Detection of X-ray Polarization in the Hard State of IGR J17091-3624: Spectro-Polarimetric Study with IXPE and NuSTAR Data

The class-transition Galactic X-ray binary IGR~J17091--3624 was simultaneously monitored by the \textit{IXPE} and \textit{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 $(11.3\pm2.35)\%$ at a polarization angle (PA) of $82^\circ.7\pm5^\circ.96$ (significant at $>4\sigma$). The model-dependent polarization analysis using the \texttt{polconst} and \texttt{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 $(29.9\pm8.46)\%$ (significant at $>3\sigma$) is detected at a PA of $88^\circ.0\pm8^\circ.15$ ($>3\sigma$). The joint spectral analysis using \textit{IXPE} and \textit{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 {\fontfamily{qcr}\selectfont kerrbb} and {\fontfamily{qcr}\selectfont TCAF} models provides an estimate of the black hole mass $M_{\rm BH} = 14.8^{+4.7}_{-3.4}~M_\odot$ and dimensionless spin parameter $a^* \sim 0.54$. The requirement of a higher hydrogen column density in the spectral fit of the second \textit{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.