Tag: Astronomy & Astrophysics

TAU-led team discovers new way black holes are “fed”

These “giant monsters” were observed suddenly devouring gas in their surroundings

Supermassive black holes weigh millions to billions times more than our sun and lie at the center of most galaxies. A supermassive black hole several million times the mass of the sun is situated in the heart of our very own Milky Way.

Despite how commonplace supermassive black holes are, it remains unclear how they grow to such enormous proportions. Some black holes constantly swallow gas in their surroundings, some suddenly swallow whole stars. But neither theory independently explains how supermassive black holes can “switch on” so unexpectedly and keep growing so fast for a long period.

A new Tel Aviv University-led study published today in Nature Astronomy finds that some supermassive black holes are triggered to grow, suddenly devouring a large amount of gas in their surroundings.

Following the light

In February 2017, the All Sky Automated Survey for Supernovae discovered an event known as AT 2017bgt. This event was initially believed to be a “star swallowing” event, or a “tidal disruption” event, because the radiation emitted around the black hole grew more than 50 times brighter than what had been observed in 2004.

However, after extensive observations using a multitude of telescopes, a team of researchers led by Dr. Benny Trakhtenbrot and Dr. Iair Arcavi, both of TAU’s Raymond & Beverly Sackler School of Physics and Astronomy, concluded that AT 2017bgt represented a new way of “feeding” black holes.

“The sudden brightening of AT 2017bgt was reminiscent of a tidal disruption event,” says Dr. Trakhtenbrot. “But we quickly realized that this time there was something unusual. The first clue was an additional component of light, which had never been seen in tidal disruption events.”

Dr. Arcavi, who led the data collection, adds, “We followed this event for more than a year with telescopes on Earth and in space, and what we saw did not match anything we had seen before.”

The observations matched the theoretical predictions of another member of the research team, Prof. Hagai Netzer, also of Tel Aviv University.

“We had predicted back in the 1980s that a black hole swallowing gas from its surroundings could produce the elements of light seen here,” says Prof. Netzer. “This new result is the first time the process was seen in practice.”

Mysterious re-activation 

Astronomers from the U.S., Chile, Poland and the U.K. took part in the observations and analysis effort, which used three different space telescopes, including the new NICER telescope installed on board the International Space Station.

One of the ultraviolet images obtained during the data acquisition frenzy turned out to be the millionth image taken by the Neil Gehrels Swift Observatory — an event celebrated by NASA, which operates this space mission.

The research team identified two additional recently reported events of black holes “switched on,” which share the same emission properties as AT 2017bgt. These three events form a new and tantalizing class of black hole re-activation.

“We are not yet sure about the cause of this dramatic and sudden enhancement in the black holes’ feeding rate,” concludes Dr. Trakhtenbrot. “There are many known ways to speed up the growth of giant black holes, but they typically happen during much longer timescales.”

“We hope to detect many more such events, and to follow them with several telescopes working in tandem,” says Dr. Arcavi. “This is the only way to complete our picture of black hole growth, to understand what speeds it up, and perhaps finally solve the mystery of how these giant monsters form.”

What Disrupted A Giant Black Hole’s Feast?

Tel Aviv University investigators identified a giant black hole being interrupted in the process of swallowing material. A star that got too close to the “dining table” might have caused the disruption, and gotten swallowed too.

