First detection of polarization in X-rays for PSR B0540-69 and its nebula

Kavli Affiliate: Herman L. Marshall

| First 5 Authors: Fei Xie, Josephine Wong, Fabio La Monaca, Roger W. Romani, Jeremy Heyl

| Summary:

We report on X-ray polarization measurements of the extra-galactic Crab-like
PSR B0540-69 and its Pulsar Wind Nebula (PWN) in the Large Magellanic Cloud
(LMC), using a ~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 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 ${pm}$ 5.3)% and polarization angle (PA) of (78.1 ${pm}$
6.2){deg} 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 polarization degree of (50.0
${pm}$ 13.1)% and polarization angle of (6.2 ${pm}$ 7.4){deg}. A
‘simultaneous’ PSR/PWN analysis finds two bins at the edges of the pulse
exceeding 3${sigma}$ PD significance, with PD of (68 ${pm}$ 20)% and (62
${pm}$ 20)%; intervening bins at 2-3${sigma}$ significance have lower PD,
hinting at additional polarization structure.

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