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
