The puzzling harmonic behavior of the cathedral QPO in XTEJ1859+226

Abstract

We present a spectral and temporal analysis of the Cathedral quasi-periodic oscillation (QPO) detected in the power density spectra of the black hole binary and microquasar XTEJ1859+226, obtained using Rossi X-Ray Timing Explorer. This type of QPO has been seen on two occasions (MJDs 51574.43 and 51575.43) during the 1998 outburst of this source. It manifests as two peaks with similar amplitudes (3% and 5% rms) and harmonically related centroid frequencies (3 and 6 Hz). The temporal properties of these two peaks are different: the amplitude of the 3-Hz feature varies, in anticorrelation with the count rate, by 50%. The 6-Hz feature, on the other hand, shows a slight increase (7%) in its amplitude with the count rate. The rms spectra of the two peaks are also quite different. The 3-Hz feature is softer than the other one, and, although its rms amplitude increases with energy, it shows a cutoff at an energy of 6 keV. The rms of the 6-Hz feature increases with energies up to at least 20 keV. We also study the bicoherence b 2(μ, ν) of both observations. At the diagonal position of the peaks, the values b 2( 3, 3) and b 2( 6, 6) are rather high and similar to those reported for type C QPOs of GRS 1915+105. In comparison with the latter source, the bicoherence of the 3-Hz feature is higher than that of the other peak, which may indicate that the 3-Hz feature is the fundamental QPO and the other is its first harmonic. The value of b 2( 3, 6) is, however, very low and therefore indicates a behavior different from that seen in the type CQPO of GRS 1915+105. We discuss the implications of these differences in the context of a harmonic relationship between the peaks, and suggest that, rather than pure harmonics, we may see different modes of the same underlying phenomenon competing to produce QPOs at different frequencies. © 2011. The American Astronomical Society. All rights reserved..