2,305 research outputs found
Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
The progenitors of Type Ia Supernovae (SNe Ia) are still unknown, despite
significant progress during the last years in theory and observations. Violent
mergers of two carbon--oxygen (CO) white dwarfs (WDs) are one candidate
suggested to be responsible for at least a significant fraction of normal SNe
Ia. Here, we simulate the merger of two CO WDs using a moving-mesh code that
allows for the inclusion of thin helium (He) shells (0.01\,\msun) on top of the
WDs, at an unprecedented numerical resolution. The accretion of He onto the
primary WD leads to the formation of a detonation in its He shell. This
detonation propagates around the CO WD and sends a converging shock wave into
its core, known to robustly trigger a second detonation, as in the well-known
double-detonation scenario for
He-accreting CO WDs. However, in contrast to that scenario where a massive He
shell is required to form a detonation through thermal instability, here the He
detonation is ignited dynamically. Accordingly the required He-shell mass is
significantly smaller, and hence its burning products are unlikely to affect
the optical display of the explosion. We show that this scenario, which works
for CO primary WDs with CO- as well as He-WD companions, has the potential to
explain the different brightness distributions, delay times and relative rates
of normal and fast declining SNe Ia. Finally, we discuss extensions to our
unified merger model needed to obtain a comprehensive picture of the full
observed diversity of SNe Ia.Comment: accepted for publication by ApJL, significant changes to first
version, including addition of merger simulatio
Characterisation of the CAFOS linear spectro-polarimeter
Aims. This research note presents a full analysis of the CAFOS polarimeter
mounted at the Calar Alto 2.2m telescope. It also provides future users of this
mode with all necessary information to properly correct for instrumental
effects in polarization data obtained with this instrument.
Methods. The standard stars BD+59d389 (polarized) and HD14069 (unpolarized)
were observed with CAFOS in November, 2010, using 16 half-wave plate angles.
The linear spectropolarimetric properties of CAFOS were studied using a Fourier
Analysis of the resulting data.
Results. CAFOS shows a roughly constant instrumental polarization at the
level of ~0.3% between 4000 and 8600 A. Below 4000 A the spurious polarization
grows to reach ~0.7% at 3600 A. This instrumental effect is most likely
produced by the telescope optics, and appears to be additive. The Wollaston
prism presents a clear deviation from the ideal behavior. The problem is
largely removed by the usage of at least 4 retarder plate angles. The
chromatism of the half-wave plate causes a peak-to-peak oscillation of ~11
degrees in the polarization angle. This can be effectively corrected using the
tabulated values presented in this paper. The Fourier analysis shows that the
k!=0,4 harmonics are practically negligible between 3800 and 7400 A.
Conclusions. After correcting for instrumental polarization and retarder
plate chromatism, with 4 half-wave plate angles CAFOS can reach an rms linear
polarization accuracy of about 0.1%.Comment: 5 pages, 5 figures, Accepted for publication in A&A (Research Note
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Vulnerability Identification Errors in Security Risk Assessments
At present, companies rely on information technology systems to achieve their business objectives, making them vulnerable to cybersecurity threats. Information security risk assessments help organisations to identify their risks and vulnerabilities. An accurate identification of risks and vulnerabilities is a challenge, because the input data is uncertain. So-called ’vulnerability identification errors‘ can occur if false positive vulnerabilities are identified, or if vulnerabilities remain unidentified (false negatives). ‘Accurate identification’ in this context means that all vulnerabilities identified do indeed pose a risk of a security breach for the organisation. An experiment performed with German IT security professionals in 2011 confirmed that vulnerability identification errors do occur in practice. In particular, false positive vulnerabilities were identified by participants.
