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The long and the short of it The tidy classification system that divided γ-ray bursts (GRBs) into long-duration busts (lasting more than two seconds) and short may have had its day. The final nail in its coffin may be GRB 060614. Discovered on 14 June 2006 by the Burst Alert Telescope on-board the Swift satellite, this burst was long, at 102 seconds, but as reported in a clutch of papers in this issue, it has a number of properties, including the absence of an accompanying supernova, that were previously considered diagnostic of a 'short' GRB. The hunt is now on for a classification system to take account of the diversity now apparent in GRBs. In the accompanying News & Views, Bing Zhang suggests that the answer may be to adopt a Type I/Type II classification similar to that used for supernovae. Deep optical observations of GRB 060614 show no emerging supernova with absolute magnitude brighter than M V = − 13.7. Any supernova associated with GRB 060614 was therefore at least 100 times fainter, at optical wavelengths, than the other supernovae associated with GRBs. Gamma-ray bursts (GRBs) are short, intense flashes of soft γ-rays coming from the distant Universe. Long-duration GRBs (those lasting more than ∼2 s) are believed to originate from the deaths of massive stars 1 , mainly on the basis of a handful of solid associations between GRBs and supernovae 2 , 3 , 4 , 5 , 6 , 7 . GRB 060614, one of the closest GRBs discovered, consisted of a 5-s hard spike followed by softer, brighter emission that lasted for ∼100 s (refs 8 , 9 ). Here we report deep optical observations of GRB 060614 showing no emerging supernova with absolute visual magnitude brighter than M V  = -13.7. Any supernova associated with GRB 060614 was therefore at least 100 times fainter, at optical wavelengths, than the other supernovae associated with GRBs 10 . This demonstrates that some long-lasting GRBs can either be associated with a very faint supernova or produced by different phenomena.

Hard evidence Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. It is now clear that the long-duration type are caused by explosions of massive stars in distant star-forming galaxies. Only in recent months, with the Swift satellite latching onto bursts as soon as they happen, has it been possible to collect data on short bursts that may lead to similar certainty as to their cause. GRB 050724 burst onto the scene on 24 July, and has all the properties needed to solve the mystery of short GRBs. The new evidence supports the merging compact object model of short GRBs, involving either a neutron star–neutron star merger, or a neutron star–black hole binary system as progenitor. Two short (< 2 s) γ-ray bursts (GRBs) have recently been localized 1 , 2 , 3 , 4 and fading afterglow counterparts detected 2 , 3 , 4 . The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5 ), coincident with the position determined by ground-based optical and radio observations 6 , 7 , 8 . The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3 , 31 ), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2–3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts—the coalescence of binary systems of neutron stars or a neutron star-black hole pair—are the progenitors of short bursts.

Two views of Christmas γ-ray burst The Christmas γ-ray burst of 25 December 2010 (GRB 101225A, first detected by the Swift orbiting observatory) was a very unusual event. It was long lasting without the typical decreasing trend, its X-ray afterglow faded rapidly and its spectrum was atypical. Two papers in this issue offer very different explanations for these puzzling properties. Sergio Campana's group favours a comet crashing onto a neutron star as the cause of the outburst. Christina Thöne's group prefers a more conventional supernova mechanism, in this case involving a merger between a helium star and a neutron star. The tidal disruption of a solar-mass star around a supermassive black hole has been extensively studied analytically 1 , 2 and numerically 3 . In these events, the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris falls into the black hole, forming an accretion disk and emitting radiation 4 , 5 , 6 . The same process may occur on planetary scales if a minor body passes too close to its star. In the Solar System, comets fall directly into our Sun 7 or onto planets 8 . If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy 9 . Here we report that the peculiarities of the ‘Christmas’ gamma-ray burst (GRB 101225A 10 ) can be explained by a tidal disruption event of a minor body around an isolated Galactic neutron star. This would indicate either that minor bodies can be captured by compact stellar remnants more frequently than occurs in the Solar System or that minor-body formation is relatively easy around millisecond radio pulsars. A peculiar supernova associated with a gamma-ray burst provides an alternative explanation 11 .

