The QPO states of 4U 1630-47

TL;DR

This paper investigates the different quasi-periodic oscillation (QPO) states in the black hole binary system 4U 1630-47, analyzing their spectral features and evolution during outbursts to understand the underlying physical mechanisms.

Contribution

It identifies and characterizes two distinct QPO states in 4U 1630-47 and proposes an empirical scenario explaining their spectral differences and evolution.

Findings

Two distinct QPO states: single and twin QPOs.

Spectral features suggest possible slim disc formation.

Different QPO states show different spectral evolution patterns.

Abstract

Among the transient black hole binary systems, 4U 1630-47 is one of the most active sources exhibiting outbursts every few hundred days, with every outburst lasting typically around hundred days. During the 2002-2004 outburst the appearance of quasi-periodic oscillations (QPOs) coincide with the onset of anomalous state. There are two distinct QPO states: namely the single QPO state with one QPO and the twin QPO state with two QPOs not related harmonically. The spectral features of this state are corroborated by a previous outburst in 1998. The evolution of the inner disc temperature and the inner disc radius suggest the possible onset of geometrically thicker, slim disc as a possible explanation of the energy spectral features. The other possibilities involving the Comptonizing cloud also exist that may explain the anomalous state. The two different QPO states exhibit different spectral features, and we provide a possible empirical physical scenario from the observation of the evolution of the spectral features.