The extraordinary X-ray spectrum of XTE J0421+560

Abstract

We report results of two BeppoSAX observations of the transient X-ray source XTE J0421+560 during the outburst that started in March 1998. The source exhibits radio jets and coincides with the binary system CI Cam. The 0.1--50 keV spectrum is unlike those of other X-ray transients, and cannot be fit with any simple model. The spectra can be represented by an absorbed two component bremsstrahlung model with narrow Gaussian emission features identified with O, Ne/Fe-L, Si, S, Ca and Fe-K. During the second observation (TOO2) the energies of the O and the Ne/Fe-L features decreased smoothly by ~9% over an interval of 30 hrs. No significant energy shift of the other lines is detected with e.g, a 90% confidence upper limit to any Fe-K line shift of 3.5%. No moving lines were detected during the first observation (TOO1) with e.g., an upper limit of <1.4% to any shift of the Fe-K line. The low-energy absorption decreased by a factor >=1.8 from ~6 10E21 atom cm/2 between TOO1 and TOO2. We propose that the time variable emission lines arise in precessing relativistic jets, while the stationary lines originate in circumstellar material.