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Type Ia supernovae (SNe Ia) arise from the thermonuclear explosion ofcarbon-oxygen white dwarfs. Though the uniformity of their light curves makesthem powerful cosmological distance indicators, long-standing issues remainregarding their progenitors and explosion mechanisms. Recent detection of theearly ultraviolet pulse of a peculiar subluminous SN Ia has been claimed as newevidence for the companion-ejecta interaction through the single-degeneratechannel. Here, we report the discovery of a prominent but red optical flash at$\sim$ 0.5 days after the explosion of a SN Ia which shows hybrid features ofdifferent SN Ia sub-classes: a light curve typical of normal-brightness SNe Ia,but with strong titanium absorptions, commonly seen in the spectra ofsubluminous ones. We argue that the early flash of such a hybrid SN Ia isdifferent from predictions of previously suggested scenarios such as thecompanion-ejecta interaction. Instead it can be naturally explained by a SNexplosion triggered by a detonation of a thin helium shell either on anear-Chandrasekhar-mass white dwarf ($\gtrsim$ 1.3 M$_{\odot}$) with low-yield$^{56}$Ni or on a sub-Chandrasekhar-mass white dwarf ($\sim$ 1.0 M$_{\odot}$)merging with a less massive white dwarf. This finding provides compellingevidence that one branch of the previously proposed explosion models, thehelium-ignition scenario, does exist in nature, and such a scenario may accountfor explosions of white dwarfs in a wider mass range in contrast to what waspreviously supposed.

A hybrid type Ia supernova with an early flash triggered by helium-shell detonation