Several weeks ago, physicists at The Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first direct detection of gravitational waves, 100 years after Albert Einstein predicted their existence. Gravitational waves are ripples in space time caused by a violent cosmic event taking place in the distant Universe.
Published by the editorial team, 7 March 2016
Research published last week describes the follow-up programme that began soon after the gravitational wave candidate was first identified by LIGO in September 2015. Within two days of the trigger, 21 teams responded to the alert and began observations with satellites and ground-based telescopes around the world. Over the next three months, observations were performed using facilities spanning the entire spectrum from radio to gamma-ray wavelengths.
Researchers from the Square Kilometre Array (SKA) community played an exciting part in the follow-up observations at radio wavelengths using a number of SKA pathfinder and precursor facilities - telescopes dotted around the globe that are important technology test beds for the SKA.
For instance, in Europe the Low Frequency Aperture Array (LOFAR) was involved in the follow-up. LOFAR is located throughout Europe with its core in The Netherlands and is coordinated by ASTRON, the Netherlands institute for radio astronomy. The facility was conceived as a multipurpose sensor network, with a vast computer and network infrastructure
LOFAR started observations approximately one week after the LIGO trigger and then repeated one and three months later to try and identify a late radio afterglow, focusing on the highest probability region identified in the northern hemisphere.
In the southern hemisphere, the SKA-linked radio telescopes used included the Australian SKA Pathfinder (ASKAP) and the Murchison Widefield Array (MWA), both located on CSIRO's Murchison Radio-astronomy Observatory (MRO) in Western Australia - the Australian SKA site. Both instruments offer a wide field of view, high sensitivity and quick response times, and are complemented by high-speed supercomputing capabilities, making them valuable additions to the LIGO/Virgo collaboration.
The challenge of conducting follow-up work for gravitational wave surveys is that the position of the source is not well known and is located somewhere within a large region of sky. While LIGO indicates the general area of sky to look at and the time the trigger started, telescopes with a large field of view, like ASKAP, MWA and LOFAR, are ideally suited to follow-up and try to hone in on the precise source of the event.
While the radio telescopes did not detect anything this time, with this infrastructure in place and now successfully tested, these SKA pathfinders will also contribute to future LIGO detections. Both the MWA and LOFAR have cooperative agreements with LIGO and other telescopes to follow up time-critical astronomical events such as gravitational wave events and gamma-ray bursts. When an alert message from other instruments like LIGO is received, these facilities can immediately initiate followup observations of the source often within seconds.
MWA and LOFAR are testing technologies to be used for SKA-low, the low frequency array of the SKA to be located in Australia. ASKAP is testing innovative instrumentation called phased array feeds that may be installed on the SKA dishes in South Africa, the other telescope host country of the SKA.
This is a great example of the complementarity of all of these science facilities and the need for international collaboration covering the full spectrum from radio waves to gamma rays, said Robert Braun, Science Director at SKA Organisation.
Having three SKA pathfinder and precursor telescopes involved in this major discovery is very exciting and gives a sense of what the SKA will unlock when it becomes operational in the early 2020s.
We have now entered the era of multi-wavelength/multi-messenger astronomy, enabling us to combine both gravitational wave and electro-magnetic observations, and it is fantastic that LOFAR played a part in the campaign to perform follow-up observations of GW 150914. LOFAR has beautifully demonstrated its capability to perform sensitive, wide field follow-up of these events at low radio frequencies and we targeted key time-scales where the faint afterglows of these events may be expected. Antonia Rowlinson (ASTRON & University of Amsterdam), member of the LOFAR follow-up team.
The SKA will look at different gravitational waves from those detected by LIGO. While LIGO has detected gravitational waves as they pass through the Earth, the SKA will detect gravitational waves as they pass through our Galaxy, helping us confirm our model of the expansion of the Universe and the formation of galaxies themselves. For more information see: https://www.skatelescope.org/news/ligo-ska-gw/