  To constrain the equation of state of super-nuclear density matter and probe
the interior composition of the X-ray pulsar in SAX J1808.4-3658. In our
estimation, we consider both its persistent 2.49 ms X-ray pulsations discovered
by Wijnands and van der Klis from using the Rossi X-ray Timing Explorer, which
is interpreted to come from an accreting-powered millisecond X-ray pulsar in
the low mass X-ray binaries, and the corresponding mass-radius data analyzed of
the light curves of SAX J1808.4-3685 during its 1998 and 2005 outbursts by
Leahy et al. from assuming a hot spot model where the X-rays are originated
from the surface of the neutron star.
