Confirm enhanced X-ray polarisation during type-I bursts

Confirm through future X-ray polarimetric observations whether the indication of enhanced polarisation during the type-I X-ray burst from SAX J1808.4–3658 is genuine and determine its physical origin.

Background

IXPE detected a tentative 2–6 keV polarisation degree of 16% ± 6% during the peak of a type-I X-ray burst from SAX J1808.4–3658, with a polarisation angle approximately orthogonal to that of the persistent emission. The detection has only 2.5σ significance, so its reality requires confirmation by future observations.

If confirmed, the enhanced polarisation could arise from scattering or reflection involving the accretion disc, a disc wind, a vertically extended structure, or an accretion flow. Establishing whether the signal is real and identifying the responsible scattering geometry would clarify how type-I X-ray bursts affect accretion inflows.

References

If confirmed by future observations, the indication of enhanced polarisation during the burst could be attributed to the scattering or reflection of the incident emission, driven by a change in either the angular profile of the incident emission or the geometry of the scatterer/reflector relative to the persistent state.

— 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  (2610.09477 - Papitto et al., 7 Oct 2026) in Section 3, Discussion and conclusions