Physical origin of the type-C QPO shoulder

Determine the physical origin of the high-frequency shoulder component accompanying the type-C QPO in GX 339-4 during the rising phase of the 2006/2007 outburst, including whether it represents an earlier, broader stage of the type-B QPO variability observed later in the outburst.

Background

The paper identifies a broad high-frequency shoulder adjacent to the narrow type-C QPO in RXTE observations of GX 339-4. The shoulder is initially detected primarily through the imaginary part of the cross spectrum and later becomes resolved in the power-density spectrum. It has a substantially larger hard phase lag than the type-C QPO and broadly similar energy-dependent rms behavior.

Because the shoulder appears shortly before a type-B QPO is detected and has broadly similar lag-energy and rms-energy properties, the paper suggests that it may be an earlier, less-developed form of the same variability. However, the observations do not establish the physical mechanism producing the shoulder or its evolutionary relationship to the later type-B QPO.