Continuous evolution of the polarization properties in the transient X-ray pulsar RX J0440.9+4431/LS V +44 17

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 Department of Physics and Astronomy, FI-20014 University of Turku, Finland
⁴ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road Oxford OX1 3RH, UK
⁵ Department of Astronomy, University of Geneva, 16 Chemin d’Ecogia, Versoix CH-1290, Switzerland

^⋆ Corresponding authors; taolian@ihep.ac.cn; sergey.tsygankov@utu.fi

Received: 4 November 2024
Accepted: 17 December 2024

Abstract

We present a detailed time-resolved and phase-resolved polarimetric analysis of the transient X-ray pulsar RX J0440.9+4431/LS V +44 17, using data from the Imaging X-ray Polarimetry Explorer (IXPE) during the 2023 giant outburst. We conducted a time-resolved analysis by dividing the data into several intervals for each observation. This analysis reveals a continuous rotation of the phase-averaged polarization angle (PA) across the observations performed during the supercritical and subcritical regimes. To investigate the origin of the PA rotation, we performed a pulse phase-resolved polarimetric analysis over four time intervals, each spanning approximately three days. Applying the rotating vector model (RVM), the geometric parameters of the system were determined for each interval. Despite the short time gap of just ∼20 days, we observed significant variation in the RVM parameters between the first interval and the subsequent three, indicating the presence of an additional polarized component alongside the RVM component. Using a two-polarized component model with the assumption that this additional component remains constant across pulse phases, we calculated the phase-averaged PA and polarized flux of both the variable and constant components. The phase-averaged PA of each component remained relatively stable over time, but the polarized flux of the constant component decreased, while that of the variable component increased. The observed rotation of the PA is attributed to the gradual shift in the polarized flux ratio between the two components and is not directly related to the different accretion regimes.