A flash of extraordinary intensity suddenly illuminated the cosmos in 2018, capturing the attention of astronomers with its unprecedented luminosity. This energetic manifestation, coming from a very distant region, immediately raised questions about its origin and nature.
The event comes from a supermassive black hole located about 10 billion light-years away, designated as J2245+3743. In 2018, its luminosity increased dramatically, reaching the equivalent of 10,000 billion suns, making it the most powerful flash ever recorded for an object of this type. The initial observations were made by the Zwicky Transient Facility (ZTF) and the Catalina Real-Time Transient Survey, two sky monitoring programs based at Caltech's Palomar Observatory.
Credit: Caltech/R. Hurt (IPAC)
Researchers identified this phenomenon as a tidal disruption event, where the intense gravity of the black hole tears apart a star that gets too close. In this case, the star involved had a mass at least thirty times that of the Sun, making this episode the most massive ever observed of its kind. This process releases a considerable amount of energy in the form of light and radiation, explaining the exceptional brightness detected.
The observation of this event is made special by the cosmological time dilation due to the expansion of the Universe. The emitted light takes billions of years to reach us, and the unfolding appears slowed down from Earth. Matthew Graham, lead researcher, specifies that seven Earth years correspond to only two years at the black hole level, allowing scientists to study the phenomenon in natural slow motion.
The rarity of such events in an active galactic nucleus enhances the interest of this discovery. Active galactic nuclei are regions where a supermassive black hole actively accumulates matter, often masking the signals of star disruptions. Here, the magnitude of the flash allowed it to be clearly distinguished, offering a precious opportunity to study the interactions between massive stars and black holes in extreme environments.
This observation opens perspectives for understanding stellar evolution and energetic phenomena in the young Universe. Programs like ZTF continue to monitor the sky, and the future Vera C. Rubin Observatory could reveal other comparable events.
The ZTF is installed on the 48-inch Samuel Oschin Telescope at Palomar Observatory.
Credit: Palomar/Caltech
The implications of this discovery extend beyond observational astronomy, touching on fundamental physics. The energy released, equivalent to the conversion of a significant proportion of the stellar mass into energy, shows the extreme violence of these processes. Such studies help refine models on the formation and destruction of stars in distant galaxies.
Tidal Disruption Event
A tidal disruption event occurs when a star gets too close to a supermassive black hole. The intense gravitational force of the black hole exerts a differential pull on the star, tearing it to shreds. This phenomenon releases a significant amount of energy in the form of radiation, often visible as a luminous flash. Astronomers study these events to understand how black holes influence their environment and accumulate matter.
The process begins with the capture of the star by the black hole's gravitational field. When the star crosses the Roche limit, the tidal forces overcome its own gravity, causing it to stretch and fragment. The stellar debris then forms an accretion disk around the black hole, heated to extreme temperatures and emitting light. This phase can last for months or years, depending on the mass of the star and the black hole.
Tidal disruption events are rare but important for testing theories of general relativity and high-energy astrophysics. They provide clues about the distribution of supermassive black holes and the life cycle of stars in various galaxies. Moreover, they help calibrate observation instruments and prepare for future space missions.
The study of these phenomena also reveals how matter behaves under extreme conditions, comparable to those of the early moments of the Universe. By observing the emitted light and spectra, scientists can deduce the composition of stars and the properties of black holes, enriching our overall understanding of the cosmos.
Cosmological Time Dilation
Cosmological time dilation is a prediction of Einstein's theory of general relativity. It stems from the expansion of the Universe itself: the more distant an astronomical object is from us (and thus the more we observe it in the distant past), the more time seems to be stretched, slowed down from our current observation point.
This phenomenon has been empirically confirmed through the observation of standardized cosmic events, such as type Ia supernovae. Astronomers have found that the duration of their light curve (the evolution of their luminosity) is stretched by a factor of (1+z), where z is the redshift of the object. This means that a supernova that explodes at a redshift z=1 will take twice as long to reach its peak luminosity and fade, compared to similar supernovae observed nearby.
More recently, in July 2023, a study on the variable activity of 190 quasars allowed this effect to be observed in the early Universe. Researchers discovered that, when the Universe was only about one billion years old, time seemed to flow five times slower than today. As astrophysicist Geraint Lewis explained, "we observe things evolving about five times slower than today. It's like watching a movie in slow motion."
It is important to note that this dilation is not an illusion due to the time it takes for light to reach us. It is a fundamental property of expanding spacetime. For a hypothetical observer present in that young Universe, one second lasted exactly one second. It is from our reference frame, several billion years in the future, that this primordial flow of time appears slowed down to us. This observation reinforces the validity of general relativity and our understanding of an expanding Universe.