Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472

TL;DR

This study compares three consecutive outbursts of the black hole candidate 4U 1630-472, revealing significant differences in high-energy emissions despite similar spectral behaviors, and explores potential causes for the regular outburst pattern.

Contribution

It provides a detailed comparative analysis of multiple outbursts, highlighting variations in high-energy properties and proposing new scenarios for the regularity of outbursts in this system.

Findings

2010 outburst extends up to 200 keV without cutoff

2006 and 2008 outbursts lack detection above 30 keV

Similar accretion disk evolution despite different corona characteristics

Abstract

4U 1630-472 is a recurrent X-ray transient classified as a black-hole candidate from its spectral and timing properties. One of the peculiarities of this source is the presence of regular outbursts with a recurrence period between 600 and 730 d that has been observed since the discovery of the source in 1969. We report on a comparative study on the spectral and timing behaviour of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years of activity. We show that, in spite of having a similar spectral and timing behaviour in the energy range between 3 and 30 keV, these three outbursts show pronounced differences above 30 keV. In fact, the 2010 outburst extends at high energies without any detectable cut-off until 150-200 keV, while the two previous outbursts that occurred in 2006 and 2008 are not detected at all above 30 keV. Thus, in spite of a very similar accretion disk evolution, these three outbursts exhibit totally different characteristics of the Compton electron corona, showing a softening in their evolution rarely observed before in a low mass X-ray binary hosting a black hole. We argue the possibility that the unknown perturbation that causes the outbursts to be equally spaced in time could be at the origin of this particular behaviour. Finally we describe several possible scenarios that could explain the regularity of the outbursts, identifying the most plausible, such as a third body orbiting around the binary system.