Neutron star crust cooling in the Terzan 5 X-ray transient Swift J174805.3-244637

When neutron stars reside in transient X-ray binaries, their crustal layers become heated during accretion outbursts and subsequently cool in quiescence. Observing and modelling this thermal response has yielded valuable insight into the physics of neutron star crusts. However, one unresolved problem is the evidence in several neutron stars for an extra energy source, located at shallow depth in the crust, that is not accounted for by standard heating models. Its origin remains puzzling, and it is currently unclear whether this additional heating occurs in all neutron stars, and if the magnitude is always the same. Here, we report on Chandra observations that cover two years after the 2012 outburst of the transient neutron star Xray binary Swift J174805.3-244637 in the globular cluster Terzan 5. The temperature of the neutron star was elevated during the first two months following its similar or equal to 8 week accretion episode, but had decayed to the pre-outburst level within similar or equal to 100 d. Interpreting this as rapid cooling of the accretion-heated crust, we model the observed temperature curve with a thermal evolution code. We find that there is no need to invoke shallow heating for this neutron star, although an extra energy release up to similar or equal to 1.4 MeV nucleon(-1) is allowed by the current data (2 sigma confidence). We also present two new data points on the crust-cooling curve of the 11-Hz X-ray pulsar IGR J17480-2446 in Terzan 5, which was active in 2010. The temperature of this neutron star remains significantly above the pre-outburst level, but we detect no change in the thermal emission since the previous measurements of 2013 February. This is consistent with the slow crust cooling expected several years post-outburst.