360 research outputs found
Polarization properties of X-ray millisecond pulsars
Radiation of X-ray bursts and of accretion shocks in weakly magnetized
neutron stars in low-mass X-ray binaries is produced in plane-parallel
atmospheres dominated by electron scattering. We first discuss polarization
produced by single (non-magnetic) Compton scattering, in particular the
depolarizing effect of high electron temperature, and then the polarization due
to multiply electron scattering in a slab. We further predict the X-ray pulse
profiles and polarization properties of nuclear- and accretion-powered
millisecond pulsars. We introduce a relativistic rotation vector model, which
includes the effect of rotation of polarization plane due to the rapid motion
of the hot spot as well as the light bending. Future observations of the X-ray
polarization will provide a valuable tool to test the geometry of the emission
region in pulsars and its physical characteristics.Comment: 8 pages, 6 figures, to appear in "X-ray Polarimetry: A New Window in
Astrophysics", edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri
(Cambridge University Press
Time Lags in Compact Objects: Constraints on the Emission Models
Accreting black holes and neutron stars in their hard (low) state show not
only very similar X/gamma-ray spectra but also that the behaviour of their
light curves is quite similar which can be quantified as having similar
power-density spectra and Fourier-frequency-dependent time/phase lags. Taken
together this argues for a common mechanism of the X/gamma-ray production in
these objects. This mechanism is probably a property of the accretion flow only
since it does not depend on the nature of the compact object. In this paper, I
review the observational data paying most attention to the properties of the
temporal variability such as the time/phase lags that hopefully can help us to
discriminate between different theoretical models. I also discuss the models
developed to account for the basic observational facts. Particularly, I show
that the commonly used Compton cloud models with constant temperature cannot
explain variable sources without violating the energy conservation law.
Alternative models where time lags are related to the spectral evolution during
X-ray flares are discussed and compared with observations. Compton reflection
from the outer edge of the accretion disc is shown to markedly affect the time
lag Fourier spectrum.Comment: 16 pages; invited talk at the meeting "X-ray Astronomy 1999: Stellar
Endpoints, AGN and the Diffuse Background", held in Bologna, Italy, September
199
On the Nature of the X-ray Emission from the Accreting Millisecond Pulsar SAX J1808.4-3658
The pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658
at different energies are studied. The two main emission component, the black
body and the Comptonized tail that are clearly identified in the time-averaged
spectrum, show strong variability with the first component lagging the second
one. The observed variability can be explained if the emission is produced by
Comptonization in a hot slab (radiative shock) of Thomson optical depth ~0.3-1
at the neutron star surface. The emission patterns of the black body and the
Comptonized radiation are different: a "knife"- and a "fan"-like, respectively.
We construct a detailed model of the X-ray production accounting for the
Doppler boosting, relativistic aberration and gravitational light bending in
the Schwarzschild spacetime. We present also accurate analytical formulae for
computations of the light curves from rapidly rotating neutron stars using
formalism recently developed by Beloborodov (2002). Our model reproduces well
the pulse profiles at different energies simultaneously, corresponding phase
lags, as well as the time-averaged spectrum. We constrain the compact star mass
to be bounded between 1.2 and 1.6 solar masses. By fitting the observed
profiles, we determine the radius of the compact object to be R~11 km if M=1.6
M_sun, while for M=1.2 M_sun the best-fitting radius is ~6.5 km, indicating
that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a
lower limit on the inclination of the system of 65 degrees.Comment: 11 pages, 7 figures, submitted to MNRA
Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state
Spectra of the spreading layers on the neutron star surface are calculated on
the basis of the Inogamov-Sunyaev model taking into account general relativity
correction to the surface gravity and considering various chemical composition
of the accreting matter. Local (at a given latitude) spectra are similar to the
X-ray burst spectra and are described by a diluted black body. Total spreading
layer spectra are integrated accounting for the light bending, gravitational
redshift, and the relativistic Doppler effect and aberration. They depend
slightly on the inclination angle and on the luminosity. These spectra also can
be fitted by a diluted black body with the color temperature depending mainly
on a neutron star compactness. Owing to the fact that the flux from the
spreading layer is close to the critical Eddington, we can put constraints on a
neutron star radius without the need to know precisely the emitting region area
or the distance to the source. The boundary layer spectra observed in the
luminous low-mass X-ray binaries, and described by a black body of color
temperature Tc=2.4+-0.1 keV, restrict the neutron star radii to R=14.8+- 1.5 km
(for a 1.4-Msun star and solar composition of the accreting matter), which
corresponds to the hard equation of state.Comment: 13 pages, 13 figures, MNRAS, in pres
Gamma-ray burst spectra from continuously accelerated electrons
We discuss here constraints on the particle acceleration models from the
observed gamma-ray bursts spectra. The standard synchrotron shock model assumes
that some fraction of available energy is given instantaneously to the
electrons which are injected at high Lorentz factor. The emitted spectrum in
that case corresponds to the spectrum of cooling electrons, F_\nu ~ \nu^{-1/2},
is much too soft to account for the majority of the observed spectral slopes.
We show that continuous heating of electrons over the life-time of a source is
needed to produce hard observed spectra. In this model, a prominent peak
develops in the electron distribution at energy which is a strong function of
Thomson optical depth \tau_T of heated electrons (pairs). At \tau_T>1, a
typical electron Lorentz factor \gamma ~ 1-2 and quasi-thermal Comptonization
operates. It produces spectrum peaking at a too high energy. Optical depths
below 10^{-4} would be difficult to imagine in any physical scenario. At \tau_T
=10^{-4}-10^{-2}, \gamma ~ 30-100 and synchrotron self-Compton radiation is the
main emission mechanism. The synchrotron peak should be observed at 10--100 eV,
while the self-absorbed low-energy tail with F_\nu ~ \nu^2 can produce the
prompt optical emission (like in the case of GRB 990123). The first Compton
scattering radiation by nearly monoenergetic electrons peaks in the BATSE
energy band and can be as hard as F_\nu ~ \nu^1 reproducing the hardness of
most of the observed GRB spectra. The second Compton peak should be observed in
the high-energy gamma-ray band, possibly being responsible for the 10-100 MeV
emission detected in GRB 941017. A significant electron-positron pair
production reduces the available energy per particle, moving spectral peaks to
lower energies as the burst progresses.Comment: 4 pages, 1 figure, Il nuovo cimento C, in press. Proceedings of the
4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 200
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