1,645 research outputs found
SN1998bw: The Case for a Relativistic Shock
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.Comment: A&A (in press), Rome GRB Symposium, Nov. 199
Constraints on Off-Axis GRB Jets in Type Ibc Supernovae From Late-Time Radio Observations
It has been suggested that the peculiar properties of the luminous Type Ic
supernova SN 1998bw and its low-energy gamma-ray burst GRB 980425 may be
understood if they originated in a standard gamma-ray burst explosion viewed
far from the axis of the relativistic jet. In this scenario, strong radio
emission is predicted from the jet on a timescale 1 to 10 years after the
explosion as it decelerates and spreads into our line of sight. To test this
hypothesis we have carried out late-time radio observations of SN 1998bw at
years, yielding upper limits which are consistent with the continued
fading of the supernova. We find these limits to be consistent with an off-axis
jet only if the progenitor mass loss rate is
M yr (for a wind velocity km s) or the
fraction of the shock energy in magnetic fields is . These values are low relative to those inferred for cosmological
GRBs. We combine the SN 1998bw measurements with existing observations for a
sample of 15 local Type Ibc supernovae to estimate that at most 6% produce
collimated, relativistic outflows.Comment: Revised version, as it appears in ApJ
Towards Validating Risk Indicators Based on Measurement Theory
Due to the lack of quantitative information and for cost-efficiency purpose, most risk assessment methods use partially ordered values (e.g. high, medium, low) as risk indicators. In practice it is common to validate risk scales by asking stakeholders whether they make sense. This way of validation is subjective, thus error prone. If the metrics are wrong (not meaningful), then they may lead system owners to distribute security investments inefficiently. Therefore, when validating risk assessment methods it is important to validate the meaningfulness of the risk scales that they use. In this paper we investigate how to validate the meaningfulness of risk indicators based on measurement theory. Furthermore, to analyze the applicability of measurement theory to risk indicators, we analyze the indicators used by a particular risk assessment method specially developed for assessing confidentiality risks in networks of organizations
The Role of Deontic Logic in the Specification of Information Systems
In this paper we discuss the role that deontic logic plays in the specification of information systems, either because constraints on the systems directly concern norms or, and even more importantly, system constraints are considered ideal but violable (so-called `soft¿ constraints).\ud
To overcome the traditional problems with deontic logic (the so-called paradoxes), we first state the importance of distinguishing between ought-to-be and ought-to-do constraints and next focus on the most severe paradox, the so-called Chisholm paradox, involving contrary-to-duty norms. We present a multi-modal extension of standard deontic logic (SDL) to represent the ought-to-be version of the Chisholm set properly. For the ought-to-do variant we employ a reduction to dynamic logic, and show how the Chisholm set can be treated adequately in this setting. Finally we discuss a way of integrating both ought-to-be and ought-to-do reasoning, enabling one to draw conclusions from ought-to-be constraints to ought-to-do ones, and show by an example the use(fulness) of this
The ATESP 5 GHz radio survey. II. Physical properties of the faint radio population
One of the most debated issues about sub-mJy radio sources, which are
responsible for the steepening of the 1.4 GHz source counts, is the origin of
their radio emission. Particularly interesting is the possibility of combining
radio spectral index information with other observational properties to assess
whether the sources are triggered by star formation or nuclear activity. The
aim of this work is to study the optical and near infrared properties of a
complete sample of 131 radio sources with S>0.4 mJy, observed at both 1.4 and 5
GHz as part of the ATESP radio survey. We use deep multi-colour (UBVRIJK)
images, mostly taken in the framework of the ESO Deep Public Survey, to
optically identify and derive photometric redshifts for the ATESP radio
sources. Deep optical coverage and extensive colour information are available
for 3/4 of the region covered by the radio sample. Typical depths of the images
are U~25, B~26, V~25.4, R~25.5, I~24.3, 19.5<K_s<20.2, J<22.2. Optical/near
infrared counterparts are found for ~78% (66/85) of the radio sources in the
region covered by the deep multi-colour imaging, and for 56 of these reliable
estimates of the redshift and type are derived. We find that many of the
sources with flat radio spectra are characterised by high radio-to-optical
ratios (R>1000), typical of classical powerful radio galaxies and quasars.
