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SN 2005E: untrue to type The novel properties of the faint supernova SN 2005E mean that it does not fit readily into the established supernova categories. Types Ib, Ic and II, core-collapse supernovae, are thought to form when a massive star explodes at the end of its life, and type Ia as a result of the thermonuclear explosion of an accreting white dwarf. From spectroscopic data, Perets et al . conclude that SN 2005E is helium rich, like a type Ib, and lacks the hydrogen, silicon and sulphur spectral lines typical of type Ia. But based on its presence in an 'old' stellar environment, and with a low derived ejected mass, they argue against a core-collapse origin and for an origin from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary system. Kawabata et al . see it differently. SN 2005E resembles SN 2005cz, they say, a type Ib supernova that is unusual in being found in an elliptical galaxy. Both SN 2005E and SN 2005cz, they suggest, are best explained as products of the core collapse of massive stars at the low (6–12 solar mass) end of massiveness. In the accompanying News & Views, David Branch discusses these two models in the context of the latest thinking on how stars explode. Supernovae are thought to arise through one of two processes. Type Ib/c and type II supernovae are produced when the cores of massive, short-lived stars undergo gravitational core collapse and eject a few solar masses. Type Ia supernovae are thought to form by the thermonuclear detonation of a carbon-oxygen white dwarf. Here a faint type Ib supernova, SN 2005E, is reported that seems not to have had a core-collapse origin, but perhaps arose from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary. Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae 1 , 2 . Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous 3 , 4 or a regular 1 type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44 Ti.

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. Radio and X-ray observations of X-ray flash XRF 060218, which is associated with supernova SN 2006aj, show that this event is 100 times less energetic but ten times more common than cosmological gamma-ray bursts (GRBs). The production of relativistic ejecta seems to be the key physical distinction between GRBs/XRFs and ordinary supernovae. Over the past decade, long-duration γ-ray bursts (GRBs)—including the subclass of X-ray flashes (XRFs)—have been revealed 1 , 2 , 3 to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated 4 with supernova SN 2006aj), the second-nearest 5 , 6 GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10 48  erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

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. Optical observations of γ-ray burst (GRB) 060614 (duration ∼100s) rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive 'collapsar' that powers a GRB without any associated supernova, or a new type of engine, as long-lived as the collapsar but without a massive star. Over the past decade, our physical understanding of γ-ray bursts (GRBs) has progressed rapidly, thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (≳2 s) GRBs are associated with the explosive deaths of massive stars (‘collapsars’, ref. 1 ), which produce accompanying supernovae 2 , 3 , 4 , 5 ; the short-duration (≲2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects 6 , 7 , 8 , 9 . Here we report optical observations of GRB 060614 (duration ∼100 s, ref. 10 ) that rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive collapsar that powers a GRB without any associated supernova, or a new type of ‘engine’, as long-lived as the collapsar but without a massive star. We also show that the properties of the host galaxy (redshift z  = 0.125) distinguish it from other long-duration GRB hosts and suggest that an entirely new type of GRB progenitor may be required.

The final chapter in the long-standing mystery of the γ-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates. Short gamma-ray bursts Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of Nature ), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole.

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. Despite a rich phenomenology, γ-ray bursts (GRBs) are divided 1 into two classes based on their duration and spectral hardness—the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing 2 their origin to star-forming galaxies, and hence the death of massive stars, and indicating 3 an energy release of about 10 51  erg. While theoretical arguments 4 suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent 5 , 6 , 7 , 8 localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift 9 z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10–1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions 5 , 6 and providing support for coalescing compact object binaries as the progenitors.

