The early phase of a H1743-322 outburst observed by INTEGRAL, RXTE, Swift, and XMM/Newton

Abstract

We investigate the early phase of the first state change during the 2008 September-November outburst of H1743-322, first detected by the INTEGRAL satellite. We analyse INTEGRAL, RXTE, Swift, and XMM/Newton observations, which provide coverage of the quiescence to outburst evolution in the 3-200 keV range every few days. The energy spectra are well fitted by a phenomenological model consisting of an exponentially cut-off power law plus a disc component. A more physical model of thermal Comptonisation (and a disc) represents the spectra equally well. In a first phase (up to MJD 54760), the photon index and temperature of the disc do not vary significantly, and have values reminiscent of the Hard State (HS). The timing analysis is also consistent with that of a HS, and shows in particular a rather high degree of variability (~30%), and a strong ~0.5-1 Hz QPO with its first harmonic. The timing and spectral characteristics of H1743-322 are similar to those of the first HS during its 2003 outburst. After MJD 54760, a change to softer spectra and a ~5-6 Hz QPO indicate that the source underwent a state transition into a Hard-Intermediate State (HIMS). We observe in both states a correlation between the QPO frequency and the photon index, which indicates a strong link between the accretion disc, generally understood to determine the QPO frequency, and the corona, which determines the QPO power. The gradual disappearance of the QPO harmonic, and the slowly decreasing hard X-ray flux, imply that the accretion disc gradually moved inwards during the HS.