The Absolute Cosmos: Neutron star merger produces magnetar with brightest kilonova ever observed

At about 5.5 billion light years away, telescopes have observed a super bright flash of a short gamma ray burst. In half a second burst released the same energy that Sun will produce in ten billion years of its life time.

After analysing this burst with X-ray, near infrared, optical and radio wavelengths, a team of scientists concluded that they may have observed the birth of a magnetar.

Team believes that merger of two neutron stars have produced this magnetar, resulting in a blazing kilonova. The light from kilonova reached Earth on 22 May 2020 and it is the brightest kilonova ever observed. The light first came as a short gamma ray burst.

According to team, the predicted outcome of two neutron star merger is a heavy neutron star which collapses in milliseconds or less to form a black hole.

Their findings show that heavy neutron star survived in the case of this particular short gamma ray burst. It became a magnetar rather than collapsing into a black hole.

NASA’s Neil Gehrels Swift Observatory was first to detect the light. This space observatory is designed to observe the gamma ray bursts as soon as possible by using its Burst Alert Telescope.

After that team was quick to aim other space and ground based telescopes on burst’s location. They used Hubble Space Telescope, the Very Large Array, the W.M. Keck Observatory and the Las Cumbres Observatory Global Telescope network to procure the electromagnetic profile of the event.

The observations revealed that the event was a short gamma-ray burst lasting for less than two seconds in duration, associated with two merging neutron stars. But Hubble’s near infrared observations showed something unexpected.

Team noticed that emission detected by Hubble in near infrared was ten times brighter than predicted by neutron star merger models, which simply doesn’t fit traditional explanations for short gamma-ray bursts.

As per team short bursts are caused by the merger of two neutron stars. With about 1.1 to 2.5 times the mass of our Sun, neutron stars are tiny and extremely dense objects, but stuffed in just twenty kilometre across sphere.

When two neutron stars happen to collide, they produce extreme amount of energy in kilonova explosion- an event 1,000 times brighter than a normal nova. The event is accompanied by a burst of high energy gamma rays.

Mostly short gamma ray bursts result in a black hole but in this case Hubble’s near infrared brightness indicates that the two neutron stars combined to form something else- a magnetar.

Magnetars are type of neutron stars with extremely powerful magnetic fields. As of today only twenty four magnetars have been confirmed within Milky Way galaxy.

Most of these magnetars are formed in the violent deaths of massive stars. However, now it seems possible that a small fraction of them could form by neutron star mergers.

This discovery is unique as birth of a magnetar due to neutron star merger was never seen before.

If the unpredictable brightness observed by Hubble telescope indeed came from a magnetar, then within few years the material that was expelled from the burst will produce light that can be seen at radio wavelengths.

Future follow up observations at radio wavelengths may prove that if this was a magnetar or something else.

For more information: The Broad-band Counterpart of the Short GRB 200522A at z=0.5536: A Luminous Kilonova or a Collimated Outflow with a Reverse Shock?

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