An extraordinarily bright flash of light seen in a distant galaxy in 2015, and dubbed ASASSN-15lh, was declared the brightest supernova ever seen. Based on new observations from several observatories, including ESO, a group of astronomers have proposed that the source was an even more extreme and very rare event — a rapidly spinning black hole ripping apart a passing star that wandered too close.
ASASSN-15lh, the Puzzling SuperNova
In June 2015, the All Sky Automated Survey for SuperNovae (ASAS-SN) detected an event named ASASSN-15lh, in a galaxy in the southern constellation Indus, 4 billion light years away, that was recorded as the brightest supernova ever — and categorized as a superluminous supernova, the explosion of an extremely massive star at the end of its life. It was twice as bright as the previous record holder, and at its peak was 20 times brighter than the total light output of the entire Milky Way. The event was reported by lead scientist Subo Dong of Peking University.
Four robotic 14-centimeter telescopes, collectively known as Cassius, stationed in Cerro Tololo, Chile, were staring at the entire visible sky when they spotted the flash.
Subsequent observations of the exciting discovery were made using the Swift and the Hubble space telescopes.
After the big flash, ASASSN-15lh faded, as supernovae generally do. But roughly three months after it began dimming, the supernova changed course. For 40-some days, its ultraviolet radiation charged up, increasing five-fold before plateauing for another couple of months and finally dropping away. Radiation at visible wavelengths did not experience this effect and continued to fade unabated. See arxiv.org/… for the paper describing the measurements.
Scientists at the time were puzzled by the observations and put forth various theories to explain the un-supernovae-like behavior of ASASSN-15lh. One hypothesis was that the star collapsed into a magnetar, a rapidly spinning neutron star with a very strong magnetic field — about 10 trillion times stronger than Earth’s magnetic field — which pumped energy into the exploding supernova.
New Observations
An international team, led by Giorgos Leloudas at the Weizmann Institute of Science, Israel, and the Dark Cosmology Centre, Denmark, has been studying ASASSN-15lh using a selection of telescopes, both on the ground and in space. Among them was the Very Large Telescope at ESO’s Paranal Observatory, the New Technology Telescope at ESO’s La Silla Observatory and the NASA/ESA Hubble Space Telescope . The observations with the NTT were made as part of the Public ESO Spectroscopic Survey of Transient Objects (PESSTO).
The New Explanation
Based on the new observations, a new tantalizing explanation has been put forth by the researchers. In the new scenario, the extreme gravitational forces of a supermassive black hole, located in the center of the host galaxy, ripped apart a Sun-like star that wandered too close — a so-called tidal disruption event, something so far only observed about 10 times. In the process, the star was “spaghettified” and shocks in the colliding debris as well as heat generated in accretion led to a massive burst of light. This gave the event the appearance of a very bright supernova explosion, even though the star would not have become a supernova on its own as it did not have enough mass.
In astrophysics, spaghettification is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces, such as those near a black hole.
The new data revealed that the event went through three distinct phases over the 10 months of follow-up observations. These data overall more closely resemble what is expected for a tidal disruption than a superluminous supernova. An observed re-brightening in ultraviolet light as well as a temperature increase further reduce the likelihood of a supernova event. Furthermore, the location of the event — a red, massive and passive galaxy — is not the usual home for a superluminous supernova explosion, which normally occur in blue, star-forming dwarf galaxies.
The mass of the host galaxy implies that the super-massive black hole at its center has a mass of at least 100 million times that of the Sun. A black hole of this mass would normally be unable to disrupt stars outside of its event horizon — the boundary within which nothing is able to escape its gravitational pull. However, if the black hole is a particular kind that happens to be rapidly spinning — a so-called Kerr black hole — the situation changes and this limit no longer applies.
The researchers conclude — “Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event. But it is by far the most likely explanation.”
Here is a fascinating animation of the event — as it occurred 4 billion years ago -
Black Holes
A black hole is a region of space where matter has collapsed in on itself. This collapse results in a huge amount of mass being concentrated in an extremely small volume. The gravitational pull of this region is so great that nothing can escape – not even light.
As a star ages, the nuclear fusion reactions stop because the fuel for these reactions gets burned up. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the outer layers of material and radiation blasts out into space. What remains is the highly compressed, and extremely massive core, with gravity so strong that even light cannot escape.
After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.
Black holes come in various sizes and need not be super massive -
Note that the smallest black hole that can be formed by natural processes at the current stage of the universe has over twice the mass of the Sun.
