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As the global demand for propene (propylene) is increasing, classic commercial production processes are becoming unable to keep up. Non-oxidative dehydrogenation, although hitherto underutilised industrially, has been put forward as a viable and green alternative, which is already used in a few commercial processes. In this work, we present detailed first-principles calculations of this reaction over a chromium oxide catalyst, which is the cornerstone of the Catofin ® process. A complete reaction pathway for the dehydrogenation of propane to propene and ultimately to propyne (methylacetylene) was considered. Cracking, which can yield C1 and C2 hydrocarbons, and the deactivation of the catalyst because of coking were also included and modelled. We used density functional theory calculations with the Hubbard model to study the structure of the involved intermediates, their adsorption and their interconversion to explain how chromium oxide catalysts facilitate this reaction and which processes cause their deactivation. We showed that the interaction of the hydrocarbons and molecular hydrogen with the catalytic surface is rather weak, resulting in low surface coverages, but increasing with multiple bonds present in hydrocarbons. Having constructed the potential energy surface with all the intermediates and the transition states linking them, we proposed a kinetic model for the reaction. Kinetic Monte Carlo simulations were performed at experimentally relevant temperatures (700–1000 °C), pressures (up to 10 bar) and inlet mixture compositions to study the kinetics of the reaction and discover the rate determining steps. As the reaction is highly endothermic, considerable conversions only occur at high temperatures. The accumulation of propene and propyne in the reaction mixture adversely affects the reaction rate and selectivity. Higher pressures increase the reaction rate but also increase the rate of coke formation, which poisons the catalyst. Deactivation of the catalyst has a strong temperature dependence and is caused by the accumulation of C∗ and CH3CC∗ on the surface, which are hard to remove even with hydrogen.

International audience; CO2 photoreduction is studied using in-house synthesized undoped and N-doped TiO2 nanoparticle photocatalysts. Comparison with the commercial P25 TiO2 powder shows that the synthesized samples are more effective. P25 produced a negligible amount of CH4, unlike the synthesized samples. N-doping of TiO2 powder caused a higher productivity rate of all products, and provided the best performance for CO2 reduction. The average production rate of CH4 was 0.191 μmol h−1 g−1, whereas that of CO was 0.111 μmol h−1 g−1. The experimental data are used to fit the micro-kinetic modelling parameters. The kinetic constant of H2O dissociation is the lowest for all tested samples, revealing that this is the rate-determining step. The kinetic constants for the H2, CO, and CH4 formation were of the same order for all catalyst samples, showing that the rates of these reactions are independent of the catalyst type.

Molecules that are difficult to convert catalytically often require a complex and specially tailored catalyst composition. Catalytic alkane dehydrogenation is an interesting example, where theoretical ab-initio and kinetic meso-scale simulations can provide the understanding of the performance improvement when the catalyst composition is altered. Herein, we study non-oxidative dehydrogenation of propane and butane over variously doped (Na, Li, K, Mg, Ca, or Cs) chromium oxide using first principles. The reaction pathway for the conversion of propane to propene/propyne and of butane to 1- and 2-butene is studied using density functional theory with the Hubbard U correction. Energies and kinetic parameters describing the adsorption, desorption, and surface reactions are used in mean-field microkinetic and kinetic Monte Carlo simulations. The process was modelled at industrially relevant temperatures, pressures, and feed gas flow velocities. Calculating the catalytic conversion, selectivity to products, and activity trends show that surface doping enhances the conversion and activity. The highest conversion for most dopants is achieved at temperatures below 850 K and atmospheric pressure, where the activity is significantly improved compared to the non-doped Cr2O3 catalyst.

Cellular biological environments are difficult to analyse on the first principle basis. In this modeling work, a novel approach is presented which transits from a broad mechanistic description of the individual metabolic pathways to a simplified macro-kinetic model. The latter is characterized within the concept of metabolic flux analysis and associated elementary modes, which obey the constraints of phenotype. Unlike the piecewise-defined kinetic expressions with a discrete separate treatment of different cell phases, the Monod equation rate law is extended herein, allowing for a dynamic reversal of macro-reactions, and thus, a uniform continuous functionality from growth to death cultivation periods. The enzymatic biochemical reactions of complete metabolic network are defined to satisfy the composition of mammalian culture structure which reflects on stoichiometry. This affords us an opportunity to handle the density of the viable cells (biomass) along with central metabolisms’ transformations, rather than as a decoupled constituent function, as habitually assumed in the literature. The methodology of the extracellular metabolite measurements for the Chinese hamster ovary (CHO) cells is confronted, confirming the applicability in multi-scale biologically-relevant systems. Temporal species’ behavior reveals inflection points that switch among stages. In further extensions metabolic models have a potential to forecast the outcomes of growth medium perturbations, feeding protocols, and process parameters in order to facilitate the productivity and quality of biosimilars in biotechnology.

