A MISSING-LINK IN THE SUPERNOVA-GRB CONNECTION: THE CASE OF SN 2012ap

Gamma Ray Bursts (GRBs) are characterized by ultra-relativistic outflows, while supernovae are generally characterized by non-relativistic ejecta. GRB afterglows decelerate rapidly usually within days, because their low-mass ejecta rapidly sweep up a comparatively larger mass of circumstellar material. However supernovae, with heavy ejecta, can be in nearly free expansion for centuries. Supernovae were thought to have non-relativistic outflows except for few relativistic ones accompanied by GRBs. This clear division was blurred by SN 2009bb, the first supernova with a relativistic outflow without an observed GRB. Yet the ejecta from SN 2009bb was baryon loaded, and in nearly-free expansion for a year, unlike GRBs. We report the first supernova discovered without a GRB, but with rapidly decelerating mildly relativistic ejecta, SN 2012ap. We discovered a bright and rapidly evolving radio counterpart driven by the circumstellar interaction of the relativistic ejecta. However, we did not find any coincident GRB with an isotropic fluence of more than a sixth of the fluence from GRB 980425. This shows for the first time that central engines in type Ic supernovae, even without an observed GRB, can produce both relativistic and rapidly decelerating outflows like GRBs.Comment: 8 pages, 5 figures, 1 table, accepted for publication in Ap

Antiferromagnetism in nanofilms of Mn-doped GaN

We theoretically study the role of surfaces in the electronic and magnetic properties of nanofilms made of wurtzite and zinc-blende (Ga,Mn)N. The studied doping reactions suggest that Mn impurities replacing Ga cations preferably stay just below the unsaturated surface rather than near the substrate. The hole-mediated ferromagnetism, typical of (Ga,Mn)N bulk, is absent from these films, and Mn moments for the most stable cationic positions become antiferromagnetically coupled. The holes ascribed to dopants in (Ga,Mn)N semiconductors are here occupied by electrons from dangling bonds. In other less-stable sites, Mn atoms are ferromagnetic; the surface moment can be then parallel to them and small, as in the wurtzite and zinc-blende (111) geometries, or antiparallel and large, as in zinc-blende (001). Hence, the magnetic interplay between surfaces and Mn impurities depends on the surface orientation, which could be useful for the design of magnetic nanodevices.This work has been supported by the Basque Government through the NANOMATERIALS project (Grant No. IE05-151) under the ETORTEK Program (iNanogune), the Spanish MCyT (FIS2010-19609-C02-02), and the UPV (Grant No. IT-366-07).Peer reviewe