Study of Neutron-Star Merger Uncovers New Mystery: The ‘Cocoon’

Astronomers have thought powerful cosmic explosions called short gamma ray bursts could be the handiwork of colliding neutron stars. A new study says not so quickly.

A hidden or 'choked' jet (white) powers a radio-emitting 'cocoon' (pink). Credit: NRAO/AUI/NSF: D. Berr

A hidden or ‘choked’ jet (white) powers a radio-emitting ‘cocoon’ (pink). Credit: NRAO/AUI/NSF: D. Berr

For the first time ever, humankind observed the dramatic collision and merger of two neutron stars on August 17, 2017. The spectacular event, whose progress was tracked by the gravitational-wave observatories in the US and Italy, produced a big explosion, released gravitational waves and unleashed a sea of radiation. Designated GW170817, the event has since been recognised as the breakthrough scientific discovery of the year by multiple science magazines.

The collision of two neutron stars is an unusual event because there are few other events in the universe that are equally energetic and also release gravitational waves and electromagnetic waves at the same time. So when they collide, astronomers doing all kinds of research want to get their hands on the data because it could tell us things there are few other opportunities to find out.

For example, GW170817 put to test one long-held idea about a kind of remarkably enormous explosion called short gamma ray bursts (SGRBs). “The current understanding of SGRBs within the scientific community is that these phenomena may be related to neutron star mergers,” Kunal Mooley, an astrophysicist at Oxford University, told The Wire.

Scientists have thought that colliding neutron stars are the main sources of SGRBs in the universe. These explosions are so bright, they temporarily outshine the light of entire galaxies.

When the SGRB observed on GW170817 turned out to be over 10,000-times weaker than expected, scientists simply assumed the fireworks had been pointed away from Earth. But a new study led by Mooley says the merger we caught was host to a different kind of GRB altogether.

When big enough stars die, their cores are left over as rapidly spinning balls made entirely of neutrons. Each such neutron star can weigh as much as the Sun but be only as big as New Delhi. They have magnetic fields of incredible strength warping their surfaces. Sometimes, scientists think their spin accelerates particles and shoots them into space as extremely fast but narrow jets called GRBs.

SGRBs last for less than two seconds at a time. They make up about a third of all GRBs that observatories on Earth have spotted. Astrophysicists have believed that they were being produced by the collision and merger of binary neutron stars – but Mooley’s results could put a dampener on this.

To help understand the events that led to GW170817, observatories around the world had kept watch on that patch of the sky to observe the radiation for about 100 days after the merger had been detected. Mooley’s analysis of the data suggested not all neutron star mergers produce conventional SGRBs. That privilege, according to him, belonged to only those collisions capable of producing a jet of radiation strong enough to penetrate the cloud of matter kicked up by the kilonova.

The alternative interpretation of the data, performed before Mooley’s study was published, said that GW170817 had simply produced an SGRB pointing away from Earth. In this case, the radiation would’ve interacted with its environment and slowed down. Astrophysicists see this in the form of a characteristic radio signal on Earth: A sharp rise corresponding to the explosion, then a plateau and then a slow decline.

But Mooley and co. saw the opposite – the radio signal was becoming stronger with time, not weaker. “What we saw was a gradual but monotonic increase in the intensity,” he said.

His team realised the data was consistent with a different scenario, described in October this year by astronomers from Caltech and Tel Aviv University. Here, a GRB runs into a dome-shaped cloud of material ejected by the kilonova and finds it impossible to break through. So it transfers a large amount of its energy to the particles. This ‘dome’ is called a cocoon, and it gradually loses the extra energy it has received as radio-waves into space.

Poonam Chandra, an astrophysicist at the National Centre for Radio Astrophysics, Pune, said this model suggests that there can be GRBs happening in the universe that are too faint for us to detect. This is because the resulting SGRBs “will transfer some of the energy to the dense cocoon of matter,” she said. “A fainter event means that we may be missing many more of them if they are far away.”

The cocoon’s low-energy emission meant that more radiation was coming in as radio was. So Chandra and her colleagues obtained and contributed data taken by the Giant Metre-wave Radio Telescope (GMRT), Pune. “Since absorption [of the gamma rays’ energy by the cocoon] depends on the density [of the cocoon], low-frequency emissions are [used] to pinpoint the density of the medium in which the merger occurred,” she said.

Their data also indicated that “a large fraction of neutron star mergers have cocoons,” Mooley said – a finding that could come in handy as gravitational wave observatories are poised to observe more neutron-star mergers in the future.

Interestingly, this might just be the beginning of a debate centred on GW170817 data. What Mooley and co. have provided is one interpretation: that instead of singular, cataclysmic explosions, there could be weaker gamma ray emissions that light up a big cloud of rocks and dust.

“Since we haven’t seen a jet, we can’t say from the radio data [that] there’s a definite link between merging neutron stars and short gamma-ray bursts,” according to Gregg Hallinan, an astronomer at Caltech and a member of Mooley’s team.

Brian Metzger of Columbia University, New York, told Sky & Telescope that many astronomers are currently working on papers delineating other interpretations. This includes ways to make sense of the data without junking the ‘pointed away from Earth’ possibility. Daryl Haggard, a physicist at McGill University, Montréal, told Gizmodo, “The tension is between whether or not you can uniquely explain the observations using this cocoony thing, or whether you can still explain the observation just with a jet with more complexity built into the physics.”

Irrespective of how this debate will conclude, it’s clear that observing GW170817 using both gravitational and electromagnetic telescopes has advanced our picture of the universe and its various features. The study was published in the journal Nature on December 21, 2017.

Vishwam Sankaran is a freelance science writer.

Featured image: An artist’s concept of a neutron star with an ultra-strong magnetic field around it. Credit: gsfc/Flickr, CC BY 2.0.