Upon realizing it is a sarcophagus, he decides to sacrifice himself (I think by disconnecting his air) and exist in it for eternity along with the original creature. It may have been in an old Omni magazine, or some other periodical. I thought it may have been in the anthology "Dead Astronauts" but I did not find the story in there.
Short story about an astronaut that enters an alien space ship and discovers it is a sarcophagus
1Sounds vaguely like something Lovecraft would write, but sounds much newer– Premier BromanovDec 23, 2014 at 20:28
duplicate of scifi.stackexchange.com/questions/38659/… (not accepted but confirmed by OP comment)– OtisSep 29, 2015 at 17:32
Ben Bova's Voyagers (1981), the first book in a four-book series. An alien ship is detected coming into the Solar System. The only astronaut able to make physical contact with the ship does so as a one-way journey; he will die inside the alien ship.
The astronaut revives because the alien sarcophagus is covered in alien nanotech robots. Nanobots infect the astronaut, bringing him back from the dead. The astronaut is recovered by other astronauts and sent back to Earth. The nanobots become the focus of the next two (three?) books, because the alien nanobots include the dead alien's consciousness, hence the titles The Alien Within (1986) and Star Brothers (1990). The nanobots give regeneration, healing, different powers, but all at a price - the alien judges each person's life up to the point it is infected into, deeming them worthy or not of the nanotech gifts. Several Earth mega-corporations try to strip the alien consciousness from the bots, to no avail. The alien nanobots are trying to educate humans into accepting aliens.
Well spotted. Since the OP was looking for a short story that he read in Omni, perhaps you should add a link to the original short story (also titled "Voyagers") by Ben Bova in the August 1981 Omni which led to that series of books, and which can be read here. Dec 24, 2014 at 1:11
This is the short story as I originally read it. I was not aware of the book series. Checking it as the correct answer. I'll let the moderator decide who gets the points. Dec 24, 2014 at 1:55
Since I was answering according to the novel, and not the original magazine short story, I don't mind giving credit to user14111 for the correct answer.– jhpace1Dec 24, 2014 at 2:38
I do want to add that it seems like it took comments from both jhpace1 and user14111 to solve this one. It appears that user14111 identified the short story after seeing the comment from jhpace1. Thanks again to both of you. Dec 25, 2014 at 2:41