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We have monitored the transient neutron star low-mass X-ray binary 1RXSJ180408.9-342058 in quiescence after its ~4.5 month outburst in 2015. Thesource has been observed using Swift and XMM-Newton. Its X-ray spectra weredominated by a thermal component. The thermal evolution showed a gradual X-rayluminosity decay from ~18 x 10^32 to ~4 x 10^32 (D/5.8 kpc)^2 erg s^{-1}between ~8 and ~379 d in quiescence, and the inferred neutron star surfacetemperature (for an observer at infinity; using a neutron star atmospheremodel) decreased from ~100 to ~71 eV. This can be interpreted as cooling of anaccretion-heated neutron star crust. Modelling the observed temperature curve(using NSCOOL) indicated that the source required ~1.9 MeV per accreted nucleonof shallow heating in addition to the standard deep crustal heating to explainits thermal evolution. Alternatively, the decay could also be modelled withoutthe presence of deep crustal heating, only having a shallow heat source (again~1.9 MeV per accreted nucleon was required). However, the XMM-Newton datastatistically required an additional power-law component. This componentcontributed ~30 per cent of the total unabsorbed flux in 0.5-10 keV energyrange. The physical origin of this component is unknown. One possibility isthat it arises from low-level accretion. The presence of this component in thespectrum complicates our cooling crust interpretation because it might indicatethat the smooth luminosity and temperature decay curves we observed may not bedue to crust cooling but due to some other process.

Potential cooling of an accretion-heated neutron star crust in the low-mass X-ray binary 1RXS J180408.9-342058