The soft quiescent spectrum of the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

We report on the quiescent X-ray properties of the recently discovered transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5. Using two archival Chandra observations, we demonstrate that the quiescent spectrum of this neutron star low-mass X-ray binary is soft and can be fit to a neutron star atmosphere model with a temperature of kT∞∼ 73 eV. A power-law spectral component is not required by the data and contributes at most ∼20 per cent to the total unabsorbed 0.5-10 keV flux of ∼9 × 10−14 erg cm−2 s−1. Such a soft quiescent spectrum is unusual for neutron stars with relatively high inferred magnetic fields and casts a different light on the interpretation of the hard spectral component, which is often attributed to magnetic field effects. For a distance of 5.5 kpc, the estimated quiescent thermal bolometric luminosity is ∼6 × 1032 erg s−1. If the thermal emission is interpreted as cooling of the neutron star, the observed luminosity requires that the system is quiescent for at least ∼100 yr. Alternatively, enhanced neutrino emissions can cool the neutron star to the observed quiescent luminosity.