We present the results of numerical simulations of the spherically symmetric gravitational collapse of supermassive stars ( SMS ) .
The collapse is studied using a general relativistic hydrodynamics code .
The coupled system of Einstein and fluid equations is solved employing observer time coordinates , by foliating the spacetime by means of outgoing null hypersurfaces .
The code contains an equation of state which includes effects due to radiation , electrons and baryons , and detailed microphysics to account for electron-positron pairs .
In addition energy losses by thermal neutrino emission are included .
We are able to follow the collapse of SMS from the onset of instability up to the point of black hole formation .
Several SMS with masses in the range 5 \times 10 ^ { 5 } M _ { \odot } -10 ^ { 9 } M _ { \odot } are simulated .
In all models an apparent horizon forms initially , enclosing the innermost 25 % of the stellar mass .
From the computed neutrino luminosities , estimates of the energy deposition by \nu \bar { \nu } -annihilation are obtained .
Only a small fraction of this energy is deposited near the surface of the star , where , as proposed recently by Fuller & Shi ( 1998 ) , it could cause the ultrarelativistic flow believed to be responsible for \gamma -ray bursts .
Our simulations show that for collapsing SMS with masses larger than 5 \times 10 ^ { 5 } M _ { \odot } the energy deposition is at least two orders of magnitude too small to explain the energetics of observed long-duration bursts at cosmological redshifts .
In addition , in the absence of rotational effects the energy is deposited in a region containing most of the stellar mass .
Therefore relativistic ejection of matter is impossible .