Long-term spectral and timing behavior of the black hole candidate XTE J1908+094

We present the long-term X-ray light curves and detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-Ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for similar to 40 days, followed by a 3 day long transition to the high/soft state. The source flux ( in 2 - 10 keV) reached similar to 100 mcrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi-periodic oscillations that evolved from similar to 0.5 to similar to 5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics led us to classify XTE J1908+094 as a black hole candidate. Here we also derive precise X-ray position of the source using Chandra observations that were performed during the decay phase of the first outburst and following the second outburst.