In 2014, a strange cloudy object called G2 approached Sagittarius A*, the supermassive black hole at the center of the Milky Way. Astronomers were pretty excited, in part because they thought Sag A*’s intense gravity might tear it apart. That didn’t happen, and the event became a cosmic uproar. G2 survived the flyby by skipping around the black hole and continued on a shortened orbit. Various observations have shown that it is not just a gas cloud. It was probably a dusty protostellar object surrounded by a dusty cloud. Or perhaps several stars have merged.
Had G2 experienced a direct encounter with Sag A*, astronomers might have captured a dazzling sight illuminating the galaxy as G2 shreds and its material heats up, producing a brilliant flare. Even if it never happens, such flare activity could explain how supermassive black holes (SMBHs) embedded in galaxies can illuminate the darkness, even if the black holes themselves do not emit light.
A new study by astronomers at Syracuse University and the University of Zurich in Switzerland has created a computer simulation that explains how the destruction of a star by a black hole can trigger flare activity. High-resolution computer models show that the star is being torn apart as it is drawn into a death spiral around SMBH. Debris from that shredding eventually begins to “drain” around the black hole in the accretion disk. The friction caused by debris collisions heats it up, making it shine brighter than the galaxy as a whole. These collisions, called “tidal disruption events” (TDEs), actually light up galaxies. But no two are exactly alike, and the team’s simulations attempt to explain why.
An artist’s depiction of a supermassive black hole tearing apart a star, sending roughly half of the star’s fragments into space, with the remainder forming a glowing accretion disk around the black hole. (Credit: DESY, Science Communication Lab)
Explore TDE
TDE offers one of the few ways to study supermassive black holes in Sagittarius A* and other galaxies in more detail, said Eric Coughlin, an assistant professor of physics at Syracuse University. “By studying tidal disruption phenomena, we can learn more about black holes that are hidden from view,” Coughlin said. This is important because many small businesses are not always easy to observe. Our own Sag A* is behind a cloud of gas and dust from our vantage point. Astronomers must use X-ray, radio, and infrared telescopes to observe it. for example.
TDE actions are an interesting area of research because each event has its own activity characteristics. How the brightness rises, when it peaks, and how long it takes to dim are all activities specific to each flare. To simulate them, the scientific team had to use special methods to simulate the star’s conditions and its interaction with the black hole. Additionally, we needed to exploit the properties of the black hole itself.
Their methodology is known as smooth particle hydrodynamics. It breaks down stars into “particles” that interact hydrodynamically (i.e. according to the same basic equations that govern the flow of water through a pipe). The simulation used tens of billions of particles to model the gas of a destroyed star to show what happens after the star is torn apart. Rather than scattering randomly, the debris forms narrow, clumped streams that follow predictable paths around the black hole before colliding with it. Collisions of debris particles brighten the scene.
A three-dimensional rendering of modeled debris flow particles highlighting the self-intersection of debris flow flows as described by a team of researchers including Syracuse physics professor Eric Coughlin. (Credit: Jean Favre, CSCS, Lucio Mayer and Noah Kubli, University of Zurich)
Contribution of black hole
The extreme gravity of the SMBH causes the star to tear apart in a TDE. But other factors are also at play. These are the mass of the black hole, the speed of rotation of the black hole, and the orientation of rotation relative to the orbital plane of the stellar debris in the accretion disk. These affect when the flare starts, how bright it gets, and how long it lasts. Therefore, if a black hole that sucks in a star is rotating, it affects changes in the space-time environment around it. It can cause something called “nodal precession.” Depending on the strength of the precession, a stream of stellar debris moves around, and the resulting flare may be present, absent, or very faint. In some cases, the flare can be delayed by several loops around the black hole.
This complexity may help explain one of the enduring mysteries of TDE research. No two events look exactly alike. Some rise quickly and some fall quickly. Others unfold more slowly. Some are brighter, some darker. Some people have behaviors that are very difficult to explain. Although differences in black hole masses could explain some of these differences, these new simulations suggest that black hole spins may be one of the main reasons for their diversity.
Future observations of the TDE and SMBH regions with telescopes such as the Rubin Observatory and the Nancy Grace Roman Observatory should provide further tests of the team’s simulations. If so, it would help us better understand the characteristics of black holes in distant galaxies.
For more information
How black holes illuminate the darkness
Tidal disruption phenomenon by SPH-EXA: solving flow return
G2 and Sgr A*: Cosmic explosion at the galactic center
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