fast moving high altitude pulsating disc

I was stargazing with 12x50 binoculars.  The sky was clear with myriads of spring stars.  
I had already observed several high altitude aircraft like airliners as well as satelites.  While scanning with the binoculars I spotted a fast moving disc shaped object moving from east to west.  It was pulsating rythmically like a heart beating at a rapid rate.  It was like light was moving from the canter to the rim in waves.  The beats were roughly four per second.  
I tracked it until it went out of sight. The whole thing lasted 30 seconds to a minute.  It appeared to be very high moving very fast.  It wasn't like any aircraft I have seen.  It gave me the creeps.
Witness indicates that the date of the incident is approximate.  PD