The accretion-powered millisecond pulsar IGR J00291 + 5934 underwent two \sim 10 d long outbursts during 2008 , separated by 30 d in quiescence .
Such a short quiescent period between outbursts has never been seen before from a neutron star X-ray transient .
X-ray pulsations at the 599 Hz spin frequency are detected throughout both outbursts .
For the first time , we derive a pulse phase model that connects two outbursts , providing a long baseline for spin frequency measurement .
Comparison with the frequency measured during the 2004 outburst of this source gives a spin-down during quiescence of - ( 4 \pm 1 ) \times 10 ^ { -15 } Hz s ^ { -1 } , approximately an order of magnitude larger than the long-term spin-down observed in the 401 Hz accretion-powered pulsar SAX J1808.4 - 3658 .
If this spin-down is due to magnetic dipole radiation , it requires a 2 \times 10 ^ { 8 } G field strength , and its high spin-down luminosity may be detectable with the Fermi Large Area Telescope .
Alternatively , this large spin-down could be produced by gravitational wave emission from a fractional mass quadrupole moment of Q / I = 1 \times 10 ^ { -9 } .
The rapid succession of the outbursts also provides a unique test of models for accretion in low-mass X-ray binaries .
Disk instability models generally predict that an outburst will leave the accretion disk too depleted to fuel a second outburst after such a brief quiescence .
We suggest a modification in which the outburst is shut off by the onset of a propeller effect before the disk is depleted .
This model can explain the short quiescence and the unusually slow rise of the light curve of the second 2008 outburst .