Roger Deane (large image); NASA Goddard (inset bottom left; modified from original)
Helical jets from one supermassive black hole caused by a very closely orbiting companion (see blue dots). The third black hole is part of the system, but farther away and therefore emits relatively straight jets.
Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.
How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.
Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.
This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.
What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.
Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
For the first time, three supermassive black holes have been discovered in a tight orbital dance inside the center of a galaxy 4 billion light-years away.
The discovery was made by radio telescopes located in Europe, Asia and South Africa, and astronomers believe that it’s extreme gravitational environments such as these that rumble spacetime, generating gravitational waves that are theorized to propagate throughout the cosmos.
“What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein’s Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth,” said Roger Deane, of the University of Cape Town, South Africa, in a press release. “Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the Universe.”
Two of the black holes are orbiting very close to one another, creating corkscrew-like jets of emissions from one of the black holes as they interact. The third black hole has a wider orbit and emits straight jets from its poles that aren’t impacted significantly by the other pair of black holes.
The observation was made possible by a global network of radio antennae that operate as one, vast array. The technique of linking radio telescopes on different continents and separated by up to 10,000 kilometers is known as Very Long Baseline Interferometry (VLBI) and, when linked, the observations can reveal detail in cosmological targets 50 times finer than the Hubble Space Telescope is capable of.
For this observation of the triple-black hole system, astronomers used data from the European VLBI Network (EVN) and correlated it at the Joint Institute for VLBI in Europe (JIVE) in Dwingeloo, the Netherlands.
Supermassive black holes are massive objects, ‘weighing-in’ at between 1 million to 10 billion times the mass of our sun. The majority of galaxies are known to contain these objects at their cores and are thought to have a key impact on galactic evolution and star formation. When galaxies merge, it is thought that the central black holes spiral in toward one another, eventually merging themselves.
It is therefore of paramount importance that astronomers study and understand supermassive black holes, so finding a triple system of supermassive black holes in tight orbits provide a privileged view into the life-cycle of these fascinating objects. And radio telescopes are the perfect tool for getting an up-close view.
The inner pair of black holes of the triple system as seen by the European VLBI Network (EVN). Contours show radio emission at 1.7 GHz, the color scale show radio emission at 5 GHz frequency.R.P. Deane et al.
“VLBI is widely recognized as one of the best ways to confirm close-pair black hole systems, but the main difficulty has always been pre-selecting the most promising candidates,” said JIVE scientist Zsolt Paragi. “Our research shows that close-pair black holes may be much more common than previously thought, although their detection require extremely sensitive and high-resolution observations.”
The researchers point out that next-generation radio telescopes, such as the Square Kilometer Array (SKA) that will be located in South Africa and Australia, will be perfect for further campaigns focused on compact black hole systems.
“We have always argued that next generation radio telescopes such as the SKA should operate in VLBI mode as well, jointly with existing radio telescope arrays,” added Paragi. “This will allow to broaden our understanding of how black holes grew and evolved together with their host galaxies.”
“It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometer Array (SKA),” said Deane.
Although gravitational waves have been in the news a lot lately, astrophysicists predict that extreme environments such as these are powerful gravitational wave sources, as predicted by Einstein’s general relativity. So as sensitive detectors — such as the Laser Interferometer Gravitational Wave Observatory (LIGO) — attempt to track down these minute spacetime ripples, it is paramount that radio observations of orbiting supermassive black holes are carried out, characterizing the possible gravitational wave signatures that could be generated.