Polarized X-rays in sources such as stellar mass black holes, neutron stars, and blazars can arise due to several mechanisms.

In the case of black holes, polarized X-rays can be generated from the hot accretion disk that forms around them. This disk consists of matter falling into the black hole, which becomes heated and emits X-rays. The X-rays can undergo scattering off electrons present in the accretion disk and in the surrounding medium. This scattering process can polarize the X-rays. There are different types of scattering interactions that contribute to the polarized X-ray emission: coherent scattering, Thomson scattering, and Compton scattering. All scattering effects polarize the X-rays so that the preferred electric field direction is perpendicular to the scattering plane (the plane spanned by the incoming and outgoing photon directions). The propagation of the photons in the strong gravitational field near the black hole can modify the polarization direction.

An artist’s impression of the Cygnus X-1 system, with the black hole appearing in the center and its companion star on the left. New measurements from Cygnus X-1, reported Nov. 3 in the journal Science, represent the first observations of a mass-accreting black hole from the Imaging X-Ray Polarimetry Explorer (IXPE) mission, an international collaboration between NASA and the Italian Space Agency. (Image: John Paice)

Neutron stars can produce polarized X-rays through various mechanisms. The emitted X-rays are polarized giving clues about the state of matter in the neutron star photosphere and atmosphere. The polarization can subsequently change owing to cyclotron resonance scattering, where charged particles in the intense magnetic field of the neutron star emit or absorb X-rays with specific energy levels, resulting in polarization. Quantum electrodynamical effects such as vacuum birefringence can further impact the polarization degrees and directions.

The Crab Nebula. By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=50096996
By Emma Alexander – [1], CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=116390507

Blazars are active galactic nuclei that contain supermassive black holes at their centers, which are surrounded by accretion disks and powerful relativistic jets. These jets can emit highly polarized X-rays due to synchrotron radiation, where relativistic electrons spiral around magnetic field lines and emit polarized X-rays in the process, and inverse Compton scattered synchrotron radiation. Extremely high energy protons can initiate particle cascades which also emit highly polarized X-rays as synchrotron emission.

The generation of polarized X-rays in sources like stellar mass black holes, neutron stars, and blazars is intimately linked to the presence of strong magnetic fields and extreme physical conditions in these objects, and depends on the geometry of the emitting and reflecting matter. 

Measuring the polarization properties of X-rays from these sources can provide valuable insights into the nature of the emission processes and the environments in which the compact objects reside.