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

¹ Dipartimento di Fisica, Universitá di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
² Istituto Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, CNR, Via Gobetti 101, 40129 Bologna, Italy
³ Astrophysics Division, Space Science Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁴ Osservatorio Astronomico di Roma, Via Frascati, 33, 00040 Monteporzio Catone, Italy
⁵ Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center of High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁶ Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899, USA

Corresponding author: F. Frontera, fontera@fe.infn.it

Received: 6 May 2002
Accepted: 8 January 2003

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  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  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, Ne/Ne). 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.