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

TL;DR

This paper reports the discovery of a soft quiescent X-ray spectrum from a transiently accreting 11 Hz pulsar in Terzan 5, challenging previous assumptions about neutron star spectra with high magnetic fields.

Contribution

It presents the first detailed analysis of the quiescent spectrum of this pulsar, showing a soft spectrum fit by a neutron star atmosphere model without a significant powerlaw component.

Findings

The spectrum is soft with a temperature of ~73 eV.

The powerlaw component contributes at most 20% to the flux.

The thermal luminosity suggests the neutron star has been quiescent for at least 100 years.

Abstract

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^inf~73 eV. A powerlaw spectral component is not required by the data and contributes at most ~20% to the total unabsorbed 0.5-10 keV flux of ~9E-14 erg/cm2/s. 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 ~6E32 erg/s. 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 years. Alternatively, enhanced neutrino emissions can cool the neutron star to the observed quiescent luminosity.