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

TL;DR

This study measures X-ray polarization from the pulsar in 4U 1626-67, finding low polarization levels and providing insights into the accretion geometry using data from IXPE, NICER, and Chandra.

Contribution

First polarization measurements of 4U 1626-67 in the 2-8 keV band, constraining accretion models with combined spectropolarimetric and spectral analysis.

Findings

Pulse-averaged polarization <4% at 95% confidence

Marginal polarization detection (~4.8%) in spectral component

No significant polarization in subdivided energy bands

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.