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 th at started early within the gamma-ray phase. The peak and power-law decay o f the early optical emission strongly indicate the presence of a decelerati ng relativistic shell during that phase. Prior to this burst, it was not kn own 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 th e external shock, then the pulse widths should reflect the observed deceler ation 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 obs erved 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 pul ses are only 1.034 +/- 0.035 longer than the earlier pulses. The lack of pu lse width evolution eliminates the only remaining kinematically acceptable external shock explanation for the gamma-ray phase and, thus, the gamma ray s must originate at a central engine.