Featured image: In the left panel: a streak of debris from a disrupted star is falling toward the disk, while the hot “corona” is still emitting X-rays (the ball of white light above the black hole). In the right panel: the debris has dispersed some of the gas, causing the corona to disappear. Credit: Robert Hurt, NASA / JPL. At the center of a far-off galaxy, a giant, or “super-massive”, black hole is slowly consuming a disk of gas that swirls around it like water circling a drain. As the gas is pulled into the black hole, it heats up and emits radiation ranging from the visible to the X-rays – radiation that is clearly seen 300 million light years away on Earth. In most such systems, it’s not unusual to see the radiation change in luminosity, getting 10 times brighter or fainter as the rate at which the black hole accretes material fluctuates. But two years ago, a team of researchers led by Dr. Benny Trakhtenbrot and Dr. Iair Arcavi, both from the Department of Astrophysics at Tel Aviv University, identified strange variations in the behavior of a black hole known as 1ES 1927+654. The ASAS-SN sky survey measured a 50-fold increase in the visible radiation emitted around the black hole, and observations taken by the researchers using the Las Cumbres network of robotic telescopes showed rapid changes in the form and source of the radiation. A few weeks later, the team pointed NASA’s Swift, NuSTAR and NICER space telescopes, as well as the European Space Agency’s XMM-Newton space telescope at the black hole, and noticed a 10,000-fold decrease in the X-ray radiation coming from the black hole’s vicinity. “We’ve never seen a black hole behave this way”, says Dr. Trakhtenbrot. “Usually, the amount of radiation from the vicinity of a black hole is directly linked to the rate at which it accretes material. So the sharp rise in the visible radiation was telling us that the accretion rate is increasing, while the decrease in X-ray radiation was telling us that the accretion rate is actually decreasing”. “It was so strange that, at first, we thought maybe there was something wrong with the data”, said Claudio Ricci, an assistant professor at Diego Portales University in Santiago, Chile. Dr. Ricci is leading a new study of the black hole. In this new study, the investigators suggest that a rogue star got too close to the black hole and was torn apart by the strong gravitational forces there. In such a scenario, the remnants of the disrupted star could crash onto the disk of gas that was there earlier, heat it up (creating more visible radiation), and cause some of it to disperse (thus reducing the X-ray emission). “We’ve seen several cases of black holes tear apart stars that got too close, but until now we’ve never seen it happen around a black hole with a pre-existing disk of material, nor the collision that ensues”, says Dr. Arcavi. Almost every galaxy contains a super-massive black hole in its center, which can have a mass of a million or even a billion times the mass of the sun, but it’s still not clear how such high masses are reached. One possibility is that black holes grow by steadily accreting gas that’s around them. Recently, the possibility that an accelerated ingestion of stars could provide enough material for the black hole is also being investigated. The recent event in 1ES 1927+654 provides a glimpse into the combination of both processes. Although a drifting star seems the most likely culprit, the authors note that there could be other explanations for the unprecedented event. One remarkable feature of the observations is the fact that the overall drop in X-ray brightness wasn’t a smooth transition: Day to day, the NICER telescope, installed on the International Space Station, detected dramatic variation, sometimes changing in brightness by a factor of 100 in a few hours. such rapid changes occurring continuously for months, have never been seen before. “This data set has a lot of puzzles in it,” said Dr. Ricci. “But that’s exciting, because it means we’re learning something new about the universe”.

When the stars aligned: A star in a distant galaxy blew up in a powerful explosion, solving an astronomical mystery from the 11th century

Las Cumbres Observatory and Hubble Space Telescope color composite of the electron-capture supernova 2018zd (the large white dot on the right) and the host starburst galaxy NGC 2146 (toward the left).

Giant Explosion in Space Illuminates Thousand-Year Mystery.

Dr. Iair Arcavi.
Credit: Israel Hadari 

Dr. Iair Arcavi, a Tel Aviv University researcher at the Raymond and Beverly Sackler Faculty of Exact Sciences, participated in a study that discovered a new type of stellar explosion – an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. Such supernovas arise from the explosions of stars 8-9 times the mass of the sun. The discovery also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was seen by ancient astronomers, before eventually becoming the Crab Nebula, that we know today.

A supernova is the explosion of a star following a sudden imbalance between two opposing forces that shaped the star throughout its life. Gravity tries to contract every star. Our sun, for example, counter balances this force through nuclear fusion in its core, which produces pressure that opposes the gravitational pull. As long as there is enough nuclear fusion, gravity will not be able to collapse the star. However, eventually, nuclear fusion will stop, just like gas runs out in a car, and the star will collapse. For stars like the sun, the collapsed core is called a white dwarf. This material in white dwarfs is so dense that quantum forces between electrons prevent further collapse.

For stars 10 times more massive than our sun, however, electron quantum forces are not enough to stop the gravitational pull, and the core continues to collapse until it becomes a neutron star or a black hole, accompanied by a giant explosion. In the intermediate mass range, the electrons are squeezed (or more accurately, captured) onto atomic nuclei. This removes the electron quantum forces, and causes the star to collapse and then explode.

Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is a core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its core collapsed, creating a black hole or a neutron star. Theoretical work suggested that electron-capture supernovae would occur on the borderline between these two types of supernovae.

