An imperfect double: probing the physical origin of the low-frequency QPO and its harmonic in black hole binaries

Understand

We extract the spectra of the strong low-frequency quasi-periodic oscillation (QPO) and its harmonic during the rising phase of an outburst in the black-hole binary XTE J1550-564.

• We compare these frequency resolved spectra to the time-averaged spectrum and the spectrum of the rapid (<0.1s) variability.
• The spectrum of the time averaged emission can be described by a disc, a Compton upscattered tail, and its reflection.
• The QPO spectrum contains no detectable disc, and the Compton spectrum is generally harder than in the time averaged emission, and shows less reflection, making it very similar to the spectrum of the rapid variability.