Interior matter estimates of the X-ray pulsar in SAX J1808.4-3658 from   mass-radius and rotation measurements
  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. 
Nana Pan, Li Zhang, Xiaoping Zheng