474 research outputs found
The Afterglows of Swift-era Gamma-ray Bursts. I. Comparing pre-Swift and Swift-era Long/Soft (Type II) GRB Optical Afterglows
We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overal
The Luminosity and Redshift Distributions of Short-Duration GRB
Using the BATSE peak flux distribution we rederive the short GRBs luminosity
function and compare it with the observed redshift distribution of long bursts.
We show that both distributions are compatible with the assumption that short
as well as long bursts follow the star formation rate. In this case the
difference between the two observed distributions can be interpreted as arising
mostly from differences in the detector's sensitivity to long and short bursts,
while the local rate of short bursts is 0.1 h_70^3 Gpc^-3 yr^-1 . We also
consider the possibility that short GRBs may be associated with binary neutron
star mergers and estimate the effect of the merging time delay on the
luminosity function and redshift distribution. We find that in this case the
local rate of short GRBs is ~0.8 h_70^3 Gpc^-3 yr^-1. Assuming that all binary
merging systems lead to short GRBs, we find a typical jet opening angle of
1.6^oComment: 7 pages, 5 figures. to be published in Astronomy and 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
Inhomogeneity in the Supernova Remnant Distribution as the Origin of the PAMELA Anomaly
Recent measurements of the positron/electron ratio in the cosmic ray (CR)
flux exhibits an apparent anomaly, whereby this ratio increases between 10 and
100 GeV. We show that inhomogeneity of CR sources on a scale of order a kpc,
can naturally explain this anomaly. If the nearest major CR source is about a
kpc away, then low energy electrons ( GeV) can easily reach us. At
higher energies ( GeV), the source electrons cool via synchrotron
and inverse-Compton before reaching Earth. Pairs formed in the local vicinity
through the proton/ISM interactions can reach Earth also at high energies, thus
increasing the positron/electron ratio. A natural origin of source
inhomogeneity is the strong concentration of supernovae in the galactic spiral
arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and
taking into account their concentration near the galactic spiral arms, we
consistently recover the observed positron fraction between 1 and 100 GeV.
ATIC's electron excess at GeV is explained, in this picture, as the
contribution of a few known nearby SNRs. The apparent coincident similarity
between the cooling time of electrons at 10 GeV (where the positron/electron
ratio upturn), Myr, and the CRs protons cosmogenic age at the same
energy is predicted by this model
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
Low rate of non-attenders to primary care providers in Israel - a retrospective longitudinal study
BACKGROUND: A model that combines reactive and anticipatory care within routine consultations has become recognized as a cost-effective means of providing preventive health care, challenging the need of the periodic health examination. As such, opportunistic screening may be preferable to organized screening. Provision of comprehensive preventive healthcare within the primary care system depends on regular attendance of the general population to primary care physicians (PCPs). Objectives: To assess the proportion of patients who do not visit a PCP even once during a four-year period, and to describe the characteristics of this population. METHODS: An observational study, based on electronic medical records of 421,012 individuals who were members of one district of Clalit Health Services, the largest health maintenance organization in Israel. RESULTS: The average annual number of visits to PCPs was 7.6 ± 8.7 to 8.3 ± 9.0 (median 5, 25%-75% interval 1–11) and 9.5 ± 10.0 to10.2 ± 10.4 (median 6, 25%-75% interval 1–14) including visits to direct access consultants) in the four years of the study. During the first year of the study 87.2% of the population visited a PCP. During the four year study period, only 1.5% did not visit a PCP even once. In a multivariate analysis having fewer chronic diseases (for each additional chronic disease the OR, 95% CI was 0.40 (0.38¬0.42)), being a new immigrant (OR, 95% CI 2.46 (2.32¬2.62)), and being male (OR, 95% CI 1.66 (1.58¬1.75)) were the strongest predictors of being a non-attender to a PCP for four consecutive years. CONCLUSIONS: The rate of nonattendance to PCPs in Israel is low. Other than new immigrant status, none of the characteristics identified for nonattendance suggest increased need for healthcare services
The polarisation of afterglow emission reveals GRB jet structure
We numerically compute light and polarisation curves of gamma-ray burst
afterglows for various configurations of the jet luminosity structure and for
different dynamical evolutions. We especially consider the standard homogeneous
``top hat'' jet and the ``universal structured jet'' with power-law wings. We
also investigate a possible more physical variation of the ``top hat'' model:
the ``Gaussian jet''. The polarisation curves for the last two jet types are
shown here for the first time together with the computation of X-ray and radio
polarised fluxes. We show that the lightcurves of the total flux from these
configurations are very similar to each other, and therefore only very high
quality data could allow us to pin down the underlying jet structure. We
demonstrate instead that polarisation curves are a powerful means to solve the
jet structure, since the predicted behaviour of polarisation and its position
angle at times around the jet break are very different if not opposite. We
conclude that the afterglow polarisation measurements provide clear footprints
of any outflow energy distribution (unlike the lightcurves of the total flux)
and the joint analysis of the total and polarised flux should reveal GRBs jet
structure.Comment: 16 pages, 18 figures, MNRAS, temp, 321. Light curves and polarisation
curves for a Gaussian jet added. Cartoon of the three jet structures adde
Spectral evolution of Fermi/GBM short Gamma-Ray Bursts
We study the spectral evolution of 13 short duration Gamma Ray Bursts (GRBs)
detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra
resolved in time at the level of 2-512 ms in the 8 keV-35 MeV energy range. We
find a strong correlation between the observed peak energy Ep and the flux P
within individual short GRBs. The slope of the Ep P^s correlation for
individual bursts ranges between ~0.4 and ~1. There is no correlation between
the low energy spectral index and the peak energy or the flux. Our results show
that in our 13 short GRBs Ep evolves in time tracking the flux. This behavior
is similar to what found in the population of long GRBs and it is in agreement
with the evidence that long GRBs and (the still few) short GRBs with measured
redshifts follow the same rest frame Ep-Liso correlation. Its origin is most
likely to be found in the radiative mechanism that has to be the same in both
classes of GRBs.Comment: 5 pages, 1 table, 3 figures. Accepted by MNRA
Survival probabilities in time-dependent random walks
We analyze the dynamics of random walks in which the jumping probabilities
are periodic {\it time-dependent} functions. In particular, we determine the
survival probability of biased walkers who are drifted towards an absorbing
boundary. The typical life-time of the walkers is found to decrease with an
increment of the oscillation amplitude of the jumping probabilities. We discuss
the applicability of the results in the context of complex adaptive systems.Comment: 4 pages, 3 figure
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