Hard X-ray emission from low mass X-ray binaries

In this paper we review our current knowledge of the hard X-ray emission properties of old accreting neutron stars in low mass X-ray binaries, with particular attention to recent results obtained for the brightest sources of this class, the so-called sources. While less luminous low mass X-ray binaries often show quite hard spectra, sometimes extending up to energies >= 100 keV, the spectra of Z sources are always very soft, dominated by thermal components with characteristic temperatures similar to 3 - 6 keV. However, recent broad band observations revealed the presence of a weak hard (power-law) component that is sometimes present in the spectra of these sources. These observations have strengthened the analogies between the spectral behavior of low mass X-ray binaries hosting neutron stars and binary systems containing black hole candidates. The physical parameters regulating the presence of this hard component are unknown yet. The first parameter may be the mass accretion rate, as indicated by the general anticorrelation between the fraction luminosity in hard X-rays and mass accretion rate apparent over different sources spanning a large range of luminosities as well as individual sources undergoing state changes. However, a second, yet unknown, parameter is probably needed to explain all the phenomenology. The broad high energy coverage and good sensitivity of the INTEGRAL mission can represent an important step forward in the understanding of the origin and properties of high energy components in accreting X-ray binaries.