In information security (IS) risk assessments, security experts analyze the organisation’s assets in order to identify vulnerabilities. Methods such as brainstorming, checklists, scenario-analysis, impact-analysis, and cause-analysis (ISO, 2009b) are used to identify vulnerabilities. These methods use uncertain input data for vulnerability identification, because the probabilities, effects and losses of vulnerabilities cannot be determined exactly (Fenz and Ekelhart, 2011). Furthermore, business security needs are not considered properly; the security checklists and standards used to identify vulnerabilities do not consider company-specific security requirements (Siponen and Willison, 2009). In addition, the intentional behaviour of an attacker when exploiting vulnerabilities for malicious purposes further increases the uncertainty, because predicting human behaviour is not just about existing vulnerabilities and their consequences (Pieters and Consoli, 2009), rather than preparing for future attacks. As a result, current approaches determine risks and vulnerabilities under a high degree of uncertainty, which can lead to errors.
This thesis proposes an approach to resolve vulnerability identification errors using security requirements and business process models. Security requirements represent the business security needs and determine whether any given vulnerability is a security risk for the business. Information assets’ security requirements are evaluated in the context of the business process model, in order to determine whether security functions are implemented and operating correctly. Systems, personnel and physical parts of business processes, as well as IT processes, are considered in the security requirement evaluation, and this approach is validated in three steps. Firstly, the systematic procedure is compared to two best-practice approaches. Secondly, the risk result accuracy is compared to a best-practice risk-assessment approach, as applied to several real-world examples within an insurance company. Thirdly, the capability to determine risk more accurately by using business processes and security requirements is tested in a quasi-experiment, using security professionals.
This thesis demonstrates that risk assessment methods can benefit from explicit evaluation of security requirements in the business context during risk identification, in order to resolve vulnerability identification errors and to provide a criterion for security
Limits on stable iron in TypeIa supernovae from NIR spectroscopy
We obtained optical and near-infrared spectra of TypeIa supernovae
(SNeIa) at epochs ranging from 224 to 496 days after the explosion. The
spectra show emission lines from forbidden transitions of singly ionised iron
and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling
of the first and second ionisation stages of iron, nickel, and cobalt to fit
the spectra using a sampling algorithm allowing us to probe a broad parameter
space. We derive velocity shifts, line widths, and abundance ratios for iron
and cobalt. The measured line widths and velocity shifts of the singly ionised
ions suggest a shared emitting region. Our data are fully compatible with
radioactive Ni decay as the origin for cobalt and iron. We compare the
measured abundance ratios of iron and cobalt to theoretical predictions of
various SNIa explosion models. These models include, in addition to
Ni, different amounts of Ni and stable Fe. We can
exclude models that produced only Fe or only Ni in addition to
Ni. If we consider a model that has Ni, Ni, and
Fe then our data imply that these ratios are Fe / Ni
and Ni / Ni .Comment: 10 pages, 7 figures, Accepted for publication in A&
Applying the expanding photosphere and standardized candle methods to Type II-Plateau supernovae at cosmologically significant redshifts: the distance to SN 2013eq
Based on optical imaging and spectroscopy of the Type II-Plateau SN 2013eq,
we present a comparative study of commonly used distance determination methods
based on Type II supernovae. The occurrence of SN 2013eq in the Hubble flow (z
= 0.041 +/- 0.001) prompted us to investigate the implications of the
difference between "angular" and "luminosity" distances within the framework of
the expanding photosphere method (EPM) that relies upon a relation between flux
and angular size to yield a distance. Following a re-derivation of the basic
equations of the EPM for SNe at non-negligible redshifts, we conclude that the
EPM results in an angular distance. The observed flux should be converted into
the SN rest frame and the angular size, theta, has to be corrected by a factor
of (1+z)^2. Alternatively, the EPM angular distance can be converted to a
luminosity distance by implementing a modification of the angular size. For SN
2013eq, we find EPM luminosity distances of D_L = 151 +/- 18 Mpc and D_L = 164
