Piergiorgio Casella: Discovery of two intermittent accreting millisecond X-ray pulsars

There are a total of 8 known millisecond X-ray pulsars, which is fewer than expected from the hundreds of candidate LMXBs. A detailed search of the entire RXTE archive revealed intemittent millisecond X-ray pulsations from Aql X-1 (550Hz) and SAX J1748.9-2121 (442Hz). In the case of Aql X-1 the pulsations were only seen in 0.01% of total observation time, which indicates that longer observations may reveal millisecond pulsations in several sources where none are presently detected.