Dec 20, 2017

Two stars merge, creating a cocoon and a mystery

An artist's illustration of a "cocoon" in the aftermath of two neutron stars merging. Credit: NRAO/AUI/NSF: D. Berry

The merger of two dense neutron stars millions of years ago produced ripples in spacetime and an afterglow of radiation detected on Earth on August 17. After monitoring the signal for months, a team of astronomers reports today that the X-rays, radio waves and gamma rays emitted from the event may be coming from a cocoon-like structure — the first to be seen.

Why it matters: The origin of short gamma-ray bursts — fast, bright and powerful flashes of radiation from events long ago and far away in the universe — is unknown. A leading theory is they are produced in the merger of neutron stars and black holes that release jets of radiation. The hope then was that observing such jets in an event like researchers did over the summer would confirm the link between neutron star mergers and gamma ray bursts.

"Our continued radio monitoring has confirmed that all this emission was produced by a cocoon. This event, therefore, does not provide direct confirmation of the link between neutron star mergers and GRBs [gamma ray bursts]," Caltech astronomer and study coauthor Gregg Hallinan tells Axios.

The evidence: A jet emission of X-rays and radio waves would be expected to get weaker over time if it was being viewed on Earth from an angle, as researchers originally proposed after observing this particular event — but astronomers report the intensity of the radio waves continued to grow up to 93 days later. They propose the jet could be colliding with surrounding material to create a cocoon that can send light in many directions. This could account for the radio and gamma ray emissions they see.

Yes, but: "The tension is between whether or not you can uniquely explain the observations using this cocooony thing, or whether you can still explain the observation just with a jet with more complexity built into the physics," Daryl Haggard, assistant professor of physics at McGill University, told Gizmodo's Ryan Mandelbaum. "That is completely unresolved."

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