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

¹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
² University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China
³ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
⁴ Institute of Astrophysics, Central China Normal University, Luoyu Road 152, Wuhan 433079, P.R. China
⁵ ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
⁶ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, China
⁷ National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Beijing 100012, China
⁸ School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo 255000, China

^⋆ Corresponding author; bu@ccnu.edu.cn

Received: 20 December 2023
Accepted: 27 January 2025

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.