The hidden X-ray breaks in afterglow light curves

Gamma-Ray Burst (GRB) afterglow observations in the Swift era have a perceived lack of achromatic jet breaks compared to the BeppoSAX, or pre-Swift era. Specifically, relatively few breaks, consistent with jet breaks, are observed in the X-ray light curves of these bursts. If these breaks are truly missing, it has serious consequences for the interpretation of GRB jet collimation and energy requirements, and the use of GRBs as standard candles. Here we address the issue of X-ray breaks which are possibly 'hidden' and hence the light curves are misinterpreted as being single power-laws. We show how a number of precedents, including GRB 990510 & GRB 060206, exist for such hidden breaks and how, even with the well sampled light curves of the Swift era, these breaks may be left misidentified. We do so by synthesising X-ray light curves and finding general trends via Monte Carlo analysis. Furthermore, in light of these simulations, we discuss how to best identify achromatic breaks in afterglow light curves via multi-wavelength analysis.Comment: 4 pages, contributed talk, submitted to the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 200

Dissecting magnetar variability with Bayesian hierarchical models

Neutron stars are a prime laboratory for testing physical processes under conditions of strong gravity, high density, and extreme magnetic fields. Among the zoo of neutron star phenomena, magnetars stand out for their bursting behaviour, ranging from extremely bright, rare giant flares to numerous, less energetic recurrent bursts. The exact trigger and emission mechanisms for these bursts are not known; favoured models involve either a crust fracture and subsequent energy release into the magnetosphere, or explosive reconnection of magnetic field lines. In the absence of a predictive model, understanding the physical processes responsible for magnetar burst variability is difficult. Here, we develop an empirical model that decomposes magnetar bursts into a superposition of small spike-like features with a simple functional form, where the number of model components is itself part of the inference problem. The cascades of spikes that we model might be formed by avalanches of reconnection, or crust rupture aftershocks. Using Markov Chain Monte Carlo (MCMC) sampling augmented with reversible jumps between models with different numbers of parameters, we characterise the posterior distributions of the model parameters and the number of components per burst. We relate these model parameters to physical quantities in the system, and show for the first time that the variability within a burst does not conform to predictions from ideas of self-organised criticality. We also examine how well the properties of the spikes fit the predictions of simplified cascade models for the different trigger mechanisms.Comment: accepted for publication in The Astrophysical Journal; code available at https://bitbucket.org/dhuppenkothen/magnetron, data products at http://figshare.com/articles/SGR_J1550_5418_magnetron_data/129242

GRB 030329: 3 years of radio afterglow monitoring

Radio observations of gamma-ray burst (GRB) afterglows are essential for our understanding of the physics of relativistic blast waves, as they enable us to follow the evolution of GRB explosions much longer than the afterglows in any other wave band. We have performed a three-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes (WSRT) and the Giant Metrewave Radio Telescope (GMRT). Our observations, combined with observations at other wavelengths, have allowed us to determine the GRB blast wave physical parameters, such as the total burst energy and the ambient medium density, as well as investigate the jet nature of the relativistic outflow. Further, by modeling the late-time radio light curve of GRB 030329, we predict that the Low-Frequency Array (LOFAR, 30-240 MHz) will be able to observe afterglows of similar GRBs, and constrain the physics of the blast wave during its non-relativistic phase.Comment: 5 pages, 2 figures, Phil. Trans. R. Soc. A, vol.365, p.1241, proceedings of the Royal Society Scientific Discussion Meeting, London, September 200

LOFAR early-time search for coherent radio emission from GRB 180706A

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3$\sigma$ limit of 1.7 mJy beam$^{-1}$ at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of $1.1 \pm 0.9$ mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.Peer reviewedFinal Accepted Versio

Representing ‘Place’: City Climate Commissions and the Institutionalisation of Experimental Governance in Edinburgh

Against the backdrop of increasingly fragmented and poly-centric urban climate governance, this article examines the establishment of city climate ‘commissions’ as an experimental means of addressing the challenge of climate change at the city-scale. In doing so it addresses the question: What constitutes diversity in voices and perspectives when trying to represent the city as a place for climate action? To answer this question, the article presents an analysis of the Edinburgh Climate Commission’s establishment, drawing on participatory ethnographic research carried out by a researcher embedded within the project team. The account of how this new mode of urban governance was both conceptualised and then put into practice offers a new institutional angle to the literature on urban ‘experimentation.’ Through our reflective analysis we argue that aspirations to ensure pre-defined ‘key’ industries (high carbon emitters) are accounted for in commissioner recruitment, and an over-emphasis on capturing discernible ‘impacts’ in the short term (by involving organisations already pro-active in sustainable development) hindered an opportunity to embrace new perspectives on urban futures and harness the innovative potential of cities to engage with the multifaceted nature of the climate challenge. Furthermore, new insight into the relationship between local authorities and other ‘place-based’ agents of change opens up important questions regarding how to balance the attainment of legitimacy within the political status quo, and the prospect of a new radical politics for urban transformation

SN 2007gr: a Normal Type Ic Supernova with a Mildly Relativistic Radio Jet?

A nearby type Ic supernova, SN 2007gr was observed with the EVN in two epochs 60 days apart (second observation also included the Green Bank Telescope). In both cases one of the EVN stations was the Westerbork Synthesis Radio Telescope (WSRT), which recorded the observational data not only in the VLBI mode, but also in its normal interferometric mode. Thus it provided an important reference observation. In the first epoch the fluxes measured by the VLBI network and the WSRT alone match well. However in the second epoch the peak brightness observed in the VLBI experiment is much lower than the total flux recorded by the WSRT. There could be multiple reasons for this discrepancy: a resolution effect, coherence losses in VLBI, or extended emission contaminating the WSRT measurement. With new WSRT observations we costrain the level of background emission and find that there is still a difference between the corrected total flux density and the VLBI peak brightness. If one assumes that this is dominated by resolution, this would correspond to an average apparent expansion speed of ~0.4c

Analytical and experimental characterization of a miniature calorimetric sensor in pulsatile flow

The behaviour of a miniature calorimetric sensor, which is under consideration for catheter-based coronary artery flow assessment, is investigated in both steady and pulsatile tube flow. The sensor is composed of a heating element operated at constant power, and two thermopiles that measure flow-induced temperature differences over the sensor surface. An analytical sensor model is developed, which includes axial heat conduction in the fluid and a simple representation of the solid wall, assuming a quasi-steady sensor response to the pulsatile flow. To reduce the mathematical problem, described by a two-dimensional advection-diffusion equation, a spectral method is applied. A Fourier transform is then used to solve the resulting set of ordinary differential equations and an analytical expression for the fluid temperature is found. To validate the analytical model, experiments with the sensor mounted in a tube have been performed in steady and pulsatile water flow with various amplitudes and Strouhal numbers. Experimental results are generally in good agreement with theory and show a quasi-steady sensor response in the coronary flow regime. The model can therefore be used to optimize the sensor design for coronary flow assessment

Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase

We explore the physics behind one of the brightest radio afterglows ever, GRB 030329, at late times when the jet is non-relativistic. We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements. We observed the GRB 030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268-1128 days after the burst. We modeled all the available radio data and derived the physical parameters. The index of the electron energy distribution is p=2.1, the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at t_NR ~ 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous findings. Our findings indicate that the blast wave is roughly spherical at t_NR, and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB 030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further.Comment: 9 pages, 2 figures; accepted for publication in Astronomy & Astrophysics after minor revisions; small changes in GMRT fluxes at 1280 MH