Astronomy and Astrophysics, volume 394, 205-211 (2002/10-4)
Strongly absorbed quiescent X-ray emission from the X-ray transient XTE J0421+56 (CI Cam) observed with XMM-Newton.
BOIRIN L., PARMAR A.N., OOSTERBROEK T., LUMB D., ORLANDINI M. and SCHARTEL N.
Abstract (from CDS):
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 ∼6.5keV. 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)x1023atom/cm2. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4±0.1keV, a width σ<0.28keV and an equivalent width of 940+650–460eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTE J0421+56 is at a distance of 5kpc, its 0.5-10keV luminosity is 3.5x1033erg/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 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.