1,654 research outputs found
Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars
The aim of this work is to study the possible effects and biases on the
radius constraints for rotation-powered millisecond pulsars when using Thomson
approximation to describe electron scattering in the atmosphere models, instead
of using exact formulation for Compton scattering. We compare the differences
between the two models in the energy spectrum and angular distribution of the
emitted radiation. We also analyse a self-generated synthetic phase-resolved
energy spectrum, based on Compton atmosphere and the most X-ray luminous
rotation-powered millisecond pulsars observed by the Neutron star Interior
Composition ExploreR (NICER). We derive constraints for the neutron star
parameters using both the Compton and Thomson models. The results show that the
method works by reproducing the correct parameters with the Compton model.
However, biases are found in size and the temperature of the emitting hot spot,
when using the Thomson model. The constraints on the radius are still not
significantly changed, and therefore the Thomson model seems to be adequate if
we are interested only in the radius measurements using NICER.Comment: 6 pages, 9 figures, published in A&
Simulations of gamma-ray burst afterglows with a relativistic kinetic code
This paper introduces a kinetic code that simulates gamma-ray burst (GRB)
afterglow emission from the external forward shock and presents examples of
some of its applications. One interesting research topic discussed in the paper
is the high-energy radiation produced by Compton scattering of the prompt GRB
photons against the shock-accelerated electrons. The difference between the
forward shock emission in a wind-type and a constant-density medium is also
studied, and the emission due to Maxwellian electron injection is compared to
the case with pure power-law electrons. The code calculates the time-evolving
photon and electron distributions in the emission region by solving the
relativistic kinetic equations for each particle species. For the first time,
the full relativistic equations for synchrotron emission/absorption, Compton
scattering, and pair production/annihilation were applied to model the forward
shock emission. The synchrotron self-absorption thermalization mechanism, which
shapes the low-energy end of the electron distribution, was also included in
the electron equation. The simulation results indicate that inverse Compton
scattering of the prompt GRB photons can produce a luminous TeV emission
component, even when pair production in the emission region is taken into
account. This very high-energy radiation may be observable in low-redshift
GRBs. The test simulations also show that the low-energy end of a pure
power-law distribution of electrons can thermalize owing to synchrotron
self-absorption in a wind-type environment, but without an observable impact on
the radiation spectrum. Moreover, a flattening in the forward shock X-ray light
curve may be expected when the electron injection function is assumed to be
purely Maxwellian instead of a power law.Comment: 16 pages, 11 figures, accepted for publication in A&
Model atmospheres of X-ray bursting neutron stars
We present an extended set of model atmospheres and emergent spectra of X-ray
bursting neutron stars in low mass X-ray binaries. Compton scattering is taken
into account. The models were computed in LTE approximation for six different
chemical compositions: pure hydrogen and pure helium atmospheres, and
atmospheres with a solar mix of hydrogen and helium and various heavy elements
abundances: Z = 1, 0.3, 0.1, and 0.01 Z_sun, for three values of gravity, log g
=14.0, 14.3, and 14.6 and for 20 values of relative luminosity l = L/L_Edd in
the range 0.001 - 0.98. The emergent spectra of all models are fitted by
diluted blackbody spectra in the observed RXTE/PCA band 3 - 20 keV and the
corresponding values of color correction factors f_c are presented. We also
show how to use these dependencies to estimate the neutron star's basic
parameters.Comment: 2 pages, 1 figure, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
X-ray burst induced spectral variability in 4U 1728-34
Aims. INTEGRAL has been monitoring the Galactic center region for more than a
decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts
from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow
burster". Our aim is to study the connection between the persistent X-ray
spectra and the X-ray burst spectra in a broad spectral range. Methods. We
performed spectral modeling of the persistent emission and the X-ray burst
emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI
instruments. Results. We constructed a hardness intensity diagram to track
spectral state variations. In the soft state the energy spectra are
characterized by two thermal components - likely from the accretion disc and
the boundary/spreading layer - together with a weak hard X-ray tail that we
detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard
state the source is detected up to 200 keV and the spectrum can be described by
a thermal Comptonization model plus an additional component: either a powerlaw
tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect
emission up to 80 keV during the X-ray bursts. We find that during the bursts
the emission above 40 keV decreases by a factor of about three with respect to
the persistent emission level. Conclusions. Our results suggest that the
enhanced X-ray burst emission changes the spectral properties of the accretion
disc in the hard state. The likely cause is an X-ray burst induced cooling of
the electrons in the inner hot flow near the neutron star.Comment: 7 pages, 5 figures, Accepted for publication in A&
Photon breeding mechanism in relativistic jets: astrophysical implications
Photon breeding in relativistic jets involves multiplication of high-energy
photons propagating from the jet to the external environment and back with the
conversion into electron-positron pairs. The exponential growth of the energy
density of these photons is a super-critical process powered by the bulk energy
of the jet. The efficient deceleration of the jet outer layers creates a
structured jet morphology with the fast spine and slow sheath. In initially
fast and high-power jets even the spine can be decelerated efficiently leading
to very high radiative efficiencies of conversion of the jet bulk energy into
radiation. The decelerating, structured jets have angular distribution of
radiation significantly broader than that predicted by a simple blob model with
a constant Lorentz factor. This reconciles the discrepancy between the high
Doppler factors determined by the fits to the spectra of TeV blazars and the
low apparent velocities observed at VLBI scales as well as the low jet Lorentz
factors required by the observed statistics and luminosity ratio of
Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a
population of high-energy leptons in agreement with the constraints on the
electron injection function required by spectral fits of the TeV blazars.
