It's fun how, even as a pretty senior person, you end up in research projects with PhD candidates or postdocs who assume you an expert while you panicky try to read up on what you are working on and at least understand the basics enough to be able to follow and usefully contribute.
In other news: X-ray polarization is weird.
@vicgrinberg I feel you!
Especially the scattering at anything resembleing a crystal.
Still not black holes in labs?
What is experimental physics up to these days?
You get "polarisation flips" because of elastic scattering with quasi particles?
@littledetritus I don't know the context of the quasi particles, so can't say, but we do get vacuum birefringence in high magnetic fields: https://arxiv.org/abs/2509.19446
(careful, not yet through peer review, but this is usually a super careful bunch of authors.)
The quantum electrodynamics (QED) theory predicts that the quantum vacuum becomes birefringent in the presence of ultra-strong magnetic fields -- a fundamental effect yet to be directly observed. Magnetars, isolated neutron stars with surface fields exceeding $10^{14}$ G, provide unique astrophysical laboratories to probe this elusive prediction. Here, we report phase- and energy-resolved X-ray polarization measurements of the radio-emitting magnetar 1E 1547.0--5408 obtained with the Imaging X-ray Polarimetry Explorer (IXPE), in coordination with the Neutron Star Interior Composition Explorer (NICER) and Parkes/Murriyang radio observations. We detect a high phase-averaged polarization degree of 65% at 2 keV, where the surface thermal emission is dominant, rising to nearly 80% at certain rotational phases, and remaining at $\gtrsim40\%$ throughout the radio beam crossing. We also observe a strong decrease in polarization from 2 keV to 4 keV. Detailed atmospheric radiative transfer modeling, coupled with geometrical constraints from radio polarization, demonstrate that the observed polarization behavior cannot be consistently explained without invoking magnetospheric vacuum birefringence (VB) influences. These observational findings combined with theoretical simulations provide evidence for quantum VB naturally occurring in magnetar magnetospheres. This work marks a significant advance toward confirming this hallmark prediction of QED and lays the foundation for future tests of strong-field quantum physics using next-generation X-ray polarimeters.
Dr. Victoria Grinberg
Little Detritus