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

Abstract

We present measurements of the polarization of X-rays in the 2-8 keV band from the pulsar in the ultracompact low mass X-ray binary 4U1626-67 using data from the Imaging X-ray Polarimetry Explorer (IXPE). The 7.66 s pulsations were clearly detected throughout the IXPE observations as well as in the NICER soft X-ray observations, which we use as the basis for our timing analysis and to constrain the spectral shape over 0.4-10 keV energy band. Chandra HETGS high-resolution X-ray spectra were also obtained near the times of the IXPE observations for firm spectral modeling. We find an upper limit on the pulse-averaged linear polarization of <4% (at 95% confidence). Similarly, there was no significant detection of polarized flux in pulse phase intervals when subdividing the bandpass by energy. However, spectropolarimetric modeling over the full bandpass in pulse phase intervals provide a marginal detection of polarization of the power-law spectral component at the 4.8 +/- 2.3% level (90% confidence). We discuss the implications concerning the accretion geometry onto the pulsar, favoring two-component models of the pulsed emission.

19 pages, 7 figures, 7 tables; accepted for publication in the Astrophysical Journal

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• X-ray polarimetry of the accreting pulsar GX 301-2
• A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375
• X-ray Polarimetry of X-ray Pulsars
• Probing the polarized emission from SMC X-1: the brightest X-ray pulsar observed by IXPE
• Discovery of Polarized X-Ray Emission from the Accreting Millisecond Pulsar SRGA J144459.2-604207
• X-ray Polarimetry of the accreting pulsar 1A~0535+262 in the supercritical state with PolarLight
• Revealing two orthogonally polarized spectral components in Vela X-1 with IXPE
• The ongoing spin-down episode of 4U 1626-67
• Exploring polarization and geometry in the X-ray pulsar 4U 1538-52