Long Term Spectral and Timing Behavior of the Black Hole Candidate XTE J1908+094

Abstract

We present the long term X-ray light curves, 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 ~40 days, followed by a three day long transition to the high/soft state. The source flux (in 2$-$10 keV) reached $\sim$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 ~0.5 Hz 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 lead 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 which were performed during the decay phase of the first outburst and following the second outburst.