Discovery of polarized X-ray emission from the accreting millisecond pulsar SRGA J144459.2–604207

¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, (RM), Italy
² INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ Department of Physics and Astronomy, 20014 University of Turku, Finland
⁴ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶ Department of Physics, PO Box 64 00014 University of Helsinki, Finland
⁷ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁸ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁹ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
¹⁰ INAF/IASF Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy
¹¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy
¹² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
¹³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
¹⁴ Dipartimento di Fisica e Chimica – Emilio Segrè, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy
¹⁵ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
¹⁶ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, (RM), Italy
¹⁷ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁸ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, (CA), Italy
¹⁹ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁰ Joint Space-Science Institute, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

^⋆ Corresponding author; alessandro.papitto@inaf.it

Received: 2 August 2024
Accepted: 7 December 2024

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 Δt_rec increased from 2 to 8 h as a function of the observed count rate C as Δt_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.).