Serendipitous discovery of a strong-lensed galaxy in integral field spectroscopy from MUSE

2MASX 704035024-0239275 is a bright red elliptical galaxy at redshift 0.0661 that presents two extended sources at 2 '' to the north-east and 1 '' to the south-west. The sizes and surface brightnesses of the two blue sources are consistent with a gravitationally-lensed background galaxy. In this paper, we present Multi-Unit Spectroscopic Explorer (MUSE) observations of this galaxy from the All-weather MIJse Supernova Integral-field Nearby Galaxies (AMUSING) survey, and report the discovery of a background lensed galaxy at redshift 0.1915, together with other 15 background galaxies at redshifts ranging from 0.09 to 0.9, that are not multiply imaged. We have extracted aperture spectra of the lens and all the sources and fit the stellar continuum with STARLIGHT to estimate their stellar and emission line properties. A trace of past merger and active nucleus activity is found in the lensing galaxy, while the background lensed galaxy is found to be star-forming, Modelling the lensing potential with a singular isothermal ellipsoid, we find an Einstein radius of 1 ''.45 +/- 0 ''.04, which corresponds to 1,9 kpc at the redshift of the lens and it is much smaller than its effective radius (r(eff) similar to 9 ''). Comparing the Einstein mass and the STARLIGHT stellar mass within the same aperture yields a dark matter fraction of 18 per cent +/- 8 per cent within the Einstein radius, The advent of large surveys such as the Large Synoptic Survey Telescope (LSST) will discover a number of strong-lensed systems, and here we demonstrate how wide-field integral field spectroscopy offers an excellent approach to study them and to precisely model lensing effects.