The non standard evolution of the Compton corona in the three 2006-2010 subsequent outbursts of the Black Hole Candidate 4U 1630-47

TL;DR

This study analyzes three outbursts of the black hole candidate 4U 1630-47, revealing non-standard evolution of the Compton corona with significant spectral differences above 20 keV and peculiar state behaviors.

Contribution

It provides the first detailed comparison of high-energy spectral evolution across multiple outbursts of 4U 1630-47, highlighting unusual corona behavior.

Findings

2010 outburst extends up to 200 keV without cutoff

2006 and 2008 outbursts lack detection above 20 keV

Final hard states show rare peculiarities

Abstract

We report on the analysis of the data collected by Swift, INTEGRAL and RXTE of the Black Hole Candidate (BHC) 4U 1630-47 during 3 consecutive outbursts occurred in 2006, 2008 and 2010, respectively. We show that, although a similar spectral and temporal behaviour in the energy range between 2-10 keV, these 3 outbursts present pronounced differences above 20 keV. In fact, the 2010 outburst extends at high energies without any detectable cut-off until 150-200 keV, while the other two previous outbursts, occurred on 2006 and 2008, are not detected at all above 20 keV. Moreover, the 2008 outburst does not show any detectable hard state in its final phases and even during the 2010 outburst, the final hard state shows some peculiarities rarely observed in other BHC. We also investigate on the peculiar huge variation of 4U 1630-47 hydrogen column density (N$_{H}$) reported in the literature using the Swift/XRT data. In fact this instrument is one of the most suitable for this purpose thanks to its lower energy coverage.