Ironing out an X-ray wrinkle

A version of this post, as written by me, originally appeared in The Copernican science blog on March 1, 2013.

One of the techniques to look for and measure the properties of a black hole is to spot X-rays of specific energies coming from a seemingly localized source. The radiation emanates from heated objects like gas molecules and dust particles in the accretion disc around the black hole that have been compressed and heated to very high temperatures as the black hole prepares to gorge on them.

However, the X-rays are often obscured by gas clouds surrounding the black hole, even at farther distances, and then other objects on the path of its long journey to Earth. And this is a planet whose closest black hole is 246 quadrillion km away. This is why the better telescopes that study X-rays are often in orbit around Earth instead of on the ground to minimize further distortions due to our atmosphere.


One of the most powerful such X-ray telescopes is NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array), and on February 28, the government body released data from the orbiting eye almost a year after it was launched in June 2012. NuSTAR studies higher-energy the sources and properties of higher-energy X-rays in space. In this task, it is also complemented by the ESA’s XMM-Newton space-telescope, which studies lower-energy X-rays.

The latest data concerns the black hole at the centre of the galaxy NGC 1365, which is two million times the mass of our Sun, earning it the title of Supermassive Black Hole (SMBH). Around this black hole is an accretion disc, a swirling vortex of gases, metals, molecules, basically anything unfortunate enough to have come close and subsequently been ripped apart. Out of this, NuSTAR and XMM-Newton zeroed in on the X-ray emissions characteristic of iron.

What the data revealed, as has been the wont of technology these days, is a surprise.

What are signatures for?

Emission signatures are used because we know everything about them and we know what makes each one unique. For instance, knowing the rise and dip of X-ray brightness coming from an object at different temperatures allows us to tell whether the source is iron or something else.

By extension, knowing that the source is iron lets us attribute the signature’s distortions to different causes.

And the NuSTAR data has provided the first experimental proof that iron’s signature’s distortion is not due to gas-obscuration, but due to another model called prograde rotation which attributes the distortion to the black hole’s gravitational pull.

A clearer picture

As scientists undertake a detailed analysis of the NASA data, they will also resolve iron’s signature. This means the plot of its emissions at different temperatures and times will be split up into different “colours” (i.e., frequencies) to see how much each colour has been distorted.

With NuSTAR in the picture, this analysis will assume its most precise avatar yet because the telescope’s ability to track higher-energy X-rays lets it penetrate well enough into the gas clouds around black holes, something that optical telescopes like the one at the Keck Observatory can’t. What’s more, the data will also be complete at the higher-energy levels, earlier left blank because XMM-Newton or NASA’s Chandra couldn’t see in that part of the spectrum.

If the prograde rotation model is conclusively proven after continued analysis and more measurements, then for the first time, scientists will have a precision-measurement tool on their hands to calculated black hole spin.

How? If the X-ray distortions are due to the black hole’s gravitational pull and nothing else, then the rate of its spin should be showing itself as the amount of distortion in the emission signature. The finer details for this can be picked out from the resolved data, and a black hole’s exact spin for the first time be pinned down.

The singularity

NGC 1365 is a spiral galaxy about 60 million light-years in the direction of the constellation Fornax and a prominent member of the much-studied Fornax galaxy cluster. Apart from the black hole, the galaxy hosts other interesting features such as a central bar of gas and dust adjacent to prominent star-forming regions and a type-1a supernova discovered as recently as October 27, 2012.

As we sit here, I can’t help but imagine us peering into our tiny telescopes, picking up on feebly small bits of information, and adding an extra line in our textbooks, but in reality being thrust into an entirely new realm of knowledge, understanding, and awareness.

Now, we know there’s a black hole out there – a veritable freak of nature – spinning as fast as the general theory of relativity will allow!