Limits to the Mass and the Radius of the Compact Star in SAX J1808.4−3658 and Their Implications

We show that a survey of equations of state and observations of X-raypulsations from SAX J1808.4-3658 give 2.27 solar mass as the upper limit of thecompact star mass. The corresponding upper limit of the radius comes out to be9.73 km. We also do a probabilistic study to estimate the lower limit of themass of the compact star. Such a limit puts useful constraints on equations ofstate. We also discuss the implications of the upper mass limit for the theevolutionary history of the source, as well as the detection of it in radiofrequencies. We envisage that the possible observation of radio-eclipse may beable to rule out several soft equation of state models, by setting a moderatelyhigh value for the lower limit of inclination angle.Comment: 7 pages, 1 table, 2 figures, accepted for publication in ApJ Letter