Discovery of Polarized X-Ray Emission from the Accreting Millisecond Pulsar SRGA J144459.2-604207

• Published in: arXiv.org
• Main Authors: Papitto, Alessandro, Alessandro Di Marco, Poutanen, Juri, Salmi, Tuomo, Illiano, Giulia, Fabio La Monaca, Ambrosino, Filippo, Bobrikova, Anna, Baglio, Maria Cristina, Ballocco, Caterina, Burderi, Luciano, Campana, Sergio, Francesco Coti Zelati, Tiziana Di Salvo, Riccardo La Placa, Loktev, Vladislav, Long, Sinan, Malacaria, Christian, Zanon, Arianna Miraval, Ng, Mason, Pilia, Maura, Sanna, Andrea, Stella, Luigi, Strohmayer, Tod, Zane, Silvia
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 01.08.2024
• Subjects: Bursts; Confidence intervals; Delta rays; Emission; Magnetic properties; Millisecond pulsars; Neutrons; Polarization; Stokes parameters; X ray imagery; XMM (spacecraft)
• ISSN: 2331-8422

We report on 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{\deg} +/- 6{\deg} (East of North; uncertainties quoted at the 1\({\sigma}\) confidence level). The polarized signal shows a significant energy dependence with a degree of 4.0% +/- 0.5% between 3 and 6 keV and < 2% (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 perform a phase-resolved polarimetric analysis of IXPE data. We did not detect any significant variability of the Stokes parameters Q and U with the spin and the orbital phases. We used the relativistic rotating vector model to show that a moderately fan-beam emission from two point-like spots at a small magnetic obliquity (\(\simeq\) 10{\deg}) is compatible with the observed pulse profile and polarization properties. IXPE also detected 52 type-I X-ray bursts, with a recurrence time \(\Delta t_{rec}\) increasing from 2 to 8 h as a function of the observed count rate \(C\) as as \(\Delta t_{rec} \simeq C^{-0.8}\) We stacked the emission observed during all the bursts and obtained an upper limit on the polarization degree of 8.5% (90% c.l.).