Astronomers Witness Birth of Neutron Star Binary System

The three panels represent moments before, during, and after the faint supernova iPTF 14gqr, visible in the middle panel, appeared in the outskirts of a spiral galaxy located 920 million light years away. The massive star that died in the supernova left behind a neutron star in a very tight binary system. These dense stellar remnants will ultimately spiral into each other and merge in a spectacular explosion, giving off gravitational and electromagnetic waves. PC: SDSS/CALTECH/W. M. KECK OBSERVATORY

A team of researchers led by Caltech with help from W.M. Keck Observatory on Maunakea have for the first time observed the death of a massive star and the birth of a compact neutron star binary.

The massive star exploded in a faint and rapidly fading supernova, suggesting the presence of an unseen companion orbiting nearby that stripped the star’s mass through its gravitational pull. The massive star then collapsed and rebounded in a relatively quick supernova explosion. The explosion is believed to have caused the birth of a dead neutron star orbiting around its dense companion, indicating scientists have witnessed the first birth of a compact neutron star binary system.

Graduate student Kishalay De led the research which is described in the Oct. 12 issue of the journal Science. The work was primarily done in Mansi Kasliwal’s laboratory. Kasliwal is an assistant professor of astronomy and the principal investigator of the Caltech-led Global Relay of Observatories Watching Transients Happen (GROWTH) project.

During a supernova, a dying star blasts away all of the material in its outer layers, usually amounting to a few times the mass of the sun. However, the event that Kasliwal and her colleagues observed, called iPTF 14gqr, ejected matter only one fifth of the mass of the sun.

“We saw this massive star’s core collapse, but we saw remarkably little mass ejected,” said Kasliwal. “We call this an ultra-stripped envelope supernova and it has long been predicted that they exist. This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter.”

Researchers are accounting for the missing mass by positing that the star must have had a dense, compact companion—either a white dwarf, neutron star or black hole—close enough to gravitationally siphon away its mass before it exploded. The neutron star that remained in the aftermath of the supernova must have been born in orbit with that dense companion. Observing iPTF 14gqr was actually observing the birth of a compact neutron star binary. Because this new neutron star and its companion are so close, they will eventually merge in a collision similar to the 2017 event that produced both gravitational waves and electromagnetic waves.

The observance of iPTF 14gqr is notable since these phenomena are both rare and short-lived.

“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.”

The event was first seen at Palomar Observatory as part of the intermediate Palomar Transient Factory (iPTF), a nightly survey of the sky to look for brief cosmic events like supernovae. Because the iPTF survey keeps such a close eye on the sky, iPTF 14gqr was observed in the very first hours after it had exploded. As the earth rotated and the Palomar telescope moved out of view, astronomers around the world collaborated to monitor iPTF 14gqr, continuously observing its evolution with a number of telescopes, including Keck Observatory on Maunakea.

The team used the Low Resolution Imaging Spectrograph (LRIS) on the Keck I telescope to characterize the astrophysical nature of the event, providing important clues about the type of supernova that was observed and its surrounding environment.

“LRIS is one of the only instruments with the kind of sensitivity we needed to conduct observations of the supernova well after it exploded,” said De. “Because it grew faint so fast, Keck Observatory, which specializes in faint-object spectroscopy, was critical in studying the late stages of IPTF 14gqr’s evolution.”

De’s team also used the DEep Imaging and Multi-Object Spectrograph (DEIMOS) on the Keck II telescope to conduct follow-up spectroscopy of the supernova event as it faded.

Researchers are continuing to examine the sky even more broadly and frequently in the hopes of catching more of these rare events, which make up only 1% of all observed explosions. Such surveys will enable astronomers to better understand how compact binary systems evolve from binary massive stars.