Probing the polarized emission from the accretion-powered pulsar 4U 1907+09 with IXPE

Abstract

We present observations of the accretion-powered X-ray pulsar (XRP) 4U 1907+09, conducted with the Imaging X-ray Polarimetry Explorer (IXPE) that delivers the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods close to the periastron. We observe a stronger polarization in the first observation, with the phase-averaged polarization degree (PD) of $6.0\pm 1.6\%$ and polarization angle (PA) of $69^{\circ }\pm 8^{\circ }$. In contrast, the second observation provides weaker constraints on the polarimetric properties, with a PD=$2.2\pm 1.6\%$ and a PA=$46^{\circ }\pm 23^{\circ }$, as determined from the spectral-polarimetric analysis. Combining the data from both observations results in a PD=$3.7\pm 1.1\%$ and a PA=$63^{\circ }\pm 9^{\circ }$. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7 $\sigma$. In the phase-resolved analyses, we observe a potential PA rotation by approximately $90^{\circ }$ between adjacent energy bands (4--5 and 5--6 keV) within the single phase bin of 0.25--0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar's geometry remains stable during flare events.

9 pages, 9 figures, 6 tables. Accepted for publication in Astronomy & Astrophysics

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