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We monitored the neutron star low-mass X-ray binary SAX J1750.8-2900 afterthe end of its 2015/2016 outburst using the X-ray Telescope (XRT) aboard Swiftto detect possible post-outburst 'rebrightenings', similar to those seen afterits 2008 outburst. We did not detect any rebrightening behaviour, suggestingthat the physical mechanism behind the rebrightening events is not alwaysactive after each outburst of the source. Any model attempting to explain theserebrightenings should thus be able to reproduce the different outburst profilesof the source at different times. Surprisingly, our Swift/XRT observations wereunable to detect the source, contrary to previous Swift/XRT observations inquiescence. We determined a temperature upper limit of $\leq$ 106 eV, muchcolder than the post 2008 outburst value of $\sim$ 145 eV. We also report on anarchival Chandra observation of the source after its 2011 outburst and found atemperature of $\sim$ 126 eV. These different temperatures, including thenon-detection very close after the end of the 2015/2016 outburst, are difficultto explain in any model assuming we observe the cooling emission from a neutronstar core or an accretion-heated crust. We discuss our observations in thecontext of a change in envelope (the outer $\sim$ 100 m of the crust)composition and (possibly in combination with) a cooling crust. Both hypothesescannot explain our results unless potentially unrealistic assumptions are made.Irrespective of what causes the temperature variability, it is clear that theneutron star in SAX J1750.8-2900 may not be as hot as previously assumed.

Variable quiescent state for the neutron-star X-ray transient SAX J1750.8-2900: not such a hot neutron star after all?