Astrophysicists solve mystery linked to neutron star merger

Astrophysicists using radio telescopes, including the one located on the outskirts of Pune, may have resolved some nagging riddles, following the first-ever detection of gravitation waves in August this year, leading to the awarding of the 2017 Nobel Prize for physics to three US scientists.

The remarkable discovery of gravitational waves emanating from the merger of two neutron stars – extremely massive celestial bodies created by the collapse of giant stars – named GW170817 (after the day of the discovery) by sophisticated LIGO and VIRGO gravitational-wave observatories in the US and Italy, has created a renewed excitement in the world of astronomy. Gravitational waves are the faint ripples in spacetime, first predicted about a century ago by Albert Einstein.

The crash of neutron stars that occurred 12.5 billion trillion km (130 million light years) away from the earth released radiation across the electromagnetic spectrum. Scientists could explain some of these radiations, particularly falling in the ranges of ultraviolet, visible and near-infrared, as the radioactive decay of heavy elements such as uranium and gold during GW170817. But the emissions of gamma-rays, X-rays and radio waves remained a mystery.

While one theory on neutron star collisions proposed that they could be the result of narrow, super-fast, jets of radiation that came out during the merger, and thus would eventually weaken over time. However, observations by Mooley and others, including Poonam Chandra at the National Centre for Radio Astronomy at Pune and Varun Bhalerao at the Indian Institute of Technology Bombay, using radio telescopes at multiple locations, detected something which is just the opposite: radio emissions from the GW170817 collision were actually gaining strength over time.

"Before GW170817 was detected, astronomers thought that all merging neutron stars produce narrow super-fast jets, similar to those seen in a short gamma-ray burst (GRB),” said Mooley, currently Hintze Fellow at the Centre for Astrophysical Surveys in the Oxford University, UK.

Such bursts arise from narrow yet powerful jets that are normally aimed straight at the earth. But when the GW170817 observational data ruled out that possibility, some astronomers said the jet could be pointed slightly away from earth, but their model could not satisfactorily explain the gamma-ray emission, said Mooley, who earlier studied in Pune and at IIT Bombay, before leaving for a PhD at the California Institute of Technology in the US.

Instead, “our radio observations suggest that the jet was not pointing towards the Earth (unlike the case of short GRBs) and more importantly that the jet transferred most of its energy to the surrounding neutron-rich material (that was ejected during the neutron star merger event), thus forming a bubble-like structure called a cocoon," Mooley, who is the first author of the study that appeared in the prestigious scientific journal Nature on Wednesday, told BusinessLine.

"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," said Tara Murphy of the University of Sydney, a co-author of the study, in a statement.

Apart from radio telescopes located in Australia and the US, the Giant Metrewave Radio Telescope near Pune provided observational data for the study published in the prestigious research journal Nature on Wednesday.

Their observations suggested that the merger of the neutron stars resulted in an outflow of material in many different directions (a wide-angle outflow), unlike an earlier-suggested narrow jet, as seen in the case of short-duration GRBs. Their observation also pointed out that it was very unlikely that all neutron star mergers would give rise to regular GRBs.