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.
On April 27, a powerful flash of radiation erupted from deep space. The flash, a gamma-ray burst (GRB), was the brightest on record, challenging some of the leading theories on how the most powerful explosions in the known Universe occur.
Triggered by the sudden collapse of a dying massive star, GRBs are thought to be energized by the resulting black hole that forms in its wake. The black hole birthing drives relativistic particles through the collapsing star material, generating a shock wave, producing a highly collimated beam of gamma-ray radiation. GRBs are considered to be the more energetic cousins of supernovae, but for the first time, this particular GRB — called GRB 130427A — was seen to occur alongside a supernova; an unprecedented observation.
“We normally detect GRBs at great distance, meaning they usually appear quite faint. In this case the burst happened only a quarter of the way across the Universe meaning it was very bright. On this occasion, a powerful supernova was also produced, something we have not recorded before alongside a powerful GRB and we will now be seeking to understand this occurrence,” said Paul O’Brien, of the University of Leicester, who collaborated on one of the five papers devoted to GRB 130427A published in the journals Science and Astrophysical Journal Letters on Thursday (Nov. 21).
Another noteworthy factor of this event was the high number of observatories in space and on the ground that were able to slew in the direction of GRB 130427A just after it occurred. The initial discovery was made almost simultaneously by NASA’s orbiting Swift Gamma-Ray Burst Mission and Fermi Gamma-ray Space Telescope, then an alert was sent out to observatories on the ground such as the Rapid Telescopes for Optical Response (RAPTOR) project to watch the gamma-ray glow brighten.
Even NASA’s newest X-ray space observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), was able to get in on the act, recording hard X-ray data in the GRB’s afterglow.
“We expect to see an event like this only once or twice a century, so we’re fortunate it happened when we had the appropriate collection of sensitive space telescopes with complementary capabilities available to see it,” said Paul Hertz, director of NASA’s Astrophysics Division in Washington.
Due to GRB 130427A’s relatively close proximity and the vast quantities of data collected from the multi-instrument campaign, it has become something of a GRB “Petri dish.”
“The rapid reaction of Swift has enabled us to discover many new and unexpected aspects of GRBs, the strong confirmation of the basic theory by this new very bright burst reassures us that we are on the right track in understanding these extraordinary explosions,” said Julian Osborne, Swift team leader at the University of Leicester.
Although astrophysicists will be picking through the data for some time to come, this event has already poked a couple of holes in our understanding of how GRBs work. For example, as highlighted in Fermi data, just as the optical light from the GRB peaked, there was an anomalous spike in highly energetic gamma-rays. The energies associated with this gamma-ray peak topped out at 95 GeV, the most powerful radiation ever seen from a GRB event.
“We thought the visible light for these flashes came from internal shocks, but this burst shows that it must come from the external shock, which produces the most energetic gamma-rays,” said Sylvia Zhu, a Fermi team member at the University of Maryland in College Park.
To probe the very limits of astrophysics, sometimes you need violent events like GRB 130427A to let us know if we really are on the right track.
Image (top): These maps show the sky at energies above 100 MeV as seen by Fermi’s LAT instrument. Left: The sky during a 3-hour interval before GRB 130427A. Right: A 3-hour map ending 30 minutes after the burst. GRB 130427A was located in the constellation Leo, near its border with Ursa Major. Credit: NASA/DOE/Fermi LAT Collaboration