Weakly cyclic graphs and delivery games

game theory

The Prompt Gamma-Ray and Afterglow Energies of Short-Duration Gamma-Ray Bursts

I present an analysis of the gamma-ray and afterglow energies of the complete sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80% of the bursts exhibit a linear correlation between their gamma-ray fluence and the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of the blast wave ($F_{X,1}~F_{gamma}^1.01). An even tighter correlation is evident between E_{gamma,iso} and L_{X,1} for the subset of 13 bursts with measured or constrained redshifts. The remaining 20% of the bursts have values of F_{X,1}/F_{gamma} that are suppressed by about three orders of magnitude, likely because of low circumburst densities (Nakar 2007). These results have several important implications: (i) The X-ray luminosity is generally a robust proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a circumburst density n>0.05 cm^{-3}; (ii) most short GRBs have a narrow range of gamma-ray efficiency, with ~0.85 and a spread of 0.14 dex; and (iii) the isotropic-equivalent energies span 10^{48}-10^{52} erg. Furthermore, I find tentative evidence for jet collimation in the two bursts with the highest E_{gamma,iso}, perhaps indicative of the same inverse correlation that leads to a narrow distribution of true energies in long GRBs. I find no clear evidence for a relation between the overall energy release and host galaxy type, but a positive correlation with duration may be present, albeit with a large scatter. Finally, I note that the outlier fraction of 20% is similar to the proposed fraction of short GRBs from dynamically-formed neutron star binaries in globular clusters. This scenario may naturally explain the bimodality of the F_{X,1}/F_{gamma} distribution and the low circumburst densities without invoking speculative kick velocities of several hundred km/s.Comment: Submitted to ApJ; 9 pages, 2 figures, 1 tabl

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

Cell adhesion molecules and hyaluronic acid as markers of inflammation, fibrosis and response to antiviral therapy in chronic hepatitis C patients.

OBJECTIVE: Cell adhesion molecules (intracellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1)) and hyaluronic acid, markers of inflammation and fibrosis were monitored in hepatitis C patients to determine whether changes in plasma levels, during antiviral treatment, can predict long-term response to therapy. METHODS: In 55 patients with chronic hepatitis C virus (HCV), 33 treated with interferon (IFN) and 22 treated with IFN + ribavirin, sera was collected prior to treatment, at 3 + 6 months of therapy and 6 months post-treatment. Levels of ICAM-1, VCAM-1 and hyaluronic acid were correlated with alanine aminotransferase levels, HCV-RNA-polymerase chain reaction status and histological fibrosis scoring. RESULTS: A decrease in ICAM-1 levels at 3 and 6 months of therapy, compared with pretreatment levels, was observed in responders to IFN + ribavirin therapy but this decrease in ICAM-1 levels was not evident following cessation of treatment. Hyaluronic acid levels, in both treatment groups, did not differ significantly between responders and non-responders. Hyaluronic acid levels did correlate, significantly, with degree of fibrosis whereas VCAM-1 levels were marginally increased only in patients with moderate (grade III) fibrosis. CONCLUSIONS: Monitoring of VCAM-1 and hyaluronic acid, during antiviral therapy, does not differentiate between responders and non-responders. A decrease in ICAM-1 levels during IFN + ribavirin treatment is associated with response to therapy, and its efficacy in predicting long-term response should be further substantiated

Modeling GRB 050904: Autopsy of a Massive Stellar Explosion at z=6.29

GRB 050904 at redshift z=6.29, discovered and observed by Swift and with spectroscopic redshift from the Subaru telescope, is the first gamma-ray burst to be identified from beyond the epoch of reionization. Since the progenitors of long gamma-ray bursts have been identified as massive stars, this event offers a unique opportunity to investigate star formation environments at this epoch. Apart from its record redshift, the burst is remarkable in two respects: first, it exhibits fast-evolving X-ray and optical flares that peak simultaneously at t~470 s in the observer frame, and may thus originate in the same emission region; and second, its afterglow exhibits an accelerated decay in the near-infrared (NIR) from t~10^4 s to t~3 10^4 s after the burst, coincident with repeated and energetic X-ray flaring activity. We make a complete analysis of available X-ray, NIR, and radio observations, utilizing afterglow models that incorporate a range of physical effects not previously considered for this or any other GRB afterglow, and quantifying our model uncertainties in detail via Markov Chain Monte Carlo analysis. In the process, we explore the possibility that the early optical and X-ray flare is due to synchrotron and inverse Compton emission from the reverse shock regions of the outflow. We suggest that the period of accelerated decay in the NIR may be due to suppression of synchrotron radiation by inverse Compton interaction of X-ray flare photons with electrons in the forward shock; a subsequent interval of slow decay would then be due to a progressive decline in this suppression. The range of acceptable models demonstrates that the kinetic energy and circumburst density of GRB 050904 are well above the typical values found for low-redshift GRBs.Comment: 45 pages, 7 figures, and ApJ accepted. Revised version, minor modifications and 1 extra figur

GRB Energetics in the Swift Era

We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known redshift that were detected by the Swift spacecraft and monitored by the satellite's X-ray Telescope (XRT). Using the bolometric fluence values estimated in Butler et al. 2007b and the last XRT observation for each event, we set a lower limit the their collimation corrected energy Eg and find that a 68% of our sample are at high enough redshift and/or low enough fluence to accommodate a jet break occurring beyond the last XRT observation and still be consistent with the pre-Swift Eg distribution for long GRBs. We find that relatively few of the X-ray light curves for the remaining events show evidence for late-time decay slopes that are consistent with that expected from post jet break emission. The breaks in the X-ray light curves that do exist tend to be shallower and occur earlier than the breaks previously observed in optical light curves, yielding a Eg distribution that is far lower than the pre-Swift distribution. If these early X-ray breaks are not due to jet effects, then a small but significant fraction of our sample have lower limits to their collimation corrected energy that place them well above the pre-Swift Eg distribution. Either scenario would necessitate a much wider post-Swift Eg distribution for long cosmological GRBs compared to the narrow standard energy deduced from pre-Swift observations. We note that almost all of the pre-Swift Eg estimates come from jet breaks detected in the optical whereas our sample is limited entirely to X-ray wavelengths, furthering the suggestion that the assumed achromaticity of jet breaks may not extend to high energies.Comment: 30 pages, 10 figures, Accepted to Ap