NuSTAR/XMM-Newton Detection of a Hard Cut-Off in Cen X-4

The low-mass X-ray binary Cen X-4 is the closest known (1.2 kpc) quiescent neutron star transient. It had large outbursts in 1969 and 1979 but has been in quiescence ever since. Previous X-ray (0.5-10 keV) observations of the quiescent spectrum identified two spectral components: soft thermal emission from the neutron star atmosphere and a hard power-law tail of unknown origin. This is typical of the X-ray spectrum measured from quiescent neutron star transients, although the hard tail is not always clearly detected in other sources. We report here on a simultaneous observation of Cen X-4 with NuSTAR (114 ks, 3-79 keV) and XMM-Newton (26 ks, 0.3-10 keV) on 2013 January 20-23, providing the first sensitive measurement of the hard X-ray spectrum of a quiescent neutron star transient. We clearly detect a turnover of the hard spectral tail above 10 keV, which is well fit by an 18 keV thermal bremsstrahlung model. This is the first indication of the temperature and spatial distribution of the electron population giving rise to the hard component in a quiescent neutron star transient. The observed emission measure suggests that there is a large amount of material far from the neutron star, with only a very small fraction accreting onto the neutron star. With the hard spectral component well measured by the NuSTAR data, we are also able to obtain tighter constraints on the neutron star atmosphere emission from the XMM-Newton data than previously possible.