Polarized emission from strongly magnetized sources

Neutron Star Astrophysics at the Crossroads: Magnetars and the Multimessenger Revolution Proceedings IAU Symposium No. 363, 2023 E. Troja & M. G. Baring, eds. doi:10.1017/S174392132200028X Polarized emission from strongly magnetized sources Roberto Taverna1 , Roberto Turolla1,2, Silvia Zane2, Valery Suleimanov3,4,5 and Alexander Y. Potekhin6 1Department of Physics and Astronomy, University of Padova, via Marzolo 8, Padova, Italy email: taverna@pd.infn.it 2Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey, RH5 6NT, UK 3Institut f ̈ur Astronomie und Astrophysik, Sand 1, D-72076 T ̈ubingen, Germany 4Kazan (Volga region) Federal University, Kremlevskaja str., 18, Kazan 420008, Russia 5Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia 6Ioffe Institute, Politekhnicheskaya 26, 194021, Saint Petersburg, Russia Abstract. Anomalous X-ray pulsars (AXPs) and Soft gamma repeaters (SGRs) form together a single class of astrophysical sources, commonly associated to magnetars. New-generation X-ray polarimeters will play a key role in assessing the nature of these sources by directly probing the star magnetic field. In the highly magnetized environment radiation is expected to be strongly polarized and such a measure will be easily within reach of IXPE and eXTP. Polarization measurements will eventually confirm the presence of ultra-strong magnetic fields, probing the magnetar scenario. In this work we will discuss theoretical expectations for the polarization signature of AXPs and SGRs and present numerical simulations for the detector response of the polarimeters currently under construction. We will also show how these sources can be used to test vacuum birefringence, a QED effect predicted by Heisemberg and Euler in the Thirties and not experimentally verified as yet