X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
We've just published the best quality IXPE data and most sophisticated phase-resolved analysis of an X-ray pulsar to date! With some luck with outburst timing we were able to catch this transient at two different luminosities, and, surprisingly, not only the pulse profiles (which is quite common for Be-transients) have changed, but also polarization properties turned out to be completely different in two observations!
Our first reaction was that something VERY strange is going on, i.e. crazy precession of the neutron star, or glitch, or something even more unexpected... However, careful analysis confirmed that phase of the pulsar does not change and there is apparently noting unusual going on with pulse profiles. On the other hand, the observed changes in polarization angle can be explained if there is additional polarized component in the observed emission. We attributed it to scattering in strong winds/outflows which have been known to be present in luminous XRPs since a while and is apparently present in first observation of RX J0440.9+4431. This allowed not only to describe what we see in IXPE data, but also to obtain very tight constrains on geometry of the pulsar AND binary system. The fact that in second observation the observed PA dependence matches predictions of rotating vector model so perfectly also strongly suggests that QED effects suggested to define polarization in all magnetic rotators observed to date (pulsars, magnetars) are, in fact, real!