Supermassive black holes are known for the chaotic way they devour a star. However, they don't always finish their meal quickly, as they can emit powerful radio emissions months or even years after their cosmic feast has seemingly ended, astronomers say.
See also: Rare structures detected at the center of the Milky Way
Now, astronomers studying these phenomena have found that there is no single model that explains how black holes “digest” stellar matter. Speaking on Monday, June 15 at the 248th meeting of the American Astronomical Society in California, astronomer Kate Alexander of the University of Arizona, who studies these phenomena, said that their behavior depends on the changing phases of their “nutrition.”.
Her recent research focuses on so-called Tidal Disruption Events (TDEs), cosmic catastrophes that occur when an unlucky star gets too close to a supermassive black hole. As the star approaches this giant object, the extremely strong gravitational forces disintegrate it into an elongated, “spaghettified” stream of gaseous debris, in a process known as “spaghettification.”.
Because these phenomena are extremely rare — occurring on average once every 100,000 years in a galaxy — astronomers must monitor a huge number of galaxies to detect even a few occurrences. Until now, targeted radio astronomy observations of these disturbances have typically stopped when no emission was detected within the first year or so, leaving their long-term behavior largely unexplored.
In the new study, published this year in The Astrophysical, the research team led by Kate Alexander attempted to explain why these systems, which appear inactive for long periods of time, suddenly reactivate.
To solve the puzzle, scientists analyzed decades of data, examining 91 potential Tidal Disturbance Events (TDEs) detected between 1990 and 2019. They then focused on a representative sample of 31 incidents, for which there were complete observations at multiple wavelengths.
See also: Huge black hole "wakes up" after 100 million years
By combining radio astronomy data from the VLA telescope with archival optical and ultraviolet observations, as well as new X-ray measurements, the team was able to calculate more precisely the amount of gas consumed by the black holes at each stage of their evolution.
By correlating the timing of the black holes' "feeding" with the times when the intense radio bursts occurred, the researchers were able to determine the rate at which they were absorbing matter while they were ejecting powerful outflows of gas and energy, as Alexander explained during the press conference.
The data revealed that these delayed radio bursts occur in two completely different phases of a black hole's activity: either when it is consuming matter at an extremely high rate, or when its "appetite" is almost exhausted and its supply has been drastically reduced.
In both cases, the researchers found that some of the gas heading toward the black hole is not ultimately absorbed, but is instead ejected into space. This material collides with the gas surrounding the black hole, creating powerful shock waves that accelerate charged particles and produce the observed radio emissions.
See also: Astronomers studied a black hole devouring a star
In other words, cosmic explosions are the result of the collision of ejected matter with the surrounding gas around the black hole, a process that can occur long after the initial catastrophic event.
