Anomalous nature of outbursts of the black hole candidate 4U 1630-472

TL;DR

This study analyzes 13 outbursts of the black hole candidate 4U 1630-472 over two decades, revealing two distinct outburst types with different durations and recurrence patterns, suggesting the presence of two companion binaries influencing the outbursts.

Contribution

It identifies and characterizes two distinct outburst types in 4U 1630-472 and explores their evolution and possible binary companions, which is a novel insight into this source's behavior.

Findings

Normal outbursts recur approximately every 500 days.

Super outbursts last 1.5-2.5 years and include multiple normal outbursts.

A linear relation exists between quiescent and outbursting periods.

Abstract

The Galactic black hole candidate (BHC) 4U~1630-472 has gone through several outbursts (13 to be particular) in the last two and a half decades starting from the RXTE era till date. Like the outbursts of other transient BHCs, the outbursts of this source show variations in duration, peak numbers, highest peak flux, etc. However, unlike any other soft X-ray transients, this source showed outbursts of two types, such as normal and super. The normal outbursts of duration $\sim 100-200$~days are observed quasi periodically at an average recurrence/quiescence period of $\sim 500$~days. The super outbursts of duration $\sim 1.5-2.5$~years contain one or more normal outbursts other than one mega outburst. We make an effort to separate flux contribution of the normal and the mega outbursts from the super outbursts, and tried to understand the nature of evolution of both types (normal and mega) of outbursts, based on the quiescent period prior to the outbursts. Archival data of RXTE/ASM from January 1996 to June 2011, and MAXI/GSC from August 2009 to July 2020 are used for our study. A possible linear relation between the quiescent and outburstsing periods for both types of outbursts are observed. This makes the BHC a special source, and it may contain two companion binaries. Two companions might be responsible for two types of outbursts.