Polarised X-ray emission from the accreting millisecond pulsar SAX J1808.4-3658 during an X-ray outburst and a type-I X-ray burst
Abstract
Modelling the X-ray pulse profiles of accreting millisecond pulsars (AMSPs) is a key technique to constrain the masses and radii of neutron stars. To this end, measuring the properties of polarised X-rays provides crucial information on the system geometry. We present the significant detection by IXPE of linearly polarised 2-8 keV X-rays during the decay of the 2025 outburst of the 2.5 ms AMSP SAX J1808.4-3658, with an average polarisation degree of 2.9%±0.5% and a polarisation angle of 46°±5°, broadly consistent with the direction observed in the optical band. This level of polarisation is similar to that found in the only other source of this class that has been targeted so far. By using a pulse-phase timing solution obtained with IXPE and NuSTAR, which confirms the long-term evolution of the pulsar spin and orbit, we did not find any significant pulse phase-resolved variability in the Stokes parameters. Interpreting the polarised X-rays as originating from hot spots on the NS surface indicates they are almost aligned with the spin axis. We also report an indication for linearly polarised emission with a degree of 16%±6% (2-6 keV) during the peak of a type-I thermonuclear X-ray burst, with an angle almost orthogonal to that of the persistent emission. If confirmed, this level would exceed expectations for X-rays reflected by a thin accretion disc.
Related
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- On the assessment of the disk truncation and detection of type-II-like bursts from the accreting millisecond X-ray Pulsar IGR J17062-6143
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- A Massive and Eccentric Binary Millisecond Pulsar in the Globular Cluster M10
BibTeX
@misc{papitto2026polarised,
title = {{Polarised X-ray emission from the accreting millisecond pulsar SAX J1808.4-3658 during an X-ray outburst and a type-I X-ray burst}},
author = {A. Papitto and A. Di Marco and M. C. Baglio and D. Baldi Tomei and C. Ballocco and G. Illiano and R. La Placa and C. Malacaria and A. Bobrikova and J. Poutanen and T. Salmi and A. Sanna and A. Veledina and F. Ambrosino and L. Burderi and S. Campana and F. Coti Zelati and D. de Martino and T. Di Salvo and G. L. Israel and F. La Monaca and A. Marino and A. Miraval Zanon and M. Pilia and L. Stella},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.09477},
eprint = {2610.09477},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.09477},
}
RIS
TY - UNPB
TI - Polarised X-ray emission from the accreting millisecond pulsar SAX J1808.4-3658 during an X-ray outburst and a type-I X-ray burst
AU - A. Papitto
AU - A. Di Marco
AU - M. C. Baglio
AU - D. Baldi Tomei
AU - C. Ballocco
AU - G. Illiano
AU - R. La Placa
AU - C. Malacaria
AU - A. Bobrikova
AU - J. Poutanen
AU - T. Salmi
AU - A. Sanna
AU - A. Veledina
AU - F. Ambrosino
AU - L. Burderi
AU - S. Campana
AU - F. Coti Zelati
AU - D. de Martino
AU - T. Di Salvo
AU - G. L. Israel
AU - F. La Monaca
AU - A. Marino
AU - A. Miraval Zanon
AU - M. Pilia
AU - L. Stella
PY - 2026
DA - 2026/10/07
PB - arXiv
DO - 10.48550/arxiv.2610.09477
UR - https://arxiv.org/abs/2610.09477
AB - Modelling the X-ray pulse profiles of accreting millisecond pulsars (AMSPs) is a key technique to constrain the masses and radii of neutron stars. To this end, measuring the properties of polarised X-rays provides crucial information on the system geometry. We present the significant detection by IXPE of linearly polarised 2-8 keV X-rays during the decay of the 2025 outburst of the 2.5 ms AMSP SAX J1808.4-3658, with an average polarisation degree of 2.9%±0.5% and a polarisation angle of 46°±5°, broadly consistent with the direction observed in the optical band. This level of polarisation is similar to that found in the only other source of this class that has been targeted so far. By using a pulse-phase timing solution obtained with IXPE and NuSTAR, which confirms the long-term evolution of the pulsar spin and orbit, we did not find any significant pulse phase-resolved variability in the Stokes parameters. Interpreting the polarised X-rays as originating from hot spots on the NS surface indicates they are almost aligned with the spin axis. We also report an indication for linearly polarised emission with a degree of 16%±6% (2-6 keV) during the peak of a type-I thermonuclear X-ray burst, with an angle almost orthogonal to that of the persistent emission. If confirmed, this level would exceed expectations for X-rays reflected by a thin accretion disc.
ER -