The micro-glitch in PSR B1821-24: a case for a strange pulsar?

The single glitch observed in PSR B1821-24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller (delta nu/nu similar to 10-11) than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with B greater than or similar to 1011 G and are young, whereas PSR B1821-24 is an old recycled pulsar with a field strength of 2.25 x 109 G. We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.