166 research outputs found
Optically thick envelopes around ULXs powered by accreating neutron stars
Magnetized neutron stars power at least some ultra-luminous X-ray sources.
The accretion flow in these cases is interrupted at the magnetospheric radius
and then reaches the surface of a neutron star following magnetic field lines.
Accreting matter moving along magnetic field lines forms the accretion envelope
around the central object. We show that, in case of high mass accretion rates
the envelope becomes closed and optically
thick, which influences the dynamics of the accretion flow and the
observational manifestation of the neutron star hidden behind the envelope.
Particularly, the optically thick accretion envelope results in a multi-color
black-body spectrum originating from the magnetospheric surface. The spectrum
and photon energy flux vary with the viewing angle, which gives rise to
pulsations characterized by high pulsed fraction and typically smooth pulse
profiles. The reprocessing of radiation due to interaction with the envelope
leads to the disappearance of cyclotron scattering features from the spectrum.
We speculate that the super-orbital variability of ultra-luminous X-ray sources
powered by accreting neutron stars can be attributed to precession of the
neutron star due to interaction of magnetic dipole with the accretion disc.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Hard X-ray emission of Sco X-1
We study hard X-ray emission of the brightest accreting neutron star Sco X-1
with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of
deadtime corrected exposure on this source. We show that hard X-ray tail in
time average spectrum of Sco X-1 has a power law shape without cutoff up to
energies ~200-300 keV. An absence of the high energy cutoff does not agree with
the predictions of a model, in which the tail is formed as a result of
Comptonization of soft seed photons on bulk motion of matter near the compact
object. The amplitude of the tail varies with time with factor more than ten
with the faintest tail at the top of the so-called flaring branch of its
color-color diagram. We show that the minimal amplitude of the power law tail
is recorded when the component, corresponding to the innermost part of
optically thick accretion disk, disappears from the emission spectrum.
Therefore we show that the presence of the hard X-ray tail may be related with
the existence of the inner part of the optically thick disk. We estimate
cooling time for these energetic electrons and show that they can not be
thermal. We propose that the hard X-ray tail emission originates as a Compton
upscattering of soft seed photons on electrons, which might have initial
non-thermal distribution.Comment: 9 pages, 7 figures, Accepted for publication in MNRA
The X-ray properties of Be/X-ray pulsars in quiescence
Observations of accreting neutron stars (NS) with strong magnetic fields can
be used not only for studying the accretion flow interaction with NS
magnetospheres, but also for understanding the physical processes inside NSs
and for estimating their fundamental parameters. Of particular interest are (i)
the interaction of a rotating neutron star (magnetosphere) with the in-falling
matter at different accretion rates, and (ii) the theory of deep crustal
heating and the influence of a strong magnetic field on this process. Here, we
present results of the first systematic investigation of 16 X-ray pulsars with
Be optical companions during their quiescent states, based on data from the
Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be
roughly divided into two distinct groups: i) relatively bright objects with a
luminosity around ~10^34 erg/s and (hard) power-law spectra, and ii) fainter
ones showing thermal spectra. X-ray pulsations were detected from five objects
in group i) with quite a large pulse fraction of 50-70 per cent. The obtained
results are discussed within the framework of the models describing the
interaction of the in-falling matter with the neutron star magnetic field and
those describing heating and cooling in accreting NSs.Comment: 18 pages, 4 figures, 3 tables, accepted by MNRA
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