A black hole tore apart a star in a galaxy far, far away

Posted on

It was an event not seen for more than a decade: a sudden burst of energy launched from the center of a distant galaxy, bright enough to be visible from 8.5 billion light-years away. With a burst of light equivalent to more than 1,000 trillion suns, the flash was first detected by the Zwicky Transient Facility, a survey of the entire night sky conducted from California’s Palomar Observatory.

“On Valentine’s Day this year, we found a source that was puzzling. It was just weird!” Igor Andreoni of the University of Maryland, lead author of one of two papers about the event, shared The edge. “And weird is good at science. It means it’s something you can learn from.”

Within days, astronomers around the world pointed their telescopes at the flash and observed it in X-ray, radio and other wavelengths. It was extremely bright and resembled a gamma-ray burst – a type of bright flash usually detected by gamma-ray or X-ray telescopes. But this one had been spotted by an optical telescope.

The tremendous brightness of the flash led astronomers to conclude that the flash must have been caused by a star torn apart. A star had wandered too close to the supermassive black hole at the heart of a galaxy and had been torn to shreds by gravity. “It can tear the star apart completely. It is literally pulled and stretched until it can no longer stand together,” explains Andreoni. This is called a tidal disturbance, and astronomers have observed dozens of these events in recent years.

“Weird is good at science. It means it’s something you can learn from.”

What’s unusual about this particular event is that it produced a massive energy jet, ejecting material from the poles of the black hole at nearly the speed of light. “We don’t know why, but sometimes a very powerful jet of material is launched when the star is disrupted,” Andreoni said. This jet is believed to have been particularly bright because it is aimed directly at Earth, making it both appear brighter and visible over a wider portion of the electromagnetic spectrum.

To discover these kinds of dramatic transient events, astronomers need telescopes that continuously scan as much of the sky as possible and detect sudden changes in brightness, such as the Zwicky Transient Facility. But thousands of changes in brightness are observed every night, so this mountain of data needs to be refined to find the most interesting objects. Andreoni’s group is working to sift through this data to find very fast events in the optical wavelength.

Sudden changes in brightness could possibly be caused by a supernova or the merger of two neutron stars. Further observations are needed to understand the specific event that caused the flash. For example, a supernova brightens over a period of weeks, which is extremely fast by astronomical standards. But this particular event cleared up even faster, within a few hours or days. That made it of immediate and urgent importance.

The group signaled this flash to the international community and encouraged researchers working with telescopes operating at other wavelengths, such as radio or X-rays, to observe it as well. A total of 21 telescopes contributed data about the event. “When all the pieces of the puzzle were found and put together, this picture emerged that was just amazing,” Andreoni said. “We didn’t expect to find such a rare source, and certainly not optically.”

Of the stars torn apart by black holes, only about 1 percent seem to produce these powerful jets, but researchers are still not sure why. As the star is being pulled apart and its material pulled towards the black hole, the energy of this matter is converted into light. It’s theorized that the black hole’s magnetic fields and spin could work together to shoot material out of the poles — like a tube of paint squeezed in the center until material flies out of either end.

“We are talking about thousands of times the mass of the Earth being pulled apart and spinning and being launched at almost the speed of light. It really is a unique opportunity to study something that is impossible to reproduce on Earth,” Andreoni said.

This was the first time such a beam had been detected in the visible light portion of the electromagnetic spectrum, known as the optical wavelength. Previously, jets from around black holes were detected by looking at X-rays, gamma rays and radio waves.

This tells astronomers something about the environment around the black hole — that it’s not that dense because it lets optical light through — and shows that looking into the optical range could be a useful way to spot these extreme events in the future.

“We’re talking about thousands of times the mass of the Earth being pulled apart and spun and launched at almost the speed of light.”

The need for telescopes to respond quickly to such events also creates an impetus for greater flexibility in telescope design and planning. Telescopes like Hubble or James Webb are massively oversubscribed, meaning far more researchers are requesting time on the telescope than is possible. Therefore, the observation time is meticulously planned years in advance and every last minute of observation time is filled as much as possible. But there is also a need for telescopes that can respond to rare events within hours or even minutes.

It is difficult to safely and quickly change the direction of a space telescope, so the Hubble and James Webb Space Telescope only occasionally contribute to this type of research. But recently built ground-based telescopes, such as the MASTER network or the GROWTH-India telescope, specialize in scanning the sky for gamma rays and moving instantaneously and autonomously to observe them.

And there is always the possibility of human intervention. “Sometimes you literally have to call people up and say, ‘Hey, can you point the telescope at this or that coordinate, please?'” Andreoni said. In other cases, researchers submit requests via online systems to make observations at available times. There is growing interest in how telescopes might respond to these short and rare but scientifically significant events.

Both the international collaboration between researchers working with different telescopes and the ability of those telescopes to respond quickly were essential to this breakthrough in black hole observations, Andreoni said. “This was extremely important for this kind of discovery. If we couldn’t do it with any telescope, we wouldn’t have realized we were on such a big discovery.”

Leave a Reply

Your email address will not be published. Required fields are marked *