Artist's impression of two black holes as they spiral towards each other before merging, releasing gravitational waves – fluctuations in the fabric of spacetime. Copyright: ESA–C.Carreau
On 14 September, the terrestrial Laser Interferometer
Gravitational-wave Observatory (LIGO) detected gravitational waves –
fluctuations in the fabric of spacetime – produced by a pair of black
holes as they spiralled towards each other before merging. The signal
lasted less than half a second.
The discovery was the first direct observation of gravitational waves, predicted by Albert Einstein a century ago.
Two days after the detection, the LIGO team alerted a number of ground-
and space-based astronomical facilities to look for a possible
counterpart to the source of gravitational waves. The nature of the
source was unclear at the time, and it was hoped that follow-up
observations across the electromagnetic spectrum might provide valuable
information about the culprit.
Gravitational waves are released when massive bodies are accelerated,
and strong emission should occur when dense stellar remnants such as
neutron stars or black holes spiral towards each other before
coalescing.
Models predict that the merging of two stellar-mass black holes would
not produce light at any wavelength, but if one or two neutron stars
were involved in the process, then a characteristic signature should be
observable across the electromagnetic spectrum.
Another possible source of gravitational waves would be an asymmetric
supernova explosion, also known to emit light over a range of
wavelengths.
It was not possible to pinpoint the LIGO source – its position could only be narrowed down to a very long strip across the sky.
Observatories searched their archives in case data had been
serendipitously collected anywhere along this strip around the time of
the gravitational wave detection. They were also asked to point their
telescopes to the same region in search for any possible 'afterglow'
emission.
INTEGRAL is sensitive to transient sources of high-energy emission over
the whole sky, and thus a team of scientists searched through its data,
seeking signs of a sudden burst of hard X-rays or gamma rays that might
have been recorded at the same time as the gravitational waves were
detected.
"We searched through all the available INTEGRAL data, but did not
find any indication of high-energy emission associated with the LIGO
detection," says Volodymyr Savchenko of the François Arago Centre
in Paris, France. Volodymyr is the lead author of a paper reporting the
results, published today in Astrophysical Journal Letters.
The team analysed data from the Anti-Coincidence Shield on INTEGRAL's
SPI instrument. The shield helps to screen out radiation and particles
coming from directions other than that where the instrument is pointing,
as well as to detect transient high-energy sources across the whole
sky.
The team also looked at data from INTEGRAL's IBIS instrument, although
at the time it was not pointing at the strip where the source of
gravitational waves was thought to be located.
"The source detected by LIGO released a huge amount of energy in
gravitational waves, and the limits set by the INTEGRAL data on a
possible simultaneous emission of gamma rays are one million times lower
than that," says co-author Carlo Ferrigno from the INTEGRAL Science Data Centre at the University of Geneva, Switzerland.
Subsequent analysis of the LIGO data has shown that the gravitational
waves were produced by a pair of coalescing black holes, each with a
mass roughly 30 times that of our Sun, located about 1.3 billion light
years away. Scientists do not expect to see any significant emission of
light at any wavelength from such events, and thus INTEGRAL's null
detection is consistent with this scenario.
Similarly, nothing was seen by the great majority of the other
astronomical facilities making observations from radio and infrared to
optical and X-ray wavelengths.
The only exception was the Gamma-Ray Burst Monitor on NASA's Fermi
Gamma-Ray Space Telescope, which observed what appears to be a sudden
burst of gamma rays about 0.4 seconds after the gravitational waves were
detected. The burst lasted about one second and came from a region of
the sky that overlaps with the strip identified by LIGO.
This detection sparked a bounty of theoretical investigations,
proposing possible scenarios in which two merging black holes of stellar
mass could indeed have released gamma rays along with the gravitational
waves.
However, if this gamma-ray flare had had a cosmic origin, either linked
to the LIGO gravitational wave source or to any other astrophysical
phenomenon in the Universe, it should have been detected by INTEGRAL as
well.
The absence of any such detection by both instruments on INTEGRAL
suggests that the measurement from Fermi could be unrelated to the
gravitational wave detection.
"This result highlights the importance of synergies between
scientists and observing facilities worldwide in the quest for as many
cosmic messengers as possible, from the recently-detected gravitational
waves to particles and light across the spectrum," says Erik Kuulkers, INTEGRAL Project Scientist at ESA.
This will become even more important when it becomes possible to
observe gravitational waves from space.
This has been identified as the
goal for the L3 mission in ESA's Cosmic Vision programme, and the
technology for building it is currently being tested in space by ESA's
LISA Pathfinder mission.
Such an observatory will be capable of detecting gravitational waves
from the merging of supermassive black holes in the centres of galaxies
for months prior to the final coalescence, making it possible to locate
the source much more accurately and thus provide astronomical
observatories with a place and a time to look out for associated
electromagnetic emission.
"We are looking forward to further collaborations and discoveries in the newly-inaugurated era of gravitational astronomy," concludes Erik.
Notes for Editors
"INTEGRAL Upper Limits On Gamma-Ray Emission Associated With The Gravitational Wave Event GW150914," by V. Savchenko et al. is published in Astrophysical Journal Letters.
For further information, please contact:
Markus Bauer
ESA Science Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer
esa.intVolodymyr Savchenko
François Arago Center
APC - Astroparticule et Cosmologie
Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire De Paris, Sorbonne Paris Cité
Paris, France
Email: savchenk
apc.in2p3.frCarlo Ferrigno
INTEGRAL Science Data Centre
University of Geneva, Switzerland
Email: Carlo.Ferrigno
unige.chErik Kuulkers
ESA INTEGRAL Project Scientist
Email: Erik.Kuulkers
esa.int
Source: ESA/INTEGRAL
