We combine electromagnetic ( EM ) and gravitational wave ( GW ) information on the binary neutron star ( NS ) merger GW170817 in order to constrain the radii R _ { ns } and maximum mass M _ { max } of NSs .
GW170817 was followed by a range of EM counterparts , including a weak gamma-ray burst ( GRB ) , kilonova ( KN ) emission from the radioactive decay of the merger ejecta , and X-ray/radio emission consistent with being the synchrotron afterglow of a more powerful off-axis jet .
The type of compact remnant produced in the immediate merger aftermath , and its predicted EM signal , depend sensitively on the high-density NS equation of state ( EOS ) .
For a soft EOS which supports a low M _ { max } , the merger undergoes a prompt collapse accompanied by a small quantity of shock-heated or disk wind ejecta , inconsistent with the large quantity \gtrsim 10 ^ { -2 } M _ { \odot } of lanthanide-free ejecta inferred from the KN .
On the other hand , if M _ { max } is sufficiently large , then the merger product is a rapidly-rotating supramassive NS ( SMNS ) , which must spin-down before collapsing into a black hole .
A fraction of the enormous rotational energy necessarily released by the SMNS during this process is transferred to the ejecta , either into the GRB jet ( energy E _ { GRB } ) or the KN ejecta ( energy E _ { ej } ) , also inconsistent with observations .
By combining the total binary mass of GW170817 inferred from the GW signal with conservative upper limits on E _ { GRB } and E _ { ej } from EM observations , we constrain the likelihood probability of a wide-range of previously-allowed EOS .
These two constraints delineate an allowed region of the M _ { max } - R _ { ns } parameter space , which once marginalized over NS radius places an upper limit of M _ { max } \lesssim 2.17 M _ { \odot } ( 90 % ) , which is tighter or arguably less model-dependent than other current constraints .