[Confirmed:] The following handbill circulating 2ch announces an anime adaptation of the To Aru Majutsu no Index side-story, 「To Aru Kagaku no Rail Gun」. More details are expected to be released in the May issue of Dengeki Daioh magazine — on sale March 27th.

[UPDATE 03.18.2009] – Confirmation came today as the final episode of To Aru Majutsu no Index aired the following notice on AT-X channel.

[UPDATE 03.25.2009] – 「To Aru Kagaku no Rail Gun」 will begin in October.

*Synopsis courtesy of Manga-Updates:

“The story is set in a world where supernatural powers exist through science, and magic exists through religion.
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University City. 2.3 million inhabitants. 80% of the students are active in the ‘Brain Development Program’ and out of those only 7 are Level 5s.
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Meet the civilian protagonist Mikoto and her friend Kuroko, maintainers of peace and order at the University district, as members of the ‘Judgement’.”

Source(s): [1], [2], [3]