Broad-band X-ray spectral evolution of GX 339-4 during a state transition
Gehrels, N.; Homan, J.; van der Klis, M.; Lewin, W. H. G.; Casella, P.; Fender, R. P.; Belloni, T. M.; Bazzano, A.; Del Santo, M.; Méndez, M.; Gallo, E.
Italy, United States, Netherlands, United Kingdom
Abstract
We report on X-ray and soft γ-ray observations of the black hole candidate GX 339-4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness-intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution in the diagram suggested that a transition from hard- to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms of the high-energy cut-off or in terms of variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX 339-4.
Based on the observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the participating institutions' countries: Denmark, France, Germany, Italy, Switzerland and Spain), Czech Republic and Poland, and with participation of Russia and the United States. E-mail: melania.delsanto@iasf-roma.inaf.it