Cell and particle activation for Radiative Transfer

Dear All

I've been entertaining the idea of disentangling the cases where we declare particles and cells as active for radiative transfer from being exactly the same as for hydro. My motivation behind this is that I expect that in many cases, initially we will have no radiation present, so we could save ourselves a lot of work by not activating RT even though hydro is active as long as there is no radiation present in the particles or the cells.

So my idea was the following:

• Give particles an individual time step for radiation, similarly to how they have one for hydro, one for gravity, etc.
• While the particle doesn't have any radiation, mark it as inactive for RT.
• Define a cell as active for RT analoguely to the way it is done for hydro: Check the minimal active time bin from all its particles

In practice, if this worked somehow, we could also "skip" a couple of hydro time steps in the sense that the hydro could keep its original time step size, and we could treat RT separately.

However, I encountered a problem that I can't really solve: If I mark particles as inactive for RT while they don't have any radiation, they will never turn active. The reason is simple: If they are never active, there is no mechanism except direct injection to propagate radiation to them. So while we could e.g. inject new energy to a star's neighbours, we could never propagate that energy away, because the star's neighbours' neighbours will never turn active.

I'm not sure how to solve this, and honestly I'm not certain how much effort to put into this before we make some sort of plan on how the subcycling is going to work. But I'd really appreciate if some of you had ideas.

Best, Mladen