Correlated spectro-polarimetric study along the Z track in XTE J1701–462 puts constraints on its coronal geometry

Abstract

Context. In September 2022, the transient neutron star low-mass X-ray binary XTE J1701–462 went into a new outburst.Aims. The objective of this work is to examine the evolution of the accretion geometry of XTE J1701–462 by studying the spectro-polarimetric properties along the Z track of this source. The simultaneous observations archived by the Insight-Hard X-ray Modulation Telescope (HXMT) and the Imaging X-ray Polarimetry Explorer (IXPE) give us the opportunity to do this.Methods. We present a comprehensive X-ray spectro-polarimetric analysis of XTE J1701–462, using simultaneous observations from IXPE, Insight-HXMT, and NuSTAR. For IXPE observations, two methods were employed to measure the polarization: a model-independent measurement with PCUBE and a model-dependent polarization-spectral analysis with XSPEC. The corresponding spectra from Insight-HXMT and NuSTAR were studied with two configurations that correspond to the eastern model and western model, respectively.Results. Significant polarization characteristics are detected in XTE J1701–462. The polarization degree shows a decreasing trend along the Z track, reducing from (4.84 ± 0.37)% to (3.76 ± 0.43)% on the horizontal branch and dropping to ≤1% on the normal branch. The simultaneous spectral analysis from Insight-HXMT and NuSTAR suggests that the evolution of the PD is closely linked to changes in the covering factor of the Comptonized corona along the Z track.