* Astronomy

Hubble observes first multiple images of explosive distance indicator

A Swedish-led team of astronomers used the NASA/ESA Hubble Space Telescope to analyse the multiple images of a gravitationally lensed type Ia supernova for the first time. The four images of the exploding star will be used to measure the expansion of the Universe. This can be done without any theoretical assumptions about the cosmological model, giving further clues about how fast the Universe is really expanding. The results are published in the journal Science.
An international team, led by astronomers from the Stockholm University, Sweden, has discovered a distant type Ia supernova, called iPTF16geu - it took the light 4.3 billion years to travel to Earth. The light from this particular supernova was bent and magnified by the effect of gravitational lensing so that it was split into four separate images on the sky. The four images lie on a circle with a radius of only about 3000 light-years around the lensing foreground galaxy, making it one of the smallest extragalactic gravitational lenses discovered so far. Its appearance resembles the famous Refsdal supernova, which astronomers detected in 2015. Refsdal, however, was a core-collapse supernova.
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Title: The discovery of the multiply-imaged lensed Type Ia supernova iPTF16geu
Author: A.Goobar, R.Amanullah, S.R.Kulkarni, P.E.Nugent, J.Johansson, C.Steidel, D.Law, E.Mortsell, R.Quimby, N.Blagorodnova, A.Brandeker, Y.Cao, A.Cooray, R.Ferretti, C.Fremling, L.Hangard, M.Kasliwal, T.Kupfer, R.Lunnan, F.Masci, A.A.Miller, H.Nayyeri, J.D.Neill, E.O.Ofek, S.Papadogiannakis, T.Petrushevska, V.Ravi, J.Sollerman, M.Sullivan, F.Taddia, R.Walters, D.Wilson, L.Yan, O.Yaron

We report the discovery of a gravitationally lensed Type Ia supernova (SN Ia) by the intermediate Palomar Transient Factor (iPTF). The light originating from SNIa iPTF16geu, at redshift z_SN=0.409, is magnified by an intervening galaxy at z_l=0.216, acting as a gravitational lens. Using Laser Guide Star Adaptive Optics (LGSAO) OSIRIS and NIRC2 observations at the Keck telescope, as well as measurements with the Hubble Space Telescope, we were able to detect the strong bending of the light path, both for iPTF16geu and its host galaxy. We detect four images of the supernova, approximately 0.3" from the center of the lensing galaxy. iPTF16geu is the first \snia for which multiple images have been observed. From the fits of the multi-colour lightcurve we derive a lensing magnification, Delta m=4.37±0.15 mag, corresponding to a total amplification of the supernova flux by a factor µ~56. The discovery of iPTF16geu suggests that lensing by sub-kpc structures may have been greatly underestimated. In that scenario, many discoveries of gravitationally magnified objects can be expected in forthcoming surveys of transient phenomena, opening up a new window to precision cosmology with supernovae.

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