Scientists observe hot plasma surrounding black holes to better understand the cosmic giants

The new Cygnus X-1 measurements, published today in a new paper in the journal Science, are the first observations of a mass-accreting black hole captured by the Imaging X-Ray Polarimetry Explorer (IXPE) mission. The IXPE mission is run as part of a collaboration between NASA and the Italian Space Agency (ASI).

"Previous X-ray observations of black holes only measured the arrival direction, arrival time, and energy of the X-rays from hot plasma spiraling toward the black holes,” explained lead author Henric Krawczynski, from the Washington University in St. Louis. "IXPE also measures their linear polarization, which carries information about how the X-rays were emitted — and if, and where, they scatter off material close to the black hole."

Though no light can escape from the event horizon of a black hole, IXPE is able to detect X-rays emitted by the hot plasma, or matter, roughly 1,240 miles (2,000 km) away from the event horizon. The researchers combined this data with observations made at the same time by NASA's NICER and NuSTAR X-ray observatories. This allowed them to carefully analyze the geometry of the plasma.

"We've found a lot of surprises"

The scientists found that their plasma extends outwards perpendicular to a long plasma outflow, or jet, imaged in previous radio observations. In their statement, they explain that "the alignment of the direction of the X-ray polarization and the jet lends strong support to the hypothesis that the processes in the X-ray bright region close to the black hole play a crucial role in launching the jet."

Their observations corroborate specific models that predict the behavior of hot plasma surrounding black holes, while ruling out one that predicts the black hole's corona is a narrow plasma column along the jet axis. The scientists believe that, though their observations are related to the nearby regions outside of the black hole's event horizon, they can help to better understand the mysterious behavior of a black hole and how it accretes mass.

“The IXPE mission uses X-ray mirrors fabricated at NASA’s Marshall Space Flight Center and focal plane instrumentation provided by a collaboration of ASI, the National Institute for Astrophysics (INAF) and the National Institute for Nuclear Physics,” said co-author Fabio Muleri of INAF-IAPS. “Beyond Cygnus X-1, IXPE is being used to study a wide range of extreme X-ray sources, including mass accreting neutron stars, pulsars and pulsar wind nebulae, supernova remnants, our galactic center and active galactic nuclei. We’ve found a lot of surprises, and we’re having a lot of fun.”