Probing the accretion geometry of black hole X-ray binaries: A multi-mission spectro-polarimetric and timing study

Abstract

We present a comprehensive spectro-polarimetric and timing analysis of twelve black hole X-ray binaries, namely Cyg X-1, 4U 1630-47, Cyg X-3, LMC X-1, 4U 1957+115, LMC X-3, Swift J1727.8-1613, GX 339-4, Swift J151857.0-572147, IGR J17091-3624, MAXI J1744-294 and GRS 1915+105, using quasi-simultaneous observations from {\it{IXPE}}, {\it{NICER}}, {\it{NuSTAR}}, and {\it{AstroSat}}. Timing analyses reveal type-B and type-C Quasi-periodic Oscillations across different spectral states, often associated with episodic radio ejections. Broadband ($0.7-60$ keV) spectral modeling, employing disc, Comptonization, and reflection components, reveals degeneracies in constraining disc-corona geometries. Polarimetric measurements in $2-8$ keV band detect significant polarization degrees (PDs) ranging from $3-20.6\%$ ($1.2-21.4\%$) in harder (softer) states, with moderate to strong energy dependence, except for LMC X$-1$, Swift J$151857.0-572147$, and MAXI J1744$-$294, where no significant polarization is detected. We report polarization detections of Cyg X$-3$ (PD $\sim 21.4\%$, SIMS), LMC X$-3$ (PD $\sim 2.4\%$, HSS) and IGR J$17091-3624$ (PD $\sim 9\%$, LHS) using recent {\it IXPE} observations. A positive correlation is found between PD and Comptonized photon fraction ($co{v}_{\mathrm{frac}}$), while an anti-correlation is observed with disc-to-Comptonized flux ratio ($F_{\mathrm{ratio}}$) across spectral states. Combined timing, spectral, and polarimetric results, together with constraints from radio jet observations, suggest a radially extended corona within a truncated disc for Cyg X$-1$, Swift J$1727.8-1613$, IGR J$17091-3624$, and GX $339-4$, whereas the disc-corona geometry remains poorly constrained for 4U $1957+115$, LMC X$-3$, and 4U $1630-47$. We discuss the implications of these findings for understanding accretion geometries and highlight prospects for future X-ray polarimetric studies.

23 pages, 8 figures, 6 tables, to appear in MNRAS