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Neutron star merger creates new mysteries

Neutron star merger creates new mysteries

Image: A hidden or ‘choked’ jet (white) powering a radio-emitting ‘cocoon’ (pink) is the best explanation for the radio waves, gamma rays and X-rays the astronomers observed.

neutron-star merger announced in October has solved one mystery—where gold comes from—but has also raised other questions, as reported by an international team of astronomers, including members of the Australian Research Council (ARC) Centre of Excellence for All-sky Astrophysics (CAASTRO).

The merger, dubbed GW170817, took place 130 million light-years away and was detected in August 2017 by the gravitational waves it created. Astronomers then followed it up with conventional telescopes.

The collision’s glowing wreckage generated radio waves, detected by international teams including an Australian one led by ARC Future Fellow Associate Professor Tara Murphy from The University of Sydney and CAASTRO. 

Astronomers from the University of Sydney, Caltech, CSIRO and institutions around the world have monitored the merger site for months with three radio telescopes—the CSIRO Australia Telescope Compact Array, the Karl G. Jansky Very Large Array in the USA and the Giant Meterwave Radio Telescope in India. 

“We expected to find evidence that merging neutron stars create something we’ve never found the cause of—short gamma-ray bursts,” Associate Professor Murphy said.

These bursts go off every few weeks, in any part of the sky. Each lasts less than two seconds. But the researchers haven’t seen one associated with GW170817.

Instead, the astronomers saw the radio glow continuing to brighten more than 100 days after the cosmic crash detected in August. This suggests not an extremely fast jet but a slower, broader outflow of radio-emitting material—a ‘cocoon’—probably matter thrown out by the explosion that’s been powered up by a jet hidden inside it.

“The cocoon scenario can explain the radio light curve of GW170817 as well as the gamma ­rays and X-­rays. It’s the one most consistent with the data,” Associate Professor Murphy said.

But the jet might later emerge from its hiding place and rescue the model, she added.

Dr Adam Deller, also an ARC Future Fellow, from the Swinburne University of Technology and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) predicts a bright future for the new field of gravitational-wave astronomy.

“By tying the information we gather across the electromagnetic spectrum to that gained from the gravitational wave detection, we can learn an enormous amount of detail about events like this one,” he said.

The research has been published in the journal Nature

Media issued by CAASTRO.

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