2,298 research outputs found
Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53
We detected millihertz quasi-periodic oscillations (QPOs) in an XMM-Newton
observation of the neutron-star low-mass X-ray binary 4U 1636-53. These QPOs
have been interpreted as marginally-stable burning on the neutron-star surface.
At the beginning of the observation the QPO was at around 8 mHz, together with
a possible second harmonic. About 12 ks into the observation a type I X-ray
burst occurred and the QPO disappeared; the QPO reappeared ~25 ks after the
burst and it was present until the end of the observation. We divided the
observation into four segments to study the evolution of the spectral
properties of the source during intervals with and without mHz QPO. We find
that the temperature of the neutron-star surface increases from the QPO segment
to the non-QPO segment, and vice versa. We also find a strong correlation
between the frequency of the mHz QPO and the temperature of a black-body
component in the energy spectrum representing the temperature of neutron-star
surface. Our results are consistent with previous results that the frequency of
the mHz QPO depends on the variation of the heat flux from the neutron star
crust, and therefore supports the suggestion that the observed QPO frequency
drifts could be caused by the cooling of deeper layers.Comment: Accepted for publication in the MNRA
Iron-line and continuum variations in the XMM-Newton and Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1636-53
We used six simultaneous XMM-Newton and Rossi X-ray Timing Explorer plus five
Suzaku observations to study the continuum spectrum and the iron emission line
in the neutron-star low-mass X-ray binary 4U 1636-53. We modelled the spectra
with two thermal components (representing the accretion disc and boundary
layer), a Comptonised component (representing a hot corona), and either a
Gaussian or a relativistic line component to model an iron emission line at
about 6.5 keV. For the relativistic line component we used either the diskline,
laor or kyrline model, the latter for three different values of the spin
parameter. The fitting results for the continuum are consistent with the
standard truncated disc scenario. We also find that the flux and equivalent
width of the iron line first increase and then decrease as the flux of the
Comptonised component increases. This could be explained either by changes in
the ionisation state of the accretion disc where the line is produced by
reflection, or by light bending of the emission from the Comptonised component
if the height at which this component is produced changes with mass accretion
rate.Comment: Accepted for publication in MNRAS, 16 pages, 10 figures, 5 table
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