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

Abstract

We have observed the X-ray transient XTE J0421+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^+0.02) by neutral material and is strongly absorbed with an \nh of ( 5 _-2⁺³) x 10²³ atom cm^-2. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4 +/- 0.1 keV, a width sigma <0.28 keV and an equivalent width of 940 ⁺⁶⁵⁰_-460 eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTE J0421+56 is at a distance of 5 kpc, its 0.5-10 keV luminosity is 3.5 x 10³³ erg s^-1. 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 XTE J0421+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.