Strongly absorbed quiescent X-ray emission from the X-ray transient XTE J0421+56 (CI Cam) observed with XMM-Newton

We have observed the X-ray transient XTEJ0421+56 in quiescence with XMM-Newton. The observed spectrum is highly unusual being dominated by an emission feature at ∼6.5 keV. The spectrum can be fit using a partially covered power-law and Gaussian line model, in which the emission is almost completely covered (covering fraction of 0.98 −0.06) by neutral material and is strongly absorbed with an NH of (5 +3 −2) × 10 23 atom cm. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4±0.1 keV, a width σ < 0.28 keV and an equivalent width of 940 −460 eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTEJ0421+56 is at a distance of 5 kpc, its 0.5–10 keV luminosity is 3.5 × 10 erg s. The Optical Monitor onboard XMM-Newton indicates a V magnitude of 11.86 ± 0.03. The spectra of X-ray transients in quiescence are normally modeled using advection dominated accretion flows, power-laws, or by the thermal emission from a neutron star surface. The strongly locally absorbed X-ray emission from XTEJ0421+56 is therefore highly unusual and could result from the compact object being embedded within a dense circumstellar wind emitted from the supergiant B[e] companion star. The uncovered and unabsorbed component observed below 5 keV could be due either to X-ray emission from the supergiant B[e] star itself, or to the scattering of high-energy X-ray photons in a wind or ionized corona, such as observed in some low-mass X-ray binary systems.