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Non pulsating neutron stars in low mass X-ray binaries largely outnumberthose that show pulsations. The lack of detectable pulses represents a big openproblem for two important reasons. The first is that the structure of theaccretion flow in the region closest to the neutron star is not well understoodand it is therefore unclear what is the mechanism that prevents the pulseformation. The second is that the detection of pulsations would immediatelyreveal the spin of the neutron star. Aql X-1 is a special source among low massX-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 twoalternatives. 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 toincrease our knowledge about the pulse formation process and understand thechances we have to detect weak pulses in other LMXBs. In this paper we performa semi-coherent search on the entire X-ray data available for Aql X-1. We findno evidence for (new) weak pulsations with the most stringent upper limitsbeing of the order of 0.3% in the 7-25 keV energy band.

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