RESUMEN
Gravitational waves (GWs) cause the apparent position of distant stars to oscillate with a characteristic pattern on the sky. Astrometric measurements (e.g., those made by Gaia) provide a new way to search for GWs. The main difficulty facing such a search is the large size of the data set; Gaia observes more than one billion stars. In this Letter the problem of searching for GWs from individually resolvable supermassive black hole binaries using astrometry is addressed for the first time; it is demonstrated how the data set can be compressed by a factor of more than 10^{6}, with a loss of sensitivity of less than 1%. This technique was successfully used to recover artificially injected GW signals from mock Gaia data and to assess the GW sensitivity of Gaia. Throughout the Letter the complementarity of Gaia and pulsar timing searches for GWs is highlighted.
RESUMEN
The massive star that underwent a collapse of its core to produce supernova (SN)1993J was subsequently identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion. It showed an excess in ultraviolet and B-band colours, suggesting either the presence of a hot, massive companion star or that it was embedded in an unresolved young stellar association. The spectra of SN1993J underwent a remarkable transformation from the signature of a hydrogen-rich type II supernova to one of a helium-rich (hydrogen-deficient) type Ib. The spectral and photometric peculiarities were best explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion, producing a 'type IIb' supernova, but the hypothetical massive companion stars for this class of supernovae have so far eluded discovery. Here we report photometric and spectroscopic observations of SN1993J ten years after the explosion. At the position of the fading supernova we detect the unambiguous signature of a massive star: the binary companion to the progenitor.
RESUMEN
We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8(+4)(-2) solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae.