[Cerium]

It appears it is "... of TNT". Wolfram Alpha converts "10^56 ergs to imperial megatons" to a value of: 1.095×10^23 long megatons (using E = mc^2). While it converts "10^56 ergs to megatons" to a value of "2.39×10^33 megatons of TNT" which is much more like that number.

Peak luminosity from https://losc.ligo.org/events/GW170104/ Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+megato... Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+imperi...