Rats are generally the responsibility of the owner.
Unless the tenant is filthy, or they have a compost bin outside etc, then you are just an unlucky investor and will have to have someone lay baits and get rid of them at your cost.
It's also advisable to do this ASAP, as Rats in walls and ceilings will chew your water pipes and electricity cables quick smart, and then you'll be up for that cost too.
Matt