Pitt researcher helps discover smallest known black hole

Out in the vast blackness of space lie two stars dancing in space. These stars are partners, known as binary stars, that exist together in stellar multiplicity. They orbit around a single large mass that cannot be seen by the naked eye or even a telescope.

However, thanks to a group of astronomers, including one at the University of Pittsburgh, we now know that our third dance partner is one of the smallest black holes ever found.

Carlos Badenes, an associate professor of physics and astronomy, recently co-authored a paper detailing the discovery of a black hole in the Auriga constellation that is significantly smaller than the average black hole. According to a press release, lead author Todd A. Thompson of Ohio State University and affiliated authors, including Badenes, published “A noninteracting low-mass black hole–giant star binary system” in the journal Science on Nov. 1. With the advent of this discovery, scientists can now establish an unbiased size distribution of black holes, a task that has been evading scientists for decades.

Badenes stated that astronomers studied black holes by analyzing the black hole’s accretion, which is the black hole’s ability to eat material from matter around it. This accretion emits light that observers can see and study via X-ray wavelengths. However, Badenes highlights a problem with this method.

“The problem is that the more massive the black hole is, the more luminous the material is going to become,” Badenes said. “When you try to take a census of the black hole population using this method, you end up with black holes that are always very massive.”

Badenes drew an analogy to a height census at the University of Pittsburgh.

“Imagine you were trying to get the distribution of heights among Pitt students, and the way you did this was to just go to the men’s locker room after the basketball team is done training,” Badenes said. “You’d end up with the mistaken impression that everyone at Pitt is very tall.”

Black holes that are extremely massive accrete material to the point of emitting light and radiation, which scientists use to find them. If scientists can only observe extremely massive black holes, observers then create the impression that the average mass of a black hole is also extremely massive. This is not the case, according to Badenes, asserting that smaller black holes exist.

“We’ve known that black holes like this should exist for a long time, it’s the fact that we can find them without going through the complicated messy physics of accretion that gives us a glimpse into what the unbiased distribution of black hole masses might be,” Badenes said.

Badenes discovered this black hole through a process of elimination. He and his collaborators utilized APOGEE, a large collection of star data that analyzes the light emitted from stars at certain wavelengths over a particular unit in time. One of the binary stars under observation would shift back and forth between emitting blue and red light, a phenomenon known as the Doppler effect. This observation can be used to determine the speed of the stars in space and determine that this is indeed a binary star. The research team then utilized physical models to determine the mass of the unknown companion.

“A more massive companion will make a larger shift … if the amount of light they make goes up and down there’s this system that pops out,” Badenes said. “The interesting thing about that is that it gives you a period, if you have a period you can use the shift to put into an equation and get a mass estimate.”

Badenes stated that the calculated mass estimate was very high and that if it was a star, the team should be able to see the light emitted from it. Upon taking more data, Badenes and collaborators realized that there simply was no light to be found on any spectrum of light such as the X-ray or infrared spectra. From this, Badenes concluded with his collaborators that the unknown mass was a black hole, as no light can escape a black hole.

The analysis and subsequent deduction of the presence of a historically small black hole opens up new dialogue for scientists and students. Benedes currently advises graduate student Christine Mazzola, who is engaged in this discovery. Benedes’ paper plays into Mazzola’s research into stellar multiplicity, the study of stars that come in multiples together, as binary stars are an example of stellar multiplicity.

“The reason why I was interested in this project was because they used radio velocity measurements from APOGEE … I’m interested in finding the likely binary stars in the data sample,” Mazzola said.

Using mathematical models, the speed at which a star spins around a mass can be determined by the light it emits, referred to as its radio velocity. Mazzola said she suspects that this discovery of a new way to find black holes will be studied further by grad students and astronomers alike.

Grad students aren’t the only ones who will study this new discovery — undergraduates have been introduced to this black hole in the lecture halls. Sandhya Rao, a research professor in physics and astronomy at Pitt, is tasking her students with reading about Badenes’ discoveries and writing about them in class.

“What I do for my students is that I ask them to write and describe one of the science articles on sciencedaily.com on black holes … a good number of [student submissions] were about this discovery,” Rao said.

Carlos Badenes’ discovery opens up a whole new way for physicists and astronomers alike to find and study black holes. These discoveries in astrophysics have been a common theme in the past few years, complementing the first image of a black hole back in April captured by a team at NASA. Rao also stated that other discoveries were made these past few years, including the biggest neutron star known to man and the presence of gravity waves, which were once only theorized to exist. All this knowledge and data lead to a better understanding of the way the universe works, yet Mazzola knows there’s more to be uncovered.

“This field is just too interesting and there’s too much you can do … I truly believe this will be a future prospect for students” Mazzola said.