While Ubuntu 15.10 (Wily Werewolf) is still using the Linux 3.19 kernel packages of Ubuntu 15.04 (Vivid Vervet), the team is working hard these days to make the switch to the 4.0 series of the Linux kernel. At the moment, they are using the upstream Linux kernel 4.0.4, but tracking of the 4.1 branch continues in the unstable repositories. So, we should expect the switch to Linux kernel 4.0.x for Ubuntu 15.10 by the end of next week.