Detection of X-Ray Polarization in the Hard State of IGR J17091-3624: Spectropolarimetric Study with IXPE and NuSTAR Data

Subham Srimani^1*, Dipak Debnath^1,2, Hsiang-Kuang Chang^1,3,4

¹Institute of Astronomy, National Tsing Hua University, Hsnchu, Taiwan
²Institute of Astronomy Space and Earth Science, Kolkata, India
³Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴Institute of Space Engineering, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Subham Srimani, email:subhamsrimani2023@gmail.com

The well-known Galactic class-transition X-ray binary black hole candidate IGR J17091–3624 showed renowned outburst in February 2025. We present a detailed spectropolarimetric study of the source using archival data from Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR satellites (March 7-10). 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 ± 2.35)% at a polarization angle (PA) of 82.7° ± 5.96°, significant at >4σ. The model-dependent polarization analysis using the polconst and polpow models yields consistent values of PD and PA with a maximum PD of (29.9 ± 8.46)% is detected at a PA of 88. 0° ±8.15° (>3σ) in 6–8keV band. In both methods, an energy-dependent increasing trend of PD is observed. Joint spectral analysis using IXPE and NuSTAR data in the 2–70keV band was performed with four different sets of phenomenological and physical models. Our results indicate a strong dominance of nonthermal photons originating from a hot Compton cloud, suggesting that the source was in a hard spectral state. The physical kerrbb and TCAF models provides an estimate of the black hole mass MΒΗ = 14.8 (+4.7, - 3.4) Msolar 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 nonthermal photons during the dip phase, likely caused by the presence of wind accreted from the companion star.

Keywords: X-ray binary stars, X-ray transient sources, Polarimetry, Black hole physics, Accretion