Surprise discovery provides new insights into stellar deaths

Artist conception of a tidal disruption event (TDE) that happens when a star passes fatally close to a supermassive black hole, which reacts by launching a relativistic jet. Image credit: Sophia Dagnello, NRAO/AUI/NSF.  Hi-res image

Astronomers, working on a project to detect supernovas, made a surprise discovery when they found that one supernova explosion was actually a star being pulled apart by a supermassive black hole. ASTRON’s Westerbork Synthesis Radio Telescope was involved in the observations.

This rare stellar death, known as a tidal disruption event, or TDE, occurs when the powerful gravity of a supermassive black hole rips apart a star that has wandered too close to the massive monster. 
Theorists have suggested that material pulled from the doomed star forms a rotating disk around the black hole, emitting intense X-rays and visible light, and launches jets of material outward from the poles of the disk close to the speed of light. 
“Never before have we been able to directly observe the formation and evolution of a jet from one of these events,” said Miguel Perez-Torres, of the Astrophysical Institute of Andalucia in Granada, Spain. 
Originally, the researchers were monitoring a pair of colliding galaxies known as Arp 299, nearly 150 million light-years from Earth. This area of space is so rich in supernova explosions it has been dubbed the “supernova factory”. However, in January 2005 the researchers discovered a bright burst of infrared emission coming from the nucleus of one of these galaxies, and in July of the same year a new, distinct source of radio emission was witnessed from the same location. 
“As time passed, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays,” said Seppo Mattila, of the University of Turku in Finland. “The most likely explanation is that thick interstellar gas and dust near the galaxy’s centre absorbed the X-rays and visible light, then re-radiated it as infrared,” he added. The researchers used the Nordic Optical Telescope on the Canary Islands and NASA’s Spitzer space telescope to follow the object’s infrared emission. 
Over the course of the next decade, the team continued to observe the radio emission using a technique known as Very Long Baseline Interferometry (VLBI). VLBI involves the remote coordination of multiple telescopes across the globe to focus on a single radio source at a given time. 
This technique provides extremely high resolution imaging when studying a radio source in space, providing the researchers with detailed data on the TDE. Telescopes in the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) were used for the observations, while the data collected was correlated at the Joint Institute for VLBI ERIC (JIVE), the Netherlands, and the Very Large Array (VLA), USA, respectively. 
This extensive monitoring revealed in 2011 that the radio-emitting portion was expanding in one direction, forming an elongation called a jet, as previously predicted by theorists. The measured expansion indicated that the material in the jet moved at an average of one-fourth the speed of light.
Most galaxies have supermassive black holes at their cores with masses that are millions to billions of times greater than the Sun. This mass is so concentrated that the resulting gravitational pull does not even allow light to escape. In this instance, the black hole is actively drawing material from its surroundings and ripping apart a star that is twice the Sun’s mass. This material forms a rotating disk around the black hole, and superfast jets of particles are launched outward – a phenomenon seen in radio galaxies and quasars. 
“Much of the time, however, supermassive black holes are not actively devouring anything, so they are in a quiet state,” Perez-Torres explained. “Tidal disruption events can provide us with a unique opportunity to advance our understanding of the formation and evolution of jets in the vicinities of these powerful objects,” he added. 
“Because of the dust that absorbed any visible light, this particular tidal disruption event may be just the tip of the iceberg of what until now has been a hidden population,” Mattila said. “By looking for these events with infrared and radio telescopes, we may be able to discover many more, and learn from them,” he said. 
Such events may have been more common in the distant Universe, so studying them could help scientists to better understand the environment in which galaxies developed billions of years ago.
Mattila and Perez-Torres led a team of 36 scientists from 26 institutions around the world in the observations of Arp 299. Their findings are published in the journal Science, which can be accessed here: http://science.sciencemag.org/lookup/doi/10.1126/science.aao4669

More information: 

The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources. 
Collectively the EVN forms the most sensitive radio telescope array at both centimetre wavelengths and millarcsecond resolution. The data collected at each of the individual stations is collated centrally at the correlator – a data processor housed at the Joint Institute for VLBI ERIC (JIVE) in Dwingeloo, the Netherlands.
 
The following EVN antennas observed at one or more epochs: Kunming, Seshan, Urumqi (China), Effelsberg, Wettzell (Germany), Medicina, Noto (Italy), Irbene (Latvia), Torun (Poland), Badary, Svetloe, Zelenchukskaya (Russia), Robledo, Yebes (Spain), Onsala (Sweden), Westerbork (The Netherlands), Cambridge and Jodrell Bank (The United Kingdom). 
Article: Mattila, S., Pérez-Torres, M., et al. 2018. A dust enshrouded tidal disruption event with a resolved radio jet in a galaxy merger. Science. DOI: 10.1126/science.aao4669 


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