Polarized X-rays indicate the shape and orientation of highly hot materials in the vicinity of a black hole

The Imaging X-Ray Polarimetry Explorer (IXPE) mission's first images of a mass-accreting black hole offer fresh insights about the configuration of extremely hot matter in the region immediately surrounding it. Researchers are using X-ray polarisation data to test and enhance models that describe how black holes devour stuff and become some of the most luminous sources of light in the universe, including X-rays.

As matter is drawn toward a black hole, it gets heated to millions of degrees. In X-rays, this heated stuff glows. Measurements of the polarisation of these X-rays are being used by researchers to test and develop models that describe how black holes swallow stuff, producing some of the most luminous sources of light — including X-rays — in the cosmos.

The latest Cygnus X-1 results, published online by the journal Science on Thursday, Nov. 3, are the first observations of a mass-accreting black hole from NASA's Imaging X-Ray Polarimetry Explorer (IXPE) project, a joint venture between NASA and the Italian Space Agency (ASI). Cygnus X-1 is one of our galaxy's brightest X-ray sources, composed of a 21 solar mass black hole in orbit with a 41 solar mass companion star.

"Previous X-ray observations of black holes only measured the arrival direction, arrival time, and energy of the X-rays from hot plasma spiralling toward the black holes," said lead author Henric Krawczynski, the Wayman Crow Professor of Physics in Arts & Sciences at Washington University in St. Louis and a faculty fellow in the McDonnell Center for the Space Sciences at the university. "IXPE also detects their linear polarisation, which tells us how the X-rays were created — and whether and where they scatter off material close to the black hole."

No light, not even X-ray light, can escape from inside a black hole's event horizon. The X-rays identified by IXPE are generated by heated matter, or plasma, in a 2,000-km-diameter zone encircling the black hole's 60-km-diameter event horizon.

The authors were able to restrict the geometry of the plasma by combining IXPE data with concurrent observations from NASA's NICER and NuSTAR X-ray observatories in May and June 2022.

The plasma stretches perpendicular to a two-sided, pencil-shaped plasma outflow, or jet, observed in previous radio measurements, according to the researchers. The alignment of the X-ray polarisation direction and the jet provides substantial support to the theory that activities in the X-ray bright region around the black hole play an important role in launching the jet.

The data are consistent with models that predict that the corona of hot plasma either sandwiches or replaces the disc of matter spiralling into the black hole. Models in which the black hole's corona is a small plasma column or cone along the jet axis are ruled out by the new polarisation data.

The scientists emphasised that a better understanding of the geometry of the plasma surrounding a black hole can disclose a lot about how black holes behave and how they gain mass.

"These new insights will allow for better X-ray examinations of how gravity twists space and time around black holes," Krawczynski added.

In terms of the Cygnus X-1 black hole, "IXPE measurements suggest that the accretion flow is viewed more edge-on than previously assumed," noted co-author Michal Dovciak of the Czech Academy of Sciences.

"This could be a symptom of a misalignment of the black hole's equatorial plane and the binary's orbital plane," or the matched duo of the black hole and its companion star, explained co-author Alexandra Veledina of the University of Turku. "When the black hole progenitor star erupted, the system may have acquired that misalignment."

"The IXPE mission employs X-ray mirrors manufactured at NASA's Marshall Space Flight Center, as well as focal plane instrumentation provided by a collaboration of ASI, the National Institute for Astrophysics (INAF), and the National Institute for Nuclear Physics," said INAF-IAPS co-author Fabio Muleri. "IXPE is being utilised to examine a wide spectrum of extreme X-ray sources, including mass accreting neutron stars, pulsars and pulsar wind nebulae, supernova remnants, our galactic centre, and active galactic nuclei, in addition to Cygnus X-1." We've discovered a lot of surprises and are having a great time."

(source : ANI)