That’s the theory that was developed in the 1980’s by Ken’ichi Nomoto of the University of Tokyo, and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova. The stars should lose a lot of mass of particular composition before exploding, and the supernova itself should be relatively weak, have little radioactive fallout, and produce neutron-rich elements.  

The new study, published in Nature Astronomy, focuses on the supernova SN2018zd, discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki. Dr. Iair Arcavi, of the astrophysics department at Tel Aviv University, also took part in the study. This supernova, located in the galaxy NGC 2146, has all of the properties expected from an electron-capture supernova, which were not seen in any other supernova. In addition, because the supernova is relatively nearby – only 31 million light years away – the researchers were able to identify the star in pre-explosion archival images taken by the Hubble Space Telescope. Indeed, the star itself also fits the predictions of the type of star that should explode as an electron-capture supernovae, and is unlike stars that were seen to explode as the other types of supernovae.

From left: Japanese amateur astronomer Koichi Itagaki (who discovered the supernova), Tel Aviv University researcher Dr. Iair Arcavi (who participated in the study), and University of California graduate student Daichi Hiramatsu (lead author of the study), at one of Itagaki’s telescopes in Japan.

While some supernovae discovered in the past had a few of the indicators predicted for electron-capture supernovae, only SN2018zd had all six – a progenitor star that fits within the expected mass range, strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity, and neutron-rich material. “We started by asking ‘what’s this weirdo?’” said Daichi Hiramatsu of the University of California Santa Barbara and Las Cumbres Observatory, who led the study. “Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.”

The new discoveries also illuminate some mysteries of one of the most famous supernovae of the past. In A.D. 1054 a supernova happened in our own Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant, the Crab Nebula, has been studied in great detail, and was found to have an unusual composition. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic 1054 supernova was an electron-capture supernova.

“It’s amazing that we can shed light on historical events in the Universe with modern instruments,” says Dr. Arcavi. “Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1000 years between one event and the next.”

Dr. Arcavi is a member of the Global Supernova Project, and makes use of the Las Cumbres telescope network to study rare transient phenomena like supernovae, neutron star mergers, and stars torn apart by black holes.

Link to the original article: https://www.nature.com/articles/s41550-021-01384-2

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Out of This World

A new star and satellite observatory is currently being set up on the roof of TAU’s Shenkar building, and is set to become one of the most sophisticated labs in the world.

If you’d like to take a look at the positioning of the receivers at the International Space Station or see how TAU’s own Nano-satellite, TAU SAT-1 (which has been orbiting the Earth for almost a year now) is doing, we’re here to tell you that you will soon be able to do so. A new state-of-the-art optical ground station is currently being built on the roof of Raymond & Beverly Sackler School of Physics & Astronomy. The new optical ground station will allow us to observe tiny details far above us.

The optical ground station will be used for advanced communication with satellites and other spacecraft and tracking of relatively close-up celestial bodies, but also stars that are millions of light-years away. At a later stage, the station will serve as a tool for quantum encryption in space, one that will allow us to best encrypt any type of information.

Moments before the most sophisticated telescope in Israel will be installed here at Tel Aviv University, we met with Prof. Yaron Oz, Head of the Quantum Center; Prof. Haim Suchowski from the School of Physics and Astronomy, and Michael Tzukran, a professional astronomy photographer who will be operating the new station, for a light conversation about, you know, the usual: quantum optical communication, space photography and surprise meetings that would lead to groundbreaking projects.  

Replacing Light Pollution

Prof. Suchowski’s department, together with the University’s Engineering and Maintenance Division and partial funding from the Quantum Center, are currently working on making the Tel Aviv University campus free of celestial light pollution. This is a side-project that was born in conjunction with the construction of the new observation station. In the coming months, all polluting lighting on campus will be replaced with ecological lighting fixtures, making Tel Aviv University the first University in Israel to be free of light pollution.

Where it all started: Michael Tzukran in the old observatory on the roof of the Shenkar building the non-linear interaction of light with various materials in nature. In recent years, I’ve also been involved in the intensive activities at the Nano-Satellite Center and the new Quantum Center that have started operating on campus. What we’re dealing with on the roof these days is a combination of all these things,” he explains.

“The field of space once ‘belonged’ exclusively to NASA and very specific bodies, such as the aerospace industry in the case of Israel. Today, even high school students can send satellites into space,” explains Suchowski. “The New Space Revolution allows private companies to send and operate relatively affordable Nano-satellites into space and has changed our lives. Over the past 15 years, universities have been sending their own Nano-satellites as well.”

