Using extensive HST imaging , combined with Chandra X-ray and Parkes radio data , we have detected the optical binary companion to a second millisecond pulsar ( MSP ) in the globular cluster 47 Tucanae .
This faint ( V = 22.3 ) blue ( V - I = 0.7 ) star shows a large amplitude ( 60–70 % ) sinusoidal variation in both V and I .
The period ( 3.19066 hr ) and phase of the variation match those of the MSP 47 Tuc W ( which does not have an accurate radio timing position ) to within 0.5 seconds and 1.2 minutes respectively , well within the 1 \sigma errors .
The phase dependence of the intensity and color implies that heating of a tidally locked companion causes the observed variations .
The eclipsing nature of this MSP in the radio , combined with the relatively large companion mass ( > 0.13 M _ { \odot } ) and the companion ’ s position in the color-magnitude diagram , suggest that the companion is a main sequence star , a rare circumstance for an MSP companion .
This system is likely to have had a complex evolution and represents an interesting case study in MSP irradiation of a close companion .
We present evidence for another optical variable with similar properties to the companion of 47 Tuc W. This variable may also be an MSP companion , although no radio counterpart has yet been detected .