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 ξ_1,cold = 0.36) and high emission measure (EM_1,cold ≈ 3 × 10⁵⁹ cm^-3) and another with high ionization (log ξ_2,hot = 3.7) and low emission measure (EM_2,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 n_c/n_i ≈ 180 is deduced, in agreement with theoretical expectations and optical-ultraviolet observations of massive star winds.