175 research outputs found
Energy spectra of X-ray quasi-periodic oscillations in accreting black hole binaries
We investigate the energy dependencies of X-ray quasi-periodic oscillations
in black hole X-ray binaries. We analyze RXTE data on both the low- and
high-frequency QPO. We construct the low-f QPO energy spectra, and demonstrate
that they do not contain the thermal disk component, even though the latter is
present in the time averaged spectra. The disk thus does not seem to
participate in the oscillations. Moreover the QPO spectra are harder than the
time averaged spectra when the latter are soft, which can be modeled as a
result of modulations occurring in the hot plasma. The QPO spectra are softer
than the time averaged spectra when the latter are hard. The absence of the
disk component in the QPO spectra is true also for the high-frequency
(hecto-Hz) QPO observed in black hole binaries. We compute the QPO spectra
expected from the model of disk resonances.Comment: 4 pages, Proc. of IAU Symposium 238, "Black Holes from Stars to
Galaxies - across the range of masses", Prague, Aug 200
Geomagnetic field and altitude effects on the performance of future IACT arrays
The performance of IACT's arrays is sensitive to the altitude and geomagnetic
field (GF) of the observatory site. Both effects play important role in the
region of the sub-TeV gamma-ray measurements. We investigate the influence of
GF on detection rates and the energy thresholds for five possible locations of
the future CTA observatory using the Monte Carlo simulations. We conclude that
the detection rates of gamma rays and the energy thresholds of the arrays can
be fitted with linear functions of the altitude and the component of the GF
perpendicular to the shower axis core. These results can be directly
extrapolated for any possible localization of the CTA. In this paper we also
show the influence of both geophysical effects on the images of shower and
gamma/hadron separation.Comment: 4 pages, 6 figures, two-column. Contribution to ICRC 2013 proceeding
Monte Carlo simulations of global Compton cooling in inner regions of hot accretion flows
Hot accretion flows such as advection-dominated accretion flows are generally
optically thin in the radial direction. Thus photons generated at some radii
can cool or heat electrons at other radii via Compton scattering. Such global
Compton scattering has previously been shown to be important for the dynamics
of accretion flows. Here, we extend previous treatments of this problem by
using accurate global general relativistic Monte Carlo simulations. We focus on
an inner region of the accretion flow (R < 600R_g), for which we obtain a
global self-consistent solution. As compared to the initial, not
self-consistent solution, the final solution has both the cooling rate and the
electron temperature significantly reduced at radii >=10 gravitational radii.
On the other hand, the radiation spectrum of the self-consistent solution has
the shape similar to that of the initial iteration, except for the high-energy
cut-off being at an energy lower by a factor of ~2 and the bolometric
luminosity decreased by a factor of ~2. We also compare the global Compton
scattering model with local models in spherical and slab geometry. We find that
the slab model approximates the global model significantly better than the
spherical one. Still, neither local model gives a good approximation to the
radial profile of the cooling rate, and the differences can be up to two orders
of magnitude. The local slab model underestimates the cooling rate at outer
regions whereas it overestimates that rate at inner regions. We compare our
modelling results to observed hard-state spectra of black-hole binaries and
find an overall good agreement provided any disc outflow is weak. We find that
general-relativistic effects in flows which dynamics is modified by global
Comptonization is crucial in approaching this agreement.Comment: 9 pages, 4 figures. Accepted to MNRAS. Add a new section to discuss
on the impact of outflow and viscous electron heatin
General-relativistic model of hot accretion flows with global Compton cooling
We present a model of optically thin, two-temperature, accretion flows using
an exact Monte Carlo treatment of global Comptonization, with seed photons from
synchrotron and bremsstrahlung emission, as well as with a fully general
relativistic description of both the radiative and hydrodynamic processes. We
consider accretion rates for which the luminosities of the flows are between
~0.001 and 0.01 of the Eddington luminosity. The black hole spin parameter
strongly affects the flow structure within the innermost 10 gravitational
radii. The resulting large difference between the Coulomb heating in models
with a non-rotating and a rapidly rotating black hole is, however, outweighed
by a strong contribution of compression work, much less dependent on spin. The
consequent reduction of effects related to the value of the black spin is more
significant at smaller accretion rates. For a non-rotating black hole, the
compressive heating of electrons dominates over their Coulomb heating, and
results in an approximately constant radiative efficiency of approximately 0.4
per cent in the considered range of luminosities. For a rapidly rotating black
hole, the Coulomb heating dominates, the radiative efficiency is ~1 per cent
and it slightly increases (but less significantly than estimated in some
previous works) with increasing accretion rate. We find an agreement between
our model, in which the synchrotron emission is the main source of seed
photons, and observations of black-hole binaries in their hard states and AGNs
at low luminosities. In particular, our model predicts a hardening of the X-ray
spectrum with increasing luminosity, as indeed observed below ~0.01 of the
Eddington luminosity in both black-hole binaries and AGNs. Also, our model
approximately reproduces the luminosity and the slope of the X-ray emission in
Cen A.Comment: 13 pages, MNRAS, accepte
A new detector concept based on the prompt gamma radiation analysis for In vivo boron monitoring in BNCT
The problem of boron concentration monitoring during the boron neutron capture therapy (BNCT) therapy is one of the main challenges of this type of radiotherapy and is directly related to the nature of the interaction of neutrons with mater. Among the available in vivo methods of boron monitoring positron emission tomography seems to be very promising but it requires a new boron carrier with a β+ emitter, which is not yet clinically available. An alternative solution may be the prompt gamma radiation analysis (PGRA) based on the secondary radiation emitted in the interaction of neutrons with the patient’s tissues. This method requires, however, compact gamma radiation detection systems sustaining high counting rates and characterized by very good energy resolution. In this contribution, we present state-of-the-art solutions for monitoring in BNCT based on PGRA. Moreover, we describe a new concept of such a system based on position-sensitive scintillator detectors equipped with an anti-Compton shield and data analysis supported with modern artificial intelligence algorithms
Beam profile investigation of the new collimator system for the J-PET detector
Jagiellonian Positron Emission Tomograph (J-PET) is a multi-purpose detector
which will be used for search for discrete symmetries violations in the decays
of positronium atoms and for investigations with positronium atoms in
life-sciences and medical diagnostics. In this article we present three methods
for determination of the beam profile of collimated annihilation gamma quanta.
Precise monitoring of this profile is essential for time and energy calibration
of the J-PET detector and for the determination of the library of model signals
used in the hit-time and hit-position reconstruction. We have we have shown
that usage of two lead bricks with dimensions of 5x10x20 cm^3 enables to form a
beam of annihilation quanta with Gaussian profile characterized by 1 mm FWHM.
Determination of this characteristic is essential for designing and
construction the collimator system for the 24-module J-PET prototype.
Simulations of the beam profile for different collimator dimensions were
performed. This allowed us to choose optimal collimation system in terms of the
beam profile parameters, dimensions and weight of the collimator taking into
account the design of the 24 module J-PET detector.Comment: 14 pages, 9 figure
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