Did Oscar-winning actor Russell Crowe really capture photos of a UFO outside his office in Australia, passing over Sydney's Royal Botanic Gardens? Or was it just a sailboat passing by?

In a series of Twitter updates, Crowe — who won the best-actor Oscar for his role in "Gladiator" and recently starred in another Oscar-nominated film, "Les Miserables" — insists that the pictures are real and that they don't show reflections or lens flare. What the YouTube video does show is a series of three timed-exposure photos, with a flat red light moving across the frame.

Crowe said the pictures were taken by a camera (a Canon 5D with no flash, to be precise) that was set up on the balcony of his office in the Sydney suburb of Woolloomooloo to capture pictures of fruit bats rising from the gardens. "This was a big surprise," Crowe wrote.

Some commenters quickly speculated that the UFO was nothing more than reflections from a light, perhaps from a beacon on a sailboat that was passing through nearby Woolloomooloo Bay. But Crowe defended the sighting: "The camera is on a balcony, not behind glass," he told one questioner. "Can't be a lens flare because it moves, camera is fixed," he said in another tweet.

Unless Crowe 'fesses up to a publicity stunt, or accepts one of the alternate explanations offered by skeptics, this sighting is likely to go into a big thick folder of unsolved celebrity UFO files. The conversation also rates a place among Crowe's most entertaining tweets. For what it's worth, here's another one from the Twitter files: "Due to a hangover of massive proportions ... anything I say on Leno tonight needs to be taken with a pinch of salt ... and a slice of lime."

I'll drink to that.

Update for 8 p.m. ET March 6: Facebook friend Tom Jorgenson came up with what seems to be the best explanation for the red light: It's reflected sunlight from a plane passing across the scene near sunset. You can make out what appears to be the outline of the plane's fuselage and tail. The exposure setting may have made the time-lapse pictures look more dramatic. To confirm that hypothesis, you'd have to check the time for the photo-taking session (at sunset) and the orientation of the camera (pointing to reflect the sun's rays into the camera lens). But I think we have a winner. What do you think?

Update for 12:13 a.m. ET March 7: OK, here's a much better explanation. ParaBreakdown's Phil Poling shows why Russell Crowe's UFO is most likely to be a series of long-exposure photos of an Unidentified Floating Object ... which now appears to have been identified. The YouTube video below breaks it down:

More from Cosmic Log's UFO files:

• Exploding UFO looks like weather balloon
• Middle East UFO linked to Russian missile test
• Cosmic Log archive on UFOs

Tip o' the Log to Huffington Post UK for the ParaBreakdown video.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

This story was originally published on Wed Mar 6, 2013 5:52 PM EST