How is an asynchronous request depicted on a sequence diagram?

An asynchronous request on a sequence diagram is depicted with an arrow that has an open (dashed) arrowhead, directed from the calling object to the called object. Unlike a synchronous call, which is represented by a solid arrow with a filled arrowhead, an asynchronous call does not block the calling object and does not require an immediate response.

Example:

Object A  --->>  Object B : asynchronous call

Here, the arrow with a double dashed arrowhead indicates an asynchronous call.