WE PROPOSE HIGH-CADENCE XRT MONITORING OBSERVATIONS (1 KS X 5 VISITS PER DAY FOR 10 DAYS) OF TWO VARIABLE HIGH-INCLINATION ULTRALUMINOUS X-RAY SOURCES (ULXS: NGC 5408 X-1 AND NGC 55 ULX). OUR MAIN GOAL IS TO SEARCH FOR ORBITAL PERIODS IN THE RANGE OF A FEW 10S OF HOURS IN THESE TWO SOURCES. THIS PARTICULAR PERIOD RANGE TYPICAL OF STELLAR-MASS BLACK HOLE BINARIES HAS NOT BEEN PROBED BEFORE. ANY DETECTED PERIODS WILL BE AN IMPORTANT STEP TOWARDS DYNAMICAL MASS MEASUREMENT OF THESE BLACK HOLES. SWIFT WITH ITS EXCELLENT X-RAY SENSITIVITY AND FAST MANEUVERING CAPABILITY IS CURRENTLY THE ONLY OBSERVATORY THAT CAN CARRY OUT THESE OBSERVATIONS. THIS PROPOSAL BUILDS ON PREVIOUS SUCCESSFUL SWIFT/XRT DETECTION OF A 12.1 HR ORBITAL PERIOD FROM THE ULX HOLMBERG IX X-1.

Massachusetts Institute Of Technology, Cambridge MA

Investigators