Evidence of magnetospheric vacuum birefringence in the polarized X-rays of a radio magnetar

Abstract

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.

Figures (13)

• Figure 1: Background-subtracted and phase-averaged polarization measurements of 1E 1547.0$-$5408. We display the PD in 4 energy bands, 2--3, 3--4, 4--8, and 2--8 keV. Contours show the $68.3\%$ ($1 \sigma$) confidence regions for Stokes Q/I and U/I (polarization degree and angle are shown in the radial and polar lines, respectively). The stars display the corresponding minimum detectable polarization at the 99% confidence level (MDP$_{99}$) for each energy bin. The 2--8 keV energy-integrated polarization characteristics are shown in dark blue. Notably, the energy-integrated PD (dark blue) is 46% and the 2--3 keV PD (magenta) reaches 59%. The PD decreases substantially in the energy range 3--4 keV. The PD remains high at 40% in the 4--8 keV band, yet it suffers from a considerably lower signal-to-noise ratio (S/N). The PA does not exhibit strong variation across energy.
• Figure 2: Phase-resolved radio and X-ray polarization characteristics of 1E 1547.0$-$5408. The uppermost row shows the radio polarization angle (two cycles shown for clarity). The second row displays the radio intensity curve (black) along with the total linear radio polarization (orange) as well as the circular radio polarization (blue). All of the shown radio data are averaged across our three Murriyang observations. The bottom three rows show the IXPE intensity, polarization degree, and polarization angle pulse profiles, respectively. The radio PA is also displayed in the bottom panel for ease of comparison to the X-ray PA. The IXPE pulse profiles are displayed according to two energy bands: 2--4 keV (left column) and 2--8 keV (right column). The shaded curves overlaying the polarization degree panels depict the MDP$_\text{99}$ at each corresponding phase bin. The vertical colored bands highlight the binning used for the IXPE phase-resolved polarization analysis.
• Figure 3: Simulated Stokes $Q/I$ (panel a), $U/I$ (panel b), and intensity (panel c) pulse profiles from MAGTHOMSCATT for a single hot spot wedge offset from the magnetic pole as a function of rotational phase (solid lines). The black dots represent the Stokes $Q/I$ (panel a), $U/I$ (panel b), and intensity (panel c) data extracted from IXPE in the 2--3 keV energy range. The case with the best statistical fit (lowest combined total $\chi^2$; see Methods) incorporates vacuum birefringence, and corresponds to a magnetic colatitude of $\theta_m = [0\degree, 17\degree]$ and longitude $\phi_m = [0\degree, 120\degree]$ (zero longitude contains the rotation and magnetic axes); it is displayed in blue. The orange solid curves on the center and right panel show the corresponding polarization profiles wherein VB is turned off – these fits are statistically worse than those with VB on (see Methods). Panel (d) displays the comparison in the Stokes Q-U space between the observed data (black dots) and the simulated results obtained with the best-fit wedge-shaped hot spot with (blue line) and without (orange line) including magnetospheric VB, plotted for one rotational cycle. The red solid line represents the best result among the VB-off cases (see Methods), corresponding to a pole-centered circular hot spot with a magnetic inclination of $\alpha = 2^{\degree}$ and a viewing angle of $\zeta = 20.5^{\degree}$.
• Figure 4: A comparative study between model-independent Stokes Q and U polarization characteristics obtained through using IXPEOBSSIM (left) and through applying a polconst*bbodyrad spectral model in Xspec (right) at four energy bins: 2--3 keV, 3--4 keV, 4--5 keV, and 2--8 keV. The resulting PD and PA for the two methods are comparable to each other. Notably, both show a non-linear trend in the energy-dependence of the PD as the 4--5 keV band approaches the value of the 2--3 keV band at $\sim1\sigma$ level. Future observations with higher count statistics are needed to complement this study to more directly probe the nature of the high-energy polarization.
• Figure 5: The simultaneous spectral $\nu F_{\nu}$ models of the NICER+IXPE observations. The upper left panel displays the best fit model, a single absorbed blackbody with a linear polarization component: constant*tbabs(pollin*bbodyrad). NB: Only the IXPE spectra are displayed here for the sake of visual clarity. The bottom left panel shows the data divided by the folded model for the absorbed BB model. The right-hand panels show the normalized Stokes Q/I (top) and U/I (bottom) spectra in linear space for the three IXPE DUs with the solid lines showing the best fit of the linear polarization component. The quasi-thermal BB is accompanied by a strong polarization signal that decreases as a function of energy.