Two separate spectral substates within Low-Hard States of NS-LMXBs

Abstract

The whole BeppoSAX archive was screened for observations of Neutron Star (NS) Low-Mass X-ray Binaries, in order to characterise the X-ray properties of this class of sources when observed in a Low Hard State (LHS). A total of 12 sources in the sample exhibited LHS spectra; half of the objects showed a "canonical" LHS spectrum, in which two separate populations of soft blackbody-distributed photons emerge. One, or possibly both the populations, is Comptonized by a hot (kT_e > 20 keV) electron plasma cloud. For the remaining 6 sources just a single population is sufficient to account for both the directly observed blackbody and the Comptonized radiation. The luminosities of the NS with canonical two-population spectra are found to be systematically higher than the ones of the sources showing the atypical, single photon population spectra. This suggests the possibility of two separate subclasses of LHS spectra. The transition from a subclass to the other is likely related to the accretion rate, which drives the presence of a hotter X-ray photon population in the close vicinity of the NS, or possibly a stronger thermal gradient in the inner, X-ray active, accretion disk.