A screenshot of a simulation of the black hole encounter — after close approach, the cloud will be a turbulent mess.
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.
In 2011, astronomers were getting excited for what promised to be a spectacular cosmic event.
Rapidly approaching our galaxy’s central supermassive black hole was a stream of gas, likely stripped from a doomed star, that seemed destined to be swallowed in the black hole’s gravitational well. But what was going to happen when the gas made contact with the black hole’s accretion disk?
It was hoped that a violent interaction would take place and observatories would see some powerful flaring events at the center of the Milky Way, providing a valuable insight to the eating habits of supermassive black holes that are thought to reside in the centers of most galaxies.
Now the time has come — what are astronomers seeing? Well… not a lot. It seems that the cloud of gas, known simply as “G2,” isn’t generating the fireworks it seemed destined to.
Flaring supermassive black holes have been observed before in other galaxies. By looking at the energetic radiation generated by these events, astronomers can surmise what the black hole has “eaten.” Stars, planets, gas clouds, even asteroids have been seen being blended by the gravitational behemoths. So to have the possibility of seeing our very own supermassive black hole — known as Sagittarius A*, or simply Sgr. A* — light up while munching on G2, astronomers were very excited for the scientific opportunity.
Sadly, just as the streamer was predicted to be interacting with Sgr. A* during an observing campaign this spring, they saw nothing.
In a new paper published to the arXiv preprint service this month, Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues gave an explanation as to why the stream of gas may not be kicking off the display astronomers hoped.
Gillessen, who used the European Southern Observatory’s Very Large Telescope in Chile to discover that G2 was approaching Sgr. A*, suggests that the streamer’s interaction with the black hole’s accretion disk is a lot more subtle than they were expecting.
It appears that G2 is composed of one gravitationally-bound clump and a less dense stream of gas that is steadily rushing into the accretion disk like a breeze and not in a clumpy, energetic manner as predicted.
The researchers have also worked out that another clump of gas that interacted with the black hole over a decade ago, known as “G1″, is related to G2 as both are following the same orbital path. It is thought that both clumps were stripped from the same star 100 to 200 years ago.
This theory was also examined by another research group headed by James Guillochon and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
But, as discussed by today’s Nature News article, it’s possible that the G2 clump is hiding a star that is gravitationally binding the majority of the material, preventing it from being pulled into the black hole’s accretion disk.
Whatever the mechanisms behind the drama unfolding in the center of our galaxy, one thing is clear: our supermassive black hole does not have the munchies (yet).