Discovery of polarized X-ray emission from the accreting millisecond pulsar SRGA J144459.2–604207
Coti Zelati, Francesco; Salmi, Tuomo; Papitto, Alessandro; Stella, Luigi; Poutanen, Juri; Zane, Silvia; Sanna, Andrea; Campana, Sergio; Malacaria, Christian; Ng, Mason; Di Marco, Alessandro; La Monaca, Fabio; Pilia, Maura; Bobrikova, Anna; Loktev, Vladislav; Strohmayer, Tod; Di Salvo, Tiziana; Burderi, Luciano; Illiano, Giulia; Ambrosino, Filippo; Cristina Baglio, Maria; Ballocco, Caterina; La Placa, Riccardo; Long, Sinan; Miraval Zanon, Arianna
Italy, Finland, Russia, Netherlands, Spain, United Kingdom, Italia, United States
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 1σ 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 Δtrec increased from 2 to 8 h as a function of the observed count rate C as Δtrec ∝ 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.).