How wobbling black holes explain blinking cosmic lights

The mystery of why some of the brightest lights in the cosmos blink on and off may lie in how big stars die, and black holes are born

Powerful jets of gas and radiation punch their way out of a dying star to generate a gamma ray burst (Ore Gottlieb/Northwestern University)

The brightest lights in the universe blink because the violent expulsions of gas from dying stars wobble.

That’s the finding of a study published Wednesday in The Astrophysical Journal Letters, which saw a team of astrophysicists at Northwestern University, in Illinois, use computational modeling to better understand collapsars — massive, dying stars in the process of collapsing to form black holes. It is believed that as the stars die, they generate gamma ray bursts, or GRBs, incredibly bright, but brief flashes of light that until now, puzzled scientists in that they also blinked on and off.

The new study not only provides scientists with a better understanding of how black holes form, and why GRBs blink, but could force them to revise their understanding of the prevalence of GRBs in the cosmos entirely.

When extremely massive stars run out of fuel to sustain their thermonuclear fires, they collapse under their own weight to form a black hole. But the entire mass of the star doesn’t disappear down the black hole all at once, and as the stellar gas compresses into a whirling disk within the star’s heart as it falls into the black hole, it generates tremendous energies, which burst outward as a jet of hot gas and radiation.

These jets are the most powerful events in the universe,” Northwestern University astrophysicist and study author Ore Gottlieb said in a statement. “Previous studies have tried to understand how they work, but those studies were limited by computational power and had to include many assumptions. We were able to model the entire evolution of the jet.”

Their model showed that the jets only generate GRBs once they punch through what remains of the star and break out into space.

“The jet generates a GRB when it reaches about 30 times the size of the star — or a million times the size of the black hole,” Mr Gottlieb said. “In other words, if the black hole is the size of a beach ball, the jet needs to expand over the entire size of France before it can produce a GRB.”

The modeling also showed that while the jets are blasting their way into space, more star stuff falls onto the whirling disk of magnetised gas falling into the black hole. This tilts the disk, causing it, and the jets, to wobble.

So rather than GRBs blinking off and back on to extreme lumenince, it turns out they actually swing in and out of view from the perspective of the observer as the disk wobbles.

But this has implications for GRBs more generally.

These short lived bursts were already considered rare, with just one per cent of collapsars producing GRBs. But the wobbling nature of the jets means there should be more opportunities for astronomers to catch GRBs as they swing into view, and the researchers conclude they should be about 10 times as observable as they actually are.

“Wobbling increases the number of detectable GRBs, so the correction from the observed to true rate [of GRBs] is smaller,” Mr Gottlieb said. “If we miss fewer GRBs, then there are fewer GRBs overall in the sky.”

This realisation could help scientists better understand the last moments in the lives of massive stars and how they form black holes, as one explanation for the rarity of GRBs is that the jets generated in most collapsars never punch through the remaining mass of the star.

“Studying jets enables us to ‘see’ what happens deep inside the star as it collapses,” Mr Gottlieb said. “Otherwise, it’s difficult to learn what happens in a collapsed star because light cannot escape from the stellar interior. But we can learn from the jet emission.”