IXPE view of the Sco-like source GX 349+2 in the normal branch

TL;DR

This study uses simultaneous IXPE and NuSTAR observations of GX 349+2, a Sco-like Z-source, to perform a comprehensive polarimetric and spectropolarimetric analysis across Z-track states. A model-independent polarimetry in the 2–8 keV band yields a polarisation degree of $1.1\% \pm 0.3\%$ and a polarisation angle of $29^\circ \pm 7^\circ$ with $\sim 4.1\sigma$ significance, and reveals an energy-dependent polarisation when analyzed in finer energy bins. Spectropolarimetric modeling with a diskbb+bbodyrad continuum and a Gauss line, extended by a relxillNS reflection component, indicates a disc-dominated soft spectrum with an inclination of $32^\circ \pm 1^\circ$ and a polarisation pattern that differs between the disc and Comptonised components, slightly favouring a spreading-layer geometry. The results show polarisation properties in GX 349+2 that align more closely with Cyg-like systems at similar inclination rather than Sco-like ones, suggesting that accretion state and orbital inclination mainly govern polarisation rather than subclass classification. These findings provide insight into the geometry of accretion and reflection in neutron-star LMXBs and demonstrate the utility of broadband spectropolarimetry for disentangling emission components in Z-sources.

Abstract

We present a detailed spectropolarimetric study of the Sco-like Z-source GX 349+2, simultaneously observed with the Imaging X-ray Polarimetry Explorer (IXPE) and Nuclear Spectroscopic Telescope Array (NuSTAR). During the observations GX 349+2 was found mainly in the normal branch. A model-independent polarimetric analysis yields a polarisation degree of $1.1\% \pm 0.3\%$ at a polarisation angle of $29° \pm 7°$ in the 2--8 keV band, with ${\sim}4.1σ$ confidence level significance. No variability of polarisation in time and flux has been observed, while an energy-resolved analysis shows a complex dependence of polarisation on energy, as confirmed by a spectropolarimetric analysis. Spectral modeling reveals a dominant disc blackbody component and a Comptonising emitting region, with evidence of a broad iron line associated with a reflection component. Spectropolarimetric fits suggest differing polarisation properties for the disc and Comptonised components, slightly favouring a spreading layer geometry. The polarisation of the Comptonised component exceeds the theoretical expectations, but is in line with the results for other Z-sources with similar inclination. A study of the reflection's polarisation is also reported, with polarisation degree ranging around 10% depending on the assumptions. Despite GX 349+2's classification as a Sco-like source, these polarimetric results align more closely with the Cyg-like system GX 340+0 of similar inclination. This indicates that polarisation is governed primarily by accretion state and orbital inclination, rather than by the subclass to which the source belongs.

Figures (8)

• Figure 1: Light curves and HRs of GX 349$+$2. Top panel: MAXI light curve including the period of the IXPE observation (orange shaded region). The MAXI data were binned in 1.5 h intervals. Middle panel: IXPE light curve and HR binned in 200 s intervals. The blue dotted line identifies the threshold for the high and low flux states used in the analysis. The green shaded regions highlight the two NuSTAR simultaneous observations. Bottom panel: NuSTAR light curves and HRs binned in 200 s intervals. The NuSTAR count rate was obtained in the 3--20 keV energy band. The time refers to hours since the start of the IXPE observation. The observation IDs are reported in Table \ref{['tab:exposure']}.
• Figure 2: IXPE Hardness-Intensity Diagram of GX 349$+$2 in 200-s time bins. The coloured points from blue to green report the elapsed time in hours since the start of the IXPE observation.
• Figure 3: NuSTAR Hardness-Intensity Diagram of GX 349$+$2 in 200-s time bins. The dark and bright blue points represent the two NuSTAR observations reported in Table \ref{['tab:exposure']} and Fig. \ref{['fig:IXPE_NuSTAR_LC']} overlapping with archival NuSTAR observations (grey points).
• Figure 4: Polar plot of the PD and PA for GX 349$+$2 in the 2--8 keV energy band obtained by pcube analysis when the IXPE observation is divided into high and low flux states. The contours represent the allowed regions at 68%, 90% and 99% CL from the innermost to the outermost.
• Figure 5: Energy-resolved polarimetric analysis of GX 349$+$2: PD (top panel) and PA (bottom panel) in 1 keV-wide energy bins. The values are also reported in Table \ref{['tab:PD_PAvsEnergy']}. Errors are at 68% CL.