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Astron. Astrophys. 340, L19-L22 (1998)

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5. Discussion

The X-ray spectra of XTE J0421+560 presented here are unlike those of any other X-ray transient. Certain properties are reminiscent of the galactic radio jet source SS 433 (e.g. the appearance of a twisted radio-jet after the outburst, the presence of shifting emission lines and the X-ray luminosities; see also Frontera et al. (1998)). The X-ray spectrum of SS 443 displays a pattern of red- and blue-shifted He- and H-like emission lines superposed on a two temperature bremsstrahlung continuum (Kotani et al. 1994). The lines most probably originate in two collimated precessing (with a period of 163 d) relativistic (0.26 c) jets which result from super-Eddington accretion onto a black hole (e.g., Rose 1995). We propose that the time variable features in XTE J0421+560 also originate in jets and that the variation in energy of these features is due to precession. Fitting a sinusoid to the variation in energy of the 0.74 keV feature during TOO2 and assuming a rest energy of 0.65 keV (ie. that the line is O VIII K[FORMULA]) implies a precessional period of [FORMULA]6 days and a velocity of [FORMULA] (1[FORMULA] uncertainty).

ASCA observations of some high mass X-ray binary pulsars such as Vela X-1 (Nagase et al. 1994) reveal spectra rich in He-like emission lines, almost certainly due to reprocessing in circumstellar material. The stationary lines (Si XIII , S XV , Fe XXV , and possibly Ca XIX ) most probably originate in circumstellar matter. The identification of the moving features with emission from H-like ions (although the rest energies are uncertain), and the stationary ones with He-like ions, is consistent with this interpretation. In the case of SS 443 two sets of moving X-ray lines are seen (Kotani et al. 1994), although earlier studies had revealed only one (e.g, Watson et al. 1986). This was explained by assuming that one beam was occulted by the accretion disk at certain precession phases (Stewart et al. 1987). This may also be the case in XTE J0421+560.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998