The first X-ray spectrum of the high-mass X-ray binary XTE J1855−026 during the compact object eclipse

We present the first XMM–Newton observation of the classical supergiant high-mass X-ray binary XTE J1855−026 taken entirely during the eclipse of the neutron star (NS), covering the orbital phases ϕ = 0.00–0.11. The analysis of the data allows us to (a) compare with the parameters obtained during the existing pre-eclipse observation and (b) explore the back-illuminated stellar wind of the B0I-type donor. The blackbody component, used to describe the soft excess during pre-eclipse, is not observed during eclipse. It must be then produced near the NS or along the donor–NS line. The 0.3–10 keV luminosity during eclipse (∼1034 erg s−1) is 70 times lower than pre-eclipse. The intensity of the Fe Kα line, in the average eclipse spectrum, is ∼7.4 times lower than the one measured during pre-eclipse. Since Kα photons cannot be resonantly scattered in the wind, the vast majority of Fe Kα emission must come from distances within 1R* from the NS. The eclipse spectrum is successfully modelled through the addition of two photoionized plasmas, one with low ionization (log ξ1,cold = 0.36) and high emission measure (EM1,cold ≈ 3 × 1059 cm−3) and another with high ionization (log ξ2,hot = 3.7) and low emission measure (EM2,hot ≈ 2 × 1056 cm−3). Assuming that the cold and hot gas phases are the clumps and the interclump medium of the stellar wind, respectively, and a clump volume filling factor of ≈0.04–0.05, typical for massive stars, a density contrast between clumps and the interclump medium of nc/ni ≈ 180 is deduced, in agreement with theoretical expectations and optical–ultraviolet observations of massive star winds