201 research outputs found
Detecting Malicious Code by Model Checking
Abstract. The ease of compiling malicious code from source code in higher programming languages has increased the volatility of malicious programs: The first appearance of a new worm in the wild is usually followed by modified versions in quick succession. As demonstrated by Christodorescu and Jha, however, classical detection software relies on static patterns, and is easily outsmarted. In this paper, we present a flexible method to detect malicious code patterns in executables by model checking. While model checking was originally developed to verify the correctness of systems against specifications, we argue that it lends itself equally well to the specification of malicious code patterns. To this end, we introduce the specification language CTPL (Computation Tree Predicate Logic) which extends the well-known logic CTL, and describe an efficient model checking algorithm. Our practical experiments demonstrate that we are able to detect a large number of worm variants with a single specification. Key words: Model Checking, Malware Detection.
The dust temperature distribution in prestellar cores
We have computed the dust temperature distribution to be expected in a
pre-protostellar core in the phase prior to the onset of gravitational
instability. We have done this in the approximation that the heating of the
dust grains is solely due to the attenuated external radiation field and that
the core is optically thin to its own radiation. This permits us to consider
non spherically symmetric geometries. We predict the intensity distributions of
our model cores at millimeter and sub-millimeter wavelengths and compare with
observations of the well studied object L1544. We have also developed an
analytical approximation for the temperature at the center of spherically
symmetric cores and we compare this with the numerical calculations. Our
results show (in agreement with Evans et al. 2001) that the temperatures in the
nuclei of cores of high visual extinction (> 30 magnitudes) are reduced to
values of below ~8 K or roughly half of the surface temperature. This has the
consequence that maps at wavelengths shortward of 1.3 mm see predominantly the
low density exterior of pre-protostellar cores. It is extremely difficult to
deduce the true density distribution from such maps alone. We have computed the
intensity distribution expected on the basis of the models of Ciolek & Basu
(2000) and compared with the observations of L1544. The agreement is good with
a preference for higher inclinations (37 degrees instead of 16) than that
adopted by Ciolek & Basu (2000). We find that a simple extension of the
analytic approximation allows a reasonably accurate calculation of the dust
temperature as a function of radius in cores with density distributions
approximating those expected for Bonnor-Ebert spheres and suggest that this may
be a useful tool for future calculations of the gas temperature in such cores.Comment: 14 latex pages, 10 ps figures, A&A accepte
[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon
Context: We analyze the NGC 2024 HII region and molecular cloud interface
using [12CII] and [13CII] observations. Aims: We attempt to gain insight into
the physical structure of the interface layer between the molecular cloud and
the HII region. Methods. Observations of [12CII] and [13CII] emission at 158
{\mu}m with high spatial and spectral resolution allow us to study the detailed
structure of the ionization front and estimate the column densities and
temperatures of the ionized carbon layer in the PDR. Results: The [12CII]
emission closely follows the distribution of the 8 mum continuum. Across most
of the source, the spectral lines have two velocity peaks similar to lines of
rare CO isotopes. The [13CII] emission is detected near the edge-on ionization
front. It has only a single velocity component, which implies that the [12CII]
line shape is caused by self-absorption. An anomalous hyperfine line-intensity
ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of
the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2
in the gas emitting the [CII] line. A large fraction of this gas has to be at a
temperature of several hundred K. The self-absorption is caused by a cooler
(T<=100 K) foreground component containing a column density of N(H)~10^22
cm^-2
Far-Infrared Properties of Spitzer-selected Luminous Starbursts
We present SHARC-2 350 micron data on 20 luminous z ~ 2 starbursts with
S(1.2mm) > 2 mJy from the Spitzer-selected samples of Lonsdale et al. and
Fiolet et al. All the sources were detected, with S(350um) > 25 mJy for 18 of
them. With the data, we determine precise dust temperatures and luminosities
for these galaxies using both single-temperature fits and models with power-law
mass--temperature distributions. We derive appropriate formulae to use when
optical depths are non-negligible. Our models provide an excellent fit to the
6um--2mm measurements of local starbursts. We find characteristic
single-component temperatures T1 ~ 35.5+-2.2 K and integrated infrared (IR)
luminosities around 10^(12.9+-0.1) Lsun for the SWIRE-selected sources.
