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W B Focke (GSFC / UMD), J H Swank (GSFC)
We present observations of the transient galactic black hole candidate XTEJ1748-288. The source was first detected in outburst on 1998 June 4, and was in a very high state the first time it was observed with the RXTE PCA. It made further transitions to a high state between 1998 June 11 and 1998 June 13, and to a low state between 1998 July 13 and 1998 July 18. Each of these states featured both a soft, quasithermal component and a hard powerlaw tail, with their relative intensities and the source timing behavior varying between states. In the very high state, both spectral components are strong, although the hard tail is stronger. The PSD of temporal variations in this state can be described by a broken powerlaw plus one or more QPO. During the high state, the hard component dimmed and the soft brightened to assume dominance, while temporal variability essentially ceased. During the low state, the two spectral components switched relative brightness, so that the tail dominated. Temporal variability showed the band-limited noise common to black hole candidates in the low state.
During the very high state a QPO was present whose frequency varied from 18--32~Hz on timescales of a few minutes. Many spectral and timing parameters were correlated with these changes, including PSD spectral index and break frequency, disk inner edge radius and temperature, and photon spectral index of the powerlaw tail.
An Iron line was observed to start and rise to a constant flux level during the high state, and persist at a constant level into the low state.
Hjellming et al. have observed two ejections of radio jets from the source. The first occurs at approximately the same time as the source enters outburst, and the second is coincident with the transition to the low state.
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