35 research outputs found
On the spreading layer emission in luminous accreting neutron stars
Emission of the neutron star surface potentially contains information about
its size and thus of vital importance for high energy astrophysics. In spite of
the wealth of data on the emission of luminous accreting neutron stars, the
emission of their surfaces is hard to disentangle from their time averaged
spectra. A recent X-ray transient source XTE J1701-462 has provided a unique
dataset covering the largest ever observed luminosity range for a single
source. In this paper, we extract the spectrum of the boundary layer between
the inner part of the accretion disc and the neutron star surface with the help
of maximally spectral model-independent method. We show compelling evidences
that the energy spectrum of the boundary layer stays virtually the same over
factor of 20 variations of the source luminosity. It is rather wide and cannot
be described by a single temperature blackbody spectrum, probably because of
the inhomogeneity of the boundary layer and a spread in the colour temperature.
The observed maximum colour temperature of the boundary/spreading layer
emission of kT~2.4-2.6 keV is very close to the maximum observed colour
temperature in the photospheric radius expansion X-ray bursts, which is set by
the limiting Eddington flux at the neutron star surface. Observed stability of
the boundary layer spectrum and its maximum colour temperature strongly
supports theoretical models of the boundary/spreading layers on surfaces of
luminous accreting neutron stars, which assume the presence of a region
emitting at the local Eddington limit. Variations in the luminosity in that
case lead to changes in the size of this region, but affect less the spectral
shape. Elaboration of this model will provide solid theoretical grounds for
measurements of the neutron star sizes using the emission of the
boundary/spreading layers of luminous accreting neutron stars.Comment: 7 pages, 7 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
RXTE Observations of an Outburst of Recurrent X-ray Nova GS 1354-644
We present the results of Rossi X-ray Timing Explorer observations of GS
1354-644 during a modest outburst in 1997-1998. The source is one of a handful
of black hole X-ray transients that are confirmed to be recurrent in X-rays. A
1987 outburst of the same source observed by Ginga was much brighter, and
showed a high/soft spectral state. In contrast the 1997-1998 outburst showed a
low/hard spectral state. Both states are typical for black hole binaries. The
RXTE All Sky Monitor observed an outburst duration of 150 to 200 days. PCA and
HEXTE observations covered ~70 days near the maximum of the light curve and
during the flux decline. Throughout the observations, the spectrum can be
approximated by Compton upscattering of soft photons by energetic electrons.
The hot electron cloud has a temperature kT ~30 keV and optical depth tau~4--5.
To fit the data well an additional iron fluorescent line and reflection
component are required, which indicates the presence of optically thick cool
material, most probably in the outer part of the accretion disk. Dramatic fast
variability was observed, and has been analyzed in the context of a shot noise
model. The spectrum appeared to be softest at the peaks of the shot-noise
variability. The shape of the power spectrum was typical for black hole systems
in a low/hard state. We note a qualitative difference in the shape of the
dependence of fractional variability on energy, when we compare systems with
black holes and with neutron stars. Since it is difficult to discriminate these
systems on spectral grounds, at least in their low/hard states, this new
difference might be important.Comment: 12 pages, 9 figures, accepted for publication in ApJ (Feb. 2000,
v.530), uses emulateapj.st