Average hard X‐ray emission from NS LMXBs

Abstract

We studied and compared the long term average hard X‐ray (>20 keV) spectra of a sample of twelve bright low‐mass X‐ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright (“GX”) bulge sources while two are weaker ones in the 2–10 keV range (H 1750‐440 and H 1608‐55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. We extend the detection of H 1750‐440 up to 150 keV and discover a hard tail in the spectrum of the Atoll source GX 13+1. We can describe all the spectral states as well as the long known variability of the hard X‐ray tails detected in some of these objects in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion, at the origin of the hard X‐ray tail, is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. Using radio data from the literature we find, in all Z sources and bright “GX” Atolls, a systematic positive correlation between the X‐ray hard tail (40–100 keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.