As the increasing concentration of the atmospheric CO2 is being progressively recognized as a global environmental problem due to its greenhouse effect, the catalytic hydrogenation of carbon dioxide to methanol has been repeatedly put forward as a way of carbon fixation. Time and again have been copper-based heterogeneous catalysts shown to be best suited for this technological purpose, but their performance must be improved with secondary metal oxides, dopants, and supports. Herein, first-principles surface simulations of a Cu phase with four prospective perovskite substrate materials were performed. Cu/CaTiO3, Cu/SrTiO3, Cu/BaTiO3, and Cu/PbTiO3 were systematically studied. After extensive density functional theory (DFT) calculations, aimed at elucidating their stable structure, mapping out a complex reaction network, and pinpointing the rate-determining mechanism steps, the results were fed into a kinetic Monte Carlo (kMC) setup at industrially relevant operating conditions (the temperature of 420–660 ...

Propane (C3H8) and butane (C4H10) are short straight-chain alkane molecules that are difficult to convert catalytically. Analogous to propane, butane can be dehydrogenated to butenes (also known as...

It is well accepted that dendrites grow on anode surface of lithium-metal based batteries, while magnesium-metal batteries do not exhibit such a behavior. However, it has been recently shown experimentally and theoretically that magnesium can form uneven deposits that lead to similar safety concerns as lithium dendrites. To investigate a complex phenomenon such as dendritic growth from a theoretical point of view, it is necessary to study it on a multiscale level, i.e. from atomistic to meso scale. Herein we investigate thermodynamics, kinetics and morphology evolution of magnesium and lithium surfaces using density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations. To be realistic for a device such as a battery, the electrochemical dimension has also to be included: this study was done within the framework of Grand canonical DFT approach that enables potential dependent DFT calculations, simulating relevant battery operating conditions. The atomistic potential dependent DFT results were upscaled using kMC simulations allowing to extract mesoscopic behavior at various operating potentials. The results of these calculations demonstrate the effect of potential on both the energetics and the dynamics of diffusion at the electrode-electrolyte interface. In particular, the equilibrium morphology of the metal electrode is strongly modified by the potential, leading to the stabilization of different surface orientations with potential. We show that the applied potential impacts atomic diffusion barriers leading to different surface mesoscopic relaxation times in the overall kinetics. The obtained results provide not only potential dependent information about systems morphology evolution, but also demonstrate the importance of using a suitable potential range (i.e. coherent with the experimental conditions): an unrealistic potential could lead to a very different behavior of the system on atomic and on meso scale.

In heterogeneous catalysis, bifunctional catalysts often outperform one-component catalysts. The activity is also strongly influenced by the morphology, size, and distribution of catalytic particle...

Context. Gamma-ray burst (GRB) afterglows originate from the interaction between the relativistic ejecta and the surrounding medium. Consequently, their properties depend on several aspects: radiation mechanisms, relativistic shock micro-physics, circumburst environment, and the structure and geometry of the relativistic jet. While the standard afterglow model accounts for the overall spectral and temporal evolution for a number of GRBs, its validity limits emerge when the data set is particularly rich and constraining, especially in the radio band. Aims. We aimed to model the afterglow of the long GRB160131A (redshift $z = 0.972$), for which we collected a rich, broadband, and accurate data set, spanning from $6\times10^{8}$ to $7\times10^{17}$ Hz in frequency, and from 330 s to 160 days post burst in time. Methods. We modelled the spectral and temporal evolution of this GRB afterglow through two approaches: the adoption of empirical functions to model optical/X-rays data set, later assessing their compatibility with the radio domain; the inclusion of the entire multi-frequency data set simultaneously through the Python package named sAGa (Software for AfterGlow Analysis), to come up with an exhaustive and self-consistent description of the micro-physics, geometry, and dynamics of the afterglow. Results. From deep broadband analysis (from radio to X-ray frequencies) of the afterglow light curves, GRB160131A outflow shows evidence of jetted emission. Moreover, we observe dust extinction in the optical spectra, and energy injection in the optical/X-ray data. Radio spectra are characterised by several peaks, that could be due to either interstellar scintillation (ISS) effects or a multi-component structure. Conclusions. The inclusion of radio data in the broadband set of GRB160131A makes a self-consistent modelling hardly attainable within the standard model of GRB afterglows., Comment: 36 pages, 16 figures, 8 tables, accepted by A&A; v2: updated Acknowledgements

