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Tidal Disruption by Black Hole?

The supernova considered to be the brightest ever recorded may have been evidence of something even more exotic. The explosion was caught by the All Sky Automated Survey for SuperNovae (ASAS-SN), the event itself dubbed ASASSN-15h. Yesterday we looked at what happens to a star roughly as massive as the Sun as it goes through a red giant phase and becomes a white dwarf, but stars significantly more massive than the Sun take no such route. A star a minimum of 8 times the mass of the Sun can explode as a Type II supernova.

But is that what ASASSN-15h really was? Detected in 2015 in a galaxy about 4 billion light years from Earth, the event has now been the subject of new work by an international team led by Giorgos Leloudas (Weizmann Institute of Science, Israel) and the Dark Cosmology Centre (Denmark). From this we get a new explanation: ASASSN-15h may have been the result of a rapidly spinning supermassive black hole tearing a relatively low mass star apart. The passing star, in other words, lacked the mass to become a supernova, but the tidal disruption caused by the black hole led to its extreme outburst.

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Image: An artist’s depiction of a rapidly spinning supermassive black hole surrounded the rotating leftovers of a star that was ripped apart by the tidal forces of the black hole.
Photo Credit: ESO, ESA/Hubble, M. Kornmesser.

A spectacular tidal event in the center of a galaxy sounds like something out of a Greg Benford novel, but in addition to the intrinsic fascination of ASASSN-15h comes a way to use it to delve into the physics of a black hole. For the mass of the host galaxy implies a black hole of at least 100 million solar masses, one that would be too large to disrupt stars outside its event horizon. As explained in the paper on this work, stars can only be disrupted outside the horizon of a supermassive black hole if the black hole is below a certain size — larger supermassive black holes, in other words, do disrupt stars, but by swallowing them whole.

We learn, though, that a spinning black hole — a so-called Kerr black hole, after Roy Kerr, the New Zealand mathematician who worked out the mathematics of such objects — would allow the disruption to occur, producing what had been thought to be a supernova. We wind up with a new tool for the exploration of extreme phenomena, as the paper explains:

… the typical tidally-disrupted star comes from the lower end of the stellar mass function, and this hypothesis [of a supernova] is further challenged by the old age of the galaxy’s stellar population. Observations of active galactic nuclei suggest that rapid SMBH [supermassive black hole] spins are common. We demonstrated here that TDEs [tidal disruption events] present a method to probe the SMBH spins of quiescent galaxies. Given the inferred rapid spin of the SMBH, the fact that we did not detect a jet at radio wavelengths implies that black hole spin alone is not sufficient to launch powerful jets.

The researchers agree that their evidence does not allow absolute certainty that ASASSN-15lh was a tidal disruption event. But there are sound reasons for thinking it so, including the location of the event in a red, massive galaxy of the kind not known to host superluminous supernova explosions, which usually occur in younger, star-forming dwarf galaxies. Moreover, their 10 months of observations from telescopes including the Hubble instrument, ESO’s Very Large Telescope and the the Las Cumbres Observatory Global Telescope showed three phases consistent with a tidal disruption event, including a re-brightening in ultraviolet light, that reduce the chances that this was a supernova.

“Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event,” adds Leloudas in this ESO news release. “But it is by far the most likely explanation.”

The paper is Leloudas et al., “The Superluminous Transient ASASSN-15lh as a Tidal Disruption Event from a Kerr Black Hole,” accepted by Nature Astronomy (preprint).

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Comments on this entry are closed.

  • RobFlores December 13, 2016, 13:14

    Safely at the center of our Galaxy, our monster: good news.

    Assuming our galaxy’s age and the fact that any Black holes formed would
    not have decayed significantly. That leaves the burning question how common are they? Most sources say 100 million.

    Doing some math on average density of BH in our galaxy.
    Galaxy Area in SqLy Pi 100,000^2 / 100Million =341
    Pythagoras (A^2 + B ^2 ) Square Root. A&B = 341
    Assuming average distribution, there should be a black hole At least 241 Ly
    distant from Earth. (no evidence BTW, there would be average distribution)

    Now as a hazard to our solar system, It is similar to the odds of any particular Star or Black Hole getting close enough to disturb the outer solar system. but 2 obvious differences:

    1) a fast moving black hole would be fairly invisible to us, until it was upon us.

    2) with a 10-20 solar mass BH, even a near miss at 100-200AU would cause
    perturbations that could disrupt orbital resonances of the outer planets.
    Hopefully new harmless orbital harmony can be achieved, but there is no guarantee of that.

    • Ashley Baldwin December 13, 2016, 18:08

      Ultimately though the effects would be purely mass/gravity related and independent of the nature of the passing stellar body , so a close pass star ( which has happened before and will again with several prospective candidates already identified) is a far more likely cause of future disruption. No such really close events are predicted for hundreds of thousands of years or more and the effects on the inner solar system are more likely to be indirect and felt through impact on the Oort Cloud than direct via gravitational resonance.

