ἐστῶτα [estōta] , perfect tense participle, while having stood. The angel is still standing on the sea and land when he raises his hand to swear.
raised up his hand to heaven
Raising the hand was a common practice when taking an oath (Gen. Gen. 14:22; Deu. Deu. 32:40; Eze. Eze. 20:5, Eze. 20:15; Eze. 36:7; Eze. 47:14). The MT and NU texts indicate the angel raised his right hand, in which case the book would be held in his left hand. Another powerful angel lifted both hands before Daniel to swear by Him who lives forever (Dan. Dan. 12:7).