NICER observation list: Target ID 668901

Accreting weakly magnetized Neutron Stars in X-ray Binaries are amongst the brightest X-ray sources. Thanks to IXPE, there is now the possibility to have X-ray polarimetric data allowing to disentangle parameters/scenarios which at present can equally describe spectral/temporal properties of these sources. IXPE, during its 2nd observing period, will observe Sco X-1, for which there are hints for a polarization above 4keV, 4U 1820-303 and Cir X-1, whose polarization has never been studied before. However, these sources are highly variable: therefore, we propose a simultaneous 100ks NICER and IXPE observation to monitor their energy spectra. NICER s better energy resolution allows obtaining a better spectral model reducing the free parameters in the joint IXPE spectro-polarimetric analysis.