Astronomers Stunned As Mysterious Flare Repeats From Same Black Hole Two Years Later

 

A star being torn apart by a black hole. Credit: Illustration by Ignacio de la Calle – Quasar Science Resources for ESA

A recent finding casts doubt on the conventional wisdom on the interactions between stars and black holes.

 While it’s often remarked that lightning never strikes the same location twice, black holes seem to defy that law.  A group of astronomers, headed by scientists from Tel Aviv University, have seen a strong flare that was caused by a star being dragged into and killed by a black hole.  This occurrence is noteworthy because, only two years prior, a virtually identical flare, known as AT 2022dbl, was observed emanating from the same location of space.

This is the first time a star has been known to survive a first encounter with a supermassive black hole and then return for a second, comparable encounter. The discovery casts doubt on conventional wisdom regarding tidal disruption events and suggests that these spectacular flares might just be the start of a more complex cosmic process.

Prof. Iair Arcavi of Tel Aviv University's Astrophysics Department oversaw the study, which was led by Dr. Lydia Makrygianni, a former postdoctoral researcher who is currently at Lancaster University in the United Kingdom.  In addition, Prof. Arcavi is the director of Mizpe Ramon's Wise Observatory.  Prof. Ehud Nakar, the department chair, students Sara Faris and Yael Dgany from Arcavi's research group, and a number of foreign partners were also contributors.  The Astrophysical Journal Letters recently published the study.

The Nature of Supermassive Black Holes

According to the experts, a black hole that is millions to billions of times the mass of the sun is located at the core of every massive galaxy.  Our own Milky Way Galaxy contains one of these supermassive black holes; its discovery earned it the 2020 Nobel Prize in Physics.  Beyond their existence, however, little is known about how these monsters form or how they impact their home galaxies.

The fact that these black holes are, well, black presents one of the biggest obstacles to comprehending them. A black hole is an area of space where light cannot escape due to the extreme gravity. The motion of nearby stars led to the discovery of the supermassive black hole in the Milky Way's center. Such movement, however, is not detectable in other, more distant galaxies.

Research team (Left to right): Sara Faris, Yael Dgany & Prof. Iair Arcavi. Credit: Tel Aviv University

Fortunately, or unfortunately, depending on your perspective, a star will get too close to its galaxy's supermassive black hole once every 10,000 to 100,000 years, tearing it to pieces.  The black hole will "swallow" half of the star and hurl the other half out.

 Similar to water flowing down a bathtub drain, stuff falls upon a black hole in a circular motion.  However, in the vicinity of black holes, the rotating material's velocity approaches that of light; it gets heated and emits a dazzling glow.  Thus, for a few weeks to months, such a bad star "illuminates" the black hole, giving scientists a fleeting chance to examine its characteristics.

An Unexpected Twist: Two Flares, Same Star?

But oddly, these flares haven't been acting like they should. Both their temperature and brilliance were far lower than expected. AT 2022dbl might have offered the solution after roughly ten years of attempting to figure out why. The fact that the first flare was repeated in a nearly identical way two years later suggests that the star was partially disrupted in the first place, with a large portion of the star surviving and returning for a nearly identical second passage. Therefore, the supermassive black hole views these outbursts as more of a "snack" than a "meal."

According to Prof. Arcavi, "the question now is whether we'll see a third flare after two more years, in early 2026." "A third flare indicates that the second one was also a partial disruption of the star," Arcavi goes on. Thus, perhaps all of these flares—which we have been attempting to interpret as complete stellar disruptions for the past ten years—are not what we initially believed.

The second flare might have been the complete disruption of the star if a third flare doesn't happen.  This implies that partial and complete disruptions appear nearly identical, as predicted by Prof. Tsvi Piran's research group at the Hebrew University prior to this discovery.  "In any case," Arcavi continues, "we'll need to revise how we understand these flares and what they can teach us about the monsters that lurk in galaxies' centers."

Reference