Molecular gas masses are estimated at 4 x 10^(10) Msun, assuming
kappa(850um)=0.15 m^2/kg and a submillimeter-selected galaxy (SMG)-like
gas-to-dust mass ratio. The best-fit models imply >~2 kpc emission scales. We
also note a tight correlation between rest-frame 1.4 GHz radio and IR
luminosities confirming star formation as the predominant power source. The
far-IR properties of our sample are indistinguishable from the purely
submillimeter-selected populations from current surveys. We therefore conclude
that our original selection criteria, based on mid-IR colors and 24 um flux
densities, provides an effective means for the study of SMGs at z ~ 1.5--2.5.Comment: 13 pages, 4 figures, edited to match published version in ApJ 717,
29-39 (2010
Accuracy of core mass estimates in simulated observations of dust emission
We study the reliability of mass estimates obtained for molecular cloud cores
using sub-millimetre and infrared dust emission. We use magnetohydrodynamic
simulations and radiative transfer to produce synthetic observations with
spatial resolution and noise levels typical of Herschel surveys. We estimate
dust colour temperatures using different pairs of intensities, calculate column
densities and compare the estimated masses with the true values. We compare
these results to the case when all five Herschel wavelengths are available. We
investigate the effects of spatial variations of dust properties and the
influence of embedded heating sources. Wrong assumptions of dust opacity and
its spectral index beta can cause significant systematic errors in mass
estimates. These are mainly multiplicative and leave the slope of the mass
spectrum intact, unless cores with very high optical depth are included.
Temperature variations bias colour temperature estimates and, in quiescent
cores with optical depths higher than for normal stable cores, masses can be
underestimated by up to one order of magnitude. When heated by internal
radiation sources the observations recover the true mass spectra. The shape,
although not the position, of the mass spectrum is reliable against
observational errors and biases introduced in the analysis. This changes only
if the cores have optical depths much higher than expected for basic
hydrostatic equilibrium conditions. Observations underestimate the value of
beta whenever there are temperature variations along the line of sight. A bias
can also be observed when the true beta varies with wavelength. Internal
heating sources produce an inverse correlation between colour temperature and
beta that may be difficult to separate from any intrinsic beta(T) relation of
the dust grains. This suggests caution when interpreting the observed mass
spectra and the spectral indices.Comment: Revised version, 17 pages, 17 figures, submitted to A&
Submillimeter to centimeter excess emission from the Magellanic Clouds. II. On the nature of the excess
Dust emission at submm to cm wavelengths is often simply the Rayleigh-Jeans
tail of dust particles at thermal equilibrium and is used as a cold mass tracer
in various environments including nearby galaxies. However, well-sampled
spectral energy distributions of the nearby, star-forming Magellanic Clouds
have a pronounced (sub-)millimeter excess (Israel et al., 2010). This study
attempts to confirm the existence of such a millimeter excess above expected
dust, free-free and synchrotron emission and to explore different possibilities
for its origin. We model NIR to radio spectral energy distributions of the
Magellanic Clouds with dust, free-free and synchrotron emission. A millimeter
excess emission is confirmed above these components and its spectral shape and
intensity are analysed in light of different scenarios: very cold dust, Cosmic
Microwave Background (CMB) fluctuations, a change of the dust spectral index
and spinning dust emission. We show that very cold dust or CMB fluctuations are
very unlikely explanations for the observed excess in these two galaxies. The
excess in the LMC can be satisfactorily explained either by a change of the
spectral index due to intrinsic properties of amorphous grains, or by spinning
dust emission. In the SMC however, due to the importance of the excess, the
dust grain model including TLS/DCD effects cannot reproduce the observed
emission in a simple way. A possible solution was achieved with spinning dust
emission, but many assumptions on the physical state of the interstellar medium
had to be made. Further studies, using higher resolution data from Planck and
Herschel, are needed to probe the origin of this observed submm-cm excess more
definitely. Our study shows that the different possible origins will be best
distinguished where the excess is the highest, as is the case in the SMC.Comment: 7 pages, 6 figures; accepted in A&
The P2 Receptor Antagonist PPADS Supports Recovery from Experimental Stroke In Vivo
BACKGROUND: After ischemia of the CNS, extracellular adenosine 5'-triphosphate (ATP) can reach high concentrations due to cell damage and subsequent increase of membrane permeability. ATP may cause cellular degeneration and death, mediated by P2X and P2Y receptors. METHODOLOGY/PRINCIPAL FINDINGS: The effects of inhibition of P2 receptors by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on electrophysiological, functional and morphological alterations in an ischemia model with permanent middle cerebral artery occlusion (MCAO) were investigated up to day 28. Spontaneously hypertensive rats received PPADS or vehicle intracerebroventricularly 15 minutes prior MCAO for up to 7 days. The functional recovery monitored by qEEG was improved by PPADS indicated by an accelerated recovery of ischemia-induced qEEG changes in the delta and alpha frequency bands along with a faster and sustained recovery of motor impairments. Whereas the functional improvements by PPADS were persistent at day 28, the infarct volume measured by magnetic resonance imaging and the amount of TUNEL-positive cells were significantly reduced by PPADS only until day 7. Further, by immunohistochemistry and confocal laser scanning microscopy, we identified both neurons and astrocytes as TUNEL-positive after MCAO. CONCLUSION: The persistent beneficial effect of PPADS on the functional parameters without differences in the late (day 28) infarct size and apoptosis suggests that the early inhibition of P2 receptors might be favourable for the maintenance or early reconstruction of neuronal connectivity in the periinfarct area after ischemic incidents
High mass star formation in the IRAS 17233-3606 region: a new nearby and bright hot core in the southern sky
We present molecular line observations of the massive star forming region
IRAS 17233-3606 aimed at studying the molecular core associated with the
source. The observations were made using the Atacama Pathfinder Experiment
telescope in the CO (3-2) and HCO^+ (4-3) transitions, and in the CH_3OH
(6_K-5_K), (7_K-6_K) and CH_3CN (16_K-15_K) bands. For the CO(3-2) and HCO^+
(4-3) transitions, we obtained maps with a size of 70''\times 70''. The typical
angular resolution of the data is ~18''. Our observations reveal an
exceptionally rich molecular spectrum, a signpost of hot core activity.
