Testing the External Shock Model of Gamma-Ray Bursts using the Late-Time Simultaneous Optical and X-ray Afterglows

We study the ``normal'' decay phase of the X-ray afterglows of gamma-ray bursts (GRBs), which follows the shallow decay phase, using the events simultaneously observed in the R-band. The classical external shock model -- in which neither the delayed energy injection nor time-dependency of shock micro-physics is considered -- shows that the decay indices of the X-ray and R-band light curves, $\alpha_{\rm X}$ and $\alpha_{\rm O}$, obey a certain relation, and that in particular, $\alpha_{\rm O}-\alpha_{\rm X}$ should be larger than -1/4 unless the ambient density increases with the distance from the central engine. For our selected 14 samples, we have found that 4 events violate the limit at more than the 3$\sigma$ level, so that a fraction of events are outliers of the classical external shock model at the ``normal'' decay phase.Comment: Accepted for publication in ApJL. 12 page, 2 figures, 2 table

Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment

Ia型超新星の爆発直後の閃光を捉えることに成功 --特異な爆発に至る恒星進化の謎に迫る--. 京都大学プレスリリース. 2021-12-10.In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hr of the supernova explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen Camera. The fast evolution of the early flash was captured by intensive intranight observations via the Tomo-e Gozen high-cadence survey. Numerical simulations show that such a prominent and fast early emission is most likely generated from an interaction between 0.01 M⊙ circumstellar material (CSM) extending to a distance of ∼10¹³ cm and supernova ejecta soon after the explosion, indicating a confined dense CSM formation at the final evolution stage of the progenitor of SN 2020hvf. Based on the CSM–ejecta interaction-induced early flash, the overluminous light curve, and the high ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf (“super-MCh WD”). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-MCh WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova