Astronomers reveal a likely second neutron star merger
A gamma ray burst from 2015 was caused by the same type of event that produced the gravitational waves measured last year. Ben Lewis reports.
In October 2017 a group of astronomers excitedly announced they had simultaneously observed light and gravitational waves emanating from a massive galactic collision between two neutron stars. A year on, a second group has revealed another event which had remarkable similarities to that ground-breaking discovery.
Called GRB150101B, the newly reported event was originally identified as a gamma ray burst (GRB) by NASA's Fermi Gamma-ray Space Telescope in January 2015. However, following more investigations carried out by a team led by Eleonora Troja from the University of Maryland in the US, astronomers have now determined that it was likely produced by the same type of event – a merger between neutron stars – that resulted in the 2017 find.
The 2017 event is officially classified as GW170817.
“It's a big step to go from one detected object to two,” says Troja.
“Our discovery tells us that events like GW170817 and GRB150101B could represent a whole new class of erupting objects that turn on and off—and might actually be relatively common.”
According to the researchers, both GRB150101B and GW170817 were most likely created from the collision of two neutron stars. During the impact, a narrow jet of high-energy particles called a gamma ray burst (GRB) shot out across the universe.
Troja and colleagues show that GW170817 and GRB150101B share striking similarities. Both produced an unusually faint and short-lived GRB, followed by bright, blue optical light lasting a few days, and X-ray emission lasted much longer again.
It was the blue light that provided the team an important clue that GRB150101B involved a titanic explosion, 1000 times larger than a supernova, known as a kilonova. Although kilonovae were first tentatively observed in 2008, and the term introduced in 2010, it wasn’t until the 2017 event that they were decisively linked to neutron star collisions.
Using the Hubble Space Telescope and the ground-based Discovery Channel Telescope in Arizona, US, the researchers also found that the host galaxies of the two events look remarkably similar. Both are bright and elliptical, with a population of stars a few billion years old, displaying no evidence for new stars forming.
However, Troja and colleagues remain unsure whether GRB150101B produced gravitational waves the way the 2017 impact did, because Advanced LIGO, the facility responsible for picking up the waves from GW170817, only became operational just after GRB150101B occurred.
Even had it been working, the researchers think it would have been unlikely to have detected waves from the 2015 collision. GW170817 occurred around 130 million light years from Earth, but GRB150101B lies about 1.7 billion light years away – a distance the researchers suggest may be beyond its capabilities.
“We have a case of cosmic look-alikes,” says Geoffrey Ryan, who was part of the team from the University of Maryland.
“They look the same, act the same and come from similar neighbourhoods, so the simplest explanation is that they are from the same family of objects.”
However, the lack of gravitational wave data means the researchers aren’t sure of the masses of the objects involved in the collision, meaning there is the possibility it could have involved a black hole and a neutron star, rather than two neutron stars.
The researchers now believe it is possible that mergers such as GW170817 and GRB150101B have actually been observed by astronomers previously, but were not identified as such.
Troja and her team hope the lessons learnt from GRB150101B will allow astronomers to more easily recognise similar events in the future.