The most energetic supernova conceivable? Free

Near the turn of this century, astronomers discovered superluminous supernovae (SLSNe), which can outshine conventional supernovae by a factor of 100 or more. The most radiant SLSN of all—with double the luminosity of the runner-up—was spotted last June by the Ohio State University–led All-Sky Automated Survey for SuperNovae (ASAS-SN, pronounced “assassin”), according to an analysis by Subo Dong (Peking University) and colleagues. The research team concluded that the object, dubbed ASASSN-15lh, was an SLSN after noting the similarity of its spectra to those of known SLSNe. The energy radiated by SLSNe is typically supplied by supersized versions of the principal mechanisms that power conventional supernova radiation: radioactive decay of nickel-56 or shock waves heating hydrogen. But those explanations don’t fly for ASASSN-15lh. The ⁵⁶Ni mechanism would require an implausible 30 or more solar masses of the isotope, and the spectra of ASASSN-15lh show no signs of hydrogen. An alternative explanation posits that the remnant of ASASSN-15lh is a magnetar, a rapidly spinning neutron star with a high magnetic field (see Physics Today, May 2005, page 19). As the magnetar spins down, its rotational energy is converted to intense radiation of the material blown off in the SLSN explosion. To be consistent with the record-setting luminosity, the magnetar would have to be born spinning so rapidly that it would be on the verge of tearing itself apart. Perhaps we’ll eventually learn that ASASSN-15lh, rather than being an SLSN, is a new kind of explosion altogether. But if it is an SLSN, it’s about as energetic as it can possibly be. (S. Dong et al., Science 351, 257, 2016).