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We report on the aperiodic X-ray timing and color behavior of the accretingmillisecond X-ray pulsar (AMXP) IGR J17511-3057, using all the pointedobservations obtained with the Rossi X-ray Timing Explorer Proportional CounterArray since the source's discovery on 2009 September 12. The source can beclassified as an atoll source on the basis of the color and timingcharacteristics. It was in the hard state during the entire outburst. In thebeginning and at the end of the outburst, the source exhibited what appear tobe twin kHz quasi periodic oscillations (QPOs). The separation \Delta \nubetween the twin QPOs is ~ 120 Hz. Contrary to expectations for slow rotators,instead of being close to the 244.8 Hz spin frequency, it is close to half thespin frequency. However, identification of the QPOs is not certain as thesource does not fit perfectly in the existing scheme of correlations ofaperiodic variability frequencies seen in neutron star low mass X-ray binaries(NS LMXBs), nor can a single shift factor make it fit as has been reported forother AMXPs. These results indicate that IGR J17511-3057 is a unique sourcediffering from other AMXPs and could play a key role in advancing ourunderstanding of not only AMXPs, but also NS LMXBs in general.

Possible twin kHz Quasi Periodic Oscillations in the accreting millisecond X-ray Pulsar IGR J17511-3057