Spitzer telescope reveals precise timing of a black hole dance

The OJ 287 galaxy is home to one of the largest black holes ever found, with more than 18 billion times the mass of our Sun. Orbiting this giant is another black hole approximately 150 million times the mass of the Sun. Twice every 12 years, the smallest black hole crashes through the huge gas disk surrounding its largest companion, creating a flash of light brighter than a trillion stars, even brighter than the entire galaxy of the Milky Way. Light takes 3.5 billion years to reach Earth.

But the black hole’s smallest orbit is oblong, non-circular, and irregular: it changes position with each loop around the largest black hole and tilts relative to the gas disk. When the smallest black hole passes through the disk, it creates two expanding hot gas bubbles moving away from the disk in opposite directions, and in less than 48 hours the system appears to quadruple in brightness.

Due to the irregular orbit, the black hole collides with the disk at different times during each 12-year orbit. Sometimes the rashes appear as little as a year apart; other times, up to 10 years apart. Attempts to model orbit and predict when eruptions would occur took decades, but in 2010, scientists created a model that could predict their occurrence in about one to three weeks. They proved their model correct by predicting an outbreak to appear in December 2015 within three weeks.

Then in 2018, a group of scientists led by Lankeswar Dey, a graduate student at the Tata Institute for Fundamental Research in Mumbai, India, released a document with an even more detailed model that they say could predict the timing of future outbreaks. . in four hours. In a new study published in the Astrophysical chartsThose scientists report that their accurate prediction of an outbreak that occurred on July 31, 2019 confirms that the model is correct.

The observation of that flare almost did not happen. Because OJ 287 was on the opposite side of Earth’s Sun, out of sight of all telescopes on the ground and in Earth’s orbit, the black hole would not be seen in sight of those telescopes again until early September, long after the flare had faded. But the system was in sight of NASA’s Spitzer Space Telescope, which the agency retired in January 2020.

After 16 years of operations, the spacecraft’s orbit had placed it 158 ​​million miles (254 million km) from Earth, or more than 600 times the distance from Earth to the Moon. From this point of view, Spitzer was able to observe the system from July 31 (the same day the flare was expected to appear) until early September, when OJ 287 would be observable by telescopes on Earth.

“When I first verified the visibility of OJ 287, I was surprised to find that it became visible to Spitzer just the day the next flare was forecast,” said Seppo Laine, associate scientist at Caltech / IPAC staff in Pasadena. , California, which oversaw Spitzer’s observations of the system. “It was extremely fortunate that we were able to capture the peak of this flare with Spitzer, because no other man-made instrument was able to accomplish this feat at that specific time.”

Waves in space

Scientists regularly model the orbits of small objects in our solar system, such as a comet that orbits the Sun, taking into account the factors that will most significantly influence its motion. For that comet, the Sun’s gravity is usually the dominant force, but the gravitational pull of nearby planets can also change its path.

Determining the motion of two huge black holes is much more complex. Scientists need to consider factors that might not noticeably affect smaller objects; Chief among them is something called gravitational waves. Einstein’s theory of general relativity describes gravity as the deformation of space by the mass of an object. When an object moves through space, distortions become waves. Einstein predicted the existence of gravitational waves in 1916, but they were not directly observed until 2015 by the Laser Interferometer Gravitational Wave Observatory (LIGO).

The larger the mass of an object, the larger and more energetic are the gravitational waves it creates. In the OJ 287 system, scientists expect gravitational waves to be so large that they can transport enough energy from the system to measurably alter the black hole’s smallest orbit, and therefore the timing of the eruptions.

While previous OJ 287 studies have taken gravitational waves into account, the 2018 model is the most detailed yet. By incorporating the information gathered from LIGO’s gravitational wave detections, it refines the window in which a flare is expected to occur in just 1 1/2 days.

To further refine the prediction of the eruptions at just four hours, the scientists gave details on the physical characteristics of the largest black hole. Specifically, the new model incorporates something called the “hairless” theorem of black holes.

Published in the 1960s by a group of physicists that included Stephen Hawking, the theorem makes a prediction about the nature of the “surfaces” of black holes. While black holes have no real surfaces, scientists know there is a limit around them beyond which nothing, not even light, can escape. Some ideas postulate that the outer edge, called the event horizon, could be irregular or irregular, but the no-hair theorem postulates that the “surface” has no such characteristics, not even hair (the name of the theorem was a joke).

In other words, if you cut the black hole in half along its axis of rotation, the surface would be symmetrical. (The Earth’s axis of rotation is almost perfectly aligned with its north and south poles. If you cut the planet in half along that axis and compare the two halves, you will see that our planet is mostly symmetrical, although characteristic like oceans and mountains create some small variations between the halves.)

Find symmetry

In the 1970s, Caltech professor emeritus Kip Thorne described how this scenario, a satellite orbiting a massive black hole, could reveal whether the surface of the black hole was smooth or irregular. By correctly anticipating the orbit of the smallest black hole with such precision, the new model supports the Hairless Theorem, which means that our basic understanding of these incredibly strange cosmic objects is correct. The OJ 287 system, in other words, supports the idea that the surfaces of black holes are symmetrical along their axes of rotation.

So how does the smoothness of the massive black hole’s surface affect the moment of the smallest black hole’s orbit? That orbit is primarily determined by the mass of the largest black hole. If it became more massive or lost any of its weight, that would change the size of the black hole’s smaller orbit. But mass distribution also matters. A massive bulge on one side of the larger black hole would distort the space around it differently than if the black hole were symmetrical. That would alter the trajectory of the smaller black hole as it orbits its partner, and would significantly change the timing of the black hole’s collision with the disk in that particular orbit.

“It is important to black hole scientists that we prove or disprove the no hair theorem. Without it, we cannot rely on black holes like those envisioned by Hawking and others,” said Mauri Valtonen, astrophysicist at the University of Turku in Finland and co-author of the article.