GRB 990123: Evidence that the gamma rays come from a central engine

GRB 990123 was a long, complex gamma-ray burst with an optical transient that started early within the gamma-ray phase. The peak and power-law decay of the early optical emission strongly indicate 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 a factor between 1.25 and 2.3, depending on the angular extent of the shell. We analyze the fine time structure observed in the gamma-ray data from BATSE and determine that the width of the peaks does not increase as expected for a decelerating shell; the later pulses are only 1.034 +/- 0.035 longer than the earlier pulses. 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.