Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40

TL;DR

This study analyzes low-frequency quasi-periodic oscillations in the 2005 outburst of GRO J1655-40, revealing simultaneous non-harmonically related QPOs and suggesting different physical mechanisms for LFQPO types.

Contribution

First detection of simultaneous non-harmonically related type-B and type-C QPOs in a black hole binary during an outburst.

Findings

Type-C LFQPOs reached record high frequencies.

Simultaneous type-B and type-C QPOs indicate different physical origins.

Broad peaked noise may evolve into a type-B QPO at high count rates.

Abstract

We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi X-ray Timing Explorer. All LFQPOs could be identified as either type B or type C using previously proposed classification schemes. In the soft state of the outburst the type-C LFQPOs reached frequencies that are among the highest ever seen for LFQPOs in black holes. At the peak of the outburst, in the ultra-luminous state, the power spectrum showed two simultaneous, non-harmonically related peaks which we identified as a type-B and a type-C QPO. The simultaneous presence of a type-C and type-B QPO shows that at least two of the three known LFQPO types are intrinsically different and likely the result of distinct physical mechanisms. We also studied the properties of a broad peaked noise component in the power spectra of the ultra-luminous state. This noise component becomes more coherent with count rate and there are strong suggestions that it evolves into a type-B QPO at the highest observed count rates.