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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.

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.

X-Rated Supernova A link between long γ-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana et al . report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian et al . report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg et al . report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali et al . modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole. Observations of the close gamma-ray burst GRB 060218 and its connection to supernova SN 2006aj reveal the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the GRB progenitor. These observation catch a supernova in the act of exploding. Although the link between long γ-ray bursts (GRBs) and supernovae has been established 1 , 2 , 3 , 4 , hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a γ-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5 ) and its connection to supernova SN 2006aj (ref. 6 ). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor 7 . We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf–Rayet star.

he Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of gamma-ray bursts (GRBs) and GRB afterglows. The X-ray telescope (XRT) enables Swift to determine GRB positions with a few arcseconds accuracy within 100 s of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM-Newton/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^sup 2^ at 1.5 keV, field of view of 23.6 × 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2×10^sup -14^ erg cm^sup -2^ s^sup -1^ in 10^sup 4^ s. The instrument is designed to provide automated source detection and position reporting within 5 s of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.[PUBLICATION ABSTRACT]

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.

Swift response The Swift satellite, launched in November last year, is designed to study γ-ray bursts (GRBs) as soon as they happen. GRBs are the most powerful explosions known in the Universe, and Swift's ability to study the early phases of the X-ray afterglow was expected to yield exciting results. Swift has now bagged its first two long GRBs: in both, the X-ray afterglow emission declined rapidly in the first few hundred seconds, then flattened out. The steep decline was unexpected, and neither it nor the spectral properties of the afterglow can be explained by current models. ‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see 1 , 2 . As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands 1 , 2 . Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

The Swift X-ray Telescope (XRT) has discovered that flares are quite common in early X-ray afterglows of gamma-ray bursts (GRBs), being observed in roughly 50% of afterglows with prompt follow-up observations. The flares range in fluence from a few per cent to approximately 100% of the fluence of the prompt emission (the GRB). Repetitive flares are seen, with more than four successive flares detected by the XRT in some afterglows. The rise and fall times of the flares are typically considerably smaller than the time since the burst. These characteristics suggest that the flares are related to the prompt emission mechanism, but at lower photon energies. We conclude that the most likely cause of these flares is late-time activity of the GRB central engine.

Long gamma-ray bursts (GRBs) are bright flashes of high-energy photons that can last for tens of minutes; they are generally associated with galaxies that have a high rate of star formation and probably arise from the collapsing cores of massive stars, which produce highly relativistic jets (collapsar model). Here we describe gamma- and X-ray observations of the most distant GRB ever observed (GRB 050904): its redshift (z) of 6.29 means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era. This means that not only did stars form in this short period of time after the Big Bang, but also that enough time had elapsed for them to evolve and collapse into black holes.

Long-duration γ-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of massive stars. Previous early optical observations of even the most exceptional GRBs (990123 and 030329) lacked both the temporal resolution to probe the optical flash in detail and the accuracy needed to trace the transition from the prompt emission within the outflow to external shocks caused by interaction with the progenitor environment. Here we report observations of the extraordinarily bright prompt optical and γ-ray emission of GRB 080319B that provide diagnostics within seconds of its formation, followed by broadband observations of the afterglow decay that continued for weeks. We show that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet. GRB 080319B: fit to burst The γ-ray burst GRB 080319B, the result of the violent collapse of a massive star to form a black hole, is the most luminous optical flash so far observed in the 40-year history of γ-ray astronomy. Discovered by the Swift satellite on 19 March 2008 and briefly visible to the naked eye, it produces energy across the entire electromagnetic spectrum. Now a reanalysis of the extraordinarily bright emissions of GRB 080319B within a few seconds of its formation, together with broadband observations of its decay over the following few weeks, provide the clearest picture yet of one of these events. The data clearly establish that the prompt optical flash was produced in the same physical region as the γ-ray burst itself. The afterglow properties cannot be explained by the standard simple models, but rather imply a multi-component jet interpretation. Long duration γ-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and provide a window into the process of black hole formation from the collapse of massive stars. Observations of the extraordinarily bright prompt optical and γ-ray emission of GRB 080319B shows that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet.