The heart of our galaxy beats with a mysterious signal

Compared to some other galaxies, the supermassive black hole at the center of the Milky Way is not very active. This is not a mass of material being sucked in and giant plasma jets blasting into space. However, even a relatively peaceful supermassive black hole is a wild, savage beast, and our own black hole, called Sagittarius A*, has been recorded doing some pretty strange things. And now there’s a new one: Astrophysicists Gustavo Magallanes-Guijón and Sergio Mendoza of the National Autonomous University of Mexico have caught its pulse.

Every 76 minutes, Sgr A*’s gamma ray flux oscillates like clockwork. According to the researchers, this is similar in periodicity to changes in the radio and X-ray emission of a black hole; This shows the orbital motion of something spinning wildly around the black hole.

Black holes themselves do not emit radiation that we can currently detect. They are darker than the dark shadows that cannot be seen through the telescopes through which we examine the light flowing through the universe. But the space around them is another matter. Many things can occur in the extreme gravitational regime beyond the event horizon of a black hole.

The Sagittarius A* region emits light of various wavelengths, and the strength of this light varies significantly over time. And at least at some of these wavelengths, astronomers have seen a pattern. According to a paper published in 2022, radio waves oscillate in about 70 minutes. And a 2017 paper showed the 149-minute periodicity underlying the black hole’s X-ray bursts. This is approximately twice the frequency of radio emissions and now gamma radiation.

It was only recently, in 2021, that gamma ray emission was revealed to be reliably associated with Sgr A*. Magallanes-Guillon and Mendoza thought there might be some secrets hidden in the gamma ray data, and they set out to analyze them. They took public data recorded by the Fermi Space Gamma Ray Telescope between June and December 2022, processed it, and looked for periodic patterns. I found one. The results show that every 76.32 minutes, Sgr A* emits a burst of gamma rays, the most energetic wavelength range of light in the universe.

The similarity between the frequency of radio and X-ray bursts suggests a common cause. Researchers say the radio burst has about the same frequency as the gamma ray burst. The X-ray burst at minute 149 has double periodicity; This is hardly a coincidence, suggesting that its periodicity is a harmonic of gamma and radio periodicities.

Since the black hole itself does not emit radiation, and such regularly repeated periodicity is usually a sign of orbital motion, the physical mechanism is likely to be something orbiting around the black hole. A 2022 paper concluded that the thing was likely a mass of hot gas held together by a strong magnetic field that subjected the particles to synchrotron acceleration, emitting radiation in the process.

This point has an orbital distance from Sgr A* similar to Mercury’s orbit around the Sun. But it moves at incredibly high speeds, about 30 percent of the speed of light, with an orbital period of 70 to 80 minutes.

Magallanes-Guillon and Mendoza say their results are consistent with this interpretation of the radio data and suggest that the gas cluster radiates at more than one wavelength. According to theory, this point emits bursts of energy as it rotates. As it cools, it shines brighter in the radio light. The discovery of gamma-ray bursts supports this model.

However, there are still some details that need to be fixed. Black holes are known to be quite difficult to study, and Sgr A* is no exception. More observations at different wavelengths could help shed more light (har-har) into the dark and mysterious heart of the Milky Way.