American Express Co. President Ed Gilligan died Friday morning after becoming ill on an overseas flight to New York. Gilligan, 55, was returning from a business trip to Tokyo on a corporate aircraft, which made an emergency landing, according to a person with knowledge of the matter. “This is deeply painful and frankly unimaginable for all of us who had the great fortune to work with Ed, and benefit from his insights, leadership and enthusiasm,” Chief Executive Officer Ken Chenault said in a letter to the staff.