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 $\mathit{\text{IXPE}}$ and $\mathit{\text{NuSTAR}}$ 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 $(11.3\pm 2.35)\mathrm{\%}$ at a polarization angle (PA) of ${82}^{\circ }.7\pm 5^{\circ }.96$ (significant at $>4\sigma$). The model-dependent polarization analysis using the $\mathtt{\text{polconst}}$ and $\mathtt{\text{polpow}}$ 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 $(29.9\pm 8.46)\mathrm{\%}$ (significant at $>3\sigma$) is detected at a PA of ${88}^{\circ }.0\pm 8^{\circ }.15$ ($>3\sigma$). The joint spectral analysis using $\mathit{\text{IXPE}}$ and $\mathit{\text{NuSTAR}}$ 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 $M_{\mathrm{B}\mathrm{H}}=14.8_{-3.4}^{+4.7}{M}_{\odot }$ and dimensionless spin parameter $a^{\ast }\sim 0.54$. The requirement of a higher hydrogen column density in the spectral fit of the second $\mathit{\text{NuSTAR}}$ 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.