TOKYO, Sept 29 Reuters The yen39;s slide to the cusp of 150 per dollar has put investors on high alert for the risk of intervention. But, Japanese authorities could find propping up their currency both difficult to achieve and hard to justify.

At its core, the yen39;s 3 slide in September to its weakest in 11 months at 149.71 on Wednesday is a result of the Bank of Japan39;s hesitancy exiting an ultraeasy monetary policy while the U.S. Federal Reserve keeps its options open for further tightening.

The dollaryen pair traditionally tracks the gap between the countries39; longterm yields, which has yawned to 380 basis points in the dollar39;s favour. U.S. Treasury yields jumped after Fed officials surprised markets last week by hinting at another rate rise this year.

On the Japanese side, BOJ Governor Kazuo Ueda has quashed expectations for a hawkish shift during coming months by repeatedly emphasising a patient approach was needed to tightening the taps on its super loose policy.

Intervention is both financially risky and politically charged. To make even a ripple in the 5 trillion currency market, the BOJ would need to draw down massive amounts of dollar reserves.

Considering the major rich democracies commitment to letting markets determine exchange rates, Tokyo could get a grudging response from Washington when it tries explaining why it needed to pour so many dollars into the open market.

You39;ve got the Fed and most other G10 countries hiking rates, while the…