In June 2015 the All-Sky Automated Survey for Supernovae snagged a huge fish: an extremely luminous object, dubbed ASASSN-15lh, whose spectrum appeared consistent with that of a supernova. The object’s luminosity peaked at about 2.2 x 1045 erg/s (2.2 x 1038 J/s), a factor of two higher than that of any other measured stellar explosion (see Physics Today, March 2016, page 14). Yet the ramifications of the supernova interpretation, including the sheer scale of nucleosynthesis necessary to generate so much energy, led scientists to consider alternative mechanisms.
Following a 10-month examination at multiple wavebands, one research team now argues that ASASSN-15lh is actually a star that got torn apart by the extreme tidal forces of its galaxy’s supermassive black hole (SMBH). The scientists, led by Giorgos Leloudas from the Weizmann Institute of Science in Israel and the University of Copenhagen in Denmark, found that ASASSN-15lh went through three distinct spectroscopic phases, including one with helium emission lines that have never been detected in the brightest supernovae. Further support for the researchers’ proposal comes from the location of ASASSN-15lh. The event took place at the center of a massive red galaxy whose star formation rate has all but petered out. Such galaxies harbor SMBHs but not the young blue stars whose lives end in supernovae. The researchers also observed a second peak in UV emissions about two months after ASASSN-15lh, which is consistent with the behavior of a previous transient that is thought to be a tidal disruption event.
In making their case, Leloudas and colleagues faced one major objection: The SMBH in question is so massive that it should swallow stars whole rather than shredding them first. The researchers circumvented that problem by proposing that the black hole is rapidly spinning (see artist’s conception above), which can extend the range of strong tidal forces by nearly an order of magnitude. (G. Leloudas et al., Nat. Astron. 1, 2, 2016.)
