The joint NASA-Italian Space Agency X-ray Polarity Explorer (IXPE) peered deep into the hot gas surrounding a black hole, in observations that help us learn how black holes swallow and spit out matter.
IXPE launched in December 2021 to study some of the most energetic objects in the universe, including accreting black holes. neutron stars and pulsar. It does this by observing the polarization of the X-rays emitted by these extreme objects. Polarization is the principle on which sunglasses work — they block all light except that which oscillates in a certain direction. Likewise, the polarized X-rays that IPXE detects are electromagnetic waves that vibrate primarily in a particular direction.
The polarization “conveys information about how the X-rays are emitted,” said lead researcher Henric Krawcynski of Washington University in St. Louis. statement (opens in new tab). Regarding black holes, polarization also tells us “if, and where, [the X-rays] scatter material near the black hole,” Krawcynski added.
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IXPE observed Cygnus X-1, which is an X-ray binary consisting of a 21-solar-mass black hole and a 41-solar-mass companion star 7,200 light years away in the Cygnus Cygnus constellation. The black hole’s gravity pulls matter away from its stellar companion, and this matter forms a stream of gas that spirals around the black hole and forms an “accretion disk.” Friction within the gas raises the temperature to millions of degrees, hot enough to emit X-rays. However, with frictional, magnetic and gravitational forces all at play within the disk, it has never been entirely clear to astronomers how some of this of matter then falls into the event horizon and into the black hole bay, and how some of the matter is funneled into dipole outflows escaping the black hole.
the IXPE observations, combined with secondary X-ray observations from NASA’s NuSTAR mission and the MORE BEAUTIFUL experiment on board International Space Stationshed light on the shape and location of the X-ray-emitting material around the black hole in Cygnus X-1.
They find that X-rays are scattered by material in a 2,000 km wide coronal region around the black hole. A black hole’s corona is formed by extremely hot plasma and is suspected to be involved in producing jets of charged particles seen by radio telescopes running away from black holes like Cygnus X-1. The X-ray polarization measured with IPXE suggests that Cygnus X-1’s corona extends away from the black hole parallel to the plane of the accretion disk and perpendicular to the jets. Therefore, the corona either squeezes matter into a spiral, or actually forms the inner part of the accretion disk.
Furthermore, the corona and inner accretion disk appear to be misaligned with respect to the orbital plane of the companion star around the black hole and the orientation of the outer accretion disk. This misalignment could have been caused as a result of the supernova that produced the black hole causing the black hole to spin at an angle to the system. This sharp rotation, and the gravity exerted by the black hole, could then have introduced torques into the inner disk, twisting and distorting it.
“These new insights will enable improved X-ray studies of how gravity curves space and time near black holes,” Krawczynski said.
The findings are published (opens in new tab) in the November 3 issue of the journal Science.