Energy-resolved pulse profiles of Vela X-1 Cross-calibrating XMM-Newton and NuSTAR to trace spectral features

Context. Pulse profiles probe the emission geometry of accreting X-ray pulsars, but their observed shapes may depend on instrumental response and observational setup. The pulsed fraction spectrum provides a compact spectro-timing observable that can both trace localized spectral features and serve as a quantitative cross-calibration diagnostic.
Aims. We assess the consistency of energy-resolved pulse profiles obtained with simultaneous XMM-Newton/EPIC-pn and NuSTAR/FPM observations of Vela X-1 and investigate the broadband pulsed fraction spectrum as a diagnostic of spectral features from 1 to 70 keV.
Methods. We constructed energy–phase matrices for both instruments and derived pulsed fraction spectra after carefully accounting for instrumental and observational effects. We quantified the residual systematics in the overlapping 3–10 keV band. We then modeled the broadband pulsed fraction spectra phenomenologically and searched for timing signatures of spectral features.
Results. After correcting for instrumental effects, the pulsed fraction spectra derived strictly over the common exposure intervals of the two instruments agree within 5% in their overlapping 3–10 keV range. Remaining discrepancies larger than 5% are confined to the iron-line region and can be attributed to the different energy resolutions of the two instruments. The broadband pulsed fraction spectrum reveals significant localized features corresponding to known emission lines in the soft band and to the fundamental and harmonic cyclotron resonant scattering features at ∼25 and ∼55 keV. An orbital-phase-resolved modeling of the pulsed fraction spectrum of EPIC-pn shows that the soft band features strongly depend on the value of the equivalent absorption column, with emission line signatures becoming progressively suppressed during highly absorbed intervals.
Conclusions. The pulsed fraction spectrum serves both as a quantitative cross-calibration diagnostic and as a powerful spectro-timing diagnostic. Its modeling provides independent constraints on spectral features, complementing traditional phase-averaged spectroscopy.