Six years after witnessing a supernova, or the bright explosion of a star, in a distant spiral galaxy, astronomers have seen a rare phenomenon, where the remnants of the same stellar explosion started glowing again.

Supernovae explosions have been observed and studied by various astronomical teams across the globe. They mark the end of life for stars and suddenly appear as bright transients in the night sky. The light from these explosions can be so bright that they could even outshine the galaxy hosting the star in question.

However, once the event occurs, the bright flash of supernovae starts fading away, gradually disappearing from sight over several weeks, months or sometimes even years. This is exactly what the researchers expected six years ago after witnessing a supernova in spiral galaxy NGC 4790.

The extremely energetic event, named SN 2012au, was observed in the constellation of Virgo and faded away slowly with time. However, a group of astronomers from Purdue University, Indiana, recently witnessed that the gaseous remains of the explosion regained luminosity and that too without any external support like that from hydrogen-rich gas which could slam into the supernova remains and brighten them up.

"We haven't seen an explosion of this type, at such a late timescale, remain visible unless it had some kind of interaction with hydrogen gas left behind by the star prior to [the] explosion," Dan Milisavljevic, a member of the team behind the observations, said in a statement. "But there's no spectral spike of hydrogen in the data — something else was energizing this thing."

Further studies of the observational data led them to conclude that this was a rare sighting of processes driven by the birth of a neutron star or small city-sized stellar objects carrying 1.5 to 2 times the mass of the sun.

A dense neutron star, as scientists have long known, forms from a supernova explosion when the core of a large star collapses and all its particles, protons and electrons, melt into each other to form neutrons. If the resulting neutron star is highly magnetized and rotating rapidly, it emits a beam of electromagnetic radiation, aka pulsars, and powers a pulsar wind nebula inside the shell of the supernova remnant. This, as the researchers explained, is exactly what happened in the case of SN 2012au and is lightning up its remains.

"We know that supernova explosions produce these types of rapidly rotating neutron stars, but we never saw direct evidence of it at this unique time frame," Milisavljevic added in the statement. "This a key moment when the pulsar wind nebula is bright enough to act like a lightbulb illuminating the explosion's outer ejecta."

The team believes looking at other extremely bright or superluminous supernovae from the past could reveal neutron star birth events and even their effect. These type of explosions are also believed to be related to black holes, gravitational waves, gamma-ray, and fast radio bursts — a fact that makes studying them even more intriguing.

"If there truly is a pulsar or magnetar wind nebula at the center of the exploded star, it could push from the inside out and even accelerate the gas," Milisavljevic concluded. "If we return to some of these events a few years later and take careful measurements, we might observe the oxygen-rich gas racing away from the explosion even faster."

The study was published Sept. 12 in the Astrophysical Journal Letters.