By Tamiyuki Kihara, Makiko Yamazaki and Leika KiharaTOKYO, Aug 2 (Reuters) - Japanese Finance Minister Satsuki Katayama will announce on Monday that Tokyo and Washington took joint action in the currency market to arrest the yen's slide to 40-year lows, two Japanese government officials told Reuters. One source, asked if Katayama would announce "joint action", said yes, adding, "The operation is still ongoing." FIRST JOINT YEN INTERVENTION IN 15 YEARSThe expected announcement follows what market sources say were rounds of yen-buying in the market by the Japanese and U.S. authorities, the first joint intervention since 2011, seeking to boost the Japanese currency from its lowest levels against the dollar since 1986. The Japanese government bought yen for dollars in New York trading hours on Thursday, a market source told Reuters, with Bank of Japan data suggesting it sold as ‌much as $58.97 billion to support the yen. Treasury Secretary Scott Bessent, who said last week the yen "seems very undervalued to me", had a notepad at a Friday cabinet meeting with the words "To Do", followed by "Buy Japanese Yen (JPY) $5-10 bil", a Reuters photo showed.