+/- 20 Mpc by making use of different sets of dilution factors taken from the
literature. Application of the standardized candle method for Type II-P SNe
results in an independent luminosity distance estimate (D_L = 168 +/- 16 Mpc)
that is consistent with the EPM estimate.Comment: 12 pages, 4 figures, accepted by A&
Early light curves for Type Ia supernova explosion models
Upcoming high-cadence transient survey programmes will produce a wealth of
observational data for Type Ia supernovae. These data sets will contain
numerous events detected very early in their evolution, shortly after
explosion. Here, we present synthetic light curves, calculated with the
radiation hydrodynamical approach Stella for a number of different explosion
models, specifically focusing on these first few days after explosion. We show
that overall the early light curve evolution is similar for most of the
investigated models. Characteristic imprints are induced by radioactive
material located close to the surface. However, these are very similar to the
signatures expected from ejecta-CSM or ejecta-companion interaction. Apart from
the pure deflagration explosion models, none of our synthetic light curves
exhibit the commonly assumed power-law rise. We demonstrate that this can lead
to substantial errors in the determination of the time of explosion. In
summary, we illustrate with our calculations that even with very early data an
identification of specific explosion scenarios is challenging, if only
photometric observations are available.Comment: 15 pages, 14 figures, 3 tables, accepted for publication in MNRA
High luminosity, slow ejecta and persistent carbon lines: SN 2009dc challenges thermonuclear explosion scenarios
Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae (SNe Ia), but is clearly overluminous, with a (pseudo-bolometric) peak luminosity of log (L) = 43.47 (erg s^(−1)). Its light curves decline slowly over half a year after maximum light [Δm_(15)(B)_true= 0.71], and the early-time near-IR light curves show secondary maxima, although the minima between the first and the second peaks are not very pronounced. The bluer bands exhibit an enhanced fading after ~200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C ii lines are present until ~2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of interaction with a circumstellar medium (CSM) are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ~1.8 M_⊙ of ^(56)Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ~2.8 M_⊙, this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies. A low metallicity of the progenitor may therefore be an important prerequisite for producing superluminous SNe Ia. We discuss a number of possible explosion scenarios, ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I(1/2) SNe to a core-collapse origin of the explosion. None of the models seems capable of explaining all properties of SN 2009dc, so that the true nature of this SN and its peers remains nebulous
Late-time spectral line formation in Type IIb supernovae, with application to SN 1993J, SN 2008ax, and SN 2011dh
We investigate line formation processes in Type IIb supernovae (SNe) from 100
to 500 days post-explosion using spectral synthesis calculations. The modeling
identifies the nuclear burning layers and physical mechanisms that produce the
major emission lines, and the diagnostic potential of these. We compare the
model calculations with data on the three best observed Type IIb SNe to-date -
SN 1993J, SN 2008ax, and SN 2011dh. Oxygen nucleosynthesis depends sensitively
on the main-sequence mass of the star and modeling of the [O I] 6300, 6364
lines constrains the progenitors of these three SNe to the M_ZAMS=12-16 M_sun
range (ejected oxygen masses 0.3-0.9 M_sun), with SN 2011dh towards the lower
end and SN 1993J towards the upper end of the range. The high ejecta masses
from M_ZAMS >= 17 M_sun progenitors give rise to brighter nebular phase
emission lines than observed. Nucleosynthesis analysis thus supports a scenario
of low/moderate mass progenitors for Type IIb SNe, and by implication an origin
in binary systems. We demonstrate how oxygen and magnesium recombination lines
may be combined to diagnose the magnesium mass in the SN ejecta. For SN 2011dh,
a magnesium mass of of 0.02-0.14 M_sun is derived, which gives a Mg/O
production ratio consistent with the solar value. Nitrogen left in the He
envelope from CNO-burning gives strong [N II] 6548, 6583 emission lines that
dominate over H-alpha emission in our models. The hydrogen envelopes of Type
IIb SNe are too small and dilute to produce any noticeable H-alpha emission or
absorption after ~150 days, and nebular phase emission seen around 6550 A is in
many cases likely caused by [N II] 6548, 6583. Finally, the influence of
radiative transport on the emergent line profiles is investigated...(abridged)Comment: Published versio
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