Suzaku Observation of the Black Hole Binary 4U 1630--47 in the Very High State

TL;DR

This study analyzes the X-ray and near-infrared properties of the black hole binary 4U 1630--47 during a very high state, revealing spectral characteristics, disc truncation, and changes in disc wind properties compared to previous states.

Contribution

First detailed multi-wavelength analysis of 4U 1630--47 in the very high state, highlighting spectral components, disc truncation, and wind variability.

Findings

X-ray spectrum shows steep power law with photon index 3.2.

Inner disc appears slightly truncated compared to high/soft state.

Disc wind properties change, with less ionization and different launch conditions.

Abstract

We report the results from an X-ray and near-infrared observation of the Galactic black hole binary 4U 1630--47 in the very high state, performed with {\it Suzaku} and IRSF around the peak of the 2012 September-October outburst. The X-ray spectrum is approximated by a steep power law, with photon index of 3.2, identifying the source as being in the very high state. A more detailed fit shows that the X-ray continuum is well described by a multi-color disc, together with thermal and non-thermal Comptonization. The inner disc appears slightly truncated by comparison with a previous high/soft state of this source, even taking into account energetic coupling between the disc and corona, although there are uncertainties due to the dust scattering correction. The near-infrared fluxes are higher than the extrapolated disc model, showing that there is a contribution from irradiation in the outer disk and/or the companion star at these wavelengths. Our X-ray spectra do not show the Doppler shifted iron emission lines indicating a baryonic jet which were seen four days previously in an XMM-Newton observation, despite the source being in a similar state. There are also no significant absorption lines from highly ionized irons as are seen in the previous high/soft state data. We show that the increased source luminosity is not enough on its own to make the wind so highly ionized as to be undetectable. This shows that the disc wind has changed in terms of its launch radius and/or density compared to the high/soft state.