Discovery of coherent millisecond X-ray pulsations in Aql X-1

TL;DR

This paper reports the discovery of coherent millisecond X-ray pulsations in Aql X-1, providing insights into neutron star spin and accretion processes in low-mass X-ray binaries.

Contribution

It presents the first detection of persistent millisecond pulsations in Aql X-1, linking pulsation frequency to neutron star spin and exploring implications for pulsation absence.

Findings

Pulsation lasted over 150 seconds with constant frequency.

Pulsation frequency is 550.27 Hz, close to burst oscillation maximum.

Pulsed fraction varies with energy from <1% to >10%.

Abstract

We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 seconds, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from <1% (3-5 keV) to >10% (16-30 keV). We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and non-pulsating sources.