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

TL;DR

This study analyzes X-ray timing data from the millisecond pulsar IGR J17511-3057, revealing potential twin kHz QPOs with unusual frequency separation, challenging existing models of neutron star variability.

Contribution

First detection of potential twin kHz QPOs in IGR J17511-3057, highlighting its unique properties and implications for neutron star variability theories.

Findings

Twin kHz QPOs observed with separation ~120 Hz

QPO frequency separation near half the spin frequency

Source exhibits atypical variability correlations

Abstract

We report on the aperiodic X-ray timing and color behavior of the accreting millisecond X-ray pulsar (AMXP) IGR J17511-3057, using all the pointed observations obtained with the Rossi X-ray Timing Explorer Proportional Counter Array since the source's discovery on 2009 September 12. The source can be classified as an atoll source on the basis of the color and timing characteristics. It was in the hard state during the entire outburst. In the beginning and at the end of the outburst, the source exhibited what appear to be twin kHz quasi periodic oscillations (QPOs). The separation \Delta \nu between 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 the spin frequency. However, identification of the QPOs is not certain as the source does not fit perfectly in the existing scheme of correlations of aperiodic 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 for other AMXPs. These results indicate that IGR J17511-3057 is a unique source differing from other AMXPs and could play a key role in advancing our understanding of not only AMXPs, but also NS LMXBs in general.