Description
Millisecond pulsars (MSPs) are the quickest-spinning neutron stars (NSs) known. The X-ray pulsations
produced by hot spots on the NS surface encode information about the mass and size of
these compact objects; in this context, accreting MSPs (AMSPs) are among the best targets because
they become relatively bright X-ray sources during X-ray outbursts. Yet, breaking the degeneracy
among the many parameters that shape their pulse profiles was not possible due to the limited
effective area of X-ray instruments flown to date. X-ray polarimetry is a potential game-changer,
providing an independent measure of the geometry of the hot spots. In this talk, the IXPE results
obtained by observations of the two AMSPs, SRGA J144459.2-604207 and SAX J1808.4-3658, are
presented. Although the spin-phase variability of the normalized Stokes parameters is not highly
significant, applying the relativistic rotating vector model, the geometry of the hot spots was determined
for SRGA J144459.2-604207. On the other hand, unexpected time variability on longer
time scales has been observed in non-pulsating NS-LMXBs, particularly in response to intensity
drop during dips. These variations could be explained by a possible contribution from an extended
accretion-disk corona or a disk-wind component. In this talk, results for GX 13+1 and XTE J1701-
462 are presented, along with possible constraints on the geometry of the extended accretion-disk
corona or the disk wind.