Shocked jets in CCSNe can power the zoo of fast blue optical transients

Evidence is mounting that recent multiwavelength detections of fast blue optical transients (FBOTs) in star-forming galaxies comprise a new class of transients, whose origin is yet to be understood. We show that hydrogen-rich collapsing stars that launch relativistic jets near the central engine can naturally explain the entire set of FBOT observables. The jet-star interaction forms a mildly relativistic shocked jet (inner cocoon) component, which powers cooling emission that dominates the high velocity optical signal during the first few weeks, with a typical energy of similar to 10(50)-10(51) erg. During this time, the cocoon radial energy distribution implies that the optical light curve exhibits a fast decay of L ((alpha)under-tilde) t(-2)(.4). After a few weeks, when the velocity of the emitting shell is similar to 0.01 c, the cocoon becomes transparent, and the cooling envelope governs the emission. The interaction between the cocoon and the dense circumstellar winds generates synchrotron self-absorbed emission in the radio bands, featuring a steady rise on a month time-scale. After a few months the relativistic outflow decelerates, enters the observer's line of sight, and powers the peak of the radio light curve, which rapidly decays thereafter. The jet (and the inner cocoon) becomes optically thin to X-rays similar to day after the collapse, allowing X-ray photons to diffuse from the central engine that launched the jet to the observer. Cocoon cooling emission is expected at higher volumetric rates than gamma-ray bursts (GRBs) by a factor of a few, similar to FBOTs. We rule out uncollimated outflows, however, both GRB jets and failed collimated jets are compatible with all observables.

Automated summary

We propose a model to explain fast blue optical transients (FBOTs) as relativistic jets launched by hydrogen-rich collapsing stars. The interaction between the jet and the star forms an inner cocoon, which produces cooling emission dominant in the high velocity optical signal. The interaction between the cocoon and the circumstellar winds generates synchrotron self-absorbed emission in the radio bands. After deceleration, the relativistic outflow powers the peak of the radio light curve and becomes optically thin to X-rays. The volumetric rates of cooling emission in FBOTs are higher than gamma-ray bursts.