Probing the Polarized Emission from SMC X-1: the Brightest X-ray Pulsar Observed by IXPE

Recent observations of X-ray pulsars (XRPs) performed by the Imaging X-rayPolarimetry Explorer (IXPE) have made it possible to investigate the intricatedetails of these objects in a new way, thanks to the added value of X-raypolarimetry. Here we present the results of the IXPE observations of SMC X-1, amember of the small group of XRPs displaying super-orbital variability. SMC X-1was observed by IXPE three separate times during the high state of itssuper-orbital period. The observed luminosity in the 2-8 keV energy band ofL=2×10^38 erg/s makes SMC X-1 the brightest XRP ever observed by IXPE.We detect significant polarization in all three observations, with values ofthe phase-averaged polarization degree (PD) and polarization angle (PA) of3.2±0.890±8 for Observation 2, and 5.5±1.1Observation 3, for the spectro-polarimetric analysis. The observed PD shows anincrease over time with decreasing luminosity, while the PA decreases indecrements of 10. The phase-resolved spectro-polarimetric analysis revealssignificant detection of polarization in three out of seven phase bins, withthe PD ranging between 2∼70 to ∼100. The pulse-phase resolved PD displays anapparent anti-correlation with the flux. Using the rotating vector model, weobtain constraints on the pulsar's geometrical properties for the individualobservations. The position angle of the pulsar displays an evolution over timesupporting the idea that we observe changes related to different super-orbitalphases. Scattering in the wind of the precessing accretion disk may beresponsible for the behavior of the polarimetric properties observed during thehigh-state of SMC X-1's super-orbital period.