On Aug 14th 2017 LIGO and the Virgo Gravitational Wave detectors based in the USA and Italy detected the fourth merging Black Hole event. This was a triumph for both groups, but especially the Virgo group which had only started taking regular data earlier that month. It did leave many astronomers asking where are all the merging neutron star binaries which had been predicted? The question was answered just three days later on Aug 17th when a fifth gravitational wave event was detected, now known as GW170817. This time the gravitational wave signal was detected was of much longer duration.  And electromagnetic radiation was also detected.

For, two gamma-ray satellites orbiting the Earth, Fermi and Integral, detected a gamma-ray burst occurring less than two seconds after the time of the gravitational wave event. This was a sign that this event was a merging neutron star binary. Unlike Black hole mergers, these neutron star mergers are predicted to result in the emission of electromagnetic radiation (such as optical radiation). The part of the sky which the event took place was being narrowed down in real time, originally being anywhere within a few thousand square degrees (the moon has a diameter of 1/2 a degree), to just a few tens of degrees.

Artist’s illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet


The gravitational wave signal allowed a rough estimate of its distance so astronomers started mapping galaxies which were known to be approximately at this distance. Amazingly, a previously unknown source of optical light was seen very close to the galaxy NGC 4993 in the constellation of Hydra in the southern sky. At 16th magnitude it was sufficiently bright to be detectable with amateur telescopes. The optical transient faded rapidly over the next few days but not before astronomers in different parts of the world had trained their telescopes on the source. The Swift satellite, which has gamma-ray, X-ray and ultra violet telescopes on it, detected very bright ultra violet emission for a few days. However, it was only after 12 days that the X-ray satellite Chandra detected the source in X-rays and it was even longer before it appeared in radio observations made by the VLA in New Mexico.

Since this event astronomers around the world have been working hard to make sense of all these data and to understand what happened. On Monday their work was announced at a press conference in Washington DC. It had been thought for many years that short duration gamma-ray bursts were caused by neutron star binaries merging: GW170817 now gives us the proof. These observations also showed that elements heavier than iron (such as gold) are created in such neutron stars mergers. There remains a good deal of controversy about the absence of X-rays for the first 12 days which could be related to the viewing angle that we see the jets, which were produced a few seconds after the initial merger. It is clear that the data from this event will be studied for many months and years.

LIGO’s first black hole merger detection. The duration of the “chirp” was just 0.2 seconds; 500 times shorter than the signal generated by the neutron stars. (Caltech/MIT/LIGO Lab)

Thirty-seconds worth of binary neutron star inspiral as it appeared in the LIGO detectors. The entire signal lasted 100 seconds.



LIGO is currently undergoing downtime for the next 10 months or so while the engineers and scientists make improvements to the instruments and detectors which will increase LIGO’s sensitivity. Virgo will continue operating until the spring next year when it will undergo further improvements to its sensitivity. Armagh Observatory and Planetarium is a partner of the Gravitational-Wave Optical Transient Observer (GOTO) which is a multi-telescope array on La Palma who main goal is to identify the optical counterpart of gravitational wave events just like GW170817. Unfortunately the event took place too far in the southern sky for us to detect this one. It makes our plans to have a sister site based in Australia even more crucial.

Gravitational Wave astronomy is now moving from birth into its infancy just like X-ray astronomy was in the 1960’s when the first X-ray sources were being detected by short lived instruments sent up in rockets. The first X-ray source discovered after the Sun was an object called Sco X-1 — this too contained an neutron star in a binary system — here its companion is a normal star with a mass half that of the Sun. We look forward to the next gravitational wave event with great interest!


Article by: Gavin Ramsey

Gavin Ramsay – Research Astronomer at Armagh Observatory and Planetarium

Further reading:

LIGO-Virgo Press Release: https://www.ligo.caltech.edu/page/press-release-gw170817

GOTO web link: https://goto-observatory.org/



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