Identify the origin of phase-dependent spectral modulation

Identify which physical quantities in the thermal components of EP J223759.5+531421 vary with rotational phase and thereby produce the observed phase-dependent X-ray spectral modulation.

Background

Phase-resolved spectroscopy yields statistically comparable fits under different parameterizations. In representative fits, the inferred radii of thermal components vary with phase, but alternative models allow different thermal parameters to vary while maintaining comparable goodness of fit.

Consequently, the present data do not establish whether the modulation is driven primarily by changes in emitting areas, temperatures, or other spectral-component properties.

References

Different choices of free parameters for the thermal components provide statistically comparable descriptions of the phase-resolved spectra, implying that the present data do not uniquely identify which physical quantities are responsible for the spectral modulation.

— Einstein Probe discovery of the magnetar EP J223759.5+531421  (2609.25990 - Rea et al., 22 Sep 2026) in Discussion, paragraph beginning “This picture is supported by phase-resolved spectroscopy”