Carl Icahn, the activist hedge fund manager, took to Twitter on Tuesday to announce he had bought a large position in Apple, Inc. (NASDAQ: AAPL). Icahn is the majority interest holder in Icahn Enterprises, L.P. (NASDAQ: IEP), a master limited partnership. Icahn said the Company was currently undervalued. Apple rose around 4.75% in trading on the news. This was Apple’s second best trading day of the year. Icahn is pushing Apple to increase its share buyback program, which would lift the share price.

We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come.

— Carl Icahn (@Carl_C_Icahn) August 13, 2013

Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly.

— Carl Icahn (@Carl_C_Icahn) August 13, 2013

Icahn set a price target of $625, saying Apple could borrow at a low interest rate and buyback a significant number of shares even without any substantial earnings growth. Apple is currently trading above $500 a share for the first time since January. The shares hit an all-time high of $705 in June of 2012, but have since fallen from that lofty valuation. The Company has a current dividend yield of 2.49%, and a P/E ratio of 12.23. The rise in Apple’s stock on the Icahn announcement is impressive, considering Apple’s massive market capitalization of $444 billion.

The news of Icahn’s position comes right before the expected announcement of new Apple products. Apple has scheduled a release date of September 10th. There are indications that there could be two new products unveiled during the release, the iPhone 5S and a budget-version iPhone 5c. The actual release for the new products may not be until October 25th. It is also believed that Apple is working on new iPad products that may be released in the next few months.