[AstroNet] gravitational waves released by the collision of two neutron stars - Kilonova

[Sam Rametse]

http://www.esa.int/Our_Activities/Space_Science/Integral_sees_blast_travelling_with_gravitational_waves



16 October 2017

ESA’s Integral satellite recently played a crucial role in discovering the
flash of gamma rays linked to the gravitational waves released by the
collision of two neutron stars.

On 17 August, a burst of gamma rays lit up in space for almost two seconds.
It was promptly recorded by Integral and NASA’s Fermi satellite.

Such short gamma-ray bursts are not uncommon: Integral catches about 20
every year. But this one was special: just seconds before the two
satellites saw the blast, an entirely different instrument was triggered on
Earth.

One of the two detectors of the Laser Interferometer Gravitational-wave
Observatory (LIGO) experiment, in the USA, recorded the passage of
gravitational waves – fluctuations in the fabric of spacetime caused by
powerful cosmic events.

[image: ESA’s Integral observatory is able to detect gamma-ray bursts, the
most energetic phenomena in the Universe.]
<http://www.esa.int/ESA_Multimedia/Images/2011/06/Integral_gamma-ray_observatory>

Integral gamma-ray observatory

“This is a ground-breaking discovery, revealing for the first time
gravitational waves and 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 on four
occasions: in all cases, they were traced back to pairs of merging black
holes as they spiralled towards each other.

The two LIGO detectors had seen the first
<http://www.esa.int/Our_Activities/Space_Science/ESA_congratulations_on_gravitational_wave_discovery>
in
September 2015, followed by two more in late 2015 and early 2017. Recently,
on 14 August, the fourth observation of gravitational waves also involved
Europe’s Virgo instrument in Italy.

These detections won the LIGO founding scientists the Nobel Prize in
physics earlier this month.

Gravitational waves are the only ‘messenger’ expected when black holes
collide. Following these four measurements, scientists across the world
began searching with ground and space telescopes for possible luminous
bursts linked to the gravitational waves.

“We had contributed to these earlier searches with Integral
<http://www.esa.int/Our_Activities/Space_Science/Integral_sets_limits_on_gamma_rays_from_merging_black_holes>,
looking for gamma- or X-ray emission and finding none, as expected from the
vast majority of theories,” says Volodymyr Savchenko from the Integral
Science Data Centre in Geneva, Switzerland.

This time, however, the story took a different turn.



Neutron star merger

Access the video
<http://www.esa.int/ESA_Multimedia/Videos/2017/10/Neutron_star_merger>

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.

[image:
http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2017/10/gamma-ray_burst_after_gravitational_waves/17207432-1-eng-GB/Gamma-ray_burst_after_gravitational_waves_large.jpg]
<http://www.esa.int/ESA_Multimedia/Images/2017/10/Gamma-ray_burst_after_gravitational_waves>

Gamma-ray burst after gravitational waves

Ordinarily, an alert from only one of the three gravitational-wave
detectors would not awaken curiosity so suddenly, but the coincidence with
the gamma-ray blast detected from space prompted the LIGO/Virgo scientists
to look again.

It later appeared that both LIGO detectors had recorded the gravitational
waves. Owing to its lower sensitivity and different orientation, Virgo
produced a smaller response, but combining all three sets of measurements
was crucial to locating the source.

The data pointed to a 28 square degree patch in the sky, equivalent to a
square spanning roughly 10 times the diameter of the full Moon on each
side. The gravitational wave signal indicated that the source lies only
about 130 millions light-years away.

Without further delay, a large number of ground and space telescopes turned
to this portion of the sky.

[image:
http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2017/10/new_source_in_galaxy_ngc_4993/17207470-1-eng-GB/New_source_in_galaxy_NGC_4993_medium.jpg]
<http://www.esa.int/ESA_Multimedia/Images/2017/10/New_source_in_galaxy_NGC_4993>

Host galaxy

About half a day after the detections, scientists at various optical
observatories, including the European Southern Observatory
<http://www.eso.org/>'s telescopes in Chile, spotted something new near the
core of galaxy NGC 4993 <http://sky.esa.int/?action=goto&target=ngc4993>.
Sitting at just the distance indicated by LIGO/Virgo, it was just what you
would expect to see in visible light as neutron stars merged.

“This is the closest short gamma-ray burst detected among the ones for
which we’ve measured the distance, and by far the dimmest one – nearly a
million times less bright than average,” says Volodymyr.

“We think that the unusual properties of this source indicate that the
powerful jets that arise in the cosmic clash of the neutron stars are not
pointing straight towards us, as happens in the majority of gamma-ray
bursts detected.”

With the position of the source known, a large number of observatories and
other sensors continued looking at it for several days and, in some cases,
weeks, searching for light and particles emitted in the aftermath of the
collision. Many are still observing it.

After the initial detection of the blast, Integral observed it
<http://blogs.esa.int/rocketscience/2017/10/16/exceptional-operations-enable-exceptional-science/>
for
five and a half days. No further gamma rays were detected, an important
fact in understanding 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 merger is impacting its surroundings, forging a wealth of
heavy elements in the process,” explains Carlo.

“This amazing discovery was made possible by a terrific collaboration of
thousands of people working in different observatories and experiments
worldwide,” says Erik.

“We are thrilled that Integral could provide a crucial contribution to
confirming the nature of such a rare phenomenon that scientists have been
seeking for decades.”

With high sensitivity to gamma rays and almost full-sky coverage for brief
events, Integral is amongst the best astronomical facilities for keeping an
eye on gamma-ray bursts.

When the LIGO/Virgo sensors start their observations again, with improved
sensitivity, in late 2018, it is crucial that as many gamma-ray satellites
as possible are active to check on the gravitational wave detections.

[image:
http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2017/06/lisa_concept/17008618-1-eng-GB/LISA_concept_medium.png]
<http://www.esa.int/ESA_Multimedia/Images/2017/06/LISA_concept>

LISA concept

Meanwhile, ESA is working on the next generation of gravitational-wave
experiments
<http://www.esa.int/Our_Activities/Space_Science/Gravitational_wave_mission_selected_planet-hunting_mission_moves_forward>,
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 lower frequency than those detected with terrestrial instruments. These
are released by the clashes of even more exotic cosmic objects:
supermassive black holes, which sit at the centre of galaxies and have
masses millions to billions of times larger than that of the stellar-mass
black holes detected by LIGO and Virgo.

“LISA will broaden the study of gravitational waves much like the first
observations at infrared and radio wavelengths have revolutionised
astronomy,” says Paul McNamara, LISA study scientist at ESA.

“Until then, we are excited that ESA’s high-energy satellites
<http://sci.esa.int/astrophysics/59663/> are contributing to the growing
field of gravitational-wave astronomy.”

*Notes for editors*
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