In heterogeneous catalysis, bifunctional catalysts often outperform one-component catalysts. The activity is also strongly influenced by the morphology, size, and distribution of catalytic particles. For CO2 hydrogenation, the size of the active copper area on top of the SrTiO3 perovskite catalyst support can affect the activity, selectivity, and stability. Here, a detailed theoretical study of the effect of bifunctionality on an important chemical CO2 transformation reaction, the reverse water gas shift (RWGS) reaction, is presented. Using density functional theory computation results for the RWGS pathway on three surfaces, namely, Cu(111), SrTiO3, and the Cu/SrTiO3 interface between both solid phases, we construct the energy landscape of the reaction. The adsorbate–adsorbate lateral interactions are taken into account for catalytic surfaces, which show a sufficient intermediate coverage. The mechanism, combining all three surfaces, is used in mesoscale kinetic Monte Carlo simulations to study the turnover and yield for CO production as a function of particle size. It is shown that the reaction proceeds faster at the interface. However, including copper and the support sites in addition to the interface accelerates the conversion even further, showing that the bifunctionality of the catalyst manifests in a more complex interplay between the phases than just the interface effect, such as the hydrogen spillover. We identify three distinct effects, the electronic, cooperative, and geometric effects, and show that the surrounded smaller Cu features on the set of supporting SrTiO3 show a higher CO formation rate, resulting in a decreasing RWGS model trend with the increasing Cu island size. The findings are in parallel with experiments, showing that they explain the previously observed phenomena and confirming the size sensitivity for the catalytic RWGS reaction.

A way to address the challenge of carbon dioxide emissions causing global warming and climate change is the heterogeneous catalytic hydrogenation of CO2. When surplus electrical energy is available, hydrogen may be produced and used for conversion of CO2 into fuels and chemicals. Industrially, a multifunctional metallic copper and zinc oxide catalyst on alumina (CZA) is commonly used. Presently, first-principles multiscale modelling was accomplished for a commercial-like catalyst (Zn3O3/Cu) and three other Cu/metal oxide combinations (Cr3O3/Cu, Fe3O3/Cu, and Mg3O3/Cu), synthesised via co-precipitation, characterised and experimentally tested. Ab initio plane wave density functional theory calculations were performed using different structural models to elucidate the adsorption of intermediates and elementary reaction steps, considering thermodynamics and kinetics. The results were fed into mean-field micro-kinetic expressions to calculate the conversion and selectivity in a continuous flow stirred-tank reactor vessel for various temperatures and pressures. Kinetic Monte Carlo simulations were used to obtain the detailed surface coverage, turnover frequency and catalytic performance. Although no experimental input besides the structure was applied for physico-chemical mechanism description, measurements were in agreement with theoretical predictions. It was shown that the formate species pathway (HCOO → H2COO → H2COOH → H2CO → H3CO) predominates on the examined Cu-based catalysts, although there are variations in the rate determining steps and the most abundant surface intermediate fractions. Whereas Zn3O3/Cu exhibited the highest conversion and a moderate CH3OH product selectivity, the former was lower for Mg3O3/Cu. Interestingly, Cr3O3/Cu was optimal in terms of yield, but with extremely low CH3OH productivity, while Fe3O3/Cu performed poorly overall. Our results highlight the superiority of Zn3O3/Cu, combining the estimations on micro-, meso- and reactor operation scales. Insights beyond traditional activity experiments may be attained and employed in full-scale reactor engineering and optimisation.

