Millisecond X-Ray Pulsar SAX J1808.4-3658: Limits to the Mass and the Radius of the Compact Star

Abstract

We predict from a survey of equations of state and observations of X-ray pulsations from SAX J1808.4-3658, that the upper limit of the mass of the compact star is 2.27 solar mass. The corresponding upper limit of the radius comes out to be 9.73 km. We also do a study to estimate the lower limit of the mass of the compact star. Such a limit is very useful to put constraints on equations of state. We also discuss the implications of the upper mass limit for the detection of the source in radio frequencies. We point out that the possible observation of radio-eclipse may be able to rule out several soft equation of state models, by setting a moderately high value for the lower limit of inclination angle.