A semi-coherent search for weak pulsations in Aql X-1

TL;DR

This paper conducts a semi-coherent search for weak pulsations in Aql X-1, aiming to detect elusive signals and better understand pulse formation in neutron stars, but finds no evidence of new pulsations within strict upper limits.

Contribution

It introduces a semi-coherent search method applied to all available data for Aql X-1, setting the most stringent upper limits on pulsation strength to date.

Findings

No new pulsations detected within upper limits of ~0.3%

Pulsations, if present, are weaker than current detection thresholds

Results inform future searches for weak signals in similar systems

Abstract

Non pulsating neutron stars in low mass X-ray binaries largely outnumber those that show pulsations. The lack of detectable pulses represents a big open problem for two important reasons. The first is that the structure of the accretion flow in the region closest to the neutron star is not well understood and it is therefore unclear what is the mechanism that prevents the pulse formation. The second is that the detection of pulsations would immediately reveal the spin of the neutron star. Aql X-1 is a special source among low mass X-ray binaries because it has showed the unique property of pulsating for only ~150 seconds out of a total observing time of more than 1.5 million seconds. However, the existing upper limits on the pulsed fraction leave open two alternatives. Either Aql X-1 has very weak pulses which have been undetected, or it has genuinely pulsed only for a tiny amount of the observed time. Understanding which of the two scenarios is the correct one is fundamental to increase our knowledge about the pulse formation process and understand the chances we have to detect weak pulses in other LMXBs. In this paper we perform a semi-coherent search on the entire X-ray data available for Aql X-1. We find no evidence for (new) weak pulsations with the most stringent upper limits being of the order of 0.3% in the 7-25 keV energy band.