497 research outputs found
Polarization and light curve variability: the "patchy shell" model
Recent advances in early detection and detailed monitoring of GRB afterglows
have revealed variability in some afterglow light curves. One of the leading
models for this behavior is the patchy shell model. This model attributes the
variability to random angular fluctuations in the relativistic jet energy.
These an-axisymmetric fluctuations should also impose variations in the degree
and angle of polarization that are correlated to the light curve variability.
In this letter we present a solution of the light curve and polarization
resulting from a given spectrum of energy fluctuations. We compare light curves
produced using this solution to the variable light curve of GRB 021004 and we
show that the main features in both the light curve and the polarization
fluctuations are very well reproduced by this model. We use our results to draw
constraints on the characteristics of the energy fluctuations that might have
been present in GRB 021004.Comment: 10 pages, 2 figures, Final version to appear in ApJ
Polarization in the prompt emission of gamma-ray bursts and their afterglows
Synchrotron is considered the dominant emission mechanism in the production
of gamma-ray burst photons in the prompt as well as in the afterglow phase.
Polarization is a characteristic feature of synchrotron and its study can
reveal a wealth of information on the properties of the magnetic field and of
the energy distribution in gamma-ray burst jets. In this paper I will review
the theory and observations of gamma-ray bursts polarization. While the theory
is well established, observations have prove difficult to perform, due to the
weakness of the signal. The discriminating power of polarization observations,
however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of
Physics focus issue on Gamma Ray Burst
The Electromagnetic Signals of Compact Binary Mergers
Compact binary mergers are prime sources of gravitational waves, targeted by
current and next generation detectors. The question "what is the observable
electromagnetic (EM) signature of a compact binary merger?" is an intriguing
one with crucial consequences to the quest for gravitational waves. We present
a large set of numerical simulations that focus on the electromagnetic signals
that emerge from the dynamically ejected sub-relativistic material. These
outflows produce on a time scale of a day macronovae - short-lived IR to UV
signals powered by radioactive decay. The interaction of this outflow with the
surrounding matter inevitably leads to a long-lasting remnant. The expected
radio signals of these remnants last longer than a year, when the
sub-relativistic ejecta dominate the emission. We discuss their detectability
in 1.4 GHz and 150 MHz and compare it with the detectability of short GRBs'
orphan afterglows (which are produced by a different component of this
outflow). Mergers with characteristics similar to those of the Galactic neutron
star binary population (similar masses and typical circum-merger Galactic disk
density of ~1cm^-3) taking place at the detection horizon of advanced GW
detectors (300 Mpc) yield 1.4 GHz [150 MHz] signals of ~50 [300] microJy. The
signal on time scales of weeks, is dominated by the mildly and/or
ultra-relativistic outflow, which is not accounted for by our simulations, and
is expected to be even brighter. Upcoming all sky surveys are expected to
detect a few dozen, and possibly more, merger remnants at any given time
thereby providing robust lower limits to the mergers rate even before the
advanced GW detectorsbecome operational. Macronovae from the same distance peak
in the IR to UV range at an observed magnitude that may be as bright as 22-23
about 10 hours after the merger but dimmer, redder and longer if the opacity is
larger.Comment: Accepted for publication in MNRAS. arXiv admin note: substantial text
overlap with arXiv:1102.102
Localizing gravitational wave sources with optical telescopes and combining electromagnetic and gravitational wave data
Neutron star binaries, which are among the most promising sources for the
direct detection of gravitational waves (GW) by ground based detectors, are
also potential electromagnetic (EM) emitters. Gravitational waves will provide
a new window to observe these events and hopefully give us glimpses of new
astrophysics. In this paper, we discuss how EM information of these events can
considerably improve GW parameter estimation both in terms of accuracy and
computational power requirement. And then in return how GW sky localization can
help EM astronomers in follow-up studies of sources which did not yield any
prompt emission. We discuss how both EM source information and GW source
localization can be used in a framework of multi-messenger astronomy. We
illustrate how the large error regions in GW sky localizations can be handled
in conducting optical astronomy in the advance detector era. We show some
preliminary results in the context of an array of optical telescopes called
BlackGEM, dedicated for optical follow-up of GW triggers, that is being
constructed in La Silla, Chile and is expected to operate concurrent to the
advanced GW detectors.Comment: 8 pages, 8 figures, Proceeding for Sant Cugat Forum for Astrophysic
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
adde
Strategy updating rules and strategy distributions in dynamical multiagent systems
In the evolutionary version of the minority game, agents update their
strategies (gene-value ) in order to improve their performance. Motivated by
recent intriguing results obtained for prize-to-fine ratios which are smaller
than unity, we explore the system's dynamics with a strategy updating rule of
the form (). We find that the strategy
distribution depends strongly on the values of the prize-to-fine ratio , the
length scale , and the type of boundary condition used. We show that
these parameters determine the amplitude and frequency of the the temporal
oscillations observed in the gene space. These regular oscillations are shown
