Papitto, AlessandroAlessandroPapittoDi Marco, AlessandroAlessandroDi Marco0000-0003-4795-7072Poutanen, JuriJuriPoutanen0000-0002-1159-4460Salmi, TuomoTuomoSalmiIlliano, GiuliaGiuliaIlliano0000-0001-5458-891XLa Monaca, FabioFabioLa Monaca0000-0001-6278-1576Ambrosino, FilippoFilippoAmbrosinoBobrikova, AnnaAnnaBobrikova0000-0001-7397-8091Baglio, Maria CristinaMaria CristinaBaglio0000-0003-2810-2394Ballocco, CaterinaCaterinaBalloccoBurderi, LucianoLucianoBurderi0000-0001-8916-4156Campana, SergioSergioCampana0000-0001-6289-7413Coti Zelati, FrancescoFrancescoCoti Zelati0000-0002-0018-1687Di Salvo, TizianaTizianaDi Salvo0000-0001-7915-996XLa Placa, RiccardoRiccardoLa PlacaLoktev, VladislavVladislavLoktevLong, SinanSinanLongMalacaria, ChristianChristianMalacariaMiraval Zanon, AriannaAriannaMiraval ZanonNg, MasonMasonNgPilia, MauraMauraPiliaSanna, AndreaAndreaSannaStella, LuigiLuigiStellaStrohmayer, TodTodStrohmayerZane, SilviaSilviaZane2024-12-092024-12-092024http://hdl.handle.net/20.500.12386/40876We 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° +/- 6° (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°) 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.).Astrophysics - High Energy Astrophysical Phenomenaeprint10.48550/arXiv.2408.006082024arXiv240800608P