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) 1.34±0.27 and polarization angle (PA) -13.69^∘±5.85^∘, while the model-dependent analysis gives PD 1.18±0.23 and PA -14.01^∘±5.80^∘. The joint spectral analysis of the broadband data constrains the average mass of the central black hole ~ 8.5±1.3 M_ and a moderate spin parameter of ~0.6±0.1 with disk inclination ~ 31^∘±8^∘. The power-law photon index and cutoff energy are 2.4±0.1 and ~ 50±9-60±8 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 ($<2$% MEdd) during these epochs. The hydrogen column density obtained from the fit is relatively high ~ 5.2±0.1× 10²² cm⁻².