48 research outputs found
Testing Autonomous Robot Control Software Using Procedural Content Generation
We present a novel approach for reducing manual effort when testing autonomous robot control algorithms. We use procedural content generation, as developed for the film and video game industries, to create a diverse range of test situations. We execute these in the Player/Stage robot simulator and automatically rate them for their safety significance using an event-based scoring system. Situations exhibiting dangerous behaviour will score highly, and are thus flagged for the attention of a safety engineer. This process removes the time-consuming tasks of hand-crafting and monitoring situations while testing an autonomous robot control algorithm. We present a case study of the proposed approach – we generated 500 randomised situations, and our prototype tool simulated and rated them. We have analysed the three highest rated situations in depth, and this analysis revealed weaknesses in the smoothed nearness-diagram control algorithm
A self-consistent hybrid Comptonization model for broad-band spectra of accreting supermassive black holes
The nature of the broad-band spectra of supermassive accreting black holes in
active galactic nuclei (AGNs) is still unknown. The hard X-ray spectra of
Seyferts as well as of Galactic stellar-mass black holes (GBHs) are well
represented by thermal Comptonization, but the origin of the seed photons is
less certain. The MeV tails observed in GBHs provide evidence in favour of
non-thermal electron tails and it is possible that such electrons are also
present in the X-ray emitting regions of AGNs. Using simulations with the
kinetic code that self-consistently models electron and photon distributions,
we find that the power-law-like X-ray spectra in AGNs can be explained in terms
of the synchrotron self-Compton radiation of hybrid thermal/non-thermal
electrons, similarly to the hard/low state of GBHs. Under a very broad range of
parameters the model predicts a rather narrow distribution of photon spectral
slopes consistent with that observed from LINERs and Seyferts at luminosities
less than 3 per cent of the Eddington luminosity. The entire infrared to X-ray
spectrum of these objects can be described in terms of our model, suggesting a
tight correlation between the two energy bands. We show that the recently found
correlation between slope and the Eddington ratio at higher luminosities can be
described by the increasing fraction of disc photons in the emitting region,
which may be associated with the decreasing inner radius of the optically thick
accretion disc. The increasing flux of soft photons is also responsible for the
transformation of the electron distribution from nearly thermal to almost
completely non-thermal. The softer X-ray spectra observed in narrow-line
Seyfert galaxies may correspond to non-thermal Comptonization of the disc
photons, predicting that no cutoff should be observed up to MeV energies in
these sources, similarly to the soft-state GBHs.Comment: MNRAS publishe
Compton scattering as the explanation of the peculiar X-ray properties of Cyg X-3
We consider implications of a possible presence of a Thomson-thick,
low-temperature, plasma cloud surrounding the compact object in the binary
system Cyg X-3. The presence of such a cloud was earlier inferred from the
energy-independent orbital modulation of the X-ray flux and the lack of high
frequencies in its power spectra. Here, we study the effect of Compton
scattering by the cloud on the X-ray energy and power spectra, concentrating on
the hard spectral state. The process reduces the energy of the high-energy
break/cut-off in the energy spectra, which allows us to determine the Thomson
optical depth. This, together with the observed cut-off in the power spectrum,
determines the size of the plasma to be 2x10^9 cm. At this size, the cloud will
be in thermal equilibrium in the photon field of the X-ray source, which yields
the cloud temperature of 3 keV, which refines the determination of the Thomson
optical depth to 7. At these parameters, thermal bremsstrahlung emission of the
cloud becomes important as well. The physical origin of the cloud is likely to
be collision of the very strong stellar wind of the companion Wolf-Rayet star
with a small accretion disc formed by the wind accretion. Our model thus
explains the peculiar X-ray energy and power spectra of Cyg X-3.Comment: MNRAS, the version as printed, the title and abstract change
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
The MeV spectral tail in Cyg X-1 and optically-thin emission of jets
We study the average X-ray and soft gamma-ray spectrum of Cyg X-1 in the hard
spectral state, using data from INTEGRAL. We compare these results with those
from CGRO, and find a good agreement. Confirming previous studies, we find the
presence of a high-energy MeV tail beyond a thermal-Comptonization spectrum;
however, the tail is much softer and weaker than that recently published by
Laurent et al. In spite of this difference, the observed high-energy tail could
still be due to the synchrotron emission of the jet of Cyg X-1, as claimed by
Laurent et al.
To test this possibility, we study optically-thin synchrotron and
self-Compton emission from partially self-absorbed jets. We develop formalisms
for calculating both emission of the jet base (which we define here as the
region where the jet starts its emission) and emission of the entire jet. We
require the emission to match that observed at the turnover energy. The
optically thin emission is dominated by that from the jet base, and it has to
become self-absorbed within it at the turnover frequency. We find this implies
the magnetic field strength at the jet base of B_0 prop. to z_0^4, where z_0 is
the distance of the base from the black-hole centre. The value of B_0 is then
constrained from below by the condition that the self-Compton emission is below
an upper limit in the GeV range, and from above by the condition that the
Poynting flux does not exceed the jet kinetic power. This yields B_0 of the
order of ~10^4 G and the location of the jet base at ~10^3 gravitational radii.
