Luminosity Functions of Gamma-Ray Burst Afterglows

Aims: Use the standard fireball model to create virtual populations of gamma-ray burst afterglows and study their luminosity functions. Methods: We randomly vary the parameters of the standard fireball model to create virtual populations of afterglows. We use the luminosity of each burst at an observer's time of 1 day to create a luminosity function and compare our results with available observational data to assess the internal consistency of the standard fireball model. Results: We show that the luminosity functions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but not very dependent on the model parameter distributions used to create the virtual populations. Comparison with observations shows that while there is good general agreement with the data, it is difficult to explain simultaneously the X-ray and optical data. Possible reasons for this are discussed and the most likely one is that the standard fireball model is incomplete and that decoupling of the X-ray and optical emission mechanism may be needed.Comment: 5 pages, 4 figures; accepted for publication in A&

Afterglow Light Curves and Broken Power Laws: A Statistical Study

In gamma-ray burst research it is quite common to fit the afterglow light curves with a broken power law to interpret the data. We apply this method to a computer simulated population of afterglows and find systematic differences between the known model parameters of the population and the ones derived from the power law fits. In general, the slope of the electron energy distribution is overestimated from the pre-break light curve slope while being underestimated from the post-break slope. We also find that the jet opening angle derived from the fits is overestimated in narrow jets and underestimated in wider ones. Results from fitting afterglow light curves with broken power laws must therefore be interpreted with caution since the uncertainties in the derived parameters might be larger than estimated from the fit. This may have implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter

Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004

Several GRB afterglow light curves deviate strongly from the power law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. As an example we consider GRB 021004 that exhibited strong light curve variations and has a reasonably well time-resolved polarimetry. We show that the light curves in the R-band, X-rays and in the radio can be accounted for by four energy injection episodes in addition to the initial event. The polarization variations are shown to be a consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ

Energy injection episodes in GRBs: The case of GRB 021004

A number of GRB afterglow light curves deviate substantially from the power law decay observed in most bursts. These variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. We show that the light curves of GRB 021004 can be accounted for by four energy injection episodes in addition to the initial event. The polarization variations are shown to be a consequence of the injections

Max-and-Smooth: a two-step approach for approximate Bayesian inference in latent Gaussian models

This is the final version. Available on open access from International Society for Bayesian Analysis (ISBA) via the DOI in this record. With modern high-dimensional data, complex statistical models are necessary, requiring computationally feasible inference schemes. We introduce Max-and-Smooth, an approximate Bayesian inference scheme for a flexible class of latent Gaussian models (LGMs) where one or more of the likelihood parameters are modeled by latent additive Gaussian processes. Max-and-Smooth consists of two-steps. In the first step (Max), the likelihood function is approximated by a Gaussian density with mean and covariance equal to either (a) the maximum likelihood estimate and the inverse observed information, respectively, or (b) the mean and covariance of the normalized likelihood function. In the second step (Smooth), the latent parameters and hyperparameters are inferred and smoothed with the approximated likelihood function. The proposed method ensures that the uncertainty from the first step is correctly propagated to the second step. Since the approximated likelihood function is Gaussian, the approximate posterior density of the latent parameters of the LGM (conditional on the hyperparameters) is also Gaussian, thus facilitating efficient posterior inference in high dimensions. Furthermore, the approximate marginal posterior distribution of the hyperparameters is tractable, and as a result, the hyperparameters can be sampled independently of the latent parameters. In the case of a large number of independent data replicates, sparse precision matrices, and high-dimensional latent vectors, the speedup is substantial in comparison to an MCMC scheme that infers the posterior density from the exact likelihood function. The proposed inference scheme is demonstrated on one spatially referenced real dataset and on simulated data mimicking spatial, temporal, and spatio-temporal inference problems. Our results show that Max-and-Smooth is accurate and fast.NER

