EDP Sciences
Free access
Volume 394, Number 1, October IV 2002
Page(s) 205 - 211
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021089

A&A 394, 205-211 (2002)
DOI: 10.1051/0004-6361:20021089

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

L. Boirin1, 2, A. N. Parmar1, T. Oosterbroek1, D. Lumb1, M. Orlandini3 and N. Schartel4

1  Astrophysics Mission Division, Research and Scientific Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2  Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
3  Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, via Gobetti 101, 40129 Bologna, Italy
4  XMM-Newton Science Operation Center, ESA, Vilspa, Apartado 50727, 28080 Madrid, Spain

(Received 24 April 2002 / Accepted 24 July 2002)

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 $N{\rm _H}$ of ( 5 -2+3$\times~$10 23 atom cm -2. This absorption is local and not interstellar. The Gaussian has a centroid energy of $6.4 \pm 0.1$ 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 \times
10^{33}$  erg s -1. The Optical Monitor onboard XMM-Newton indicates a V magnitude of $11.86~\pm~0.03$. 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.

Key words: accretion, accretion disks -- stars: individual: XTE J0421+56 -- X-rays: general

Offprint request: L. Boirin, lboirin@rssd.esa.int

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