One of the crucial steps for the development of batteries is understanding the interface stability and morphological changes occurring during continuous stripping and deposition. In order to investigate the dependence of morphology evolution on surface orientation, we examine the energetics and growth mechanism on magnesium (0001), (101[combining macron]0), (101[combining macron]1), (112[combining macron]0) and (112[combining macron]1) surface orientations using density functional theory and kinetic Monte Carlo simulations. Workfunctions, surface, adsorption and interaction energies, diffusion barriers and k-rates for diffusion via hopping and exchange mechanisms are studied. The results provide a comprehensive relationship between these properties and morphology evolution. The latter shows strong dependence on the surface orientation, demonstrating the need for all commonly present facets to be studied, instead of focusing only on the most stable surface orientation.

The effect of the structure of copper catalyst surface on CO2 conversion has been a long-standing research effort, both theoretically and experimentally. In this paper, flat Cu(111) and stepped Cu(533) surfaces are theoretically studied for catalytic CO2 hydrogenation to methanol, an important carbon utilization process for tackling modern environmental pollution problems. First-principles multiscale modeling using density functional theory (DFT) and mesoscopic kinetic Monte Carlo (kMC) is employed to simulate the complex reaction pathway, focusing on the role of surface defects on the overall catalytic selectivity and activity at laboratory operating conditions. The contribution of the electronic (interaction energy) and geometric (distortion energy) effect to the adsorbates binding energy is evaluated and discussed. The results show that a stepped Cu(533) surface enhances the selectivity and activity of the methanol synthesis, with selectivity up to 84% and turn-over frequency (TOF) up to 10−4 s−1, which is over 4 orders of magnitude better than a flat Cu(111) surface. It is found that the surface defect, compared to the Cu(111) surface and other Cu/metal oxide alloys, promotes the H2COH hydrogenation pathway, resulting in higher total CH3OH yields. The study provides an insight into surface defects engineering, serving as a step towards the rational design of multifaceted copper catalysts.

We present a catalog of early-time (~10^2-10^4s) photometry and polarimetry of all Gamma-Ray Burst (GRB) optical afterglows observed with RINGO2 imaging polarimeter on the Liverpool Telescope. For the 19 optical afterglows observed, the following 9 were bright enough to perform photometry and attempt polarimetry: GRB 100805A, GRB 101112A, GRB 110205A, GRB 110726A, GRB 120119A, GRB 120308A, GRB 120311A, GRB 120326A and GRB 120327A. We present multi-wavelength light curves for these 9 GRBs, together with estimates of their optical polarization degrees and/or limits. We carry out a thorough investigation of detection probabilities, instrumental properties and systematics. Using two independent methods, we confirm previous reports of significant polarization in GRB 110205A and 120308A, and report new detection of $P=6^{+3}_{-2}\%$ in GRB101112A. We discuss the results for the sample in the context of the reverse and forward shock afterglow scenario, and show that GRBs with detectable optical polarization at early time have clearly identifiable signatures of reverse-shock emission in their optical light curves. This supports the idea that GRB ejecta contain large-scale magnetic fields and highlights the importance of rapid-response polarimetry., Comment: Accepted for Publication in ApJ (12th June 2017)

We present multi-wavelength observations and modeling of the exceptionally bright long $\gamma$-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 $\theta_j\approx 3.6^\circ$ and kinetic energy of $E_K\approx 2\times10^{51}$ erg, propagating into a low density ($n\approx 5\times10^{-5}$ 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 $\Gamma_0\gtrsim 100$ and an ejecta magnetization of $R_B\approx 1-100$. 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., Comment: 17 pages, 8 figures, 3 tables. Accepted for publication in ApJ

Aims. We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate possible causes of the observed early-time colour variations. Methods. We collected data from various instruments and telescopes in X-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times. Results. The ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 10 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. At early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction.© 2017 ESO., A.M., S.Co., S.Ca., B.S., P.D.A., and G.T. acknowledge support from the ASI grant I/004/11/3. E.Z. acknowledges the support by the International Cooperation Program CAPES-ICRANet financed by CAPES Brazilian Federal Agency for Support and Evaluation of Graduate Education within the Ministry of Education of Brazil. D.M. acknowledges support from the Instrument center for Danish Astrophysics (IDA). C.G.M. acknowledges support from the Royal Society, the Wolfson Foundation, and the Science and Technology Facilities Council. L.H. acknowledges support from SFI (07-RFP-PHYF295, 11/RFP.1/AST/3188) and the EU-FP7/GLORIA (grant No. 283783). M.I. and J.W.K. acknowledge the support from the National Research Foundation of Korea grant Nos. 2017R1A3A3001362 and 2016R1D1A1B03934815. D.K. acknowledges the financial support from the Slovenian Research Agency (P1-0188). This work has been supported by ASI grant I/004/11/0 and by PRIN-MIUR 2009 grants.

