We observed SAX J1808.4-3658 ( 1808 ) , the first accreting millisecond pulsar , in deep quiescence with XMM-Newton and ( near-simultaneously ) Gemini-South .
The X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and 2006 , describable by an absorbed power-law with photon index 1.74 \pm 0.11 and unabsorbed X-ray luminosity L _ { X } = 7.9 \pm 0.7 \times 10 ^ { 31 } ergs s ^ { -1 } , for N _ { H } = 1.3 \times 10 ^ { 21 } cm ^ { -2 } .
Fitting all the quiescent XMM-Newton X-ray spectra with a power-law , we constrain any thermally emitting neutron star with a hydrogen atmosphere to have a temperature less than 30 eV and L _ { NS } ( 0.01-10 keV ) < 6.2 \times 10 ^ { 30 } ergs s ^ { -1 } .
A thermal plasma model also gives an acceptable fit to the continuum .
Adding a neutron star component to the plasma model produces less stringent constraints on the neutron star ; a temperature of 36 ^ { +4 } _ { -8 } eV and L _ { NS } ( 0.01-10 keV ) = 1.3 ^ { +0.6 } _ { -0.8 } \times 10 ^ { 31 } ergs/s .
In the framework of the current theory of neutron star heating and cooling , the constraints on the thermal luminosity of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these neutron stars .

We compile data from the literature on the mass transfer rates and quiescent thermal flux of the largest possible sample of transient neutron star LMXBs .
We identify a thermal component in the quiescent spectrum of the accreting millisecond pulsar IGR J00291+5934 , which is consistent with the standard cooling model .
The contrast between the cooling rates of IGR J00291+5934 and 1808 suggests that 1808 may have a significantly larger mass .
This can be interpreted as arising from differences in the binary evolution history or initial neutron star mass in these otherwise similar systems .