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

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
² ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ Department of Physics and Astronomy, 20014 University of Turku, Finland
⁴ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁵ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁷ Department of Physics, P.O. Box 64 00014 University of Helsinki, Finland

^⋆ Corresponding author: menglei.zhou@astro.uni-tuebingen.de

Received: 2 April 2025
Accepted: 23 July 2025

Abstract

We present observations of the accretion-powered X-ray pulsar 4U 1907+09 conducted with the Imaging X-ray Polarimetry Explorer, which has delivered 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 a phase-averaged polarization degree (PD) of 6.0 ± 1.6% and a polarization angle (PA) of 69° ±8°. The second observation provides weaker constraints on the polarimetric properties, PD = 2.2 ± 1.6% and PA = 46° ±23°, as determined from the spectro-polarimetric analysis. Combining the data from the two observations results in PD = 3.7 ± 1.1% and PA = 63° ±9°. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7σ. In the phase-resolved analyses, we observe a potential PA rotation of approximately 90° 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.