On November 22, 2014, astronomers made an exciting discovery using the All Sky Automated Survey for Supernovae (ASASSN). They detected a remarkable event, not a supernova, but something that happened in a galaxy about 290 million light-years away. A star got too close to a supermassive black hole, which is millions of times heavier than our Sun, and was torn apart. This dramatic event is known as a Tidal Disruption Event (TDE).
Tidal Disruption Events are rare, occurring once every 10,000 to 100,000 years in a galaxy. When a star gets too close to a black hole, the side of the star nearest to the black hole feels a much stronger gravitational pull than the far side. This difference in gravitational force can rip the star apart. The star’s matter spirals into the black hole, forming an accretion disk—a ring of gas and dust that heats up and emits visible light, ultraviolet (UV), and X-rays, which we can observe from Earth.
After the TDE, scientists used three X-ray telescopes to study the area for years. They found a strong and regular pulse of X-rays that brightened and dimmed every 131 seconds. This signal was consistent across all three telescopes and lasted for over 450 days, getting stronger over time.
Black holes are mainly defined by two features: mass and spin. Mass is relatively easy to measure by observing how the black hole’s gravity affects nearby objects. However, measuring spin is more challenging. Black holes are expected to rotate because they form from collapsing stars, and any additional matter they consume adds to their angular momentum.
In black hole physics, the innermost stable circular orbit (r-isco) is a crucial concept. According to general relativity, there is a minimum radius at which an object can orbit a black hole stably. This radius depends on the black hole’s spin; the faster it spins, the smaller r-isco becomes. By observing the behavior of matter in the accretion disk, scientists can infer the black hole’s spin.
There are three main methods to measure the spin of black holes:
In this particular TDE, researchers proposed that a white dwarf star had been orbiting the black hole for years before the event. When another star was torn apart, its mass added to the accretion disk, surrounding the white dwarf in glowing matter and creating an X-ray hotspot. The measured spin parameter from this event was at least 0.7, possibly reaching the theoretical maximum of 0.998, indicating that matter in the accretion disk was moving at half the speed of light.
This discovery marks the first time a black hole’s spin was measured using a tidal disruption event. Understanding the spin of supermassive black holes is crucial for unraveling their origins. If these black holes mainly grow by consuming matter from their galaxies, they are expected to have high spins due to the aligned angular momentum. Conversely, if they grow mainly through mergers with other black holes, their spins may be lower due to random orientations.
As scientists continue to measure the spins of more black holes, especially those that have been dormant, we can gain deeper insights into their growth and the evolution of galaxies over billions of years. This research not only enhances our understanding of black holes but also sheds light on the formation and development of the universe itself.
Using a computer simulation tool, explore how a star interacts with a supermassive black hole during a Tidal Disruption Event (TDE). Observe how the star’s matter forms an accretion disk and emits light. Discuss with your classmates how the gravitational forces at play lead to the star’s destruction.
Using the formula for the innermost stable circular orbit, calculate the ISCO for a black hole with varying spin parameters. Discuss how the spin of the black hole affects the ISCO and what this implies for the behavior of matter in the accretion disk.
Examine real or simulated X-ray data from a TDE. Identify the periodic oscillations and discuss what these oscillations reveal about the black hole’s spin and the dynamics of the accretion disk. Present your findings to the class.
Research the three main methods of measuring black hole spin: accretion disk radius, iron emission lines, and periodic oscillations. Create a presentation that explains each method, its advantages, and its limitations. Use examples from recent discoveries to illustrate your points.
Engage in a class debate on whether supermassive black holes primarily grow by consuming matter or through mergers with other black holes. Use evidence from the study of black hole spins and TDEs to support your arguments. Conclude with a discussion on the implications for galaxy evolution.
Tidal – Relating to the gravitational forces exerted by celestial bodies that cause periodic changes in the shape and motion of another body, such as the Earth or a star. – The tidal forces near a black hole can stretch objects into long, thin shapes in a process known as spaghettification.
Disruption – A disturbance or interruption in the normal structure or function of a system, often caused by external forces. – A tidal disruption event occurs when a star gets too close to a supermassive black hole and is torn apart by its gravitational forces.
Event – An occurrence or phenomenon, especially one of significance in a scientific context, such as a specific interaction or transformation. – The detection of gravitational waves from a neutron star collision was a groundbreaking event in the field of astrophysics.
Black – Referring to a black hole, an astronomical object with a gravitational pull so strong that nothing, not even light, can escape from it. – The black hole at the center of our galaxy is known as Sagittarius A*.
Hole – A region in space where the gravitational field is so intense that it prevents anything from escaping, often formed from the remnants of a massive star. – Scientists use the term “event horizon” to describe the boundary around a black hole beyond which nothing can return.
Spin – The intrinsic angular momentum of a particle or astronomical object, which affects its behavior and interactions. – The spin of a black hole can influence the shape and dynamics of its surrounding accretion disk.
Mass – A measure of the amount of matter in an object, which determines its gravitational influence and resistance to acceleration. – The mass of a star determines its lifecycle, from its formation to its eventual fate as a white dwarf, neutron star, or black hole.
Accretion – The process by which matter is drawn in and accumulated onto a celestial body, often forming a disk around it. – The accretion of gas and dust onto a young star can lead to the formation of planets in the surrounding disk.
Disk – A flat, circular region of matter orbiting around a central body, such as a star or black hole, often formed from accreted material. – The accretion disk around a black hole emits intense radiation as the material spirals inward and heats up.
Galaxy – A massive system of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way is the galaxy that contains our solar system, and it is just one of billions in the universe.
