Black Hole's Rejected Snack Becomes Science Goldmine
Stefan Gillessen and the Max-Planck Institute fur Extraterrestrische Physik
This observation shows a false-color infrared image of the region around the nucleus of our Milky Way, the supermassive black hole Sag A*. The marker shows the location of the black hole, which glows faintly due to accretion of material; the other objects are stars or dense clouds either orbiting the black hole or in its general vicinity. The size scale of the image end is about one light-year.
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.
Remember that mysterious cloud of gas that was supposed to be on a collision course with the supermassive black hole in the center of our galaxy? Well, astronomers are still trying to work out why it wasn’t sucked in, and why it didn’t spark the mother of all cosmic fireworks displays.
But in doing so, researchers have revealed some fascinating new things about the monster singularity that lurks over 25,000 light years from Earth.
In 2011, astronomers noticed a cloud of gas speeding through the innermost reaches of the galactic bulge. Ahead of the object, known (rather un-romantically) as “G2″, was supermassive black hole Sagittarius A* (or, simply, Sgr A*). After some caluculations, it was realized that this cloud would come within 250 sun-Earth distances of the black hole, close enough to be sucked in by the black hole’s powerful gravitational pull.
This was really exciting: for the first time in human history we’d be able to study material before it fell into a black hole, from approach to dazzling finale.
At the time, it was assumed G2 was composed of a nebulous collection of stellar gases. It was also assumed that, while undergoing extreme tidal warping, the cloud would be stretched out like a long noodle, with tendrils being sucked into the black hole’s accretion disk. Somewhere along the line it was hoped that the emissions from knots of this gas interacting with the extreme spacetime environment surrounding Sgr A*’s event horizon would be detected as X-ray flashes — possibly the biggest eruptions we’d ever see come from Sgr A*. We’d witness our black hole in action; from discovery of an in-falling object to that object’s ultimate doom — when matter gets transformed to energy and the black hole has a cosmic feast.
But… nothing happened.
Well, stuff did happen, but the destruction of G2 became something of a non-event and astrophysicists have been trying to work out exactly what happened… or, more accurately, why something didn’t happen.
The current hypothesis is that G2 isn’t the loose collection of gas it was assumed to be; it could be a star enveloped in a cloud of gravitationally-bound gas. During its close encounter with Sgr A*, the cloud maintained its integrity and very little gas was stripped away from the cloaked star. No infalling matter; no cosmic fireworks; disappointed astronomers.
In new research published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS), astronomers Michael McCourt and Ann-Marie Madigan of the Harvard-Smithsonian Center for Astronomy (CfA) described their study of G2, revealing that although it was a bit of a dud, the event did probe the extreme environment surrounding Sgr A*. Of particular interest: they may have tracked down where the black hole finds its regular feast.
McCourt and Madigan tracked G2, and another gas cloud called “G1″, travel through the vicinity of Sgr A*. It just so happens that the clouds passed so close, that they would have traveled through the black hole’s “accretion flow” — in other words, these clouds could be used as tracers to see the structure of the matter that regularly falls into the black hole.
As both clouds follow a similar trajectory around the black hole, small changes in the objects’ gas could be measured. And the evolution of these clouds revealed characteristics of the interstellar material surrounding Sgr A*.
“Although it is not yet clear whether these objects contain embedded stars, their extended gaseous envelopes evolve independently as gas clouds,” they write. “We find evolution consistent with the G-clouds (G1 and G2) originating in the clockwise disc. Our analysis enables the first unique determination of the rotation axis of the accretion flow: we localize the rotation axis to within 20 degrees, finding an orientation consistent with the parsec-scale jet identified in X-ray observations and with the circumnuclear disc, a massive torus of molecular gas (approximately) 1.5 parsecs (5 light-years) from Sgr A*.”
Basically, observations of G1 and G2 show the direction that material travels as it falls into the black hole, thereby tracing out the rotation of the black hole’s accretion disk. Also, they found that rather than the black hole being fed by the stellar winds of nearby stars, material is being pulled from a massive ring of material, some 5 light-years away.
So G2 didn’t spark the exciting eruption of flares and X-ray emissions that astronomers predicted in 2011, but it turns out that G2 (and it’s orbiting buddy G1) have been far more useful in not being eaten by the black hole; instead being flung around the center of the galaxy, providing tantalizing clues as to the nature of the gravitational monster living in the center of the Milky Way.