Observations of Pulsars and Magnetars using the Imaging X-ray Polarimetry Explorer (IXPE)

We summarize many observations of highly magnetized neutron stars that have been obtained with the Imaging X-ray Polarimetry Explorer (IXPE). Observations of the magnetars 4U0142+14 and RX J1708-40 yielded somewhat different results, as the polarization angle of X-rays below 4-5 keV differed by 90° relative to those above 5 keV for 4U 0142+61 but did not differ for the other magnetar, indicating somewhat different states of the surface emission and system geometry. In both cases, the polarization fractions are over 30% for a wide energy range. Pulsars in X-ray binaries that have been observed include Her X-1, X Persei, Vela X-1, Cen X-3, and 4U1626-67, with a variety of results. In several cases, the polarization fraction reaches 20-25% and there is sufficient signal to determine the orientation of the neutron star’s rotation axis as well as the angle between the magnetic dipole and the spin axis. With several observations of Her X-1, there is strong evidence that the 35 day intensity cycle is set by free precession and an indication that the crust and core are loosely coupled.

IXPE is a joint US and Italian mission. The US contribution is supported by NASA and led and managed by its Marshall Space Flight Center with industry partner Ball Aerospace. The Italian contribution is supported by the Italian Space Agency and its Space Science Data Center and by the Istituto Nazionale di Astrofisica and the Istituto Nazionale di Fisica Nucleare in Italy.