Bursting at high redshift Two groups present redshift determinations and other spectroscopic data for the γ-ray burst GRB 090423 — now the earliest and most distant astronomical object known. Salvaterra et al . report its initial detection with the Swift satellite on 23 April 2009, and a redshift determination with the Telescopio Nazionale Galileo on La Palma 14 hours after the burst, obtaining z ≈ 8.1. Tanvir et al . used the United Kingdom Infrared Telescope, Hawaii, from about 20 minutes after the burst and arrive at z ≈ 8.2. The previous highest redshift known for any object was z = 6.96 for a Lyman-α emitting galaxy. These measurements imply that massive stars were being produced and were dying as γ-ray bursts as early as about 600 million years after the Big Bang, and that their properties are very similar to those stars producing γ-ray bursts 10 billion years later. Long-duration γ-ray bursts (GRBs), thought to result from the explosions of certain massive stars, are bright enough that some of them should be observable out to redshifts of z > 20. So far, the highest redshift measured for any object has been z = 6.96, for a Lyman-α emitting galaxy. Here, and in an accompanying paper, GRB 090423 is reported to lie at a redshift of z ≈ 8.2, implying that massive stars were being produced and dying as GRBs approximately 620 million years after the Big Bang. Gamma-ray bursts (GRBs) are produced by rare types of massive stellar explosion. Their rapidly fading afterglows are often bright enough at optical wavelengths that they are detectable at cosmological distances. Hitherto, the highest known redshift for a GRB was z = 6.7 (ref. 1 ), for GRB 080913, and for a galaxy was z = 6.96 (ref. 2 ). Here we report observations of GRB 090423 and the near-infrared spectroscopic measurement of its redshift, z = . This burst happened when the Universe was only about 4 per cent of its current age 3 . Its properties are similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors that gave rise to this burst about 600,000,000 years after the Big Bang are not markedly different from those producing GRBs about 10,000,000,000 years later.

X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

The only supernovae (SNe) to show gamma-ray bursts (GRBs) or early x-ray emission thus far are overenergetic, broad-lined type Ic SNe (hypernovae, HNe). Recently, SN 2008D has shown several unusual features: (i) weak x-ray flash (XRF), (ii) an early, narrow optical peak, (iii) disappearance of the broad lines typical of SN Ic HNe, and (iv) development of helium lines as in SNe Ib. Detailed analysis shows that SN 2008D was not a normal supernova: Its explosion energy (E [almost equal to] 6x10⁵¹ erg) and ejected mass [~7 times the mass of the Sun ([Formula: see text])] are intermediate between normal SNe Ibc and HNe. We conclude that SN 2008D was originally a ~30 [Formula: see text] star. When it collapsed, a black hole formed and a weak, mildly relativistic jet was produced, which caused the XRF. SN 2008D is probably among the weakest explosions that produce relativistic jets. Inner engine activity appears to be present whenever massive stars collapse to black holes.

Segmentation is a typical task in image processing having as main goal the partitioning of the image into multiple segments in order to simplify its interpretation and analysis. One of the more popular segmentation model, formulated by Chan-Vese, is the piecewise constant Mumford-Shah model restricted to the case of two-phase segmentation. We consider a convex relaxation formulation of the segmentation model, that can be regarded as a nonsmooth optimization problem, because the presence of the l1-term. Two basic approaches in optimization can be distinguished to deal with its non differentiability: the smoothing methods and the nonsmoothing methods. In this work, a numerical comparison of some first order methods belongs of both approaches are presented. The relationships among the different methods are shown, and accuracy and efficiency tests are also performed on several images.

Laser–plasma interaction (LPI) at intensities$10^{15}{-}10^{16}~\\text{W}\\cdot \\text{cm}^{-2}$is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity${\\sim}1.2\\times 10^{16}~\\text{W}\\cdot \\text{cm}^{-2}$with a${\\sim}100~\\unicode[STIX]{x03BC}\\text{m}$scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature (${\\sim}4~\\text{keV}$) expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed. Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results, beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A developing application of laser-driven currents is the generation of magnetic fields of picosecond–nanosecond duration with magnitudes exceeding$B=10~\\text{T}$. Single-loop and helical coil targets can direct laser-driven discharge currents along wires to generate spatially uniform, quasi-static magnetic fields on the millimetre scale. Here, we present proton deflectometry across two axes of a single-loop coil ranging from 1 to 2 mm in diameter. Comparison with proton tracking simulations shows that measured magnetic fields are the result of kiloampere currents in the coil and electric charges distributed around the coil target. Using this dual-axis platform for proton deflectometry, robust measurements can be made of the evolution of magnetic fields in a capacitor coil target.