We report the discovery of the second accreting millisecond X-ray pulsar ( AMXP ) in the globular cluster NGC 6440 .
Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th , October 1st and October 28th , 2009 , during the decays of \lesssim 4 day outbursts of a newly X-ray transient source in NGC 6440 .
By studying the Doppler shift of the pulsation frequency , we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds .
Based on the mass function , we estimate a lower limit to the mass of the companion to be 0.0067 M _ { \odot } ( assuming a 1.4 M _ { \odot } neutron star ) .
This new pulsar shows the shortest outburst recurrence time among AMXPs ( \sim 1 month ) .
If this behavior does not cease , this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve .
Furthermore , the characteristics of this new source indicate that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors .
NGC 6440 is the only globular cluster to host two known AMXPs , while no AMXPs have been detected in any other globular cluster .