45 research outputs found
Magnetic fields of neutron stars in X-ray binaries
A substantial fraction of the known neutron stars resides in X-ray binaries
-- systems in which one compact object accretes matter from a companion star.
Neutron stars in X-ray binaries have magnetic fields among the highest found in
the Universe, spanning at least the range from to several 10
G. The magnetospheres around these neutron stars have a strong influence on the
accretion process, which powers most of their emission. The magnetic field
intensity and geometry, are among the main factors responsible for the large
variety of spectral and timing properties observed in the X-ray energy range,
making these objects unique laboratories to study the matter behavior and the
radiation processes in magnetic fields unaccessible on Earth. In this paper we
review the main observational aspects related to the presence of magnetic
fields in neutron star X-ray binaries and some methods that are used to
estimate their strength.Comment: 16 pages, 9 figures, invited topical review, to be published in The
Strongest Magnetic Fields in the Universe (Space Sciences Series of ISSI,
Springer), Space Science Reviews, accepte
Short term aperiodic variability of X-ray binaries: its origin and implications
In this review I briefly describe the latest advances in studies of aperiodic
variability of accreting X-ray binaries and outline the model which currently
describe the majority of observational appearances of variability of accreting
sources in the best way. Then I concentrate on the case of luminous accreting
neutron star binaries (in the soft/high spectral state), where study of
variability of X-ray emission of sources allowed us to resolve long standing
problem of disentangling the contribution of accretion disk and
boundary/spreading layer components to the time average spectrum of sources.
The obtained knowledge of the shape of the spectrum of the boundary layer
allowed us to make estimates of the mass and radii of accreting neutron stars.Comment: 11 pages, 5 figures. Proceedings article of the conference "Cool
Discs, Hot Flows: The Varying Faces of Accreting Compact Objects", Ed. M.
Axelsson, AIP Conference Proceedings 105
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
Do the Spectra of Soft X-ray Transients Reveal Bulk Motion Inflow Phenomenon?
We present our analysis of the high-energy radiation from black hole (BH)
transients, using archival data obtained primarily with RXTE observatory, and a
comprehensive test of the bulk motion Comptonization (BMC) model for the
high-soft state continuum. The emergent spectra of over 30 separate
measurements of GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE
J1755-32, and EXO~1846-031 X-ray sources are successfully fitted by the BMC
model, which has been derived from basic physical principles in previous work.
This in turn provides direct physical insight into the innermost observable
regions where matter impinging upon the event horizon can effectively be
directly viewed. The BMC model is characterized by three parameters: the disk
color temperature, a geometric factor related to the illumination of the black
hole (BH) site by the disk and a spectral index related to the efficiency of
the bulk motion upscattering. For the case of GRO J1655-40, where there are
distance and mass determinations, a self consistency check of the BMC model has
been made. Using model parameters we present new, independent constraints on
the black hole mass, mass accretion rate and the distance for the
aforementioned sources. Notable is the relationship between the color
temperature and flux, which for GRO J1655-40 is entirely distinct from a simple
T^4 dependence, and consistent with the disk model we have invoked - standard
Shakura-Sunyaev's disk. This allows us to impose an important estimation of the
hardness parameter, the ratio of the color temperature to the effective
temperature - we find T_h~2.6, higher than previous estimates used in the
literature.Comment: 50 pages, 8 figures, accepted for publication in the Astrophysical
Journal (scheduled for the May 20, 1999 issue
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