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This illustration of material swirling around a black hole highlights a particular feature, called the “corona,” that shines brightly in X-ray light. In this depiction, the corona can be seen as a purple haze floating above the underlying accretion disk, and extending slightly inside of its inner edge. The material within the inner accretion disk is incredibly hot and would glow with a blinding blue-white light, but here has been reduced in brightness to make the corona stand out with better contrast. Its purple color is purely illustrative, standing in for the X-ray glow that would not be obvious in visible light. The warp in the disk is a realistic representation of how the black hole’s immense gravity acts like an optical lens, distorting our view of the flat disk that encircles it. Credit: NASA\/Caltech-IPAC\/Robert Hurt<\/p>\n
An international team of astronomers using NASA’s IXPE (Imaging X-ray Polarimetry Explorer<\/a>), has challenged our understanding of what happens to matter in the direct vicinity of a black hole.<\/p>\n With IXPE, astronomers can study incoming X-rays and measure the polarization<\/a>, a property of light that describes the direction of its electric field.<\/p>\n The polarization degree is a measurement of how aligned those vibrations are to each other. Scientists can use a black hole’s polarization degree to determine the location of the corona\u2014a region of extremely hot, magnetized plasma that surrounds a black hole\u2014and how it generates X-rays.<\/p>\n In April, astronomers used IXPE to measure a 9.1% polarization degree for black hole IGR J17091-3624, much higher than they expected based on theoretical models.<\/p>\n “The black hole IGR J17091-3624 is an extraordinary source that dims and brightens with the likeness of a heartbeat, and NASA’s IXPE allowed us to measure this unique source in a brand-new way,” said Melissa Ewing, based at Newcastle University in Newcastle upon Tyne, England, and the lead author of the study<\/a> published in the Monthly Notices of the Royal Astronomical Society.<\/p>\n In X-ray binary systems, an extremely dense object, like a black hole, pulls matter from a nearby source, most often a neighboring star. This matter can begin to swirl around, flattening into a rotating structure known as an accretion disk.<\/p>\n The corona, which lies in the inner region of this accretion disk, can reach extreme temperatures up to 1.8 billion degrees Fahrenheit and radiate very luminous X-rays. These ultra-hot coronas are responsible for some of the brightest X-ray sources in the sky.<\/p>\n Despite how bright the corona is in IGR J17091-3624, at some 28,000 light-years from Earth, it remains far too small and distant for astronomers to capture an image of it.<\/p>\n “Typically, a high polarization degree corresponds with a very edge-on view of the corona. The corona would have to be perfectly shaped and viewed at just the right angle to achieve such a measurement,” said Giorgio Matt, professor at the University of Roma Tre in Italy and a co-author on this paper. “The dimming pattern has yet to be explained by scientists and could hold the keys to understanding this category of black holes.”<\/p>\n The stellar companion of this black hole isn’t bright enough for astronomers to directly estimate the system’s viewing angle, but the unusual changes in brightness observed by IXPE suggest that the edge of the accretion disk was directly facing Earth.<\/p>\n The researchers explored different avenues to explain the high polarization degree.<\/p>\n In one model, astronomers included a “wind” of matter lifted from the accretion disk and launched away from the system, a rarely seen phenomenon. If X-rays from the corona were to meet this matter on their way to IXPE, Compton scattering would occur, leading to these measurements.<\/p>\n “These winds are one of the most critical missing pieces to understand the growth of all types of black holes<\/a>,” said Maxime Parra, who led the observation and works on this topic at Ehime University in Matsuyama, Japan. “Astronomers could expect future observations to yield even more surprising polarization degree measurements.”<\/p>\n