Numerical Simulations of
A Jet in a Star

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. During this time, the cocoon radial energy distribution implies that the optical lightcurve exhibits a fast decay. After a few weeks, when the velocity of the emitting shell is ∼ 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 timescale. After a few months the relativistic outflow decelerates, enters the observer’s line of sight, and powers the peak of the radio lightcurve, which rapidly decays thereafter. The jet (and the inner cocoon) become optically thin to X-rays ∼ 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.
Shocked jets in CCSNe can power the zoo of fast blue optical transients, Gottlieb, Tchekhovskoy & Margutti (2022)

3D visualizations from the first general-relativistic magnetohydrodynamic simulation of jet-cocoon inside stars (top), escaping the star (middle), and after the jet is choked and the cocoon is left as the source of emission (bottom).

A jet that successfully breaks out from a star

The jet is launched into the stellar envelope following the collapse of the stellar core. The jet propagation inflates a hot pressurized bubble, known as "the cocoon". The jet-cocoon structure breaks out from the star. The cocoon spreads sideways while the jet remains narrowly collimated. The jet produces a luminous gamma-ray burst that can be seen only by an observer within the opening angle of the jet. The cocoon releases a wider and fainter gamma-ray flare upon breakout, followed by a cooling emission in X-ray, UV and optical bands. After breaking out, the cocoon spreads sideways and the supernova emission must go through the cocoon material on its way to the observer. As a result, broad absorption features can be seen in the supernova spectrum during the first several days.

Mass density colormap of a 3D simulation. Units are arbitrary and colorbar scale is logarithmic.

A choked jet with a cocoon breakout

The jet launching is terminated while the jet is still inside the star, and the jet is choked, depositing all its energy into the cocoon. The cocoon keeps propagating in the star until it breaks out. Since the jet is chocked, there is no luminous gamma-ray burst. Similarly to the successful jet, the cocoon radiates a faint flare of soft gamma rays upon breakout, followed by X-ray, UV and optical emission. After breaking out the cocoon spreads sideways in a similar way to the case where the jet is successful. The supernova emission must go through the cocoon material on its way to the observer, and broad absorption features can be seen in the supernova spectrum during the first several days. By detecting the signature of the cocoon, we can learn on the hidden jet that was launched following the core collapse. We can place constraints on the jet properties such as its energy and opening angle.

Mass density colormap of a 2D simulation. Units are arbitrary and colorbar scale is logarithmic.

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