Comparisons with two other prominent southern hot cores were made through
observations in the same frequency setups. We also detected a bipolar outflow
in CO (3-2) and HCO^+ (4-3) lines. Modelling reveals a hot core of size ~3''
and a temperature of 150 K in the IRAS17233-3606 region. The parameters of the
molecular outflow are derived through the analysis of the CO (3-2) emission,
and are typical of outflows driven by high-mass young stellar objects.Comment: 9 pages, 10 figures (plus 8 figures as Online material), accepted by
A&
GMRT 333 MHz observations of 6 nearby normal galaxies
We report Giant Meterwave Radio Telescope (GMRT) continuum observations of
six nearby normal galaxies at 333 MHz. The galaxies are observed with angular
resolutions better than ~20" (corresponding to a linear scale of about 0.4 - 1
kpc). These observations are sensitive to all the angular scales of interest,
since the resolution of the shortest baseline in GMRT is greater than the
angular size of the galaxies. Further, for five of these galaxies we show that
at 333 MHz, the mean thermal fraction is less than 5%. Using archival data at
about 1 GHz, we estimate the mean thermal fraction to be about 10% at that
frequency. We also find that the nonthermal spectral index is generally steeper
in regions with low thermal fraction and/or located in the outer parts of the
galaxy. In regions of high thermal fraction, the nonthermal spectral index is
flatter, and has a narrow distribution peaking at ~ -0.78 with a spread of
0.16, putting stringent constraints on the physical conditions for generation,
diffusion and energy losses of cosmic ray electrons at scales of ~ 1 kpc.Comment: 18 pages, 11 figures, Accepted for publication in MNRA
Analysis of galaxy SEDs from far-UV to far-IR with CIGALE: Studying a SINGS test sample
Photometric data of galaxies covering the rest-frame wavelength range from
far-UV to far-IR make it possible to derive galaxy properties with a high
reliability by fitting the attenuated stellar emission and the related dust
emission at the same time. For this purpose we wrote the code CIGALE (Code
Investigating GALaxy Emission) that uses model spectra composed of the Maraston
(or PEGASE) stellar population models, synthetic attenuation functions based on
a modified Calzetti law, spectral line templates, the Dale & Helou dust
emission models, and optional spectral templates of obscured AGN. Depending on
the input redshifts, filter fluxes are computed for the model set and compared
to the galaxy photometry by carrying out a Bayesian-like analysis. CIGALE was
tested by analysing 39 nearby galaxies selected from SINGS. The reliability of
the different model parameters was evaluated by studying the resulting
expectation values and their standard deviations in relation to the input model
grid. Moreover, the influence of the filter set and the quality of photometric
data on the code results was estimated. For up to 17 filters between 0.15 and
160 mum, we find robust results for the mass, star formation rate, effective
age of the stellar population at 4000 A, bolometric luminosity, luminosity
absorbed by dust, and attenuation in the far-UV. A study of the mutual
relations between the reliable properties confirms the dependence of star
formation activity on morphology in the local Universe and indicates a
significant drop in this activity at about 10^11 M_sol towards higher total
stellar masses. The dustiest sample galaxies are present in the same mass
range. [abridged]Comment: 22 pages, 21 figures, accepted for publication in A&
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