A large lunar impact blast on 2013 September 11

On 2013 September 11 at 20(h)07(m)28.(s)68 +/- 0.(s)01 UTC, two telescopes operated in the framework of our lunar impact flashes monitoring project recorded an extraordinary flash produced by the impact on the Moon of a large meteoroid at selenographic coordinates 17 degrees.2 +/- 0 degrees.2 S, 20 degrees.5 +/- 0 degrees.2 W. The peak brightness of this flash reached 2.9 +/- 0.2 mag in V and it lasted over 8 s. The estimated energy released during the impact of the meteoroid was 15.6 +/- 2.5 tons of TNT under the assumption of a luminous efficiency of 0.002. This event, which is the longest and brightest confirmed impact flash recorded on the Moon thus far, is analysed here. The likely origin of the impactor is discussed. Considerations in relation to the impact flux on Earth are also made.