to be the main factor which determines the strategy distribution of the
population. In addition, we find that agents characterized by
(a coin-tossing strategy) have the best chances of survival at asymptotically
long times, regardless of the value of and the boundary conditions
used.Comment: 4 pages, 7 figure
InsĂ©curitĂ© Alimentaire et stress politique au GuĂ©ra, Tchad 1965-2006: Ătude de lâimpact Ă long terme des violences sur la production agricole, lâorganisation sociale et lâaccĂšs aux services de santĂ©
A landlocked country in Central Africa, Chad has been involved in armed conflicts and civil war since independence in 1960. After the coup dâĂ©tat in 1990, a certain stability prevailed under a military democratic regime despite frequent rebelsâ incursions, civil unrest, and violations of human rights. Over six decades, structural violence, insecurity, and oppression continue to hamper sociocultural, economic, educational and health improvements. Despite oil revenues and international investments into security and poverty reduction, Chad remains a highly fragile poor country with an alarming hunger situation. Its development is constrained by internal conflicts, instability of neighbouring countries, and perpetual threats by extremists. The weakness of infrastructures, public institutions and bad governance constrain the access of communities to basic goods, services, rights and means of livelihood.In rural areas, harassment and neglect by public authorities increase the vulnerabilities of households to food insecurity and conflicts. This research insighted into the long-term impacts of persistent violence on food production, social organization and access to health services in GuĂ©ra in central Chad. Rebels and government authorities exert political stress that is a major driver of sociocultural, demographic and economic change. These acts undermine spheres of livelihoods across the Sahelian region and needs attention.WOTROColonial and Global Histor
Implications of the -ray Polarization of GRB 021206
We compare two possible scenarios for the producing of high level of
polarization within the prompt emission of a GRB: synchrotron emission from a
relativistic jet with a uniform (in space and time) magnetic field and
synchrotron emission from a jet with a random magnetic field in the plane of
the shock. Somewhat surprisingly we find that both scenarios can produce a
comparable level of polarization (% for the uniform field and % for a random field). Uniform time independent field most naturally
arises by expansion of the field from the compact object. It requires a
G field at the source and a transport of the field as . It {\it does not} imply Poynting flux domination of the energy of the
wind. There is a serious difficulty however, within this scenario, accounting
for particle acceleration (which requires random magnetic fields) both for
Poynting flux and non-Poynting flux domination. Significant polarization can
also arise from a random field provided that the observer is located within
orientation from a narrow () jet. While most
jets are wider, the jet of GRB 021206 from which strong polarization was
recently observed, was most likely very narrow. GRB 021206 is among the
strongest bursts ever. Adopting the energy-angle relation we find an estimated
angle of rad or even smaller. Thus, for this particular burst the
required geometry is not unusual. We conclude that the RHESSI observations
suggest that the prompt emission results from synchrotron radiation. However,
in view of the comparable levels of polarizations predicted by both the random
field and the homogeneous field scenarios these observations are insufficient
to rule out or confirm either one.Comment: 14 pages, 4 figure
A New Discriminator for Gamma-Ray Burst Classification: The Epeak-Fluence Energy Ratio
Using the derived gamma-ray burst E_peak and fluences from the complete BATSE
5B Spectral Catalog, we study the ensemble characteristics of the
E_peak-fluence relation for GRBs. This relation appears to be a physically
meaningful and insightful fundamental discriminator between long and short
bursts. We discuss the results of the lower limit test of the E_peak-E_iso
relations in the E_peak-fluence plane for BATSE bursts with no observed
redshift. Our results confirm the presence of two GRB classes as well as
heavily suggesting two different GRB progenitor types.Comment: 8 pages, 3 figures, accepted for publication in Ap
An audit of discharge summaries
Background: In the continuum of patient care, admission to the department of medicine constitutes a brief yet critical period. Subsequent patient care depends on the discharge summary (DS) and its implementation. Aim: To evaluate the department of medicine -family physician interface by a discharge summaries audit. Method: A retrospective study analyzing all admissions and discharges between a department of medicine and a primary care clinic over a period of ten months. Results: 129 DS were evaluated and compared to 97 available primary care medical charts. Most admissions were due to a medical emergency (95%), the patients were often elderly and 23% lived alone. Hospital stay averaged 4.0±2.4 days, readmission rate was 15.8%. In 73% of the DS at least one new drug was prescribed. The family physician was the one expected to continue treatment in most of the cases, but in over a third of the patients, a referral to further consultation was deemed necessary. The DS was found in 82% of the primary care charts. Median time interval between discharge and consultation with the family physician was three days (range 1-30). Home visits by physicians were documented in eight cases only. Conclusion: Most discharged patients require further evaluation and newly prescribed medications, making a timely and coordinated continuous care in the community mandatory. A high quality, rapidly available DS is therefore important for the family physician. Whether improved communication will reduce readmissions and improve patient prognosis and quality of care should be clarified by further study.peer-reviewe
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