This image shows the black hole, accretion disk (orange), and corona (blue). As light from the corona hits the accretion disk, it is absorbed and later emitted as echoes that allow astronomers to map where in the disk the light came from.



NASA's Goddard Space Flight Center

Closing in

The bigger picture

NICER “has this incredibly fast, high time resolution,” Kara said, which means it takes separate measurements quickly so that astronomers can watch what’s happening in greater detail. That greater detail meant “we were able to measure shorter light echoes than had ever been seen before in a stellar-mass black hole system. And that means we’re probing closer to the black hole with these light echoes than ever before possible,” Kara said.But something else was happening. “As we watched the system over several weeks, we saw that the light echoes got closer and closer together. That was indicating that something in the system was getting smaller – so either the accretion disk coming inwards, or the corona shrinking,” Kara said.How could they differentiate one from the other? Light echoes from parts of the accretion disk closer to the black hole are redder, because the light is stretched out as it tries to escape the black hole’s immense gravity. This is called gravitational redshifting. Looking at the maximum stretching that occurred told the team how close the accretion disk got to the black hole, as well as whether that distance changed over time.“We noticed that the most gravitationally redshifted emission – the emission coming from the innermost regions – the amount of that emission did not change at all. So that suggested to us that the disk itself is close to the central black hole and does not evolve over time. But because the light echoes got shorter and shorter, closer and closer together, that must mean that the corona was shrinking,” she said.Her team estimated that over several weeks, “the corona shrinks from something like 100 miles [160 kilometers] to only 10 [16 km],” Kara said.“This is the first time that we've seen this kind of evidence that it's the corona shrinking during this particular phase of outburst evolution,” study co-author Jack Steiner at the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research said in a press release . “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that's evolving in the system is the structure of the corona itself.”This first-of-its kind observation will not only help astronomers better understand the workings of stellar-mass black holes, which are tens of times the mass of the Sun, but can also be applied to supermassive black holes, which are millions or billions of times the mass of the Sun.“In stellar-mass black holes [these outbursts] evolve over timescales of several weeks to months. So we can watch this evolution happening in real time and we find now that it’s the corona that’s driving that evolution,” Kara said. But in supermassive black holes, she added, such outbursts occur over billions of years. By scaling up what’s going on around smaller black holes over shorter time periods, she explained, astronomers can learn more about how material falls in and causes changes in the region around supermassive black holes on much larger scales. That will, in turn, allow astronomers to better understand how these behemoths affect their local environment, which in turn affects the evolution of the galaxy around them.“These stellar-mass black holes are a great analogue for studying the evolution of the corona and the accretion geometry in real time,” she said.