We search the literature for reports on the spectral properties of neutron-star lowmass X-ray binaries when they have accretion luminosities between 10 34 and 10 36 ergs s −1 , corresponding to roughly 0.01% -1% of the Eddington accretion rate for a neutron star. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with blackhole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 10 34 − 10 35 erg s −1 . Despite a limited number of data points, there are indications that these spectral differences also extend to the 10 35 − 10 36 erg s −1 range, but above a luminosity of 10 35 erg s −1 the separation between neutron-star and black-hole systems is not as clear as below. In addition, the black-hole spectra only become softer below luminosities of 10 34 erg s −1 compared to 10 36 erg s −1 for the neutron-star systems. This observed difference between the neutron-star binaries and black-hole ones suggests that the spectral properties (between 0.5-10 keV) at 10 34 − 10 35 erg s −1 can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. More observations in this luminosity range are needed to determine how robust this diagnostic tool is and whether or not there are (many) systems that do not follow the general trend. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron-star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion onto the neutronstar surface for luminosities below a few times 10 34 erg s −1 .

The recently discovered accreting X-ray pulsar IGR J17480−2446 spins at a frequency of ∼11 Hz. We show that Type I X-ray bursts from this source display oscillations at the same frequency as the stellar spin. IGR J17480−2446 is the first secure case of a slowly rotating neutron star (NS) which shows Type I burst oscillations (BOs), all other sources featuring such oscillations spin at hundreds of Hertz. This means that we can test BO models in a completely different regime. We explore the origin of Type I BOs in IGR J17480−2446 and conclude that they are not caused by global modes in the NS ocean. We also show that the Coriolis force is not able to confine an oscillation-producing hot spot on the stellar surface. The most likely scenario is that the BOs are produced by a hot spot confined by hydromagnetic stresses.