Sorry, I thought this was clear.
I want to use a simple astable circuit which oscillates with 56KHz. Like the second one on this page: The 555 Astable Circuit - More Detail.
The output drives a MOSFET, which switches the LED on an off.
Now I'm looking for a way to rapidly switch on and off the whole timer circuit. The Arduino makes an output high for 600us, and the LED should be pulsing with 56KHz during this time. The Arduino makes it's output low and the 555 should stop the oscillating.