The transition from prompt to afterglow emission is one of the most exciting and least understood phases in gamma-ray bursts (GRBs). Correlations among optical, X-ray, and gamma-ray emission in GRBs have been explored, to attempt to answer whether the earliest optical emission comes from internal and/or external shocks. We present optical photometric observations of GRB 180325A collected with the TAROT and RATIR groundbased telescopes. These observations show two strong optical flashes with separate peaks at ~50 and ~120 s, followed by a temporally extended optical emission. We also present X-rays and gamma-ray observations of GRB 180325A, detected by the Burst Alert Telescope and X-ray Telescope, on the Neil Gehrels Swift observatory, which both observed a narrow flash at ~80 s. We show that the prompt gamma-ray and X-ray early emission shares similar temporal and spectral features consistent with internal dissipation within the relativistic outflow (e.g., by internal shocks or magnetic reconnection), while the early optical flashes are likely generated by the reverse shock that decelerates the ejecta as it sweeps up the external medium.
Bibliographical noteFunding Information:
We thank the referee for useful comments that helped to improve this article. We thank Fr?deric Daigne for useful discussions. We thank the staff of the Observatorio Astron?mico Nacional on Sierra San Pedro M?rtir. RATIR is a collaboration between the University of California, the Universidad Nacional Auton?ma de M?xico, NASA Goddard Space Flight Center, and Arizona State University, benefiting from the loan of an H2RG detector and hardware and software support from Teledyne Scientific and Imaging. RATIR, the automation of the Harold L. Johnson Telescope of the Observatorio Astron?mico Nacional on Sierra San Pedro M?rtir, and the operation of both are funded through NASA grants NNX09AH71G, NNX09AT02G, NNX10AI27G, and NNX12AE66G, CONACyT grants INFR- 2009-01-122785 and CB-2008-101958, UNAM PAPIIT grants IG100414, IA102917, UC MEXUS-CONACyT grant CN 09- 283, and the Instituto de Astronom?a of the Universidad Nacional Auton?ma de M?xico. We acknowledge the vital contributions of Neil Gehrels and Leonid Georgiev to the early development of RATIR. TAROT has been built with the support of the Institut National des Sciences de l'Univers, CNRS, France. TAROT is funded by the CNES and is thankful for the help of the technical staff of the Observatoire de Haute Provence, OSU-Pytheas. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. R.L.B. acknowledges support from the DGAPA-UNAM postdoctoral fellowship. We acknowledge support from the UNAM-PAPIIT grant AG100820 and AG100317. J.C., S.L., and R.F.G. acknowledge support from PAPIIT/UNAM IG100218, IN101418, IN107120. J.G. acknowledges support from the ISF-NSFC joint research program (grant No. 3296/19). A.T.A. is thankful for the support of the Czech Science Foundation under grant GA?R 20-19854S. The work of F.D.C., Y.G., and E.T. was partially performed at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611.
© 2021. The Author(s). Published by the American Astronomical Society.