Birth of a Black Hole

[mashshare]

It was one of the greatest mysteries in modern science: a series of brief but extremely bright flashes of ultra-high energy light coming from somewhere out in space. These gamma ray bursts were first spotted by spy satellites in the 1960s. It took three decades and a revolution in high-energy astronomy for scientists to figure out what they were.

Far out in space, in the center of a seething cosmic maelstrom. Extreme heat. High velocities. Atoms tear, and space literally buckles. Photons fly out across the universe, energized to the limits found in nature. Billions of years later, they enter the detectors of spacecraft stationed above our atmosphere. Our ability to record them is part of a new age of high-energy astronomy, and a new age of insights into nature at its most extreme. What can we learn by witnessing the violent birth of a black hole?

The outer limits of a black hole, call the event horizon, is subject to what Albert Einstein called frame dragging, in which space and time are pulled along on a path that leads into the black hole. As gas, dust, stars or planets fall into the hole, they form into a disk that spirals in with the flow of space time, reaching the speed of light just as it hits the event horizon. The spinning motion of this so-called “accretion disk” can channel some of the inflowing matter out into a pair of high-energy beams, or jets.

How a jet can form was shown in a supercomputer simulation of a short gamma ray burst. It was based on a 40-millisecond long burst recorded by Swift on May 9, 2005. It took five minutes for the afterglow to fade, but that was enough for astronomers to capture crucial details. It had come from a giant galaxy 2.6 billion light years away, filled with old stars.

Scientists suspected that this was a case of two dead stars falling into a catastrophic embrace. Orbiting each other, they moved ever closer, gradually gaining speed. At the end of the line, they began tearing each other apart, until they finally merged. NASA scientists simulated the final 35 thousandths of a second, when a black hole forms.

Chaos reigns. But the new structure becomes steadily more organized, and a magnetic field takes on the character of a jet. Within less than a second after the black hole is born, it launches a jet of particles to a speed approaching light.

A similar chain of events, in the death of a large star, is responsible for longer gamma ray bursts. Stars resist gravity by generating photons that push outward on their enormous mass. But the weight of a large star’s core increases from the accumulation of heavy elements produced in nuclear fusion. In time, its outer layers cannot resist the inward pull… and the star collapses. The crash produces a shock wave that races through the star and obliterates it.

In the largest of these dying stars, known as collapsars or hypernovae, a black hole forms in the collapse. Matter flowing in forms a disk. Charged particles create magnetic fields that twist off this disk, sending a portion out in high-speed jets.

Simulations show that the jet is powerful enough to plow its way through the star. In so doing, it may help trigger the explosion. The birth of a black hole does not simply light up the universe. It is a crucial event in the spread of heavy elements that seed the birth of new solar systems and planets.

But the black hole birth cries that we can now register with a fleet of high-energy telescopes are part of wider response to gravity’s convulsive power.