Bright prospect Many important astronomical discoveries began with the sighting of a transient light or radio source. The unusually brilliant optical transient OT2006-1, spotted in the galaxy Messier 85 in the Virgo Cluster in January 2006, is therefore of great interest. Kulkarni et al . report on the discovery and the first few months of life of the new source, which was brighter than a nova, but fainter than a supernova. A trawl through the archives of the Hubble Space Telescope, the Spitzer Space Telescope and the Chandra X-ray Observatory reveal no signs of a progenitor. This and the nature of the host galaxy point to a merger between two old stars as a possible cause for what has been dubbed a 'luminous red nova'. S. R. Kulkarni and colleagues report the discovery of a mysterious optical transient called M85 OT2006-1 in the outskirts of the lenticular galaxy Messier 85 in the Virgo Cluster. Brighter than novae and fainter than supernovae, it is unlikely to be a giant eruption from a luminous blue variable star because no such star is known to be there, but a possible origin is a stellar merger. Historically, variable and transient sources have both surprised astronomers and provided new views of the heavens. Here we report the discovery of an optical transient in the outskirts of the lenticular galaxy Messier 85 in the Virgo cluster. With a peak absolute R magnitude of -12, this event is distinctly brighter than novae, but fainter than type Ia supernovae (which are expected in a population of old stars in lenticular galaxies). Archival images of the field do not show a luminous star at that position with an upper limit in the g filter of about -4.1 mag, so it is unlikely to be a giant eruption from a luminous blue variable star. Over a two-month period, the transient source emitted radiation energy of almost 10 47  erg and subsequently faded in the optical sky. It is similar to, but six times more luminous at peak than, an enigmatic transient in the galaxy M31 (ref. 1 ). A possible origin of M85 OT2006-1 is a stellar merger. If so, searches for similar events in nearby galaxies will not only allow study of the physics of hyper-Eddington sources, but also probe an important phase in the evolution of stellar binary systems.

Over the six years since the discovery 1 of the γ-ray burst GRB 980425, which was associated 2 with the nearby (distance ∼40 Mpc) supernova 1998bw, astronomers have debated fiercely the nature of this event. Relative to bursts located at cosmological distance (redshift z ≈ 1), GRB 980425 was under-luminous in γ-rays by three orders of magnitude. Radio calorimetry 3 , 4 showed that the explosion was sub-energetic by a factor of 10. Here we report observations of the radio and X-ray afterglow of the recent GRB 031203 (refs 5–7 ), which has a redshift of z = 0.105. We demonstrate that it too is sub-energetic which, when taken together with the low γ-ray luminosity 7 , suggests that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic γ-ray burst. Such sub-energetic events have faint afterglows. We expect intensive follow-up of faint bursts with smooth γ-ray light curves 8 , 9 (common to both GRB 031203 and 980425) to reveal a large population of such events.

Stellar occultations--the passing of a relatively nearby body in front of a background star--can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Pluto's atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of approximately 1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Pluto's atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Pluto's atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

Despite the use of various combined chemotherapeutic regimens for advanced-stage intermediate- and high-grade lymphomas, roughly half of patients treated do not have a complete remission or eventually have a relapse after a remission. This situation has not improved noticeably in almost two decades 1 , 2 . Treatment with standard-dose salvage chemotherapy rarely results in durable remissions and often has serious toxicity. Although the use of high-dose chemotherapy with bone marrow transplantation has shown promise, not all patients derive long-term benefit from this treatment 3 . Furthermore, a curative treatment for patients with advanced low-grade lymphoma still remains to be clearly established 4 . . . .

The purpose of this review was to critically evaluate the multiple sclerosis (MS) literature that has examined physical and cognitive function in relation to ability to work. Although numerous factors may be considered when determining work ability, physical and/or cognitive functional limitations associated with MS are presumably the primary determinants of work capacity. An exhaustive search of the literature produced 20 research articles that described 18 studies. Findings from these studies support that limitations in physical or cognitive function can hinder one's ability to work; however, ability to work could not be based solely on these measures of function. Work ability among individuals extended beyond measures of impairment to include level of education, job characteristics, and disease symptoms such as fatigue. In summary, measures of physical and cognitive function can guide physicians when clinically evaluating an individual with MS, but are poor indicators for precluding an individual from working.