What Is a Tidal Disruption Event Around a Black Hole?
Black holes are likely the most interesting object in science. So much research has been done on their relativity aspects as well as their quantum implications. Sometimes it can be hard to relate to the physics around them and occasionally we may seek a more digestible option. So let’s talk about when a black hole eats a star by destroying it, also known as a tidal disruption event (TDE).
Mechanics of the Event
The first work proposing these events occurred in the late 1970s when scientists realized that a star getting too close to a black hole could get torn apart as it crosses the Roche limit, with the star flinging around, undergoing spaghettification, and some material falling into the black hole and around as a brief accretion disc while other portions fly out into space. This all creates a rather luminous event as the in falling material can form jets that might point to a black hole unknown to us, then the brightness drops as the material disappears. Much of the data would come to us in high energy positions of the spectrum such as UV or X-rays. Unless something is present for a black hole to feed on, they will be (mostly) undetectable to us, so looking for a TDE can be a challenge, especially because of the close proximity the passing star needs to achieve a TDE. Based on stellar motions and statistics, a TDE should only happen in a galaxy once every 100,000 years, with a better chance near the center of galaxies because of the population density (Gezari, Strubble, Cenko 41-3, Sokol).
As the star is devoured by the black hole, energy is released around it as UV rays and X-rays, and as is the case for many black holes, dust surrounds them. The dust also comes into collision from actual star material being flung out of the event. The dust can absorb this flow of energy via collisions and then echo it out into space as infrared radiation at its perimeter. Evidence for this was gathered by Dr. Ning Jiang (University of Science and Technology in China) and Dr. Sjoert van Velze (John Hopkins University). The infrared readings came much later than the initial TDE and so by measuring this difference in time and using the speed of light, scientist can get a distance reading on the dust around those black holes (Gray, Cenko 42).
Searching for the Event and Notable Examples
Many candidates were found in the 1990-91 search by ROSAT, and archival databases pointed to many more. How did scientists find them? The locations had no activity before or after the TDE, indicating a short-term event. Based on the number seen and the span of time they were spotted, it matched theoretical models for TDEs (Gezari).
The first one spotted at a previously known black hole was on May 31, 2010, when scientists at John Hopkins watched a star fall into a black hole and go through the TDE event. Dubbed PS1-10jh and located 2.7 billion light-years away, initial results were interpreted as a supernova or a quasar. But after the length of brightening didn’t wane down (in fact, it lasted until 2012), a TDE was the only possible explanation left. Lots of forewarning was sent out about the event at the time so observations in optical, X-rays, and radio were achieved. They found that the brightening (200 times more than normal) seen was not a result of an accretion disc based on the lack of such feature in prior readings, but jets did occur here just as a TDE would result in. The temperature was cooler than expected by a factor of 8 for accretion disc models, with a recorded temp of 30,000 C. Based on the lack of hydrogen but strength in He II lines in the spectrum, the star that fell in was likely a red giant with its outer hydrogen layer eaten by…a black hole, possibly the one that eventually ended its life. However, a mystery was left when the He II lines were found to be ionized. How did this happen? It is possible that dust between us and the TDE could have affected the light, but it’s unlikely and thus far unresolved. When examining prior observations with the brightness seen from the TDE, scientists were at least confident to conclude that the black hole is about 2 million solar masses (John Hopkins, Strubble, Cenko 44).
In a rare event, a TDE was spotted with high jet activity. Arp 299, about 146 million light-years away, was first spotted in January of 2005 by Mattila (University of Turku). As a galaxy collision, the infrared readings were high as temperatures rose but later that year the radio waves rose as well and after a decade jet features were present. This is a sign of a TDE (in this case labeled Arp 299-B AT1), and scientists were able to study the shape and behavior of the jets in the hopes of uncovering more of these rare events, possibly 100-1000 times more so than a supernova (Carlson, Timmer "Supermassive").
In November 2014, ASASSN-14li was spotted by Chandra, Swift, and XXM-Newton. Located 290 million light-years away, 14li was a post TDE observation, with the expected drop in light occurring as the observation progressed. Light spectrum readings indicate that the trailing material that was initially blown away is slowly falling back in to create a temporary accretion disc. That disc size implies that the black hole is rotating fast, possibly up to 50% the speed of light, because of its snack (NASA, Timmer "Imaging").
TDEs as a Tool
TDEs have many useful theoretical properties including being a way to find the mass of a black hole. An important class of black holes that requires more evidence for their existence are intermediate black holes (IMBHs). They are important for black hole models but few (if any) have been seen. That is why events such as the one spotted in 6dFGS gJ215022.2-055059, a galaxy roughly 740 million light-years away, are critical. In that galaxy, a TDE was observed in the X-ray portion of the spectrum and based on the readings seen the only thing massive enough to produce it would be a black hole that is 50,000 solar masses - which can only be an IMBH (Jorgenson).
Carlson, Erika K. "Astronomers Catch a Black Hole Devouring Star." Astronomy.com. Kalmbach Publishing Co., 14 Jun. 2018. Web. 13 Aug. 2018.
Cenko, S. Bradley, and Neils Gerkess. “How to Swallow a Sun.” Scientific American Apr. 2017. Print. 41-4.
Gezari, Suvi. “The tidal disruption of stars by supermassive black holes.” Physicstoday.scitation.org. AIP Publishing, Vol.
Gray, Richard. “Echoes of a Stellar Massacre.” Dailymail.com. Daily Mail, 16 Sept. 2016. Web. 18 Jan. 2018.
Jorgenson, Amber. "Rare intermediate-mass black hole found tearing apart star." Astronomy.com. Kalmbach Publishing Co., 19 Jun. 2018. Web. 13 Aug. 2018.
NASA. “Tidal Disruption.” NASA.gov. NASA, 21 Oct. 2015. Web. 22 Jan. 2018.
Sokol, Joshua. "Star-Swallowing Black Holes Reveal Secrets in Exotic Light Shows." quantamagazine.com. Quanta, 08 Aug. 2018. Web. 05 Oct. 2018.
Strubble, Linda E. “Insights into Tidal Disruption of Stars from PS1-10jh.” arXiv: 1509.04277v1.
Timmer, John. "Imaging ever closer to the event horizon." arstechnica.com. Conte Nast., 13 Jan. 2019. Web. 07 Feb. 2019.
---. "Supermassive black hole swallows star, lights up galaxy core." arstechnica.com. Conte Nast., 15 Jun. 2018. Web. 26 Oct. 2018.
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© 2018 Leonard Kelley