First Detection of Polarization in X-Rays for PSR B0540-69 and Its Nebula

We report on X-ray polarization measurements of the extragalactic Crab-like PSR B0540-69 and its Pulsar Wind Nebula (PWN) in the Large Magellanic Cloud, using a similar to 850 ks Imaging X-ray Polarimetry Explorer (IXPE) exposure. The PWN is unresolved by IXPE. No statistically significant polarization is detected for the image-averaged data, giving a 99% confidence polarization upper limit (MDP99) of 5.3% in the 2-8 keV energy range. However, a phase-resolved analysis detects polarization for both the nebula and pulsar in the 4-6 keV energy range. For the PWN defined as the off-pulse phases, the polarization degree (PD) of (24.5 +/- 5.3)% and polarization angle (PA) of (78.1 +/- 6.2)degrees is detected at 4.6 sigma significance level, consistent with the PA observed in the optical band. In a single on-pulse window, a hint of polarization is measured at 3.8 sigma with PD of (50.0 +/- 13.1)% and PA of (6.2 +/- 7.4)degrees. A "simultaneous" PSR/PWN analysis finds two bins at the edges of the pulse exceeding 3 sigma PD significance, with PD of (68 +/- 20)% and (62 +/- 20)%; intervening bins at 2-3 sigma significance have lower PD, hinting at additional polarization structure.