BeppoSAX observations of two unclassified LMXBs: X1543-624 and X1556-605

Abstract

Observations of two unclassified Low Mass X-ray Binaries, X1543-624 and X1556-605, are presented. In the 2-10 keV band the first of the two sources is a factor of two stronger than the other. Both sources do not show X-ray bursts, dips or eclipses in their X-ray light curves. We find that both spectra are described by a two-component model consisting of emission from a cool accretion disk plus a Comptonized blackbody with kTbb ∼ 1.5 keV in a low opacity plasma. The spectrum of X1543-624 hardens from the first to the second observation, when the source slowly moves from right to left in the colour-colour diagram. The spectrum of X1556-605 can also be described by a model consisting of a blackbody plus an unsaturated Comptonization with electron energy kTe ∼ 4 keV. In the first observation, X1543-624 shows evidence of a Fe K emission line at 6.4 keV. Moreover, in both observations, the source spectrum exhibits an emission feature around 0.7 keV, which is interpreted as due to the superposition of the K edge absorption features of O and Ne elements with uncommon relative abundances with respect to the solar one (O/O⊙ ∼ 0.3, Ne/Ne⊙ ∼ 2.5). In the spectrum of X1556-605 no emission lines are observed. We discuss these results and their implications for the source classification and the accretion geometry of the compact object.