Spectro-polarimetric study of a newly discovered Galactic transient Swift J151857.0-572147 using IXPE and NuSTAR observations

We study the spectro-polarimetric properties of a newly discovered black hole X-ray binary Swift\, J151857. 0-572147 jointly using IXPE and NuSTAR observations during March 2024. The analysis of IXPE data reports mode-independent polarization degree (PD) 1. 340. 27 and polarization angle (PA) -13. 69⁵. 85^, while the model-dependent analysis gives PD 1. 180. 23 and PA -14. 01⁵. 80^. The joint spectral analysis of the broadband data constrains the average mass of the central black hole 8. 51. 3 M_ and a moderate spin parameter of 0. 60. 1 with disk inclination 31⁸^. The power-law photon index and cutoff energy are 2. 40. 1 and 509-608 keV, suggesting a transition to the soft spectral state of the source during the observation period. Additionally, the best-fitted halo mass accretion is less compared to the disk mass accretion rate and a relatively lower corona size of 10 rg might be indicative of the same spectral state. The mass outflow rate was also low (<2\% M ₄₃₃) during these epochs. The hydrogen column density obtained from the fit is relatively high 5. 20. 1 10^22 cm^-2.