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T. Belloni (University of Amsterdam), S. Dieters (University of Alabama in Huntsville)
A bright X-ray outburst was detected from XTE~J0421+560/CI~Cam last year, lasting less than two weeks. Despite the observational effort in the X rays and the brightness of its optical counterpart, the nature of the system is still unclear. We present the results of the analysis of RXTE, BATSE and optical/infrared data of the 1998 outburst of this transient. The X-ray spectrum in the 2-25 keV band is thermal, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron line at around 6.7 keV is observed. No fast time variability is detected (<0.5% rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust and places the system at a distance of ~2 kpc. At this distance the peak 2-25 keV luminosity is ~ 5\times 1037 erg/s. We compare the properties of this peculiar system with those of the Be/NS LMC transient A~0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.
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