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Redback millisecond pulsars (MSPs) typically show pronounced orbitalvariability in their X-ray emission due to our changing view of the intrabinaryshock (IBS) between the pulsar wind and stellar wind from the companion. Someredbacks ("transitional" MSPs) have shown dramatic changes in theirmultiwavelength properties, indicating a transition from a radio pulsar stateto an accretion-powered state. The redback MSP 47 Tuc W showed clear X-rayorbital variability in the Chandra ACIS-S observations in 2002, which were notdetectable in the longer Chandra HRC-S observations in 2005-06, suggesting thatit might have undergone a state transition. However, the Chandra observationsof 47 Tuc in 2014-15 show similar X-ray orbital variability as in 2002. Weexplain the different X-ray light-curves from these epochs in terms of twocomponents of the X-ray spectrum (soft X-rays from the pulsar, vs. harderX-rays from the IBS), and different sensitivities of the X-ray instrumentsobserving in each epoch. However, when we use our best-fit spectra with HRCresponse files to model the HRC light-curve, we expect a more significant andshorter dip than that observed in the 2005-06 Chandra data. This suggests anintrinsic change in the IBS of the system. We use the ICARUS stellar modellingsoftware, including calculations of heating by an IBS, to model the X-ray,optical, and UV light-curves of 47 Tuc W. Our best-fitting parameters pointtowards a high-inclination system (i~60 deg), which is primarily heated by thepulsar radiation, with an IBS dominated by the companion wind momentum.

On the vanishing orbital X-ray variability of the eclipsing binary millisecond pulsar 47 Tuc W