Birth of Black Hole Captured for First Time

A Northwestern University-led international team of astronomers on the Big Island of Hawai‘i is getting closer to understanding the mysterious bright object that burst in the northern sky this summer, dubbed AT2018cow or ‘The Cow.’

With the help of W. M. Keck Observatory on Maunakea, Hawai‘i and the University of Hawai‘i Institute for Astronomy’s ATLAS twin telescopes, the multi-institutional team now has evidence that they likely captured the exact moment a star collapsed to form a compact object, such as a black hole or neutron star.

The stellar debris, approaching and swirling around the object’s event horizon, caused the remarkably bright glow.

The research, which will be appearing in The Astrophysical Journal, was announced Thursday, Jan. 10, 2019, during a press conference at the 233rd Meeting of the American Astronomical
Society in Seattle.

This rare event will help astronomers better understand the physics at play within the first moments of the creation of a black hole or neutron star.

“Based on its X-ray and UV emission, ‘The Cow’ may appear to have been caused by a black hole devouring a white dwarf,” said lead author Raffaella Margutti, assistant professor of physics and astronomy at Northwestern University and faculty member of Northwestern’s CIERA (Center for Interdisciplinary Exploration and Research for Astrophysics). “But further observations of other wavelengths across the spectrum led to our interpretation that ‘The Cow’ is actually the formation of an accreting black hole or neutron star. We know from theory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born. Never.”

An image of AT2018cow and its host galaxy obtained on August 17, 2018 using W. M. Keck
Observatory’s instrument, the DEep Imaging and Multi-Object Spectrograph (DEIMOS). PC: R. MARGUTTI/W. M. KECK OBSERVATORY

The Curious Cow

The Cow was first spotted on June 16 after the ATLAS telescopes on Haleakala and Maunaloa captured a spectacularly bright anomaly 200 million light years away in the Hercules constellation. The object quickly flared up, then vanished almost as quickly.

The event captured immediate international interest and left astronomers scratching their heads. “We thought it must be a supernova,” Margutti said. “But what we observed challenged our current notions of stellar death.”

For one, the anomaly was unnaturally bright—10 to 100 times brighter than a typical supernova. It also flared up and disappeared much faster than other known star explosions, with particles flying at 30,000 kilometers per second (or 10% of the speed of light).

Within just 16 days, the object had already emitted most of its power. In a universe where some phenomena last for millions and billions of years, two weeks amounts to the blink of an eye.

“We knew right away that this source went from inactive to peak luminosity within just a few days,” said co-author Ryan Chornock, assistant physics and astronomy professor at Ohio University. “That was enough to get everybody excited because it was so unusual and, by astronomical standards, it was very close by.”

Using Northwestern’s access to Keck Observatory’s twin telescopes, Margutti’s team took a closer look at the object’s makeup with the Low Resolution Imaging Spectrometer (LRIS) on the Keck I telescope as well as the DEep Imaging and Multi- Object Spectrograph (DEIMOS) on Keck II.

“Keck was instrumental in determining the chemical composition and geometry of AT2018cow,” said co-author Nathan Roth, a JSI postdoctoral fellow at the University of Maryland. “Keck’s unique niche is its capability to monitor the late-time behavior of The Cow. This can be difficult; as more time passes after the event, the fainter it becomes. But with Keck’s late time spectroscopy, we were able to pierce through the ‘interiors’ of the explosion. This revealed AT2018cow’s red-shifted spectral features very nicely.”

“The Cow is a great example of a type of observation that’s becoming critical in astronomy: rapid response to transient events,” says Keck Observatory Chief Scientist John O’Meara. “Looking forward, we are implementing new observational policies and telescope instrumentation that allow us to be as quick on the sky and to the science as we can.”

The team also obtained optical images from the MMT Observatory in Arizona, as well as the Southern Astrophysical Research SOAR Telescope in Chile.

When Margutti and her team examined The Cow’s chemical composition, they found clear evidence of hydrogen and helium, which excluded models of compact objects merging—like those that produce gravitational waves.

“It took us a while for us to realize what we were looking at, I would say months,” said co-author Brian Metzger, associate physics professor at Columbia University. “We tried several possibilities and were forced to go back to the drawing board multiple times. We were finally able to interpret the results, thanks to the hard work of our incredibly dedicated team.”

In addition to Chornock, Roth, and Metzger, Margutti’s core team also includes Indrek Vurm, senior research fellow at Tartu Observatory, as well as Northwestern CIERA postdoctoral researchers Giacomo Terreran, Deanne Coppejans, and Kate Alexander. Comprehensive Strategy Astronomers have traditionally studied stellar deaths in the optical wavelength, which uses telescopes to capture visible light.

Margutti’s team, on the other hand, used a more comprehensive approach. After ATLAS spotted the object, Margutti’s team quickly conducted follow-up observations using multiple observatories to analyze The Cow in different wavelengths.

The researchers viewed the object in hard X-rays using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s (ESA) INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), in soft X-rays (which are 10 times more powerful than normal X-rays) using ESA’s X-ray Multi-Mirror Mission (XMM- Newton), and in radio waves using National Radio Astronomy Observatory’s (NRAO) Very Large Array (VLA).

This enabled them to continue studying the anomaly long after its initial visible brightness faded.

Margutti attributes The Cow’s relative nakedness to potentially unraveling this intergalactic mystery. Although stars might collapse into black holes all the time, the large amount of material around newly born black holes blocks astronomers’ vision.

Fortunately, about 10 times less ejecta swirled around The Cow compared to a typical stellar explosion. The lack of material allowed astronomers to peer straight through to the object’s “central engine,” which revealed itself as a probable black hole or neutron star.

“A ‘lightbulb’ was sitting deep inside the ejecta of the explosion,” Margutti said. “It would have been hard to see this in a normal stellar explosion. But The Cow had very little ejecta mass, which allowed us to view the central engine’s radiation directly.”

Galactic Neighbor

Margutti’s team also benefited from the star’s relative closeness to Earth. Even though it was nestled in the distant dwarf galaxy called CGCG 137-068, astronomers consider that to be “right around the corner.”

“Two hundred million light-years is close for us,” Margutti said. “This is the closest transient object of this kind that we have ever found.”

“Being given the opportunity to contribute to something as cutting edge and international as understanding AT2018cow as an undergrad is a surreal experience,” said co-author Daniel Brethauer, a first-year undergraduate student at Northwestern. “To have helped the world’s experts figure out what AT2018cow is even in the smallest way was beyond my wildest expectations at the beginning of the summer and something that I will remember for the rest of my life.”