Another pulsar with strange companion in NGC 6440

We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globularcluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-RayTiming Explorer on August 30th and October 1st, 2009, during the decay of . 4 days outburst of anewly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequencywe find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and aprojected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lowerlimit to the mass of the companion to be 0.0067 M_Sun (assuming a 1.4 M_Sun neutron star). This newpulsar shows the shortest outburst recurrence time among AMXPs ( 1 month). If this behavior doesnot cease, this AMXP has the potential to be one of the best sources in which to study how the binarysystem and the neutron star spin evolve. Furthermore, the characteristics of this new source indicatesthat there might exist a population of AMXPs undergoing weak outbursts which are undetected bycurrent 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.

Lazarus wrote:Well given the object was likely a white dwarf before it got down to that mass, it wasn't doing much in the way of stellar processing anyway.