Little supernova is big discovery: The origin of binary neutron stars

An global research team discovered the first recorded "ultra-stripped supernova", a rare, faint type of supernova that is believed to play a role in the formation of binary neutron star systems.

Mansi Kasliwal, Caltech assistant professor of astronomy, and graduate student Kishalay De observed the star's faint supernova, which quickly faded. The explosion blasts away mass in the star's outer layers, leaving behind a dense neutron star.

When a massive star-at least eight times the mass of the sun-runs out of fuel to burn in its core, the core collapses inwards upon itself and then rebounds outward in a powerful explosion called a supernova.

A neutron star is a dense, compact object left behind when a massive star sheds most of its outer mass in a supernova explosion.

The fact that the star exploded at all implies that it must have previously been enveloped in lots of material, or its core would never have become heavy enough to collapse.

"We call this an ultra-stripped envelope supernova and it has always been predicted that they exist", but, De added, "This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter".

This has been modelled as a plausible scenario with a helium star-neutron star binary; and, indeed, the team also found an extended helium-rich envelope in the space around the star - and a lack thereof in its ejecta - following its death. "This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter". But where, then, was the missing mass?

The researchers inferred that the mass must have been stolen by some kind of dense, compact companion, either a white dwarf, neutron star, or black hole, which is close enough to gravitationally siphon away its mass before it exploded.

The dead neutron star and its companion are expected to eventually merge. The iPTF 14gqr was so careful and in details observed that it was possible to see it from the beginning of its explosion.

Not only is iPTF 14gqr a notable event, the fact that it was observed at all was fortuitous since these phenomena are both rare and short-lived. Thus a neutron star binary system must have started out as a binary system of two massive stars.

"You need fast transient surveys and a well-coordinated network of astronomers worldwide to really capture the early phase of a supernova", says De.

"Without data in its infancy, we could not have concluded that the explosion must have originated in the collapsing core of a massive star with an envelope about 500 times the radius of the Sun".

Using the Intermediate Palomar Transient Factory (iPTF) at Palomar Observatory, astronomers were able to observe the supernova in the very first hours after it exploded.

"In addition to the observations of binary neutron stars by gravitational and electromagnetic waves, the detections of ultra-stripped supernovae will play an important role in understanding the birthplace of elements".

Moriya explains, "This is the first clear detection of a supernova which can result in the formation of a binary neutron star system".

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