An article published in the journal “Science” describes the discovery of a star destroyed by a supermassive black hole in what in jargon is called a tidal disruption event. A team of astronomers used various telescopes searching for supernovae in Arp 299, an object generated by two merging galaxies, but in one case they came to realize that the phenomenon in progress was not an explosion but the destruction of the star under observation.
About 150 million light years away from Earth, Arp 299 is a galaxy in formation through the merger between Arp 299A, on the left side of the top image, and Arp 299B, which we still see as quite distinct. In 2005 an infrared burst from the nucleus of Arp 299B was detected and a few months later a new source of radio emissions was detected in the same area.
Several radio telescopes were used over the next few years to investigate those emissions using the technique called Very Long Baseline Interferometry (VLBI), which consists of coordinating several radio telescopes that can be very distant to observe the same target at a much higher resolution than a single radio telescope could achieve. The radio telescopes of the European network called the European VLBI Network (EVN) and the American network Very Long Baseline Array (VLBA) were combined with observations by other radio telescopes for an extensive monitoring.
In 2011, the astronomers noted that the area containing the source of radio emissions was expanding in one direction, forming a jet in which materials moved at an average speed about a quarter of the speed of light. The phenomenon was interpreted as the consequence of a tidal disruption event, the destruction of a star by a supermassive black hole.
The observations of the event collected over the years showed in the bottom animate image (Mattila, Perez-Torres, et al.; Bill Saxton, NRAO/AUI/NSF) and others indicate that the supermassive black hole in the nucleus of the galaxy Arp 299B has a mass around 20 million the Sun’s. The star destroyed by that black hole’s enormous gravity had a mass slightly more than twice the Sun’s.
The top image (Sophia Dagnello, NRAO/AUI/NSF, NASA, STScI) shows an image of the two interacting galaxies Arp 299A and Arp 299B taken by the Hubble space telescope and in the box an artistic representation of the tidal disruption event with the jet of materials ejected at very high speeds.
According to the theoretical models, during the tidal disruption event, X-rays and visible light should also be emitted, but they weren’t detected. Most likely they were absorbed by the dust near the center of the galaxy Arp 299B, which was heated up emitting infrared light instead.
Very few tidal disruption events have been observed, for example the one called ASASSN-14li, described between October and November 2015 in two articles published in the journals “Science” and “Nature”, and ASASSN-15lh, described in an article published in December 2016 in the journal “Nature Astronomy”.
In the event in Arp 299B, for the first time the jet of radio waves was observed with its evolution. This research has been going on for years involving 36 scientists from 26 institutions all over the world led by Miguel Pérez-Torres of the Instituto de Astrofisica de Andalucia in Spain and by Seppo Mattila of the University of Turku in Finland.