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HomeScienceScientists at LIGO detect heaviest binary neutron star merger ever known

Scientists at LIGO detect heaviest binary neutron star merger ever known

With about 3.4 times the mass of the Sun, this source is heavier than any previously known neutron star binary, says astrophysicist & LIGO team member Karan Jani.

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Bengaluru: Astrophysicists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) have confirmed that they have detected the heaviest binary neutron star merger ever known. 

From the data observed on 25 April 2019, the total mass of the two neutron stars combined was between 3.3 and 3.7 times the mass of the Sun, which is unusually large for binary neutron stars.

The event that originated 520 million light years away has been named GW190425. The discovery was announced at the 235th annual meeting of the American Astronomical Society in Honolulu Monday local time. 

There has been only one other merger of neutron stars, and they were 1.4 and 1.2 times the mass of our Sun, respectively. The masses of neutron stars typically tend to cap at 1.4 times the mass of the Sun each, or about 2.8 times the mass of the Sun totally. 

“Our discovery of GW190425 is very special,” Karan Jani, a LIGO team member and astrophysicist at Vanderbilt University, told ThePrint. “With about 3.4 times the mass of our Sun, this source is heavier than any previously known neutron star binary. This is a powerful testimony of the invisible universe we are discovering with gravitational-wave astronomy.”

GW190425 was detected in the project’s third observing run, known as O3. It commenced on 1 April 2019 and will end on 30 April 2020. There have been two previous observing runs before: O1, from September 2015 to January 2016, and O2, from November 2016 to August 2017. 

After each observation run, the detectors are upgraded with new technologies to increase their sensitivity. 

The previous neutron star merger, called GW170817, was observed during the O2 run. Typically, mergers of such compact objects can be seen across the electromagnetic spectrum. With that merger, there was an afterglow that was visible for 10 days in X-ray, optical, radio frequency, and infrared light. 

Neutron stars are some of the smallest stars in the universe and are formed as a remnant after a massive star explodes in a supernova. They are barely several kilometres across but are several times more massive than the Sun, making them extremely dense. 

In this binary system, the astronomers don’t yet know how the stars formed: they could either have been two stars in a binary system that reached the end of their lives or these two formed separately and were pulled together by their intense gravitational pull.  


Also read: Meet Bala Iyer, theoretical physicist leading advanced gravitational-wave project for India


GW190425 difficult to locate

GW190425 did not report any afterglow, which means we can’t narrow down the exact location of these neutron stars. 

“No electromagnetic counterpart was reported for GW190425,” said Jani, from the meeting in Honolulu. “So unlike our 2017 discovery, we cannot pinpoint the host galaxy of these stars. But we do know these colliding neutron stars are cosmic factories for gold, uranium, and other rare elements.”

The signal was detected only by the Livingston detector, as LIGO’s other detector in Hanford was temporarily offline. The Virgo detector in Italy was not sensitive enough to pick up this gravitational wave. 

Had either of the other detectors also detected the merger, its location would have been pinpointed better in the sky, saving precious time and helping us zero in on a source location faster. 

The astronomers were able to determine that of the two neutron stars, one was between 1.61 and 2.52 times the mass of the sun while the other was between 1.12 and 1.68 times the mass. While this was very unusual, it still as good as ruled out the chance the two are black holes, as black holes are at least five times the mass of the Sun.  


Also read: Unique exoplanets, moonquakes, underground lakes on Mars — the big space stories of 2019 


LIGO detectors

The LIGO detectors are a part of an international collaboration that discovered the first gravitational wave on 16 February 2016, and the first wave from a neutron star collision in 2017. These detectors have kickstarted a completely new method of astronomy in observing gravitational waves, providing another means of observation for previously unobservable high-energy phenomena.

“When LIGO-India will be online by 2026, such astrophysical events will be located in an even tiny area of the sky,” said Jani. “That is extremely important for mega telescopes around the world to follow up on gravitational-wave alerts.” 

Apart from LIGO-India, another detector in Germany and the KAGRA detector in Japan will go online shortly as well, providing a global network of gravitational wave observatories. 


Also read: Black hole image, 1kg redefinition, Greenland’s lost ice — 2019 science stories to remember 


 

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