Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

Abstract

Discovery of pulsations from a number of ULXs proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of 15-40 deg, the magnetic obliquity of 60-80 deg, and the pulsar spin position angle of -50 deg, which significantly differs from the constant component PA of about 10 deg. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30 deg misalignment between the pulsar angular momentum and the binary orbital axis.

13 pages, 10 figures, version published in A&A

References in corpus (35)

• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• The critical accretion luminosity for magnetized neutron stars
• Analyzing the Data from X-ray Polarimeters with Stokes Parameters
• ixpeobssim: a Simulation and Analysis Framework for the Imaging X-ray Polarimetry Explorer
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Ultraluminous X-ray sources
• Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
• Handling Background in IXPE polarimetric data
• A weighted analysis to improve the X-ray polarization sensitivity of IXPE
• Determination of X-ray pulsar geometry with IXPE polarimetry
• Insight-HXMT discovery of the highest energy CRSF from the first Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124
• Super-Eddington accretion discs with advection and outflows around magnetized neutron stars
• Relativistic rotating vector model for X-ray millisecond pulsars
• The X-ray polarimetry view of the accreting pulsar Cen X-3
• Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE
• Pulsating ULXs: large pulsed fraction excludes strong beaming
• Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer
• IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance
• X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?
• Complex variations of X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
• IXPE observations of the quintessential wind-accreting X-ray pulsar Vela X-1
• Super-Eddington Emission from Accreting, Highly Magnetised Neutron Stars with a Multipolar Magnetic Field
• A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375
• X-ray polarimetry of the accreting pulsar GX 301-2
• Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars
• X-ray Polarimetry of X-ray Pulsars
• X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei : NGC 1068
• RX J0440.9+4431: another supercritical X-ray pulsar
• Bright X-ray Pulsars: how outflows influence beaming, pulsations and pulse phase lags
• Probing the polarized emission from SMC X-1: the brightest X-ray pulsar observed by IXPE
• ULX pulsar Swift J0243.6+6124 observations with NuSTAR -- dominance of reflected emission in the super-Eddington state

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