We use high–quality, multi-band observations of Swift GRB120404A, from γ-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al.\ud (2012) to test the standard synchrotron shock model. The evolution of the radio and optical afterglow, with its prominent optical rebrightening at trest � 260–2600 s, is\ud remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re–energization of the shock. We thus constrain the geometry of the\ud jet with half–opening and viewing angles of 23◦ and 21◦ respectively and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters ǫB = 2.4 × 10−4 and ǫe = 9.3 × 10−2 and a circumburst density of n = 240 cm−3. The ability to self–consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X–ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasising the need for a more realistic treatment of cooling in future developments of\ud theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high quality multi–band data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.

We present the second multi-frequency radio detection of a reverse shock in a $\gamma$-ray burst. By combining our extensive radio observations of the Fermi-LAT GRB 160509A at $z = 1.17$ up to $20$ days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at $\lesssim10~$days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of $n_0\approx10^{-3}~$cm$^{-3}$, supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, $N_{\rm H} \approx 1.5\times10^{22}~$cm$^{-2}$, and a high rest-frame optical extinction, $A_{\rm V}\approx3.4~$mag. We identify a jet break in the X-ray light curve at $t_{\rm jet}\approx6~$d, and thus derive a jet opening angle of $\theta_{\rm jet}\approx4~$deg, yielding a beaming-corrected kinetic energy and radiated $\gamma$-ray energy of $E_{\rm K}\approx4\times10^{50}~$erg and $E_{\gamma}\approx1.3\times10^{51}~$erg ($1$-$10^4~$keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of $t_{\rm dec} \approx 460~$s$~\approx T_{90}$, a Lorentz factor of $\Gamma(t_{\rm dec})\approx330$, and a reverse shock to forward shock fractional magnetic energy density ratio of $R_{\rm B}\equiv\epsilon_{\rm B,RS}/\epsilon_{\rm B,FS}\approx8$., Comment: 8 pages, 2 tables, 4 figures. Submitted to ApJ

In this paper we present a sample of 10 short gamma-ray bursts (GRBs) with a robust redshift determination, discovered by the Swift satellite up to January 2011. We measure their X-ray absorbing column densities and collect data on the host galaxy offsets. We find evidence for intrinsic absorption and no correlation between the intrinsic absorbing column density and the projected offset of the GRB from its host galaxy center. We find that the properties in the gamma regime (T90, fluence and 1-s peak photon flux) of short GRBs with "bright" and "faint" X-ray afterglow likely disfavour different prompt emission mechanisms. Host galaxy offset and GRB duration (T90) do not correlate. Instead, there is a hint of anti-correlation between the effective radius normalised host galaxy offset and T90. Finally, we examine the properties of short GRBs with short-lived and long-lived X-ray afterglows, finding that some short GRBs with short-lived X-ray afterglows have their optical afterglow detected. In light of this, the X-ray afterglow duration does not seem to be an unique indicator of a specific progenitor and/or environment for short GRBs.

We use gamma-ray burst (GRB) afterglow spectra observed with the VLT/X-Shooter spectrograph to measure rest-frame extinction in GRB lines-of-sight by modelling the broadband near-infrared (NIR) to X-ray afterglow spectral energy distributions (SEDs). Our sample consists of nine Swift GRBs, of which eight belong to the long-duration and one to the short-duration class. Dust is modelled using the average extinction curves of the Milky Way and the two Magellanic Clouds. We derive the rest-frame extinction of the entire sample, which fall in the range 0 ≲ AV ≲ 1.2. Moreover, the SMC extinction curve is the preferred extinction curve template for the majority of our sample, a result that is in agreement with those commonly observed in GRB lines of sights. In one analysed case (GRB 120119A), the common extinction curve templates fail to reproduce the observed extinction. To illustrate the advantage of using the high-quality, X-Shooter afterglow SEDs over the photometric SEDs, we repeat the modelling using the broadband SEDs with the NIR-to-UV photometric measurements instead of the spectra. The main result is that the spectroscopic data, thanks to a combination of excellent resolution and coverage of the blue part of the SED, are more successful in constraining extinction curves and therefore dust properties in GRB hosts with respect to photometric measurements. In all cases but one the extinction curve of one template is preferred over the others. We show that themodelled values of the extinction AV and the spectral slope, obtained through spectroscopic and photometric SED analysis, can differ significantly for individual events, though no apparent trend in the differences is observed. Finally we stress that, regardless of the resolution of the optical-to-NIR data, the SED modelling gives reliable results only when the fit is performed on a SED covering a broader spectral region (in our case extending to X-rays). Based on observations collected at the European Southern Observatory, Paranal, Chile, under programs 084.A-0260(B), 085.A-0009(B), 088.A-0051(B), 089.A-0067(B) and 091.C-0934(C). Appendix A is available in electronic form at http://www.aanda.org/10.1051/0004-6361/201525665/olm

