NASA X-Ray Observatories Reveal Black Hole Spin
February 27, 2013

NASA X-Ray Observatories Help Solve Riddle Of Black Hole Spin

John P. Millis, PhD for — Your Universe Online

One of the challenges of studying black holes — incredibly dense stellar remnants arising from massive supernova explosions — is that it is extremely difficult to measure the spin of such objects. And it is this motion that is of particular interest, since Einstein´s theory of General Relativity predicts that the gravitational waves produced from their rotation can distort the very fabric of space-time around these massive objects.

The rotational dynamics are even more complicated and dramatic in systems driven by supermassive black holes — objects millions or billions of times more massive than our Sun. As these objects are only found in the cores of galaxies, only the closest of these systems can be studied with any reliability.

And even the most ideal candidates present problems. Because black holes are such unique systems which emit no thermal radiation themselves, researchers must rely on the interactions between these massive compact objects and their surroundings to learn even the most basic information about them. More subtle interactions, like those linked to black hole rotation, are even more challenging to study.

But scientists affiliated with NASA´s NuSTAR and XMM-Newton X-ray observatories have devised a clever way to directly measure the rotation of nearby supermassive black holes. The powerful jets from the supermassive black holes, driven by their intense magnetic fields as they twist the matter from the rotating accretion disk, accelerate electrons from near the black hole´s edge and produce broad spectrum X-rays.

Most of the low energy X-rays (aka soft X-rays) will be absorbed by the disk, while much of the high energy X-rays (aka hard X-rays) will be reflected. However, some of the atoms — iron in particular —absorb and re-emit the radiation, creating a peak in the X-ray spectrum.

If the black role is rotating, then the iron peak will appear spread out, correlating with its rotational speed and direction. Thus by measuring the soft X-ray emission, the angular motion of the black hole can be calculated.

There is one problem, however. If the black hole is partially obscured by the presence of gas or dust, a similar line broadening effect can be observed. In order to eliminate this possibility, a secondary indicator must also be present.

Luckily, this so-called “obscuration” will have a dramatic effect on the hard X-rays emission. But previous experiments, such as NASA´s Swift and ESA´s INTEGRAL, had such poor angular resolution that the supermassive black holes could not be individually studied.

With the NuSTAR Observatory, launched in 2012, an unprecedented angular resolution has now been achieved. This instrument is able target the region around a supermassive black hole. And combined with the XMM-Newton observatory, the two instruments are able to cover the entire X-ray spectrum.

The duo observed the nearby galaxy NGC 1365 and was able to isolate the broadband X-ray emission from around the 2 million solar mass black hole. Adding the spectral data together, researchers were able to determine that the broadening of the iron peak was due to a gravitational distortion caused by the rotation of the supermassive black hole, while the gas obscuration model was unable to fit the data with the same statistical significance.

According to Guido Risaliti, an astronomer with the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, the result is still a bit puzzling. The derived rotation speed of the black hole is about 84 percent of its maximum for its mass. Researchers believe that the black hole was likely created with a lower rotational energy, but processes over time have accelerated its spin rate.

Dr. Arvind Parmar, Head of Astrophysics and Fundamental Physics Missions Division, European Space Agency, noted that, as is often the case, “when one question is answered, a series of new ones arise.” So the next step will be to accumulate more data to ascertain the cause of this “spin-up.”

There are currently two models that are thought to be the most likely candidates: A smaller black hole may have been initially created and, over time, accumulated mass through accretion which led to the increase in its angular momentum. Alternatively, galaxy mergers, which are thought to be very common in the Universe, may have led to smaller supermassive black holes merging, resulting in faster rotation of the combined object.

To find the solution, researchers plan to study more galaxies in order to catalog the various spin rates and look for evidence supporting one model over the other. Fiona Harrison, NuSTAR principal investigator and professor of physics and astronomy at the California Institute of Technology, noted that researchers will study about a dozen more galaxies using XMM-Newton and NuSTAR.

But unfortunately, the ideal situation would be to get a broader temporal study of these systems, which would require taking similar observations of very distant galaxies and their supermassive black holes. Such studies, however, are currently outside the abilities of our instrumentation. It may be decades before technology allows us to accurately measure the X-rays with sufficient resolution and sensitivity to study such distant systems.

In either case, this initial offering from XMM-Netwon and NuSTAR is a significant confirmation of Einstein´s theory of general relativity and shows that the gravitational distortion of space-time, caused by the rapid rotation of these supermassive black holes, can in fact be measured.