First detection of X-ray polarization in Galactic ULX pulsar Swift J0243. 6+6124 with IXPE

We report the results of first ever spectro-polarimetric analyses of the Galactic ultra-luminous X-ray pulsar Swift J0243. 6+6124 during the 2023 outburst using quasi-simultaneous IXPE, NICER and NuSTAR observations. A pulsation of period 9. 79 s is detected in IXPE and NuSTAR observations with pulse fractions (PFs) 18\% (2-8 keV) and 28\% (3-78 keV), respectively. Energy-dependent study of the pulse profiles with NuSTAR indicates an increase in PF from 27\% (3-10 keV) to 50\% (40-78 keV). Further, epoch-dependent polarimetric measurements during decay phase of the outburst confirm the detection of significant polarization degree (PD) varying as 2-3. 1\% with polarization angle 8. 6^-10. 8^ in 2-8 keV energy range. We also observe that PD increases up to 4. 8\% at higher energies (5 keV) with dominating bbodyrad flux contribution (1. 5 F ₁₁/F ₏₋ 3. 4) in IXPE spectra. The phase-resolved polarimetric study yields PD as 1. 7-3. 1\% suggesting a marginal correlation with the pulse profiles. Moreover, the broad-band (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 ₁₁) 0. 86 0. 03 keV and photon index () 0. 98 0. 01. With the above findings, we infer that the observed `low' PD in Swift J0243. 6+6124 is resulted possibly due to `vacuum resonance' effect occurred between the overheated and relatively cooler regions of the neutron star boundary layer.