For some neutron stars ( NSs ) in the binary systems , the masses have been accurately measured .
While for the isolated neutron stars ( INSs ) , no mass measurement has been reported yet .
The situation will change soon thanks to the successful performance of the Neutron Star Interior Composition Explorer ( NICER ) , with which the radius and mass of the isolated \objectname PSR J0030+0451 can be simultaneously measured .
Anyhow , for most INSs , no mass measurements are possible for NICER because of the observational limitations .
Benefited from the recent significant progress made on constraining the equation of state ( EoS ) of NSs , in this work we propose a way to estimate the masses of the INSs with the measured gravitational redshifts .
We apply our method to \objectname RX J1856.5-3754 , \objectname RX J0720.4-3125 , and \objectname RBS 1223 , three members of “ The Magnificent Seven ” ( M7 ) , and estimate their masses to be 1.24 _ { -0.29 } ^ { +0.29 } M _ { \odot } , 1.23 _ { -0.05 } ^ { +0.10 } M _ { \odot } , and 1.08 _ { -0.11 } ^ { +0.20 } M _ { \odot } , respectively .
These masses are consistent with that of binary neutron star ( BNS ) systems , suggesting no evidence for experiencing significant accretion of these isolated objects .