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

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.).

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Journal
Astronomy and Astrophysics
Published
2025-01-09
DOI
https://doi.org/10.1051/0004-6361/202451775
Citations
12
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
13.86

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article

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

Tuomo Salmi, Juri Poutanen, F. Coti Zelati, Silvia Zane et al.
12 citations
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
13.86
article

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

Tuomo Salmi, Juri Poutanen, F. Coti Zelati, Silvia Zane, L. Stella, M. Pilia, Anna Bobrikova, Riccardo La Placa, Arianna Miraval Zanon, Alessandro Di Marco, A. Sanna, T. Di Salvo, Alessandro Papitto, L. Burderi, Mason Ng, Christian Malacaria, Tod E. Strohmayer, Giulia Illiano, S. Campana, Filippo Ambrosino, Vladislav Loktev, Fabio La Monaca, M. C. Baglio, Caterina Ballocco, Sinan Long
article en
12 citations

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.).

Astronomy and AstrophysicsVol. 694
University of Rome Tor Vergata (IT), Goddard Space Flight Center (US), Institute of Space Sciences (ES), University of Helsinki (FI), University of Turku (FI), University of Cagliari (IT), Astronomical Observatory of Rome (IT), Agenzia Spaziale Italiana (IT), Space Research Institute (RU), Brera Astronomical Observatory (IT), Institut d'Estudis Espacials de Catalunya (ES), Institute for Space Astrophysics and Planetology (IT), Joint Space Science Institute (US), Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari (IT), Osservatorio Astronomico di Cagliari (IT), Deleted Institution, University College London (GB), Massachusetts Institute of Technology (US), Sapienza University of Rome (IT), National Institute for Astrophysics (IT), University of Amsterdam (NL), University of Palermo (IT)
National Aeronautics and Space Administration, California Institute of Technology, Academy of Finland, Fondazione Cariplo, Suomen Kulttuurirahasto, Nuclear Safety and Security Commission, Agenzia Spaziale Italiana, Istituto Nazionale di Astrofisica, Ministry of Science and Higher Education of the Russian Federation, Istituto Nazionale di Fisica Nucleare, Ministero dell'Università e della Ricerca, Jet Propulsion Laboratory, Marshall Space Flight Center, Goddard Space Flight Center
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