Gamma-Ray Bursts hint at birth of Massive Neutron Stars

Astrophysicists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University have provided an analysis of 72 short-duration Gamma-Ray Bursts (GRBs) observed by NASA’s Neil Gehrels Swift Satellite and found in 18 cases, the resultant object from the merger produced a massive neutron star – before this later collapsed into a black hole.

By looking at the data of these 18 cases – researchers have been able to describe the physical properties of neutron stars with results indicating that these neutron stars are consistent with having a freely-moving ‘quark’ composition and a composition like regular matter, i.e. composed of atomic nuclei—the building blocks of the Universe.

Quarks are elementary particles that contain protons, neutrons, and atomic nuclei. In regular matter, these quarks are confined inside protons and neutrons, but in the high density and high-temperature regimes seen in neutron stars, they may move around freely.

“Our observations show a slight preference for freely-moving quarks. We look forward to getting more observations to definitively solve this puzzle” said OzGrav Ph.D. student and the lead author on the paper.

In August 2017, gravitational-wave interferometers detected a signal corresponding to the merger of two neutron stars – the first detection of its kind. Scientists had long suspected that these merger events would also produce an electromagnetic (EM) counterpart signal – and 1.7 seconds after the event, global telescopes registered the GRB from the resultant Kilonova event.

It was during this event, and detailed within the EM light signature that scientists were able to determine that heavier elements like Gold, Platinum and Strontium are created in these violent mergers – along with providing the most accurate record of the speed of gravity.

However, astrophysicists wanted to understand what the result of such mergers would produce – would they create a black hole (as expected) or something more exotic?

Nikhil, along with OzGrav colleagues Paul Lasky, and Gregory Ashton set out to explore this by reviewing the properties of GRBs. When a short-duration GRB occurs, it usually includes a lower, broadband emission – the result of the interaction of the jet from the Kilonova colliding with the surrounding medium.

The X-ray signals within the GRBs often include two features that are not explained by the lower, broadband emission – a plateau and a steep decay in the signal that occurs for a long period after the event. It is these two features within the x-ray data that indicate that the result of the merger is not an immediate black hole, but rather a long-lived, rapidly rotating, highly magnetized neutron star. The steep decay itself is the sign that the neutron star is collapsing into a black hole.

When these new super-sized neutron stars are born, they are above a non-rotating limit known as the Tolman-Oppenheimer-Volkoff mass – and as they lose their centrifugal forces from their massive spin rate, gravity takes hold causing the object to collapse in on itself and into a black hole.

Another outcome of the findings indicated that just before the super-sized neutron stars collapse under their own gravity into black holes, they unleash tiny gravitational wave signals – so small, they are even outside the range of current detectors like LIGO.

“With the construction of more sensitive gravitational-wave detectors, such as the Einstein Telescope in Europe of the Cosmic Explorer in the US, we are confident that we’ll eventually detect individual gravitational waves from these systems,” explained Sarin.

The new research has shown that the data measured as a result of the X-ray afterglow from Kilonova events, can be used to confine and determine what makes up the inside of neutron stars – one of the densest objects in the universe (a single teaspoon would way as much as a cube of Earth with sides 800m in length).

Combining this data with the gravitational-wave signals from such an event can provide scientists with further information about these violent mergers – where so many elements we find here on Earth – were once forged.