which are at the center of galaxies and have masses millions of billions of times larger than the stellar mass black holes detected by LIGO and Virgo .
"LISA will expand the study of gravitational waves in a manner similar to how early infrared observations and radio wavelengths have revolutionized astronomy," says Paul McNamara, a scientist at ESA's LISA study.
"Until then, we are excited that ESA's high-energy satellites are contributing to the growing field of gravitational wave astronomy."
bibliographic reference :
“Integral detection of the first prompt gamma-ray signal coincident with the gravitational wave event GW170817” by V. Savchenko et al., “Gamma rays and gravitational waves from a binary neutron star merger: GW170817 and GRB170817A” and “Multi-messenger observations of a binary neutron star merger” by B.P. Abbott et al. are published in Astrophysical Journal Letters.
New Gravitational Wave Discovery (Press Conference and Online Q&A Session)
Imagens Source:
http://www.esa.int/spaceinimages/Images/2017/10/Colliding_neutron_stars
http://www.esa.int/spaceinimages/Images/2017/10/New_source_in_galaxy_NGC_4993