How do I include timestamps for documents in my search results

Elastic Stack Elasticsearch
1

I tried to create an index with _timestamp enabled

POST http://localhost:9200/my_index/

"mappings" : {
    "_default_":{
        "_timestamp" : {
            "enabled" : true,
            "store" : true
        }
    }
  }
}

I then stored a bunch of documents
Then tried to search for the documents while includeing the _timestamp field

POST http://localhost:9200/my_index/_search

{
"fields" : ["_timestamp","_source"],
"query" : {
    "match_all" : {}
}

}

The results returned don't have the timestamp field though:

hits: [10]
0:  {
_index: "my_index"
_type: "my_type"
_id: "AU8KjAlJFCjkzvnECh9d"
_score: 1
_source: { ...

What's missing?

Thank you for any support!