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We present a multi-wavelength analysis of gamma-ray burst GRB 090727, forwhich optical emission was detected during the prompt gamma-ray emission by the2-m autonomous robotic Liverpool Telescope and subsequently monitored for afurther two days with the Liverpool and Faulkes telescopes. Within the contextof the standard fireball model, we rule out a reverse shock origin for theearly time optical emission in GRB 090727 and instead conclude that the earlytime optical flash likely corresponds to emission from an internal dissipationprocesses. Putting GRB 090727 into a broader observational and theoreticalcontext, we build a sample of 36 gamma-ray bursts (GRBs) with contemporaneousearly time optical and gamma-ray detections. From these GRBs, we extract asub-sample of 18 GRBs, which show optical peaks during prompt gamma-rayemission, and perform detailed temporal and spectral analysis in gamma-ray,X-ray, and optical bands. We find that in most cases early time opticalemission shows sharp and steep behaviour, and notice a rich diversity ofspectral properties. Using a simple internal shock dissipation model, we showthat the emission during prompt GRB phase can occur at very differentfrequencies via synchrotron radiation. Based on the results obtained fromobservations and simulation, we conclude that the standard external shockinterpretation for early time optical emission is disfavoured in most cases dueto sharp peaks ($\Delta t/t < 1$) and steep rise/decay indices, and thatinternal dissipation can explain the properties of GRBs with optical peaksduring gamma-ray emission.

GRB 090727 and gamma-ray bursts with early time optical emission