At the center of many large galaxies, there is a supermassive black hole (SMBH). Our Milky Way is home to Sagittarius A*, a mostly dormant supermassive black hole with a mass of approximately 4.3 million times that of the Sun. However, deeper in the universe, there are even more massive SMBHs, with masses up to tens of billions of times that of the sun. Black holes increase their mass by gravitational swallowing of nearby celestial bodies (including stars).
This is a catastrophically devastating outcome for stars unlucky enough to be swallowed by a supermassive black hole, but fortunate for scientists, as they now have the opportunity to probe the otherwise dormant centers of galaxies.
TDE illuminates the path of exploration
As their name implies, black holes don't emit any light themselves, making them difficult for researchers to observe. But when a star gets close enough to a supermassive black hole, it will be destroyed by the black hole's huge tidal gravitational field. This interaction is actually an extreme example of the tidal interaction between the Earth and the moon. Some of the material destroyed by the tides will fall into the black hole, forming a disk of very hot, very bright material. This process, known as a tidal disruption event (TDE), provides a source of light that scientists can observe and analyze with powerful telescopes.
TDEs are relatively rare - predicted to occur approximately every 10,000 to 100,000 years in a given galaxy. Typically one to twenty TDEs are detected each year, but with the advent of new technologies, such as the Vera C Rubin Observatory currently under construction in Chile, hundreds of TDEs are expected to be observed in the coming years. These powerful observatories scan the night sky for rising and falling light sources to "survey" time-varying astronomical phenomena in the universe. Using these observations, astrophysicists can study TDEs to estimate the properties of SMBHs and the stars they destroy. One of the things researchers are trying to understand is the mass of stars and SMBHs. While one model is frequently used, a new model has recently been developed and is currently being tested.
The emergence of analytical models
The accretion rate—that is, the rate at which a star's stellar material falls back into the SMBH during a TDE—reveals important characteristics of the star and the SMBH, such as their mass. The most accurate calculation method is to perform numerical simulations of fluid dynamics, using computers to analyze the gas dynamics when the tidally disrupted material in the TDE rains down on the black hole. This technique, while accurate, is expensive, and it can take researchers anywhere from weeks to months to calculate a TDE.
In recent decades, physicists have devised analytical models for calculating accretion rates. These models provide an efficient and economical way to understand the properties of disrupted stars and black holes, but uncertainty remains about the accuracy of their approximations.
There are only a handful of analytical models in existence, the most famous of which is probably the "frozen in black hole" approximation model; this name comes from the fact that the orbital period of debris falling on a black hole is determined at a specific distance from the black hole (called the tidal radius), or is "frozen in the black hole". This model, proposed by Lacey, Townes, and Hollenbach in 1982 and extended by Lodato, King, and Pringle in 2009, suggests that the accretion rate of massive stars peaks on a time scale of 1 to 10 years, depending on the star's mass. This means that if observed in the night sky, a star source may initially brighten, peak in brightness, and then gradually fade over time, on time scales of up to several years.
new way forward
Eric Coughlin, professor of physics at Syracuse University, and Chris Nixon, associate professor of theoretical astrophysics at the University of Leeds, proposed a new model in 2022, referred to as the CN22 model, which determines the peak time scale of TDE as a function of the properties of the star and the mass of the black hole. Based on this new model, they recovered the TDE's peak timescale and accretion rate, consistent with results from some hydrodynamic simulations, but the model's broader implications—and its predictions for a wider range of star types, including the stars' masses and ages—have not yet been fully elucidated.
To better describe and understand this model's predictions in a broader context, a Syracuse University research team led by Ananya Bandopadhyay, a doctoral student in the Department of Physics, conducted a study to analyze the impact of the CN22 model and test it against different types of stars and different masses of SMBHs. The team's findings were published in the Astrophysical Journal Letters. In addition to first author Bandopadhyay, co-authors include Coughlin, Nixon, undergraduate and graduate students in the Department of Physics, and students from the Syracuse City School Department (SCSD). Syracuse School Board student participation is made possible through the Syracuse University Research in Physics (SURPh) Program, a six-week paid internship program in which local high school students participate in cutting-edge research with faculty and students in the College of Arts and Sciences' physics department.
In the summers of 2022 and 2023, USC students collaborated with physicists at Syracuse University on computational projects to test the validity of the CN22 model. They used a stellar evolution code called the Stellar Astrophysics Experiment Module to study the evolution of stars. Using these profiles, they compared the "frozen" approximation and the CN22 model's predictions of accretion rates for a range of stellar masses and ages. They also performed a hydrodynamic numerical simulation of the destruction of a Sun-like star by a supermassive black hole to compare model predictions with numerically derived accretion rates.
Research results
According to Bandopadhyay, the team found that the CN22 model was in good agreement with fluid dynamics simulations. Furthermore, and perhaps most importantly, the study found that the peak timescale of the accretion rate in the TDE is very insensitive to the properties (mass and age) of the destroyed star, with the peak timescale being about 50 days for a star like the Sun that was destroyed by a black hole with a mass of Sagittarius A*.
What's most striking and surprising about this result is that the "frozen in" model made very different predictions. According to the "frozen in" model, the proliferation rate produced by the same TDE will peak on a two-year time scale, which is clearly inconsistent with the results of the hydrodynamic simulation.
"This overturns previous notions about how TDEs work, and the types of transients that can result from outright destruction of stars," said Bandopadhyay. "By confirming the accuracy of the CN22 model, we demonstrate that this analytical approach can significantly speed up inferences about the observable properties of star destruction with different masses and ages."
Their study also overturned earlier ideas that TDEs could be used to explain long-duration light curves that peak and decay over multi-year spans. In addition, Coughlin also pointed out that this paper verified that the peak fallback rate is actually independent of the mass and age of the destroyed star, and is almost entirely determined by the mass of the SMBH.
"If you measure the rise time, what you can directly peek into is actually the properties of the supermassive black hole. This is the goal of TDE physics, which is to use TDE to explain some conditions of the black hole," Coughlin said.
Given the paper's impact on the field, the American Astronomical Society invited Bandopadhyay to present the team's research results at the society's 243rd meeting in New Orleans on January 11, 2024.
Looking forward, the team says that by confirming the accuracy of the CN22 model, this study opens a window for researchers to make observable predictions of TDEs and test them against existing and upcoming detections. Through collaboration and ingenuity, Syracuse University researchers are revealing the details of black hole physics and helping explore once untraceable reaches of the distant universe.
Compiled source: ScitechDaily