The University’s own Nano-satellite, TAU SAT-1, was devised, developed, assembled and tested under the leadership of Dr. Meir Ariel, Dr. Ofer Amrani of The Iby and Aladar Fleischman Faculty of Engineering and Prof. Colin Price of the Porter School of the Environment and Earth Sciences. The satellite, which carries scientific experiments, was launched about a year ago.

Up until now, the project has consisted in building a standard radio communication ground station to communicate with the launched satellite. According to Suchowski, one of the next projects will be to create optical communication through space, and thereafter quantum optical communication through space, which is a new and evolving field.

Quantum-Encrypted Communication Satellites

Information encryption is an essential subject with many applied meanings, and quantum mechanics is changing the rules of the game in this regard.

“Today, we encrypt our information based on complex mathematical algorithms, and assume that computers will take a long time to solve these problems and therefore the information is secure,” explains Prof. Yaron Oz, Chairman of the Tel Aviv University Quantum Science and Technology Center. “Quantum computers, however, are based on a different computational paradigm and can change the picture. Decomposing an integer into its primary factors – the complexity of which protects encryption algorithms that are widely used today – will be quickly solved by a quantum computer. Therefore, it is important to depict what the encrypted methods will be in the age of quantum computers.”

“Quantum systems have exceptional encrypted information transfer capabilities due to the fact that quantum mechanics do not allow information to be copied. Any attempt to copy or modify it destroys the original information. As a result, a quantum communication line is completely safe from eavesdropping. Transmitting a cipher key in a quantum communication network is completely secure, and indeed quantum optical encryption already exists via fiber optics,” he says.

Today, this type of encryption is possible, but limited to a distance of 150-200 km. Prof. Oz tells us that such communication networks already aid financial sectors in Switzerland. However, the transfer of information between continents (for example from New York to London) in this way is not yet possible. 

 

Prof. Yaron Oz

Prof. Oz explains that in Israel there’s an understanding of the need to move in the direction of encrypting information on a satellite quantum communications network, and here at Tel Aviv University we have decided to take steps at the operational and research level. The new lab with the telescope on the roof is thereby about to take part in the future satellite project of the Nano-Satellite center.

With the help of various bodies here at TAU, the Quantum Center in particular, and with the support of Prof Erez Etzion, Head of the School of Physics and Astronomy, budget and space was ensured to build the advanced observatory and buy the massive equipment. With a telescope with a 24-inch mirror, the precise and huge robot will be able to track stars, galaxies, nebulae and other bodies. The robot, which weighs 300 kg, can move at an angular speed of up to 50 degrees per second and accurately track moving satellites at low altitudes, as well as lower flying aircraft. “We are already doing preliminary experiments in optical communication. With the level of accuracy of the new telescope we’ll be the only ones in Israel with such equipment,” promises Prof. Suchowski.

 

The construction of the new ground station, as documented in Michael Tzukran’s Instagram account

The Stargazer

Quite by chance, another actor entered the picture and helped Prof. Suchowski leverage the idea into practice: Michael Tzukran, a world expert in astronomical and satellite photography and research observatory construction consultant.

“As a seasoned astronomy photographer, I wanted to challenge myself and photograph the International Space Station. I needed an open roof close to the space station’s orbit as it passes over the skies of Israel. And so I simply asked whether it could be done here.” Tzukran brought his own equipment and took one of the most detailed photos ever taken of the space station from Earth. During the photography, the space station was flying at a speed of close to 28,000 km per hour. No big deal.

 

Passing at a speed of close to 28,000 km/h. The space station, photographed by Michael Tzukran

Michael’s specialty is to adjust and control the sophisticated robot, monitor the satellites and photograph them according to requests from researchers. With the new equipment, he plans to document satellites like they’ve never been observed before from Earth.

Prof. Ady Arie from the Faculty of Engineering and doctoral students Dolev Bashi, Georgi Gary Rosenman, Yonatan Piasetski, Sahar Shahaf, Tomer Nahum and Yuval Reches are also working on the establishment of the technological system for laboratory quantum optical communication.

Prof. Suchowski estimates that various industries, such as security and other universities, will be interested in using the new platform in the future: “This is a national resource. I believe it will become instrumental in promoting applied and basic research in Israel and the world,” he concludes.

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