  • Robert G December 13, 2016, 13:30

    … Or maybe weapons blink? :)

  • Geoffrey Hillend December 13, 2016, 18:11

    A 10 to 20 solar mass black hole would have to travel through the Oort cloud which extends beyond one light year from the Sun, so it would most likely have many collisions with dust, meteors, and even a comet which would emit the entire spectrum of electromagnetic radiation. We would have a good chance of seeing it coming towards us. I hope one never does since we could not do anything about it. We could have a collision of planets or even the destruction of our Sun and end of life on Earth.

    I agree with the above article. Current astrophysics supports the idea of supermassive black holes get jets only when there are collisions between two galaxies and the result it galactic jets from the larger black hole of the two galaxies which feed on the debris of the breaking apart of the smaller galaxy.

    According to Astronomy magazine May 2010, page 25, In NGC 1399, an elliptical galaxy, a 1,000 solar mass black hole in a globular cluster tore apart a white dwarf which shows a spectral lines of oxygen and nitrogen but no hydrogen. The accretion disk from the shattered star emits x-rays which hit the debris in the outer region of the disk. The debris re emits light in the optical spectrum so a spectral lines can be seen. This is an example of a smaller black hole with an accretion disk.

  • Ashley Baldwin December 13, 2016, 18:12

    Actually I’m far more worried about 2-300m near Earth asteroids impacting in an ocean than anything else , as they are a real threat in mere centuries. It doesn’t look as if the up coming Discovery programme selection will have funds for the usual two choices so i fear the astronomically unsexy NEOCAM will be selected though from a practical ( and risk) perspective it should be top of the list. Worrying given the ESA pulling funds on the Asteroid Redirect Mission too .

  • Geoffrey Hillend December 13, 2016, 19:15

    The diameter of a 100 million solar mass black hole is larger than the diameter of most stars, that is most of the ones that might fall into it. Sagittarius A is between 3 and 4 and a half million solar masses with a diameter of just inside the orbit of Mercury but a 100 million solar mass black hole is probably larger than the orbit of Mars if put in place of the Sun. It makes sense that it might swallow stars whole.

  • Geoffrey Hillend December 15, 2016, 18:01

    I don’t believe a black hole is going to destroy our Earth, solar system also, but I like the thought experiments of astrophysicists which show what might happen if it did just to see the power of the physical forces in our universe such as gravitation, nuclear physics etc.

  • RobFlores December 22, 2016, 12:12

    No a BH, is not heading our way,

    Getting back to the point G. Hillend made above however:, that a black hole traversing the Oort cloud would leave footprints (comets dying a fiery x-ray death)

    Isn’t there a gap between KBO and the Ort Cloud?
    if the gap is 1000AU then an object moving through it at 50mi/s
    would take around 50 yrs to traverse. In this sparse enviroment a black hole traversing said gap would leave few traces of itself. and it’s traverse through a more crowded Ort cloud would have occurred just before the age of space based x-ray telescopes.

    • Alex Tolley December 22, 2016, 13:07

      That is a good premise for a disaster scifi story.

  • ljk March 2, 2017, 23:33

    WHEN GALAXIES COLLIDE, STARS SUFFER THE CONSEQUENCES

    Published: 2 March 2017

    by Matt Williams

    When galaxies collide, the result is nothing short of spectacular. While this type of event only takes place once every few billion years (and takes millions of years to complete), it is actually pretty common from a cosmological perspective. And interestingly enough, one of the most impressive consequences – stars being ripped apart by supermassive black holes (SMBHs) – is quite common as well.

    This process is known in the scientific community as stellar cannibalism, or Tidal Disruption Events (TDEs). Until recently, astronomers believed that these sorts of events were very rare. But according to a pioneering study conducted by leading scientists from the University of Sheffield, it is actually 100 times more likely than astronomers previously suspected.

    Full article here:

    http://www.universetoday.com/134046/galaxies-collide-stars-suffer-consequences/

    To quote:

    Since the gravitational force of black holes is so strong that even light cannot escape their surfaces (thus making them invisible to conventional instruments), TDEs can be used to locate SMBHs at the center of galaxies and study how they accrete matter. Previously, astronomers have relied on large-area surveys to determine the rate at which TDEs happen, and concluded that they occur at a rate of once every 10,000 to 100,000 years per galaxy.

    However, using the William Herschel Telescope at the Roque de los Muchachos Observatory on the island of La Palma, the team of scientists – who hail from Sheffield’s Department of Physics and Astronomy – conducted a survey of 15 ultra-luminous infrared galaxies that were undergoing galactic collisions. When comparing information on one galaxy that had been observed twice over a ten year period, they noticed that a TDE was taking place.

    Their findings were detailed in a study titled “A tidal disruption event in the nearby ultra-luminous infrared galaxy F01004-2237“, which appeared recently in the journal Nature: Astronomy.