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

TL;DR

This study presents a multi-mission timing, spectroscopy and X-ray polarimetry analysis of BH-XRBs using IXPE, NICER, NuSTAR and AstroSat to probe accretion geometry. Spectral modeling with disk plus Comptonization components shows degeneracy in disc–corona configurations, while IXPE polarization measurements in the 2–8 keV band reveal significant, state-dependent PD with notable energy dependence. A robust PD–cov_frac correlation and an anti-correlation between PD and F_ratio emerge, supporting a scenario where a radially extended corona coexists with a truncated disc in hard states for several sources, though some systems remain geometrically unconstrained. The results underscore the value of combining timing, spectroscopy and polarimetry to constrain BH-XRB accretion-ejection physics and point to future polarimetric missions and wide-band campaigns as essential for breaking current degeneracies.

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 ($cov_{\rm frac}$), while an anti-correlation is observed with disc-to-Comptonized flux ratio ($F_{\rm ratio}$) 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.

Figures (8)

• Figure 1: Long-term X-ray monitoring of the sources with MAXI/GSC ($2-20$ keV), Swift/BAT ($15-50$ keV) and the corresponding hardness ratio obtained with MAXI/GSC are presented in panel (a), (b) and (c), respectively. Each data point for a given source represents averaged out respective quantity over the exposure of corresponding IXPE observation. In panel (d), the variation of radio flux is shown from the quasi-simultaneous radio observations available from various facilities. The $1\sigma$ errorbars of the respective quantities are small and remain within the markers for most of the cases. For a given source, the data points plotted with MJD, correspond to the respective epochs as mentioned in Table \ref{['tab:log']}.
• Figure 2: Results of temporal (top panel) and spectral (bottom panel) analyses of ten BH-XRBs in different spectral states (color coded) obtained using quasi-simultaneous IXPE, NICER, NuSTAR and AstroSat observations. Top panels: PDS obtained from NICER ($0.5-10$ keV) and/or NuSTAR/ AstroSat observations in different energy ranges are depicted for the respective sources in various epochs. The power spectrum of GX $339-4$ from NICER observation is rescaled by a factor of $5$. Bottom panels: Energy spectra of the sources from NICER and/or NuSTAR data are shown during the respective epochs. PDS and energy spectra in LHS, HIMS, SIMS, HSS and SPL states are presented using red, blue, magenta, orange and green colors, respectively. PDS from NuSTAR and AstroSat data are plotted using circle and triangle, irrespective of the spectral states. See the text for details.
• Figure 3: Variation of the data to model ratio of the HSS/HIMS spectra of Cyg X$-1$, GX $339-4$ and Swift J$1727.8-1613$ fitted with model combination M1 and M3. The improvement in the respective fits by incorporating the reflection model component over M1 is depicted in panels (b), (d) and (f). See the text for details.
• Figure 4: Confidence contours ($3\sigma$) in the PD–PA plane obtained from the model-independent polarization measurements using $2-8$ keV IXPE data, combining all DUs across various spectral states of the sources. The epoch of each observation is indicated in parentheses next to the corresponding spectral state. For Swift J$151857.0-572147$, LMC X$-1$ and MAXI J$1744-294$, null-detection of polarization is observed, whereas all other sources exhibit significant polarization. Inset highlights a zoomed-in region of PD–PA space for better clarity.
• Figure 5: Variation of polarization degree (PD) with energy. In each panel, source name and spectral state (observed during different epoch) are marked. The significance of energy variation of PD for the respective cases are mentioned in units of $\sigma$ in the plot legends. The histograms represent the minimum detectable polarization at $99\%$ confidence ($\rm MDP_{99}\%$). See the text for details.