Black hole X-ray binaries are systems where a star and a black hole of a few solar masses orbit each other. Matter accreted from the star by the intense gravitational field of the black hole is heated to several hundred million degrees and radiates in X-rays. Some of this material is also ejected out of the system forming powerful jets that are usually observed in radio. Cygnus X-1 is one of the brightest black hole X binaries in our Galaxy. It consists of a 21 solar mass black hole orbiting a 41 solar mass companion star.
Recent observations of Cygnus X-1 by an international team of researchers, including CNRS-INSU, using the Imaging X-Ray Polarimetry Explorer (IXPE) satellite, reveal new details about the properties of this hot material and its link to ejection processes. The IXPE satellite, an international collaboration between NASA and the Italian Space Agency (ASI), makes it possible for the first time to measure the polarisation of X-rays, which provides information on how the X-rays were emitted and on the geometry of the emission regions.
By combining data obtained in May and June 2022 by IXPE with simultaneous observations from NASA’s NICER and NuSTAR observatories, the researchers deduced that the X-rays originated from a region ~2000 km in diameter surrounding the black hole, which is itself ~60 km in diameter. Moreover, the direction of the polarisation of the Cygnus X-1 X-ray is aligned with that of the radio jet already known in this source. This alignment shows very clearly, and for the first time, the link between the processes producing the hot plasma and the jet itself. It also allows to exclude models where the hot plasma is located along the axis of the jet and favours instead a geometry where this plasma is extended perpendicular to the jet.
Article : Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1, Science, 2022.
ESA press release : https://esahubble.org/images/cygx1_illust_orig/
Contacts :
- Frédéric Marin, Chercheur CNRS à l’Observatoire astronomique de Strasbourg (ObAS) frederic.marin@astro.unistra.fr
- Pierre-Olivier Petrucci, Chercheur CNRS à l’Institut de planétologie et d’astrophysique de Grenoble (IPAG) pierre-olivier.petrucci@univ-grenoble-alpes.fr