Angling for x-ray pulsar geometry with polarimetry

Kavli Affiliate: Herman L. Marshall

| First 5 Authors: Victor Doroshenko, Juri Poutanen, Sergey Tsygankov, Valery Suleimanov, Matteo Bachetti

| Summary:

Using observations of X-ray pulsar Her X-1 by the Imaging X-ray Polarimetry
Explorer, we report on a highly significant detection of the polarization
signal from an accreting neutron star. The observed degree of the polarization
of $sim10$% is found to be far below theoretical expectations for this object,
and stays low throughout the spin cycle of the pulsar. Both the polarization
degree and the angle exhibit variability with pulse phase, which allowed us to
measure the pulsar spin position angle and magnetic obliquity of the neutron
star, which is an essential step towards detailed modeling of intrinsic
emission of X-ray pulsars. Combining our results with the optical polarimetric
data, we find that the spin axis of the neutron star and the angular momentum
of the binary orbit are misaligned by at least $sim$20 deg, which is a strong
argument in support of the models explaining stability of the observed
super-orbital variability with the precession of the neutron star.

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