Using our formalism, we find the MeV tail can be due to jet synchrotron
emission, but this requires the electron acceleration at a rather hard
power-law index, p~1.3-1.6. For acceleration indices of p> 2, the amplitude of
the synchrotron component is much below that of MeV tail, and its origin is
likely to be due to hybrid Comptonization in the accretion flow.Comment: MNRAS, in press, 13 page
Superorbital variability of X-ray and radio emission of Cyg X-1. I. Emission anisotropy of precessing sources
We study theoretical interpretations of the 150-d (superorbital) modulation
observed in X-ray and radio emission of Cyg X-1 in the framework of models
connecting this phenomenon to precession. Precession changes the orientation of
the emission source (either disc or jet) relative to the observer. This leads
to emission modulation due to an anisotropic emission pattern of the source or
orientation-dependent amount of absorbing medium along the line of sight or
both. We consider, in particular, anisotropy patterns of blackbody-type
emission, thermal Comptonization in slab geometry, jet/outflow beaming, and
absorption in a coronal-type medium above the disc. We then fit these models to
the data from the RXTE/ASM, CGRO/BATSE, and the Ryle and Green Bank radio
telescopes, and find relatively small best-fit angles between the precession
and orbital planes, ~10-20 degrees. The thermal Comptonization model for the
X-ray emission explains well the observed decrease of the variability amplitude
from 1 to 300 keV as a result of a reduced anisotropy of the emission due to
multiple scatterings. Our modeling also yield the jet bulk velocity of
~(0.3-0.5)c, which is in agreement with the previous constraint from the lack
of an observed counterjet and lack of short-term X-ray/radio correlations.Comment: 10 pages, 9 figures and 2 tables, accepted to MNRA
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
An additional soft X-ray component in the dim low/hard state of black hole binaries
We test the truncated disc models using multiwavelength (optical/UV/X-ray)
data from the 2005 hard state outburst of the black hole SWIFT J1753.5-0127.
This system is both fairly bright and has fairly low interstellar absorption,
so gives one of the best datasets to study the weak, cool disc emission in this
state. We fit these data using models of an X-ray illuminated disc to constrain
the inner disc radius throughout the outburst. Close to the peak, the observed
soft X-ray component is consistent with being produced by the inner disc, with
its intrinsic emission enhanced in temperature and luminosity by reprocessing
of hard X-ray illumination in an overlap region between the disc and corona.
This disc emission provides the seed photons for Compton scattering to produce
the hard X-ray spectrum, and these hard X-rays also illuminate the outer disc,
producing the optical emission by reprocessing.
However, the situation is very different as the outburst declines. The
optical is probably cyclo-synchrotron radiation, self-generated by the flow,
rather than tracing the outer disc. Similarly, limits from reprocessing make it
unlikely that the soft X-rays are directly tracing the inner disc radius. This
is seen more clearly in a similarly dim low/hard state spectrum from XTE
J1118+480. The very small emitting area implied by the relatively high
temperature soft X-ray component is completely inconsistent with the much
larger, cooler, UV component which is well fit by a truncated disc. We
speculate on the origin of this component, but its existence as a clearly
separate spectral component from the truncated disc in XTE J1118+480 shows that
it does not simply trace the inner disc radius, so cannot constrain the
truncated disc models.Comment: 11 pages, 10 figures, submitted to MNRA
On the X-ray spectra of luminous, inhomogeneous accretion flows
We discuss the expected X-ray spectral and variability properties of black
hole accretion discs at high luminosity, under the hypothesis that radiation
pressure dominated discs are subject to violent clumping instabilities and, as
a result, have a highly inhomogeneous two-phase structure. After deriving the
full accretion disc solutions explicitly in terms of the parameters of the
model, we study their radiative properties both with a simple two-zones model,
treatable analytically, and with radiative transfer simulations which account
simultaneously for energy balance and Comptonisation in the hot phase, together
with reflection, reprocessing, ionization and thermal balance in the cold
phase. We show that, if not only the density, but also the heating rate within
these flows is inhomogeneous, then complex reflection-dominated spectra can be
obtained for a high enough covering fraction of the cold phase. In general,
large reflection components in the observed X-ray spectra should be associated
with strong soft excesses, resulting from the combined emission of ionized
atomic emission lines. The variability properties of such systems are such
that, even when contributing to a large fraction of the hard X-ray spectrum,
the reflection component is less variable than the power-law like emission
originating from the hot Comptonising phase, in agreement with what is observed
in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls
within the family of those trying to explain the complex X-ray spectra of
bright AGN with ionized reflection, but presents an alternative, specific,
physically motivated, geometrical setup for the complex multi-phase structure
of the inner regions of near-Eddington accretion flows.Comment: 15 pages, 9 figures. MNRAS, in pres