First Detection of X-Ray Polarization in Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124 with IXPE

We report the results of first ever spectropolarimetric analyses of the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 during the 2023 outburst using quasi-simultaneous IXPE, NICER, and NuSTAR observations. A pulsation of period similar to 9.79 s is detected in IXPE and NuSTAR observations with pulse fractions (PFs) similar to 18% (2-8 keV) and similar to 28% (3-78 keV), respectively. Energy-dependent study of the pulse profiles with NuSTAR indicates an increase in PF from similar to 27% (3-10 keV) to similar to 50% (40-78 keV). Further, epoch-dependent polarimetric measurements during the decay phase of the outburst confirm the detection of significant polarization, with the polarization degree (PD) and polarization angle ranging between similar to 2%-3.1% and similar to 8.degrees 6-10.degrees 8, respectively, in the 2-8 keV energy range. We also observe that the PD increases up to similar to 4.8% at higher energies (greater than or similar to 5 keV) with dominating bbodyrad flux contribution (1.5 less than or similar to F-BB/F-PL less than or similar to 3.4) in the IXPE spectra. The phase-resolved polarimetric study yields PD as similar to 1.7%-3.1% suggesting a marginal correlation with the pulse profiles. Moreover, the broadband (0.6-70 keV) energy spectrum of combined NICER and NuSTAR observations is well described by the combination of bbodyrad and cutoffpl components with seed photon temperature (kT(bb)) similar to 0.86 +/- 0.03 keV and photon index (Gamma) similar to 0.98 +/- 0.01. With the above findings, we infer that the observed "low" PD in Swift J0243.6+6124 is attributed possibly due to the "vacuum resonance" effect between the overheated and relatively cooler regions of the neutron star boundary layer.