X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch

TL;DR

This study uses IXPE, NICER, NuSTAR, and Einstein Probe data to perform X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch (NB) and compares it to the horizontal branch (HB). The authors detect significant polarisation, with PD ≈ $1.4\%$ in NB and ≈ $3.7\%$ in HB, and find an energy-dependent polarisation, particularly in the NB, across $2$–$8$ keV. Broadband spectral modeling with disk emission and Comptonisation, plus a Gaussian or relativistic reflection component, reveals that the Comptonised component maintains a ~4% polarisation across states, while disc and reflected components contribute differently depending on the assumed model, implying additional polarised emission from a wind or an accretion disc corona (ADC). The results support a geometry in which HB is dominated by an extended corona or wind and NB by disc/BL emission, highlighting the role of polarimetry in constraining accretion flow and emission region geometry in WMNS X-ray binaries. This work advances our understanding of accretion physics in low-mass X-ray binaries and demonstrates the diagnostic power of spectropolarimetric analysis when combined with broadband spectroscopy.

Abstract

This study presents an X-ray spectropolarimetric characterisation of the Z-source GX 340+0 during the normal branch (NB) and compares it with that obtained for the horizontal branch (HB), using IXPE, NICER and NuSTAR observations. The analysis reveals significant polarisation, with polarisation degrees (PD) of ${\sim}1.4$\% in the NB and ${\sim}3.7$\% in the HB, indicating a notable decrease in polarisation when transitioning from the HB to the NB. The polarisation angles show a consistent trend across the states. Spectropolarimetric analysis favours a dependence of the polarisation on the energy. The Comptonised component shows similar polarisation in both the HB and NB and is higher than the theoretical expectation for a boundary or spreading layer. This suggests a contribution from the wind or the presence of an extended accretion disc corona (ADC) to enhance the polarisation. The results obtained here highlight the importance of using polarimetric data to better understand the accretion mechanisms and the geometry of this class of sources, providing insights into the nature of the accretion flow and the interplay between different spectral components. Overall, the findings advance our understanding of the physical processes governing accretion in low-mass X-ray binaries.

Figures (8)

• Figure 1: Light curves, HRs, and polarisation properties of GX 340$+$0 during the second IXPE observation. (a): IXPE light curve binned in 600 s intervals overlapped with the EP-WXT light curve (dodger blue points). The highlighted coloured intervals correspond to the NICER simultaneous observations reported in Table \ref{['tab:exposure']} and Fig. \ref{['fig:NICER_NuSTAR_HID']}. (b): IXPE HR binned in 600 s obtained as the ratio of the IXPE counting rates in the 4--8 and 2--4 keV energy bands. (c) and (d): Time-resolved PD and PA in equal time bins of about 9.5 h. The errors are reported at 68% CL, while the upper limits are at 90% CL.
• Figure 2: IXPE HID of the second IXPE observation of GX 340$+$0 in 600 s time bins. The grey points represent the first IXPE observation, while the coloured ones, from dark blue to yellow, report the elapsed time in hours since the start of the second IXPE observation. The dotted black line defines the region we used to select the NB.
• Figure 3: HIDs of GX 340$+$0 in 150 s time bin. Top panel: HID of GX 340$+$0 from the NICER data archive (grey points). The coloured points represent the five NICER observations performed during the IXPE one and also reported in Table \ref{['tab:exposure']} and Fig. \ref{['fig:IXPE_LC_HR_PD_PA']}-top as shaded coloured regions in the IXPE light curve. Bottom panel: HID of GX 340$+$0 from the NuSTAR archival data (grey points). The olive green points represent the NuSTAR observation selected to perform the spectral and spectropolarimetric analysis.
• Figure 4: Energy-dependent polarimetric properties of GX 340$+$0 in the NS. PD (top panel) and PA (bottom panel) as a function of the energy in 1 keV energy bins. The errors are reported at 68% CL, while the upper limit is at 90% CL, see text.
• Figure 5: Polarisation in the HB (blue) and in the NB (purple) of GX 340$+$0. The contours represent the allowed regions at 68%, 90%, and 99% CL.