An international team of astronomers has reported new findings about the behavior of matter near a black hole, using NASA’s Imaging X-ray Polarimetry Explorer (IXPE). The study focuses on the black hole candidate IGR J17091-3624, located about 28,000 light-years from Earth, and challenges existing ideas about how black holes generate X-rays.
IXPE allows scientists to measure the polarization of incoming X-rays. Polarization describes the direction of light’s electric field, and its degree indicates how aligned those vibrations are. By studying polarization, astronomers can infer the structure and behavior of the corona, a region of extremely hot, magnetized plasma surrounding a black hole.
A related study has revealed that the powerful jets of wind blasting out of a black hole only occur after it has built up a hot outer layer of plasma, known as its corona. Researchers observed the behavior of a black hole in our galaxy and developed a cosmic echocardiogram graph to display its “heartbeat,” a pattern of dimming and brightening similar to that seen in IGR J17091-3624.
“The black hole IGR J17091-3624 is an extraordinary source which 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, lead author of the study from Newcastle University in England.
In X-ray binary systems, a black hole draws matter from a nearby star. This material forms a rotating accretion disc, which feeds the black hole. The corona, located in the inner disc region, can reach temperatures up to 1.8 billion degrees Fahrenheit and emit intense X-rays. Despite its brightness, the corona remains too small and distant to be imaged directly.
Earlier study has revealed that the X-ray radiation collected from the corona contains more energy than can be explained by temperature alone. Researchers suspect that a magnetic field provides this extra energy. A chaotic magnetic field could heat the corona, while a more ordered field might allow material to escape along field lines, forming jets. This mechanism may also apply to heavier black holes, including the supermassive black hole at the center of the Milky Way.
“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,” explained Giorgio Matt, professor at the University of Roma Tre in Italy and co-author of the paper. He added, “The dimming pattern has yet to be explained by scientists and could hold the keys to understanding this category of black holes.”
IGR J17091-3624 itself exhibits a wide range of complex light curve patterns, including strong flares alternating with quiet intervals. These resemble the variability classes first defined for the unique black hole system GRS 1915+105. In comparison, IGR J17091-3624 shows the ν, ρ, α, λ, β, and μ classes, along with quiet periods resembling the χ class, but at count rate levels 10–50 times lower than those observed in GRS 1915+105. The so-called ρ class “heartbeats” occur as quickly as every few seconds or as slowly as about 100 seconds, tracing a loop in the hardness-intensity diagram. While GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the opposite direction.
To explain the unusually high polarization degree, scientists tested different models. One proposed that a “wind” of matter is lifted from the accretion disc and expelled from the system. If X-rays from the corona scatter off this outflowing material, the polarization measurements could be explained. “These winds are one of the most critical missing pieces to understand the growth of all types of black holes,” said Maxime Parra of Ehime University in Japan.
Another model considered plasma in the corona streaming outward at speeds up to 20 percent of the speed of light, or about 124 million miles per hour. Such relativistic motion could enhance the observed polarization.
Both scenarios successfully reproduced the measurements without requiring a precise edge-on view. The team plans to continue simulations and further observations to refine their understanding of polarization in black hole systems
Source: NASA, Astronomie
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