BeppoSAXobservations of two unclassified LMXBs: X1543–624 and X1556–605

Abstract

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 kT bb ∼ 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 kT e ∼ 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.

On the ultra-compact nature of the neutron star system 1RXS J170854.4−321857: insights from X-ray spectroscopy

“…On the other hand, similar features have been observed in several (candidate) UCXBs (e.g. Juett et al 2001;Juett & Chakrabarty 2003;Farinelli et al 2003;in 't Zand et al 2008). These are proposed to result from over-abundances in the absorbing material, generally enhanced O, Ne and Fe, which is probably intrinsic to the sources.…”

“…In this context, it is important to emphasise that the Ne/O ratio was questioned as UCXB marker due to the hydrogen and helium lines found in the optical spectrum of the NS system 4U 1556-60 (Nelemans et al 2006;Nelemans & Jonker 2010). However, 4U 1556-60 has not shown any of the X-ray features classically associated with uncommon abundances (Farinelli et al 2003), and therefore, it was not (and has never been as far as we know) an UCXB candidate.…”

Rapid X-ray variability

“…It may be an SS433-like super-Eddington accretor (72b). Objects showing properties not clearly characteristic for either low magnetic-field neutron stars or black holes include 4U 1957+11 (76a-c), possibly a black hole always in the HS, and 4U 1543-62 and 4U 1556-60, which might be atoll sources (Table 2. 6;Farinelli et al 2003).…”

Optical spectroscopy of (candidate) ultracompact X-ray binaries: constraints on the composition of the donor stars

“…The source 4U 1556−60 was selected as a candidate UCXB based on the suggested 'Ne feature' in its X-ray spectrum (Farinelli et al 2003). This feature was discussed in the context of finding UCXBs by Juett et al (2001), who noted a similarity in the X-ray spectra of several X-ray binaries, among which the known 20-min UCXB 4U 1850−087.…”