X-ray Polarization Reveals the Precessions of the Neutron Star in Hercules X-1
A follow-up of our first IXPE paper on Her
X-1, a beatiful
analysis mostly done by Jeremy Heyl, but
also with yours truly participation. With more IXPE data and more careful
analysis, we show that there's direct evidence that the 35-day-period is set by
the free precession of the neutron star crust, which has the important
implication that its crust is somewhat asymmetric fractionally by a few parts
per ten million. Furthermore, we find indications that the basic spin geometry
of the neutron star is altered by torques on timescale of a few hundred days.
