Counting black holes like fireflies in the night

Thursday, December 15, 2016

Stock image of space background with gravitational lensing.Waterloo Astrophysicist Avery Broderick and PhD student Mansour Karami, along with colleagues from the United States and Iran, have developed a method that will shine a light on the elusive nature of black holes.

By combining two standard astrophysical tools in use today, the researchers estimate that within two years they will double the number of known galactic black holes. And within ten years they will have accumulated data on enough black holes that they can statistically analyze their properties as a population, allowing them to study stellar mass black holes at various stages that often extend billions of years.

Their approach, published today in The Astrophysical Journal, has implications for the emerging field of gravitational wave astronomy and the way in which we search for black holes and other dark objects in space.

Avery BroderickWhat LIGO heard were the last cries from a doomed binary black hole system,” said Broderick, a professor in Waterloo’s Department of Physics and Astronomy. “Who knows how rare these events are and how many black holes are generally distributed across the galaxy? For the first time we’ll be placing all the amazing dynamical physics that LIGO’s sees into a larger astronomical context.

One of the most elusive dark objects in the universe, black holes absorb all light and matter and emit zero radiation making them impossible to image, let alone detect against the black background of space. Although very little is known about the inner workings of black holes, we do know they play an integral part in the lifecycle of stars and regulate the growth of galaxies.

The first direct proof of their existence was announced earlier this year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) when gravitational waves were detected from the collision of two black holes merging into one. Telescopes like NASA’s Chandra X-ray Observatory and the Event Horizon Telescope can study the super-hot matter swirling towards a black hole, but they can’t quite peer into the abyss itself.

But now, Broderick and his colleagues propose a bolder approach to detect and study black holes – not as single entities, but in large numbers as a system by combining microlensing with radio wave interferometry.

“The key feature here is that our method is unbiased. It’s not going to be limited by a particular formation scenario like a binary black hole merger,” said Broderick, also a faculty member at the Perimeter Institute for Theoretical Physics. “With this, it’s possible to find Galactic black holes of all shapes and sizes and relationship statuses.”

Gravitational microlensing occurs when a dark object such as a black hole passes between us and another light source, such as a star. The star’s light bends around the object’s gravitational field to reach Earth, making the background star appear much brighter, not darker as in an eclipse.

Scientists typically use microlensing to survey large sections of the galaxy for planets, waiting for stars to flare like fireflies in the night. But even the largest telescopes that observe microlensing events in visible light have a limited resolution, yielding a single blurred blob from which only the total brightening can be measured, telling astronomers very little about the object that passed by.

Instead of using visible light, Broderick and his team propose using radio waves to take multiple snapshots of the microlensing event in real time.

When you look at the same event using a radio telescope – interferometry – you can actually resolve more than one image. That’s what gives us the power to extract all kinds of parameters, like the object’s mass, distance and velocity,” said Waterloo astrophysics doctoral student Karami.

Taking a series of radio images over time and turning them into a “movie” of the event will allow them to extract another level of information about the black hole itself.

"We’re taking microlensing and doing it in a slightly different way that allows us to get at a great deal more information than you can traditionally,” said Broderick. “Taking multiple images is the secret sauce that allows us to say so much more.

Normally, stars are too dim in the radio frequency band for this approach to work, but Broderick and his team plan to focus on a particular type of star called Mira variable stars, which are bright in the radio band. Broderick and Karami will use these Mira variable stars to establish their proof-of-concept.

With modest improvements, they hope to take advantage of existing radio wave detection networks such as the Very Long Baseline Array (VLBA). Instead of stars as the background “light” source, they could perform radio microlensing surveys with other radio-bright objects, such as quasars. Broderick and Karami estimate their method will detect about ten black holes per year, doubling what is currently known within two years, and unlocking the Galactic history of black holes in over a decade.

This project was partially supported by a Natural Sciences and Engineering Council Discovery Grant. Co-authors on the paper include Sohrab Rahvar of the Perimeter Institute and Sharif University of Technology in Iran and Mark Reid of Harvard-Smithsonian Center for Astrophysics in the United States.