Mechanical Model for Relativistic Blast Waves

Relativistic blast waves can be described by a mechanical model. In this model, the "blast" -- the compressed gas between the forward and reverse shocks -- is viewed as one hot body. Equations governing its dynamics are derived from conservation of mass, energy, and momentum. Simple analytical solutions are obtained in the two limiting cases of ultra-relativistic and non-relativistic reverse shock. Equations are derived for the general explosion problem.Comment: 8 pages, accepted to ApJ Letter

Was the "naked burst" GRB 050421 really naked ?

A few long gamma-ray bursts such as GRB 050421 show no afterglow emission beyond the usual initial steep decay phase. It has been suggested that these events correspond to "naked" bursts that occur in a very low density environment. We reconsider this possibility in the context of various scenarios for the origin of the afterglow. In the standard model where the afterglow results from the forward shock as well as in the alternative model where the afterglow comes from the reverse shock, we aim to obtain constraints on the density of the environment, the microphysics parameters, or the Lorentz factor of the ejecta, which are imposed by the absence of a detected afterglow. For the two models we compute the afterglow evolution for different values of the external density (uniform or wind medium) and various burst parameters. We then compare our results to the Swift data of GRB 050421, which is the best example of a long burst without afterglow. In the standard model we show that consistency with the data imposes that the external density does not exceed 1E-5 cm-3 or that the microphysics parameters are very small with epsilon_e <~ 1E-2 and epsilon_B <~ 1E-4. If the afterglow is caused by the reverse shock, we find that its contribution can be strongly reduced if the central source has mainly emitted fast-moving material (with less than 10 - 30 % of the kinetic energy at Gamma<100 and was located in a dense environment. The two considered scenarios therefore lead to opposite constraints on the circumburst medium. The high-density environment, favored by the reverse shock model, better corresponds to what is expected if the burst progenitor was a massive star.Comment: 6 pages, 3 figures, 1 table, to appear in A&

Time-resolved spectral correlations of long-duration Gamma-Ray Bursts

For a sample of long GRBs with known redshift, we study the distribution of the evolutionary tracks on the rest-frame luminosity-peak energy Liso-Ep' diagram. We are interested in exploring the extension of the `Yonetoku' correlation to any phase of the prompt light curve, and in verifying how the high-signal prompt duration time, Tf, in the rest frame correlates with the residuals of such correlation (Firmani et al. 2006). For our purpose, we analyse separately two samples of time-resolved spectra corresponding to 32 GRBs with peak fluxes >1.8 phot cm^-2 s^-1 from the Swift-BAT detector, and 7 bright GRBs from the CGRO-BATSE detector previously processed by Kaneko et al. (2006). After constructing the Liso-Ep' diagram, we discuss the relevance of selection effects, finding that they could affect significantly the correlation. However, we find that these effects are much less significant in the Liso x Tf-Ep' diagram, where the intrinsic scatter reduces significantly. We apply further corrections for reducing the intrinsic scatter even more. For the sub-samples of GRBs (7 from Swift and 5 from CGRO) with measured jet break time, we analyse the effects of correcting Liso by jet collimation. We find that (i) the scatter around the correlation is reduced, and (ii) this scatter is dominated by the internal scatter of the individual evolutionary tracks. These results suggest that the time, integrated `Amati' and `Ghirlanda' correlations are consequences of the time resolved features, not of selection effects, and therefore call for a physical origin. We finally remark the relevance of looking inside the nature of the evolutionary tracks.Comment: 11 pages, 6 figures, 4 tables. Submitted to MNRAS (Sept 8th), after referee comment

Differentiating criminal networks in the illegal wildlife trade: organized, corporate and disorganized crime

Historically, the poaching of wildlife was portrayed as a small-scale local activity in which only small numbers of wildlife would be smuggled illegally by collectors or opportunists. Nowadays, this image has changed: criminal networks are believed to be highly involved in wildlife trafficking, which has become a significant area of illicit activity. Even though wildlife trafficking has become accepted as a major area of crime and an important topic and criminologists have examined a variety of illegal wildlife markets, research that specifically focusses on the involvement of different criminal networks and their specific nature is lacking. The concept of a ‘criminal network’ or ‘serious organized crime’ is amorphous – getting used interchangeably and describes all crime that is structured rather than solely reflecting crime that fits within normative definitions of ‘organized’ crime. In reality, criminal networks are diverse. As such, we propose categories of criminal networks that are evidenced in the literature and within our own fieldwork: (1) organized crime groups (2) corporate crime groups and (3) disorganized criminal networks. Whereas there are instances when these groups act alone, this article will (also) discuss the overlap and interaction that occurs between our proposed categories and discuss the complicated nature of the involved criminal networks as well as predictions as to the future of these networks

Differential expression and localization of TIMP-1 and TIMP-4 in human gliomas

Studies have suggested that an imbalance of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) may contribute to the malignant phenotype of gliomas. In this study, we have undertaken a detailed analysis of expression of the TIMP family in normal human brain and malignant gliomas at both the mRNA and protein level. Reverse transcription-PCR (RT-PCR) analyses of total RNA from surgical tumour specimens revealed unique expression patterns for the 4 members of the TIMP family, with TIMP-1 and -4 showing positive and negative correlations, respectively, with glioma malignancy. By RT-PCR, TIMP-2 and TIMP-3 expression did not change with tumour grade. In situ hybridization localized TIMP-1 to glial tumour cells and also to the surrounding tumour vasculature. TIMP-4 transcripts were predominantly localized to tumour cells, though minor expression was found in vessels. Recombinant TIMP-4 reduced invasion of U251 glioma cells through Matrigel, and U87 clones overexpressing TIMP-4 showed reduced invasive capacity in vitro. TIMP-4, but not TIMP-1, blocked Membrane Type-1-MMP-mediated progelatinase-A (MMP-2) activation in human umbilical vein endothelial cells. The differential expression and localization of individual TIMPs may contribute to the pathophysiology of human malignant gliomas, particularly with regard to tumour vascularization. © 2001 Cancer Research Campaign http://www.bjcancer.co

Eigenmodes and growth rates of relativistic current filamentation instability in a collisional plasma

I theoretically found eigenmodes and growth rates of relativistic current filamentation instability in collisional regimes, deriving a generalized dispersion relation from self-consistent beam-Maxwell equations. For symmetrically counterstreaming, fully relativistic electron currents, the collisional coupling between electrons and ions creates the unstable modes of growing oscillation and wave, which stand out for long-wavelength perturbations. In the stronger collisional regime, the growing oscillatory mode tends to be dominant for all wavelengths. In the collisionless limit, those modes vanish, while maintaining another purely growing mode that exactly coincides with a standard relativistic Weibel mode. It is also shown that the effects of electron-electron collisions and thermal spread lower the growth rate of the relativistic Weibel instability. The present mechanisms of filamentation dynamics are essential for transport of homogeneous electron beam produced by the interaction of high power laser pulses with plasma.Comment: 44 pages, 12 figures. Accepted for publication in Phys. Rev.

The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?

We present a wide dataset of -ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814 the best

A Reverse Shock and Unusual Radio Properties in GRB 160625B

We present multi-wavelength observations and modeling of the exceptionally bright long γ-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of {\theta }_{j}\approx 3\buildrel{\circ}\over{.} 6 and kinetic energy of ${E}_{K}\approx 2\times {10}^{51}$ erg, propagating into a low-density ($n\approx 5\times {10}^{-5}$ cm−3) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of ${{\rm{\Gamma }}}_{0}\gtrsim 100$ and an ejecta magnetization of {R}_{B}\approx 1\mbox{--}100. The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly