The X-ray emission of RXJ1856.5-3754 has been found to coincide to unprecedented accuracy with that of a blackbody , of radius 5.8 \pm 0.9 km for the measured parallax distance of 140 pc ( Burwitz et al .
2001 , Drake et al .
2002 ) .
If the emission is uniform over the whole surface of a non-rotating star , the mass of the star can not exceed 0.75 \pm 0.12 M _ { \odot } regardless of its composition .
If the compact object is a quark star described by the MIT-bag equation of state ( a “ strange star ” ) , the mass is no more than 0.3 M _ { \odot } .
Comparably small masses are also obtained for the X-ray bursters Aql X-1 and KS1731-260 for some fits to their spectra .