Watcher: How to extract @timestamp using transform map?

[kelk (kin)]

In Watcher, i'm trying to extract the @timestamp field from payload

        return [
            'my_group_array': ctx.payload.hits.hits,
            'my_group_fields': ctx.payload.hits.hits.stream()
            .map(t-> {return ['timestamp': t._source.@timestamp, 'eventSource': t._source.eventSource, 'eventName': t._source.eventName, 'recipientAccountId': t._source.recipientAccountId, 'userIdentity': t._source.userIdentity.principalId]})
            .collect(Collectors.toList())
        ]

the @ symbol causes the problem as the above works if I replace a similar eventtime field. Any idea how to extract variable which has @ in it within a watcher transform? I've tried enclosing with double quotes, but still fails

Error in case if anyone have seen this before

[script_exception] compile error, with { script_stack={ 0="... ['timestamp': t._source.@timestamp, 'eventSourc ..." & 1=" ^---- HERE" } & script="\r\n return [\r\n 'my_group_array': ctx.payload.hits.hits,\r\n 'my_group_fields': ctx.payload.hits.hits.stream()\r\n .map(t-> {return ['timestamp': t._source.\"@timestamp\",

Incoming data is

            {
              "_index": "myindex",
              "_type": "_doc",
              "_source": {
                "@timestamp": "2020-10-28T16:02:19.676Z",
                "eventSource": "kms.amazonaws.com",
                "eventName": "blahal",
                "recipientAccountId": "12345",
                "userIdentity": {
                  "principalId": "xxxxxxxxxyyyyyyyy"
                }