Superfast jets emerge from star mergers

Debarati Chatterjee
Oct 1, 2018
4 min read

On 17 August 2017, an event in a galaxy 130 Million light years away changed the way we look at astronomy forever. As we step into the era of Multi Messenger Astronomy, we now know that we need more collaborations instead of competitions, we need to work together and not against each other, in order to reach scientific breakthroughs, i.e. we need share: knowhow, perspectives, information.

Neutron stars are extremely dense objects left behind after the death of massive stars. Their properties are so extreme that they surpass our current knowledge of physics, way beyond our limits of existing technology on earth. In order to understand neutron stars, scientists need to work together to understand the interplay of many different disciplines of physics at all their extremities. Which is why it is such a challenge for physicists.. but in the process what humanity will learn from nature will pave the way for the technologies for tomorrow.

Mergers of neutron stars is one such spectacular phenomenon that, when detected, can produce a wealth of information about how nature works. It was already established back in 1974 by Hulse and Taylor that the orbit of neutron stars going around in a binary gradually shrinks. The explanation came from the theory of General Relativity that neutron stars, being very dense possess strong gravity, and therefore warp space time around them. As the neutron stars move through the warped space time, they lose energy and gradually come closer and closer, like balls rolling down a funnel. The lost energy is carried in the form of tiny ripples through the fabric of space time, called gravitational waves. These waves are so feeble, that it was believed until recently to be impossible to detect. The breakthrough came on 14 September 2015, when the first gravitational waves were detected from a couple of black holes. Soon after, came a few more discoveries of black hole gravitational waves.

Theoretical calculations and simulations suggest such a merger of neutron stars to be associated with another cataclysmic astrophysical event, the short gamma-ray burst (SGRB). One of the most energetic explosions in the universe, a "prompt" (less than 2 secs) burst of gamma ray should accompany a SGRB. This would be associated with a sharp brightening in the optical or infra-red frequencies, also known as a "kilonova".

The nature of the remnant following the merger is still a matter of debate. According to existing models, the merger would end in an explosion, propelling a shell of debris outward into the interstellar medium. A meta-stable neutron star could be produced, eventually collapsing into a black hole surrounding by a rapidly spinning disk, or in an alternative scenario a strongly magnetized stable neutron star could be produced. However, it is expected that a highly energetic collimated jet would be generated, but the mechanism is the launching of such a jet is highly complex and not yet fully understood.

Finally on 17 Aug 2017, gravitational waves were detected from a binary of neutron stars.

What is fascinating about this discovery was that, the event was followed closely by observers from every possible domain of astrophysics, that concurrently led to the discovery of many different clues that helped reconstruct the entire picture of the event.

Not only were gravitational waves detected by LIGO/VIRGO collaboration of gravitational wave detectors, the Fermi mission in space observed the prompt gamma ray burst associated with the event. Optical/IR observations next detected the "kilonova" event, that further confirmed the conjecture that it is the merger event that is the site of formation of heavy-elements such as gold and platinum.

Artists' image of the structure of the jet emerging from the neutron star merger. Courtesy: NRAO

While the central remnant of the merger still remains a mystery, the answer could come from follow-up observations in other frequency bands over the coming months. While observations of the merger in X-ray/radio and optical afterglows suggested a mildly energetic outflow immediately afterwards, the explanation was most likely that this afterglow was produced by the interaction of the ejecta through the surrounding interstellar medium. The jet would then have to "break" through the medium in order to be detected, given that the collimation axis is not very far away from the direction of the earth.

Current evidences from NRAO radio telescopes (VLBA/VLA/GBT) indicate this to be the case. During the initial observation 75 days after the merger, the jet was still interacting with the debris, forming a "cocoon" of material expanding outwards. Later observations 230 days after the merger indicate a narrow jet 5 degrees wide pointed 20 deg away from the earth, with material blasting outwards close to the speed of light. While such observations are exciting and encouraging for the entire astrophysics community, we need more detailed self-consistent descriptions of the entire evolution in order to understand the process of jet launching from neutron star mergers. Although this is just the beginning of a long journey, we are no doubt one step closer to our destination, understanding nature and the universe around us.