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The structure and composition of the crust of neutron stars plays animportant role in their thermal and magnetic evolution, hence in setting theirobservational properties. One way to study the crust properties is to measurehow it cools after it has been heated during an accretion outburst in alow-mass X-ray binary (LMXB). Such studies have shown that there is atantalizing source of heat, of currently unknown origin, that is located in theouter layers of the crust and has a strength that varies between differentsources and different outbursts. With the aim of understanding the mechanismbehind this "shallow heating", we present Chandra and Swift observations of theneutron star LMXB Aql X-1, obtained after its bright 2016 outburst. We findthat the neutron star temperature was initially much lower, and started todecrease at much later time, than observed after the 2013 outburst of thesource, despite the fact that the properties of the two outbursts were verysimilar. Comparing our data to thermal evolution simulations, we infer that thedepth and magnitude of shallow heating must have been much larger during the2016 outburst than during the 2013 one. This implies that basic neutron starparameters that do not change between outbursts, do not play a strong role inshallow heating. Furthermore, it suggests that outbursts with a similaraccretion morphology can give rise to very different shallow heating. We alsodiscuss alternative explanations for the difference in quiescent evolutionafter the 2016 outburst.

Crust cooling of the neutron star in Aql X-1: Different depth and magnitude of shallow heating during similar accretion outbursts