Determination of X-ray pulsar geometry with IXPE polarimetry

Abstract

We report on IXPE observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 at two luminosity levels during the giant outburst in January-February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in superand sub-critical states with significantly different emission region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations of the spin axis inclination and the position angle and the magnetic colatitude by tens of degrees within just a few days separating the observations. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of a polarized unpulsed component in addition to the polarized radiation associated with the pulsar itself. We show that the observed PA phase dependence in both observations can then be explained with a single set of RVM parameters defining the pulsar's geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation off the equatorial disk wind.

Beam Pattern Evolution of Accreting X-Ray Pulsar 1A 0535+262 during Its 2020 Giant Outburst

“…The advantage of the method is that interpretation of the reconstructed intrinsic beam patterns associated with individual poles is arguably easier than full modeling of the pulse profiles from a theoretical perspective. It is also worth noting that Her X-1ʼs geometry estimated by the pulse profile decomposition (Blum & Kraus 2000) appears to be consistent with the recent polarization study performed by the Imaging X-ray Polarimetry Explorer (IXPE; Doroshenko et al 2022). This indicates that the decomposition method might indeed be a good approximation for reconstructing the beam pattern of pulsars.…”

Observations of 4U 1626–67 with the Imaging X-Ray Polarimetry Explorer

“…For Her X-1 and Cen X-3, the pulse-phased X-ray polarization averaged 7%-10%, much lower than many of the predictions. These two sources yielded sufficient signals to obtain significant polarization detections (Doroshenko et al 2022;Tsygankov et al 2022). While 4U 1626-67 is fainter than these two pulsars, the low polarization is consistent with the general finding that X-ray polarizations are lower than expected.…”

“…The higher values of θ might indicate that a single-geometry picture is insufficient to characterize the accretion geometry around the pulsar. The IXPE observations of Her X-1 support a two-component model (Doroshenko et al 2022). In fact, a complex nature of the accretion geometry around accreting pulsars has been suggested for a long time.…”

“…In this situation, the rotatingvector model (RVM) of Radhakrishnan & Cooke (1969) holds to a good approximation. In the RVM, the position angle (measured from north to east) is given by (Poutanen 2020;Doroshenko et al 2022)…”

“…In two previous IXPE observations, polarizations were lower than expected, showing that the two models are not clearly distinguished. In Her X-1, there is the possibility that both emission geometries may be operating (Doroshenko et al 2022). In Cen X-3, complications from atmosphere modeling and surface reflection are suggested (Tsygankov et al 2022).…”