Polarized X-ray Radiation from Magnetars and X-ray Pulsars

With the launch of IXPE at the end of 2021, we have entered the era of X-ray polarization. IXPE is more than one thousand times more sensitive that previous observations giving us “first (polarized) light” images of hundreds of X-ray sources. Looking as these objects in an essentially new way for the very first time has been exhilarating. I will focus in particular on the observations of magnetar and accreting X-ray pulsars.

Magnetars harbour the strongest magnetic fields in the Universe. IXPE observations of magnetars have probed the properties of the surface and magnetospheres of these objects in unprecedented detail. In particular IXPE observations indicate that the surface layers of this objects may be solid or liquid (not gaseous even though their temperatures exceed one million Kelvin). Observations also provide an independent verification of magnetic fields near the surface of these objects exceeding ten gigatesla and the role of resonant cyclotron scattering in the processing of their emission.

Accreting pulsars which despite complicated magnetic field and emission geometries with only a single exception exhibit very simple changes in the polarization direction as the stars rotate. This straightforward evolution with spin results from the first (yet still unverified) prediction of QED that a magnetic field even in vacuum induces an index of refraction: vacuum birefringence. The observations of the prototypical X-ray pulsar Hercules X-1 reveal the interior of the neutron star, while observations of the supercritical accretor LS V +44 17/RX J0440.9+4431 show complexity of the emitting accretion flow can produce more complicated polarization patterns.