We present predictions of centimeter and millimeter radio emission from reverse shocks in the early afterglows of gamma-ray bursts with the goal of determining their detectability with current and future radio facilities. Using a range of GRB properties, such as peak optical brightness and time, isotropic equivalent gamma-ray energy and redshift, we simulate radio light curves in a framework generalized for any circumburst medium structure and including a parametrization of the shell thickness regime that is more realistic than the simple assumption of thick- or thin-shell approximations. Building on earlier work by Mundell et al. (2007) and Melandri et al. (2010) in which the typical frequency of the reverse shock was suggested to lie at radio, rather than optical wavelengths at early times, we show that the brightest and most distinct reverse-shock radio signatures are detectable up to 0.1 -- 1 day after the burst, emphasizing the need for rapid radio follow-up. Detection is easier for bursts with later optical peaks, high isotropic energies, lower circumburst medium densities, and at observing frequencies that are less prone to synchrotron self-absorption effects - typically above a few GHz. Given recent detections of polarized prompt gamma-ray and optical reverse-shock emission, we suggest that detection of polarized radio/mm emission will unambiguously confirm the presence of low-frequency reverse shocks at early time., 9 pages, 5 figures, accepted for publication in ApJ

In the version of this Letter originally published, in the third paragraph of the text Kyutoku et al. were not correctly cited and the sentence should have read: “As pointed out by Kyutoku at al.28, in the case of high optical depth to electron scattering (~1) and assuming spectral lines do not significantly depolarize the global emission, the linear polarization observed from the equatorial plane could be as high as a few per cent.” Also, in the Author contributions section, the final sentence should have read: “C.G.M. contributed to the writing of the paper.”

The long ${\it Swift}$ gamma-ray burst GRB 120326A at redshift $z=1.798$ exhibited a multi-band light curve with a striking feature: a late-time, long-lasting achromatic rebrightening, rarely seen in such events. Peaking in optical and X-ray bands $\sim 35$ ks ($\sim 12.5$ ks in the GRB rest frame) after the 70-s GRB prompt burst, the feature brightens nearly two orders of magnitude above the underlying optical power-law decay. Modelling the multiwavelength light curves, we investigate possible causes of the rebrightening in the context of the standard fireball model. We exclude a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward shock peak due to the passage of the maximal synchrotron frequency through the optical band, late central engine optical/X-ray flares, interaction between the expanding blast wave and a density enhancement in the circumburst medium and gravitational microlensing. Instead we conclude that the achromatic rebrightening may be caused by a refreshed forward shock or a geometrical effect. In addition, we identify an additional component after the end of the prompt emission, that shapes the observed X-ray and optical light curves differently, ruling out a single overall emission component to explain the observed early time emission., 7 pages, 4 figures, accepted for publication in Astronomy & Astrophysics

Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($L\sim 3 \times 10^{53}$ erg s$^{-1}$) and its relative proximity ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies., 64 pages, 11 figures; published in Science. Update for a few references and correction of a typo in the name of one of the authors

