RESUMO
The accretion of hydrogen onto a white dwarf star ignites a classical nova eruption1,2-a thermonuclear runaway in the accumulated envelope of gas, leading to luminosities up to a million times that of the Sun and a high-velocity mass ejection that produces a remnant shell (mainly consisting of insterstellar medium). Close to the upper mass limit of a white dwarf3 (1.4 solar masses), rapid accretion of hydrogen (about 10-7 solar masses per year) from a stellar companion leads to frequent eruptions on timescales of years4,5 to decades6. Such binary systems are known as recurrent novae. The ejecta of recurrent novae, initially moving at velocities of up to 10,000 kilometres per second7, must 'sweep up' the surrounding interstellar medium, creating cavities in space around the nova binary. No remnant larger than one parsec across from any single classical or recurrent nova eruption is known8-10, but thousands of successive recurrent nova eruptions should be capable of generating shells hundreds of parsecs across. Here we report that the most frequently recurring nova, M31N 2008-12a in the Andromeda galaxy (Messier 31 or NGC 224), which erupts annually11, is indeed surrounded by such a super-remnant with a projected size of at least 134 by 90 parsecs. Larger than almost all known remnants of even supernova explosions12, the existence of this shell demonstrates that the nova M31N 2008-12a has erupted with high frequency for millions of years.
RESUMO
Short-duration γ-ray bursts are intense flashes of cosmic γ-rays, lasting less than about two seconds, whose origin is unclear. The favoured hypothesis is that they are produced by a relativistic jet created by the merger of two compact stellar objects (specifically two neutron stars or a neutron star and a black hole). This is supported by indirect evidence such as the properties of their host galaxies, but unambiguous confirmation of the model is still lacking. Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint transient, known as a 'kilonova', in the days following the burst. Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe. Recent calculations suggest that much of the kilonova energy should appear in the near-infrared spectral range, because of the high optical opacity created by these heavy r-process elements. Here we report optical and near-infrared observations that provide strong evidence for such an event accompanying the short-duration γ-ray burst GRB 130603B. If this, the simplest interpretation of the data, is correct, then it confirms that compact-object mergers are the progenitors of short-duration γ-ray bursts and the sites of significant production of r-process elements. It also suggests that kilonovae offer an alternative, unbeamed electromagnetic signature of the most promising sources for direct detection of gravitational waves.
