  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.
