Discovery of polarized X-ray emission from the accreting millisecond pulsar SRGA J144459.2–604207
Alessandro Papitto, Alessandro Di Marco, Juri Poutanen, Tuomo Salmi, Giulia Illiano, Fabio La Monaca, Filippo Ambrosino, Anna Bobrikova, M. C. Baglio, Caterina Ballocco, L. Burderi, S. Campana, F. Coti Zelati, T. Di Salvo, Riccardo La Placa, Vladislav Loktev, Sinan Long, Christian Malacaria, Arianna Miraval Zanon, Mason Ng, M. Pilia, A. Sanna, L. Stella, Tod E. Strohmayer, Silvia Zane
Abstract
We report the discovery of polarized X-ray emission from an accreting millisecond pulsar. During a 10-day-long coverage of the February 2024 outburst of SRGA J144459.2−604207, the Imaging X-ray Polarimetry Explorer (IXPE) detected an average polarization degree of the 2–8 keV emission of 2.3%±0.4% at an angle of 59° ±6° (east of north; the uncertainties quoted are at the 1 σ confidence level). The polarized signal shows a significant energy dependence with a degree of 4.0%±0.5% between 3 and 6 keV and < 1.5% (90% c.l.) in the 2–3 keV range. We used NICER, XMM–Newton, and NuSTAR observations to obtain an accurate pulse-timing solution and to perform a phase-resolved polarimetric analysis of IXPE data. We did not detect any significant variability in the Stokes parameters Q and U with the spin and orbital phases. We used the relativistic rotating-vector model to show that a moderately fan-beam emission from two point-like spots at low magnetic obliquity (≃10°) is compatible with the observed pulse profile and polarization properties. IXPE also detected 52 type I X-ray bursts whose recurrence time Δ t rec increased from 2 to 8 h as a function of the observed count rate C as Δ t rec ∝ C −0.8 . We stacked the emission observed during all the bursts and obtained an upper limit on the polarization degree of 8.5% (90% c.l.).