The immediate optical afterglow of the γ-ray burst GRB 120308A is highly polarized, showing that γ-ray bursts contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion. On 8 March 2012 NASA's Swift satellite identified the γ-ray burster GRB120308A, initially as a single pulse of γ-rays lasting about 100 seconds. The LaPalma-based Liverpool Telescope, responding automatically to the Swift discovery, identified the optical afterglow and was able to track the evolution of polarized light during the crucial early minutes of the outburst. Those results, presented here, provide the tightest constraints yet on the physics of the jets of material ejected in a γ-ray burst explosion. After the initial burst of γ-rays that defines a γ-ray burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, during which a forward shock travels outwards and a reverse shock propagates backwards into the oncoming collimated flow, or ‘jet’1,2. Light from the reverse shock should be highly polarized if the jet’s magnetic field is globally ordered and advected from the central engine3,4, with a position angle that is predicted to remain stable in magnetized baryonic jet models5 or vary randomly with time if the field is produced locally by plasma or magnetohydrodynamic instabilities6,7. Degrees of linear polarization of P ≈ 10 per cent in the optical band have previously been detected in the early afterglow6,8, but the lack of temporal measurements prevented definitive tests of competing jet models9,10,11,12,13,14. Hours to days after the γ-ray burst, polarization levels are low (P

We present a broadband study of gamma-ray burst (GRB) 091024A within the context of other ultra-long-duration GRBs. An unusually long burst detected by Konus-Wind, Swift, and Fermi, GRB 091024A has prompt emission episodes covering ~1300 s, accompanied by bright and highly structured optical emission captured by various rapid-response facilities, including the 2-m autonomous robotic Faulkes North and Liverpool Telescopes, KAIT, S-LOTIS, and SRO. We also observed the burst with 8- and 10-m class telescopes and determine the redshift to be z = 1.0924 \pm 0.0004. We find no correlation between the optical and gamma-ray peaks and interpret the optical light curve as being of external origin, caused by the reverse and forward shock of a highly magnetized jet (R_B ~ 100-200). Low-level emission is detected throughout the near-background quiescent period between the first two emission episodes of the Konus-Wind data, suggesting continued central-engine activity; we discuss the implications of this ongoing emission and its impact on the afterglow evolution and predictions. We summarize the varied sample of historical GRBs with exceptionally long durations in gamma-rays (>~ 1000 s) and discuss the likelihood of these events being from a separate population; we suggest ultra-long GRBs represent the tail of the duration distribution of the long GRB population., Updated abstract, acknowledgements. Accepted for publication in ApJ; 21 pages, 10 figures, 6 tables

Because of their cosmological origin, gamma-ray burst (GRB) optical afterglows are attenuated when they pass intergalactic absorbers in the GRB line-of-sight. Without the knowledge of the number of absorbers and their physical properties, the effect of absorption on the observed magnitudes can not be determined precisely. Different methods have been applied in order to correct for this effect statistically, either using semi-analytical calculations or numerical simulations. We follow these works and present the expected magnitude corrections as a function of redshift for a set of filters most commonly used in the scientific community. The results are publically available on the web (http://igmac.fmf.uni-lj.si)., Comment: 4 pages, 2 figures; to appear in Proceedings of the conference "Gamma-Ray Bursts 2012 Conference"

The standard theoretical model of Gamma‐Ray Bursts (GRBs) predicts that high energy gamma‐ray emission is produced by internal shocks and that afterglow emission at longer wavelengths is produced later by external shocks, i.e., prompt emission and afterglow are coming from different regions. This is usually the case in typical light curves (LCs), where gamma‐ray/hard X‐ray data evolve differently with time than soft X‐ray and optical data. But there are cases like GRB 090727 which show that X‐ray/optical emission could be produced simultaneously with gamma‐ray emission. Some of these GRBs show very similar properties in LCs, for instance peaks occurring at the same time in optical, X‐ray and gamma‐ray regime. This favours the idea that we could be dealing with radiation at very different frequencies all coming from single emission region. We focus our attention on GRB 090727 which shows what looks like correlated optical, X‐ray and gamma‐ray emission, but we also discuss similar GRBs and possible scenarios and implications for the standard model.

Gamma-ray burst GRB 140430A was detected by the Swift satellite and observed promptly with the imaging polarimeter RINGO3 mounted on the Liverpool Telescope, with observations beginning while the prompt $\gamma$-ray emission was still ongoing. In this paper, we present densely sampled (10-second temporal resolution) early optical light curves in 3 optical bands and limits to the degree of optical polarization. We compare optical, X-ray and gamma-ray properties and present an analysis of the optical emission during a period of high-energy flaring. The complex optical light curve cannot be explained merely with a combination of forward and reverse shock emission from a standard external shock, implying additional contribution of emission from internal shock dissipation. We estimate an upper limit for time averaged optical polarization during the prompt phase to be as low as P < 12% (1$\sigma$). This suggests that the optical flares and early afterglow emission in this GRB are not highly polarized. Alternatively, time averaging could mask the presence of otherwise polarized components of distinct origin at different polarization position angles., Comment: 13 pages, 8 figures, 4 tables. Accepted for publication in ApJ

We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t − t0 = 16.1 d, which covers rest-frame 3000-6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000-6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs.

