Mysteries of Mass: Some Contrarian Views From an Experimenter

We report on the X-ray aperiodic timing analysis of two accreting millisecondpulsars: XTE J1807-294 and IGR J00291+5934. On the one hand, we discovered inXTE J1807-294 seven pairs of simultaneous kilohertz quasi-periodic oscillations(kHz QPOs) separated in frequency by nearly the spin frequency of the neutronstar. This confirms the suspected dichotomy in the frequency separation of kHzQPOs: sometimes once and sometimes half the spin frequency. On the other hand,we found an extreme behavior in the power spectra of IGR J00291+5934: verystrong variability at very low frequencies. Namely, the fractional amplitude ofthe variability was ~50%, the highest value found so far in a neutron starsystem.Comment: To appear in the proceedings of the conference `The Multicoloured Landscape of Compact Objects and their Explosive Origins' (Cefalu, Sicily, June 2006), AI