Flat-spectrum sources with low R values are preferentially identified with
early type galaxies, where the radio emission is most probably triggered by
low-luminosity active galactic nuclei. Considering both early type galaxies and
quasars as sources with an active nucleus, such sources largely dominate our
sample (78%). Flat-spectrum sources associated with early type galaxies are
quite compact (d<10-30 kpc), suggesting core-dominated radio emission.Comment: 15 pages, 13 figures, accepted for pubblication in A&
The ATESP Radio Survey II. The Source Catalogue
This paper is part of a series reporting the results of the Australia
Telescope ESO Slice Project (ATESP) radio survey obtained at 1400 MHz with the
Australia Telescope Compact Array (ATCA) over the region covered by the ESO
Slice Project (ESP) galaxy redshift survey. The survey consists of 16 radio
mosaics with ~8"x14" resolution and uniform sensitivity (1sigma noise level ~79
microJy) over the whole area of the ESP redshift survey (~26 sq. degrees at
decl. -40 degr). Here we present the catalogue derived from the ATESP survey.
We detected 2960 distinct radio sources down to a flux density limit of ~0.5
mJy (6sigma), 1402 being sub-mJy sources. We describe in detail the procedure
followed for the source extraction and parameterization. The internal accuracy
of the source parameters was tested with Monte Carlo simulations and possible
systematic effects (e.g. bandwidth smearing) have been quantified.Comment: 14 pages, 14 Postscript figures, Accepted for publication in A&A
Suppl. Corrected typos and added Journal Referenc
"Hiccup" accretion in the swinging pulsar IGR J18245-2452
IGR J18245-2452 is the fifteenth discovered accreting millisecond X-ray
pulsar and the first source of this class showing direct evidence for
transition between accretion and rotational powered emission states. These
swing provided the strongest confirmation of the pulsar recycling scenario
available so far. During the two XMM-Newton observations that were carried out
while the source was in outburst in April 2013, IGR J18245-2452 displayed a
unique and peculiar variability of its X-ray emission. In this work, we report
on a detailed analysis of the XMM- Newton data and focus in particular on the
timing and spectral variability of the source. IGR J18245-2452 continuously
switches between lower and higher intensity states, with typical variations in
flux up to a factor of about 500 in time scales as short as few seconds. These
variations in the source intensity are sometimes associated to a dramatic
spectral hardening, during which the power-law photon index of the source
changes from Gamma=1.7 to Gamma=0.9. The pulse profiles extracted at different
count rates and energies show a complex variability. These phenomena are not
usually observed in accreting millisecond X-ray pulsars, at least not on such a
short time scale. Fast variability was also found in the ATCA radio
observations carried out for about 6 hours during the outburst at a frequency
of 5.5 and 9 GHz. We interpret the variability observed from IGR J18245-2452 in
terms of a "hiccup" accretion phase, during which the accretion of material
from the inner boundary of the Keplerian disk is reduced by the onset of
centrifugal inhibition of accretion, possibly causing the launch of strong
outflows. Changes across accretion and propeller regimes have been long
predicted and reproduced by MHD simulations of accreting millisecond X-ray
pulsars but never observed to produce an extreme variability as that shown by
IGR J18245-2452.Comment: A&A in press. Revised versio
A hot cocoon in the ultralong GRB 130925A: hints of a PopIII-like progenitor in a low density wind environment
GRB 130925A is a peculiar event characterized by an extremely long gamma-ray
duration (7 ks), as well as dramatic flaring in the X-rays for
20 ks. After this period, its X-ray afterglow shows an atypical soft
spectrum with photon index 4, as observed by Swift and Chandra,
until s, when XMM-Newton observations uncover a harder spectral
shape with 2.5, commonly observed in GRB afterglows. We find that
two distinct emission components are needed to explain the X-ray observations:
a thermal component, which dominates the X-ray emission for several weeks, and
a non-thermal component, consistent with a typical afterglow. A forward shock
model well describes the broadband (from radio to X-rays) afterglow spectrum at
various epochs. It requires an ambient medium with a very low density wind
profile, consistent with that expected from a low-metallicity blue supergiant
(BSG). The thermal component has a remarkably constant size and a total energy
consistent with those expected by a hot cocoon surrounding the relativistic
jet. We argue that the features observed in this GRB (its ultralong duration,
the thermal cocoon, and the low density wind environment) are associated with a
low metallicity BSG progenitor and, thus, should characterize the class of
ultralong GRBs.Comment: 6 pgs, 3 figs, fig1 revised, ApJL in pres
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