However, until now, measurements of the X-ray emission spectrum have been limited to low energies and there were two explanations for the broadening (red-shifting) of the iron emission spectrum.
One theory was that the X-rays were being red-shifted by the extreme relativistic environment near the event horizon of a spinning black hole. The other theory was that the X-rays were being obscured by gas blocking our view of the central black hole, adding complexity to the detected X-ray signal. Through lack of convincing evidence supporting either model, an astrophysical controversy erupted.
NuSTAR, which detects more energetic X-ray emissions, has now definitively ended this controversy. The orbiting X-ray observatory has detected previously undetectable high-energy X-rays and provided conclusive evidence that NGC 1365’s black hole is spinning -- the line broadening is not therefore caused by absorption by intervening clouds of gas.
“It was my expectation, and the main scientific rationale for the project. Of course many colleagues would rather expect absorption as the right explanation ... but the whole project has been conceived to solve this puzzle,” said Risaliti.
“The interesting thing, especially in the system we looked at is that we know there’s partial absorbing clouds -- we see them going in front of the (galactic) nucleus causing time-variable absorption ... it’s not unreasonable to suppose that could be distorting the spectrum in a way that gives you broad lines,” said Harrison. “But when you add the NuSTAR data that can just be ruled out. Yes, there is absorption, but it’s not explaining the iron line.
“What they tell us is that the black hole HAS to be spinning. Now there’s a maximum rate a black hole can spin given by general relativity and that is telling us that this black hole is spinning close to that rate.”
According to Risaliti and Harrison’s team’s research, NGC 1365’s black hole is spinning at a breakneck rate 84 percent of its theoretical maximum.
So, although previous X-ray observatories have detected this iron line broadening for low-energy X-rays, NuSTAR's data of high-energy X-rays conclusively shows that the broadening is caused by relativistic effects, thereby proving it's a measure of the black hole’s spin. Why is this important?
“Well, first off it’s just cool that we’re seeing the effects of general relativity in the ‘strong field’ regime. Most tests of general relativity are done in the ‘weak field,’” said Harrison, referring to the fact that most tests of general relativity are done in “weak” gravitational fields like Earth’s. NuSTAR is probing the edge of the most extreme gravitational field possible.