Study of medications use of elderly admitted to acute care hospital

Neðst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinn View/OpenObjective: The prevalence of diseases increases with age and so does use of medications. Thus illness related to medications use does also become more prevalent. This study aims at evaluating medications of elderly people admitted to an acute care hospital, and demonstrate adverse reactions and quality indicators. Material and methods: This is a study of patients 75 years of age and older admitted acutely to internalmedicine at the Reykjavik Hospital over a three month period in the spring of 1995. All medications and diagnoses were registered and the medical records reviewed. The contribution of adverse medication effects to the admission was assessed. Quality of treatment was evaluated according to evidence based medicine for the diagnoses chosen. The study included 208 individuals, 133 women and 75 men with the mean age of 82.4 years. Results: Mean length of stay for women was 19.9 days and men 15.2 days. Number of drugs on admission and discharge ranged from 0 to 18. The mean number of drugs were for women 5.8 and 6.9 and men 6.6 and 7.7 on admission and discharge, respectively. In 16 cases or 7.7% it was judged that there was a high likelihood of the admission being due to an adverse effect. Potential drug interactions according to a computer software package were not judged to be of clinical importance in any case. Conclusions: Treatment for coronary heart disease, heart failure, osteoporosis, insomnia and long term prednisolon treatment is not completely optimal according to evidence based medicine. The results of this study indicate that treatment could be improved for example with use of clinical guidelines.Tilgangur: Með hækkandi aldri vex algengi sjúkdóma og jafnframt lyfjanotkun. Veikindi sem tengjast lyfjanotkun verða því tíðari meðal aldraðra. Rannsóknin lýsir lyfjanotkun aldraðra á bráðasjúkrahúsi, aukaverkunum og gæðavísum. Efniviður og aðferðir: Þessi rannsókn tekur til allra aldraðra, 75 ára og eldri, sem lögðust inn brátt á lyflækningadeildir Borgarspítalans á þriggja mánaða tímabili vorið 1995. Öll lyf og allar sjúkdómsgreiningar voru skráð og sjúkraskrár yfirfarnar. Lagt var mat á það hvort lyfjanotkun ætti þátt í innlögn. Gæði lyfjameðferðar voru metin meðal annars með tilliti til fyrri og núverandi sjúkdómsgreininga. Niðurstöður: Könnunin náði til 208 einstaklinga, 133 kvenna og 75 karla á aldrinum 75 til 98 ára, meðalaldur 82,4 ár. Meðallegudagar kvennanna voru 19,9 dagar og karlanna 15,2 dagar. Skráður fjöldi lyfja við innskrift var á bilinu 0 til 18 lyf og við útskrift mest 18. Meðalfjöldi lyfja hjá konum við innskrift var 5,8 lyf og við útskrift 6,9. Karlar höfðu 6,6 lyf við innskrift að meðaltali og 7,7 við útskrift. Í 16 tilvikum eða 7,7% voru taldar miklar líkur á að sjúklingur hefði lagst inn vegna aukaverkana lyfja. Engin vísbending um milliverkun samkvæmt tölvuútskrift reyndist hafa klíníska þýðingu að mati höfunda. Ályktanir: Aldraðir sem leggjast inn á bráðasjúkrahús eru á fjölda lyfja og eiga aukaverkanir lyfja nokkurn þátt í innlögnum. Hvað varðar meðferð á kransæðasjúkdómi, hjartabilun, beinþynningu, svefntruflunum og langtímameðferð með sykursterum kom fram að all nokkuð vantar upp á að bestu meðferð, samkvæmt niðurstöðum rannsókna, sé beitt. Niðurstöður rannsóknarinnar, ásamt þeim grófu gæðavísum sem beitt var, benda til að gera megi betur til dæmis með klínískum leiðbeiningum

Geology of Tindfjallajökull volcano, Iceland

The geology of Tindfjallajökull volcano, southern Iceland, is presented as a 1:50,000 scale map. Field mapping was carried out with a focus on indicators of past environments. A broad stratocone of interbedded fragmental rocks and lavas was constructed during Tindfjallajökull’s early development. This stratocone has been dissected by glacial erosion and overlain by a variety of mafic to silicic volcanic landforms. Eruption of silicic magma, which probably occurred subglacially, constructed a thick pile of breccia and lava lobes in the summit area. Mafic to intermediate flank eruptions continued through to the end of the last glacial period, producing lavas, hyaloclastite-dominated units and tuyas that preserve evidence of volcano-ice interactions. The Thórsmörk Ignimbrite, a regionally important chronostratigraphic marker, is present on the SE flank of the volcano. The geological mapping of Tindfjallajökull gives insights into the evolution of stratovolcanoes in glaciated regions and the influence of ice in their development

Modelling the 20th and 21st century evolution of Hoffellsjökull glacier, SE-Vatnajökull, Iceland

The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsjökull, a south-flowing outlet glacier of the ice cap Vatnajökull, which is located close to the southeastern coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1989, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by kinematic and differential GPS surveys and correlation of SPOT5 images. The approximately 20% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios derived from 10 global and 3 regional climate model simulations using the A1B emission scenario. If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century. If the climate warms, as suggested by most of the climate change scenarios, the model projects this glacier to almost disappear by the end of the 21st century. Runoff from the glacier is predicted to increase for the next 30–40 yr and decrease after that as a consequence of the diminishing ice-covered area

Climatic variability during the last millennium in Western Iceland from lake sediment records

The aim of this research was to create a decadal-scale terrestrial quantitative palaeoclimate record for NW Iceland from lake sediments for the last millennium. Geochemical, stable isotope and chironomid reconstructions were obtained from a lake sequence constrained by tephra deposits on the Snæfellsnes peninsula, western Iceland. Obtaining a quantitative record proved problematic, but the qualitative chironomid record showed clear trends associated with past summer temperatures, and the sedimentological records provided evidence for past changes in precipitation, mediated through catchment soil in-wash. When the full range of chronological uncertainty is considered, four clear phases of climatic conditions were identified: (1) a relatively warm phase between AD 1020 and 1310; (2) a relatively stable period between AD 1310 and 1510, cooler than the preceding period but still notably warmer than the second half of the millennium; (3) a consistent reduction of temperatures between AD 1560 and 1810, with the coolest period between AD 1680 and 1810; and (4) AD 1840–2000 has temperatures mainly warmer than in the preceding two centuries, with a rising trend and increased variability from c. AD 1900 onwards. The reconstructions show clearly that the first half of the millennium experienced warmer climatic conditions than the second half, with a return to the warmer climate only occurring in the last c. 100 years. Much of the variability of the chironomid record can be linked to changes in the North Atlantic Oscillation (NAO). The reconstructions presented can track low-frequency and long-term trends effectively and consistently but high-resolution and calibrated quantitative records remain more of a challenge – not just in finding optimal sedimentary deposits but also in finding the most reliable proxy. It is this that presents the real challenge for Holocene climate reconstruction from this key area of the North Atlantic. Keywords : iceland, palaeolimnology, chironomids, little ice age, medieval climate anomaly, north atlantic oscillatio