Radiative diffusion in a time-dependent outflow: a model for fast blue optical transients

Fast Blue Optical Transients (FBOTs) are luminous transients with fast evolving (typically trise < 12 days) light curve and blue color (usually−0.2 > g−r > −0.3)that cannot be explained by a supernova-like explosion. We propose a radiative diffusion in a time-dependent outflow model to interpret such special transients. In this model, we assume a stellar-mass black hole is formed from stellar core-collapse. As a central engine, the black hole accretes the infalling stellar envelope material via an accretion disk. Due to the extremely super- Eddington accretion rate, the disk ejects continuous outflow during a few days. We consider the ejection of the outflow to be time-dependent. The outflow is optically thick initially and photons are frozen in it. As the outflow expands over time, photons gradually escape, and our work is to model such an evolution. Numerical and analytical calculations are considered separately, and the results are consistent. We apply the model to three typical FBOTs: PS1-10bjp, ZTF18abukavn, and ATLAS19dqr. The modeling finds the total mass of the outflow (∼ 1M⊙), and the total time of the ejection (∼ a few days) for them, leading us to speculate that they may be the result of the collapse of massive stars.

THE ZADKO TELESCOPE RESULTS: TEN YEARS OF SCIENCE

The 1.0 meter f/4 fast-slew Zadko telescope is located in Western Australia, approximately seventy kilometers north of Perth at Yeal in the Shire of Gingin in a dedicated "low-luminosity" area. It is the only meter class optical research facility at this southern latitude between the east coast of Australia and South Africa and can rapidly image optical transients at a longitude not monitored by other similar facilities. We review here the main results achieved during the last decade and give some points toward the goals set for future years. Finally we discuss the modifications and improvements we had to perform in the facility to reach these new goals.