The class-transition Galactic X-ray binary IGR~J17091--3624 was simultaneously monitored by the IXPE and NuSTAR satellites. We present a detailed spectro-polarimetric study of the source using data from both satellites covering the period from March~7--10, 2025. A polarimetric analysis in the 2--8~keV band using a model-independent method reveals a significant detection of polarization degree (PD) of (11. 32. 35) \% at a polarization angle (PA) of 82^. 75^. 96 (significant at >4σ). The model-dependent polarization analysis using the polconst and polpow models yields consistent values of PD and PA. In both methods, an energy-dependent increasing trend of PD is observed. In the 6--8~keV band, a maximum PD of (29. 98. 46) \% (significant at >3σ) is detected at a PA of 88^. 08^. 15 (>3σ). The joint spectral analysis using IXPE and NuSTAR data in the 2--70~keV band was performed with four different sets of phenomenological and physical models. Our results indicate a strong dominance of non-thermal photons originating from a `hot' Compton cloud, suggesting that the source was in a hard spectral state. Spectral fitting with the physical qcr kerrbb and qcr TCAF models provides an estimate of the black hole mass M ₁₇ = 14. 8^+4. 7-₃. ₄~M_ and dimensionless spin parameter a^* 0. 54. The requirement of a higher hydrogen column density in the spectral fit of the second NuSTAR observation is attributed to the obscuration of non-thermal photons during the dip phase, likely caused by the presence of wind accreted from the companion star.
Debnath et al. (Wed,) studied this question.