The first X-ray spectrum of the HMXB XTE J1855-026 during the compact object eclipse

Abstract

We present the first {\it 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 $\phi= 0.00-0.11$. 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 black body 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 ($\sim 10^{34}$ erg s$^{-1}$) is 70 times lower than pre eclipse. The intensity of the Fe K$\alpha$ line, in the average eclipse spectrum, is $\sim 7.4$ times lower than the one measured during pre eclipse. Since K$\alpha$ photons can not be resonantly scattered in the wind, the vast majority of Fe K$\alpha$ 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 ($\log\xi_{\rm 1,cold}=0.36$) and high emission measure ($EM_{\rm 1,cold}\approx 3\times 10^{59}$ cm$^{-3}$) and another with high ionization ($\log\xi_{\rm 2,hot}=3.7$) and low emission measure ($EM_{\rm hot}\approx 2\times 10^{56}$ 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 $\approx [0.04-0.05]$, typical for massive stars, a density contrast between clumps and the interclump medium of $n_{\rm c}/n_{\rm i}\approx 180$ is deduced, in agreement with theoretical expectations and optical-UV observations of massive star winds.