Tang, SM (reprint author), Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China.
Aims. The prompt optical emission contemporaneous with the gamma-rays from gamma-ray bursts (GRBs) carries important information on the central engine and explosion mechanism. We study the time lag between prompt optical emission and gamma-rays in GRB 990123 and GRB 041219a, which are the only two GRBs detected at optical wavelengths during the ascending burst phase. Methods. Assuming profiles of prompt optical light curves are the same as the prompt gamma-rays, we simulate optical light curves with different time lags and compare them with the observed optical flux. Then the best fit time lag and its error are determined by chi-squared values. Results. We find that time lags between prompt optical emission and gamma-rays in GRB host galaxy rest-frames are consistent in the two GRBs, which is 5 similar to 7 s for GRB 990123 and 1 similar to 5 s for GRB 041219a. This result is consistent with a common origin of prompt optical and gamma-ray emissions in the two GRBs. Based on synchrotron cooling model, we also derive the parameters for the two GRBs.