Nov 15 2018
Astronomers have detected for the first time gravitational waves from a combined, hyper-massive neutron star. The researchers, Maurice van Putten of Sejong University in South Korea, and Massimo della Valle of the Osservatorio Astronomico de Capodimonte in Italy, report their results in Monthly Notices of the Royal Astronomical Society: Letters.
Graph showing data points from the LIGO gravitational wave observatory, plotting frequency against time. The GW170817 chirp in gravitational waves produced by the coalescence of two neutron stars is clearly visible as a sequence of dots in an ascending curve. (Credit: LIGO/M.H.P.M van Putten & M. Della Valle.)
Gravitational waves were foretold by Albert Einstein in his General Theory of Relativity in 1915. The waves are disturbances in space time produced by quickly moving masses, which transmit from the source. By the time the waves touch the Earth, they are extremely weak and their detection necessitates exceptionally sensitive equipment. It took researchers until 2016 to report the first observation of gravitational waves using the Laser Interferometer Gravitational Wave Observatory (LIGO) detector.
Since that seminal outcome, gravitational waves have been noticed on six more occasions. One of these, GW170817, came about from the merger of two stellar remnants called neutron stars. These objects form after stars much more enormous than the Sun burst as supernovae, leaving behind a core of material packed to unusual densities.
Simultaneously as the spurt of gravitational waves from the merger, observatories sensed emission in X-rays, gamma rays, ultraviolet, infrared, visible light, and radio waves—an unparalleled observing campaign that established the nature and location of the source.
The preliminary observations of GW170817 indicated that the two neutron stars combined into a black hole, an object with a gravitational field so strong that not even light can travel rapidly enough to escape its grip. Van Putten and della Valle embarked to check this, using a novel method to examine the data from LIGO and the Virgo gravitational wave detector sited in Italy.
Their comprehensive analysis reveals the H1 and L1 detectors in LIGO, which are separated by over 3,000 km, concurrently picked up a descending ‘chirp’ enduring for around five seconds. Importantly, this chirp began between the end of the first burst of gravitational waves and a following burst of gamma rays. Its low frequency (less than 1 KHz, reducing to 49 Hz) indicates the merged object spun down to instead develop into a larger neutron star, rather than a black hole.
There are other objects similar to this, with their total mass matching identified neutron star binary pairs. But van Putten and della Valle have now verified their origin.
We’re still very much in the pioneering era of gravitational wave astronomy. So it pays to look at data in detail. For us this really paid off, and we’ve been able to confirm that two neutron stars merged to form a larger one.
Maurice van Putten, Researcher, Sejong University.
Gravitational wave astronomy, and laboriously acquiring the data from every detection will take another step forward in the coming year when the Japanese Kamioka Gravitational Wave Detector (KAGRA) comes online.