We present a multi-wavelength analysis of gamma-ray burst GRB 090727, for which optical emission was detected during the prompt gamma-ray emission by the 2-m autonomous robotic Liverpool Telescope and subsequently monitored for a further two days with the Liverpool and Faulkes telescopes. Within the context of the standard fireball model, we rule out a reverse shock origin for the early time optical emission in GRB 090727 and instead conclude that the early time optical flash likely corresponds to emission from an internal dissipation processes. Putting GRB 090727 into a broader observational and theoretical context, we build a sample of 36 gamma-ray bursts (GRBs) with contemporaneous early time optical and gamma-ray detections. From these GRBs, we extract a sub-sample of 18 GRBs, which show optical peaks during prompt gamma-ray emission, and perform detailed temporal and spectral analysis in gamma-ray, X-ray, and optical bands. We find that in most cases early time optical emission shows sharp and steep behaviour, and notice a rich diversity of spectral properties. Using a simple internal shock dissipation model, we show that the emission during prompt GRB phase can occur at very different frequencies via synchrotron radiation. Based on the results obtained from observations and simulation, we conclude that the standard external shock interpretation for early time optical emission is disfavoured in most cases due to sharp peaks ($\Delta t/t < 1$) and steep rise/decay indices, and that internal dissipation can explain the properties of GRBs with optical peaks during gamma-ray emission., Comment: 21 pages, 12 figures, accepted for publication in ApJ

We present ground-based and HST optical and infrared observations of XRF 100316D / SN 2010bh. It is seen that the optical light curves of SN 2010bh evolve at a faster rate than the archetype GRB-SN 1998bw, but at a similar rate to SN 2006aj, a supernova that was spectroscopically linked with XRF 060218, and at a similar rate to non-GRB associated type Ic SN 1994I. We estimate the rest-frame extinction of this event from our optical data to be E(B-V)=0.18 +/- 0.08 mag. We find the V-band absolute magnitude of SN 2010bh to be M_{V}=-18.62 +/- 0.08, which is the faintest peak V-band magnitude observed to-date for a spectroscopically-confirmed GRB-SNe. When we investigate the origin of the flux at t-t_{o}=0.598 days, it is shown that the light is not synchrotron in origin, but is likely coming from the supernova shock break-out. We then use our optical and infrared data to create a quasi-bolometric light curve of SN 2010bh which we model with a simple analytical formula. The results of our modeling imply that SN 2010bh synthesized a nickel mass of M_{Ni} \approx 0.10 M_{sun}, ejected M_{ej} \approx 2.2 M_{sun} and has an explosion energy of E_{k} \approx 1.4 x 10^{52} erg. Finally, for a sample 22 GRB-SNe we check for a correlation between the stretch factors and luminosity factors in the R band and conclude that no statistically-significant correlation exists., Second version. Accepted for publication at ApJ (18-July-2011). 18 pages, 11 figures and 8 tables

We use a parent sample of 118 gamma-ray burst (GRB) afterglows, with known redshift and host galaxy extinction, to separate afterglows with and without signatures of dominant reverse-shock emission and to determine which physical conditions lead to a prominent reverse-shock emission. We identify 10 GRBs with reverse shock signatures - GRBs 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424 and 130427A. By modeling their optical afterglows with reverse and forward shock analytic light curves and using Monte Carlo simulations, we estimate the parameter space of the physical quantities describing the ejecta and circumburst medium. We find that physical properties cover a wide parameter space and do not seem to cluster around any preferential values. Comparing the rest-frame optical, X-ray and high-energy properties of the larger sample of non-RS-dominated GRBs, we show that the early-time ($$ 10 ks). We find that GRBs with an identifiable reverse shock component show high magnetization parameter $R_{\mathrm{B}} = \varepsilon_{\rm B,r}/\varepsilon_{\rm B,f} \sim 2 - 10^4$. Our results are in agreement with the mildly magnetized baryonic jet model of GRBs., Comment: 26 pages, 10 figures, 3 tables; corrected typos; accepted to ApJ