Chandra Spots Four Cavities in Center of Galaxy Cluster

Astronomers using NASA’s Chandra X-ray Observatory have found a quartet of pronounced X-ray cavities in RBS 797, a cluster of galaxies located about 3.9 billion light-years away in the constellation of Draco. This suggests there is a pair of closely orbiting supermassive black holes, both of which erupted and generated jets at about the same time. If confirmed, these black holes would be among the closest pair ever discovered, with a separation of about 250 light-years.

“We think we know what a pair of cavities represents, but what is going on when a galaxy cluster has two pairs in very different directions?” said Dr. Francesco Ubertosi, an astronomer at the University of Bologna.

Astronomers previously observed a pair of cavities in the east-west direction in RBS 797, but the pair in the north-south direction was only detected in a new, much longer Chandra observation.

The deeper image uses almost five days of Chandra observing time, compared to about 14 hours for the original observation.

NSF’s Karl G. Jansky Very Large Array had already observed evidence for two pairs of jets, which line up with the cavities.

“Our best idea is that one pair of supermassive black holes has led to a pair of a pair of cavities,” said Dr. Myriam Gitti, also from the University of Bologna.

“While we think supermassive black holes can form binary systems, it is extremely rare that both of them are observed in an active phase — in this sense the discovery of two close active black holes inflating cavities in RBS 797 is extraordinary.”

Indeed, previously a radio observation with the European VLBI Network discovered two radio point sources separated by only about 250 light-years in RBS 797.

If both sources are supermassive black holes, they are among the closest pair ever detected.

The two black holes should continue to spiral toward each other, generating huge amounts of gravitational waves, and eventually merge.

According to the astronomers, there is another possible explanation for the four cavities seen in RBS 797.

“This scenario involves only one supermassive black hole — with jets that somehow manage to flip around in direction quite quickly,” they said.

Their analysis of the Chandra data shows that the age difference for the east-west and north-south cavities is less than 10 million years.

“If there is only one black hole responsible for these four cavities, then we will have to trace the history of its activity,” said Dr. Fabrizio Brighenti, also from the University of Bologna.

“Key aspects are how the jets’ orientation changed quickly, and whether this is related to the galaxy cluster environment or to the physics of the black hole itself — or even a combination of both.”

The results were published in the Astrophysical Journal Letters.

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F. Ubertosi et al. 2021. The Deepest Chandra View of RBS 797: Evidence for Two Pairs of Equidistant X-ray Cavities. ApJL 923, L25; doi: 10.3847/2041-8213/ac374c

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