A continuous transition from Type-C Quasi Periodic Oscillations to the Heartbeat state in the Black hole X-ray binary 4U 1630-47

TL;DR

This study documents a rare, rapid transition in a black hole X-ray binary from a type-C QPO state to a Heartbeat state, revealing new insights into accretion dynamics and state transitions.

Contribution

First to observe a continuous transition from QPO to Heartbeat state in 4U 1630-47, expanding understanding of state evolution in black hole binaries.

Findings

Transition occurs within less than a day.

QPO features weaken and disappear during the transition.

Heartbeat state shows a broad peak at ~25 mHz.

Abstract

We present a timing analysis of the black hole X-ray binary (BHXRB) 4U 1630-47 using AstroSat observations from 10-19 March 2023, for the first time capturing a rare and rapid transition in variability properties. Within less than a day, the source evolved from a type-C quasi-periodic oscillation (QPO) state, with centroid frequencies between 3-5 Hz, to the Heartbeat state, characterized by a broad peak in the power density spectrum at ~25 mHz, corresponding to a ~40 s modulation period. As the source evolved, it passed through a transition track where the QPO features weakened and ultimately disappeared in the Heartbeat state. In the hardness-intensity Diagram, the QPOs occur at higher hardness and lower intensity, followed by a brightening phase as the source moved towards the soft intermediate state, and finally reached the Heartbeat state through a transition towards lower hardness. In the power-color diagram, this transition is marked by a clear shift to a distinct region of power color space, separate from the range occupied by other observed states. This work establishes 4U 1630-47 as another system, apart from GRS 1915+105, where a continuous transition from QPO to Heartbeat state has been observed. Notably, 4U 1630-47 is the only system where the QPO is absent during the heartbeat state. This provides us with another probe to understand the physical mechanism governing this transition and the overall accretion mechanism in BHXRBs.