Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67   The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic
oscillations (QPOs) and red noise variability as well as coherent pulsations at
the 0.130 Hz neutron star spin frequency. Power density spectra of observations
made with the Rossi X-ray Timing Explorer show significant sidebands separated
from the pulsar spin frequency (and its harmonics) by the QPO frequency. These
show that the instantaneous amplitude of the coherent pulsations is modulated
by the amplitude of the QPOs. This phenomenon is expected in models such as the
magnetospheric beat frequency model where the QPOs originate near the polar
caps of the neutron star. In the 4--8 keV energy range, however, the
lower-frequency sidebands are significantly stronger than their
higher-frequency complements; this is inconsistent with the magnetospheric beat
frequency model. We suggest that the 0.048 Hz QPOs are instead produced by a
structure orbiting the neutron star at the QPO frequency. This structure
crosses the line of sight once per orbit and attenuates the pulsar beam,
producing the symmetric (amplitude modulation) sidebands. It also reprocesses
the pulsar beam at the beat frequencies between the neutron star spin frequency
and the QPOs, producing the excess variability observed in the lower-frequency
sidebands. Quite independently, we find no evidence that the red noise
variability modulates the amplitude of the coherent pulsations. This is also in
contrast to the expectations of the magnetospheric beat frequency model and
differs from the behavior in some high-mass X-ray binary pulsars.

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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: math-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: q-bio
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The categories of this paper are: nucl-th
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: funct-an
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The categories of this paper are: cmp-lg
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The categories of this paper are: stat
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The categories of this paper are: chao-dyn
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The categories of this paper are: comp-gas
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The categories of this paper are: physics
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-ex