Black Hole Event Horizon
The boundary of the region from which no escape is possible is called the event horizon. Particles, including photons, that pass through the event horizon are swallowed by the black hole. Inside the event horizon, all "events" (points in space-time) stop, and nothing (not even light) can escape. Our current theories of physics do not apply inside a black hole.
The radius of the event horizon is called the Schwarzschild radius, named after astronomer Karl Schwarzschild, whose work led to the theory of black holes. The Schwarzschild radius = 2GM/c2, where G is the gravitational constant, M is the object mass and c is the speed of light.
Black Hole Observation
Blacks holes cannot be directly observed since they do not generate or reflect electromagnetic radiation. The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location.
A black hole, due to its massive gravity, creates the gravitational lens effect, which bends electromagnetic waves, including light, from other celestial objects that pass near the black hole. The following gif shows an animated simulation of gravitational lensing caused by a Schwarzschild black hole going past a galaxy in the background. A secondary image of the galaxy can be seen within the black hole Einstein ring on the opposite direction of that of the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole.
Types of Black Holes
There are two types of black holes:
- Schwarzschild - Non-rotating black hole
- Kerr - Rotating black hole
The Schwarzschild black hole is the simplest black hole, in which the core does not rotate. This type of black hole only has a singularity and an event horizon.
The Kerr black hole, which is probably the most common form in nature, rotates because the star from which it was formed was rotating. The Kerr black hole has the following parts:
- Singularity - The collapsed core
- Event horizon
- Ergosphere - An oval region of distorted space around the event horizon (The distortion is caused by the spinning of the black hole, which "drags" the space around it.)
- Static limit - The boundary between the ergosphere and normal space
If an object passes into the ergosphere, but not the event horizon, it can still be ejected from the black hole by gaining energy from the hole's rotation, perhaps with greater energy than before.
A rotating black hole is a solution of Einstein's field equation, first published in 1915. It is amazing how so many properties of the large scale Universe are predicted by Einstein’s theoretical work.
Does Nothing Escape a Black Hole?
Hawking radiation is blackbody radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974.
An explanation of the process is that vacuum fluctuations cause a particle–antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole while the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). This causes the black hole to lose mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunnelling effect, whereby particle–antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.
Rotating black holes can lose energy as described by the Penrose process; the black hole loses some of its angular momentum in the process. The energy loss is made possible because the rotational energy of the black hole is located not inside the event horizon, but on the outside of it in the ergosphere, in which a particle is propelled with the rotating spacetime. See en.wikipedia.org/… for details.
Spaghettification
In astrophysics, spaghettification is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces, such as those near a black hole.
The reason this happens is because the gravity exerted from the black hole singularity is much stronger at one end of the object from the other, causing the object to be stretched in the direction of the singularity. Along with that, the right side of the object is pulled to the left, and the left side of the body is pulled to the right, horizontally compressing the object.
Here in an interesting video on black holes and spaghettification, as a probe enters a black hole.
The ESO Observatories
Here is an interesting video of the awe-inspiring ESO observatories and telescopes in the high desert mountains of Chile. ESO and the teams of international scientists who use the facilities, have been trailblazers in modern astronomy and the discovery and study of distant celestial objects. www.eso.org/...
Closing Remarks
The Universe is a marvelous place. There is so much we do not understand yet, there is so much more to explore. Understanding the Universe, finding our place in it and searching for life is our destiny, not flipping hamburgers, driving delivery trucks or digging for coal.
There is a lot of fascinating information on black holes and other exotic celestial objects; I recommend browsing through some of the references, searching for other articles and watching some of the excellent videos on YouTube. The movie Interstellar is worth a watch too.
References
- eso1644 — Science Release — eso.org/…
- The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole — www.eso.org/…
- ESO, the European Southern Observatory — www.eso.org/...
- ASASSN-15lh: A Highly Super-Luminous Supernova — arxiv.org/…
- ASASSN-15lh: A Superluminous Ultraviolet Rebrightening Observed by Swift and Hubble — arxiv.org/…
- Black hole wiki — en.wikipedia.org/...
- How Black Holes Work - science.howstuffworks.com/…
- Hawking Radiation — en.wikipedia.org/...
- Artificial black hole creates its own version of Hawking radiation — www.nature.com/...
Other diaries on space -
- Space Debris, Tethers and Fishing Nets — www.dailykos.com/…
- List of recent diaries — www.dailykos.com/...