This is achievable with a simple photoresistor and transistor arrangement - real fireflies actually synchronize this way. Adding a CdS photocell in series with

This is achievable with a simple photoresistor and transistor arrangement - real fireflies actually synchronize this way. Adding a CdS photocell in series with a resistor to the base of your existing trigger transistor would make the circuit flash in response to external light pulses while maintaining its autonomous oscillation.