Relativistic pairs created outside the jet and emitting gamma-rays by inverse
Compton process might explain the relatively high level of the TeV emission
from the misaligned jet in the radio galaxies. The mechanism reproduces basic
spectral features observed in blazars including the blazar sequence (shift of
the spectral peaks towards lower energies with increasing luminosity). The
mechanism is very robust and can operate in various environments characterised
by the high photon density.Comment: 6 pages, 3 figures, to appear in the proceedings of the HEPRO
conference, September 24-28, 2007, Dublin, Irelan
Joint spectral-timing modelling of the hard lags in GX 339-4: constraints on reflection models
The X-ray variations of hard state black hole X-ray binaries above 2 keV show
'hard lags', in that the variations at harder energies follow variations at
softer energies, with a time-lag \tau depending on frequency \nu approximately
as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain
this time delay, including fluctuations propagating through an accretion flow,
spectral variations during coronal flares, Comptonisation in the extended hot
corona or a jet, or time-delays due to large-scale reflection from the
accretion disc. In principle these models can be used to predict the shape of
the energy spectrum as well as the frequency-dependence of the time-lags,
through the construction of energy-dependent response functions which map the
emission as a function of time-delay in the system. Here we use this approach
to test a simple reflection model for the frequency-dependent lags seen in the
hard state of GX 339-4, by simultaneously fitting the model to the
frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and
2009. Our model cannot simultaneously fit both the lag and spectral data, since
the relatively large lags require an extremely flared disc which subtends a
large solid angle to the continuum at large radii, in disagreement with the
observed Fe K\alpha emission. Therefore, we consider it more likely that the
lags > 2 keV are caused by propagation effects in the accretion flow, possibly
related to the accretion disc fluctuations which have been observed previously.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
Probing the possibility of hotspots on the central neutron star in HESS J1731-347
The X-ray spectra of the neutron stars located in the centers of supernova
remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models.
These fits yield plausible neutron star sizes for the known or estimated
distances to these supernova remnants. The evidence in favor of the presence of
a pure carbon envelope at the neutron star surface is rather indirect and is
based on the assumption that the emission is generated uniformly by the entire
stellar surface. Although this assumption is supported by the absence of
pulsations, the observational upper limit on the pulsed fraction is not very
stringent. In an attempt to quantify this evidence, we investigate the
possibility that the observed spectrum of the neutron star in HESS J1731-347 is
a combination of the spectra produced in a hydrogen atmosphere of the hotspots
and of the cooler remaining part of the neutron star surface. The lack of
pulsations in this case has to be explained either by a sufficiently small
angle between the neutron star spin axis and the line of sight, or by a
sufficiently small angular distance between the hotspots and the neutron star
rotation poles. As the observed flux from a non-uniformly emitting neutron star
depends on the angular distribution of the radiation emerging from the
atmosphere, we have computed two new grids of pure carbon and pure hydrogen
atmosphere model spectra accounting for Compton scattering. Using new hydrogen
models, we have evaluated the probability of a geometry that leads to a pulsed
fraction below the observed upper limit to be about 8.2 %. Such a geometry thus
seems to be rather improbable but cannot be excluded at this stage.Comment: 8 pages, 14 figures. Accepted for publication in A&
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