NASA-ASI X-ray observatory reveals how black holes swallow and spit out stars
The NASA–Italian Space Agency (ASI) Imaging X-ray Polarimetry Explorer (IXPE) has peered deep into the hot gas surrounding a black hole, teaching us how black holes swallow and spit out matter. IXPE launched in December 2021 to study some of the most highly energetic objects in the universe, including accreting black holes, neutron stars and pulsars. It does this by observing the polarisation of the X-rays emitted by these extreme objects. Polarisation is the principle by which sunglasses work – they block all light except that which oscillates in a specific direction. Similarly, the polarised X-rays that IPXE detects are electromagnetic waves vibrating mostly in a particular direction. The polarisation “carries information about how the X-rays were emitted,” said lead researcher Henric Krawczynski of Washington University in St Louis. In regards to black holes, the polarisation also tells us “if, and where, [the X-rays] scatter off material close to the black hole,” Krawczynski added
IXPE observed Cygnus X-1, which is an X-ray binary system consisting of a 21-solar-mass black hole and a 41-solarmass companion star 7,200 light years away in the constellation of Cygnus, the Swan. The black hole’s gravity is tearing matter from its stellar companion, and this matter is forming a stream of gas that spirals around the black hole and forms an accretion disc. 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 in play within the disc, it has never been entirely clear to astronomers how some of that matter then falls across the event horizon and into the black hole’s maw and how some of the matter is funnelled into bipolar outflows that escape the black hole.
IXPE’s observations, combined with secondary X-ray observations by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron star Interior Composition Explorer (NICER) on board the International Space Station, shed light on the shape and location of the material emitting the X-rays around the black hole. They found the X-rays are being scattered off material in a coronal region around the black hole.
A black hole’s corona is formed of ultrahot plasma and is suspected to be involved in the production of jets of charged particles that are seen by radio telescopes racing away from black holes. The polarisation of the X-rays measured by IPXE suggests that Cygnus X-1’s corona extends away from the black hole parallel to the plane of the accretion disc and perpendicular to the jets. Hence the corona is either sandwiching the in-spiralling matter, or actually forms the inner part of the accretion disc.
Furthermore, the corona and inner accretion disc seem to be misaligned relative to the orbital plane of the companion star around the black hole and the orientation of the outer accretion disc. 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 acute spin, and the gravity the black hole wields, could then have introduced torques in the inner disc, twisting and warping it. “These new insights will enable improved X-ray studies of how gravity curves space and time close to black holes,” said Krawczynski.