Nonlinear Variability of Quasi-Periodic Oscillations in GX 339-4. (arXiv:1904.05895v1 [astro-ph.HE])

Nonlinear Variability of Quasi-Periodic Oscillations in GX 339-4. (arXiv:1904.05895v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Arur_K/0/1/0/all/0/1">K. Arur</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maccarone_T/0/1/0/all/0/1">T. J. Maccarone</a>

We examine the nonlinear variability of quasi-periodic oscillations (QPOs)
using the bicoherence, a measure of phase coupling at different Fourier
frequencies. We analyse several observations of RXTE/PCA archival data of the
black hole binary GX 339-4 which show QPOs. In the type C QPOs, we confirm the
presence of the ‘hypotenuse’ pattern where there is nonlinear coupling between
low frequencies that sum to the QPO frequency as well as the ‘web’ pattern
where in addition to the hypotenuse, nonlinear coupling between the QPO
frequency and the broadband noise is present. We find that type B QPOs show a
previously unknown pattern. We also show that the bicoherence pattern changes
gradually from ‘web’ to ‘hypotenuse’ as the source moves from a hard
intermediate state to a soft intermediate state. Additionally, we reconstruct
the QPO waveforms from six observations using the biphase. Finally, we present
a scenario by which a moderate increase in the optical depth of the hard X-ray
emission region can explain the changes in the non-linearity seen during this
transition.

We examine the nonlinear variability of quasi-periodic oscillations (QPOs)
using the bicoherence, a measure of phase coupling at different Fourier
frequencies. We analyse several observations of RXTE/PCA archival data of the
black hole binary GX 339-4 which show QPOs. In the type C QPOs, we confirm the
presence of the ‘hypotenuse’ pattern where there is nonlinear coupling between
low frequencies that sum to the QPO frequency as well as the ‘web’ pattern
where in addition to the hypotenuse, nonlinear coupling between the QPO
frequency and the broadband noise is present. We find that type B QPOs show a
previously unknown pattern. We also show that the bicoherence pattern changes
gradually from ‘web’ to ‘hypotenuse’ as the source moves from a hard
intermediate state to a soft intermediate state. Additionally, we reconstruct
the QPO waveforms from six observations using the biphase. Finally, we present
a scenario by which a moderate increase in the optical depth of the hard X-ray
emission region can explain the changes in the non-linearity seen during this
transition.

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