A Compact Star Rotating at 1122 Hz and the r-Mode Instability

We show that r-mode instabilities severely constrain the composition of a compact star rotating with a submillisecond period. In particular, the only viable astrophysical scenario for such an object, present inside the low-mass X-ray binary associated with the X-ray transient XTE J1739–285, is that it has a strangeness content. Since previous analyses indicate that hyperonic stars or stars containing a kaon condensate are not good candidates, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star. We also discuss under which conditions submillisecond pulsars are rare.