GRB990123 was a long complex gamma-ray burst with an optical transient that started early within the gamma-ray phase ( [ Akerlof et al .
1999a , Akerlof et al .
1999b ] ) .
The peak and power law decay of the early optical emission strongly indicates the presence of a decelerating relativistic shell during that phase .
Prior to this burst , it was not known if the shell decelerated during the burst , so an external shock origin for the gamma rays was still possible .
If the gamma-rays are produced in the external shock , then the pulse widths should reflect the observed deceleration of the shell and increase by about 2.3 .
We analyze the fine time structure observed in the gamma-ray data from BATSE and determine that the width of the peaks do not increase as expected for a decelerating shell ; the later pulses are , at most , a factor of 1.15 longer than the earlier pulses .
We also analyze the variability to determine what fraction of the shell ’ s surface could be involved in the production of the gamma rays , the so-called surface filling factor .
For GRB990123 we find a filling factor of 0.008 .
The lack of pulse width evolution eliminates the only remaining kinematically acceptable external shock explanation for the gamma-ray phase and , thus , the gamma rays must originate at a central engine .