These are flowers of a Persian silk tree or pink silk tree (Albizia julibrissin). Persian silk trees fold their leaves during the night, which makes them look asleep, so they're called in Japanese "nemu no ki," which translates to "sleeping tree."
A Persian silk tree is popular in Japan as suggested that it's a kigo* for summer. It's also often used as a theme for paintings, such as the one in the last photo "Sparrow in a silk tree" by Keibun Matsumura at Tokyo National Museum.
* Kigo are words or phrases that are associated with a particular season in haiku or Japanese poetry.