Earlier this year I wrote about the Parkes/Arecibo radio bursts, very old radio signals—probably dating back to the early universe—that originated outside of the Milky Way galaxy, but can’t be identified. Now astronomers at McGill University have published a followup study on the bursts, and—depending on how you look at it—we now know either a little more about them, or a whole lot less.

To begin with, these were discovered over a very small patch of sky. Astronomers estimate that unless we were really lucky about where we pointed our radio telescopes, there are about 10,000 of these radio bursts being sent out at any given time. This assumes they’re a natural phenomenon, of course, but they probably are, given their age; we’re talking about signals being sent from various locations 9 billion years ago, less than 5 billion years after the universe was formed. The odds that intelligent beings could have sent these signals—that a solar system, habitable planet, life, intelligence, and radio technology all came about in those same first five billion years—are slim. Greater than zero, but slim.

So assuming they do represent natural phenomena, what kind of natural phenomena are they? A few working theories from the McGill study:

• They’re just really bright pulsars. It’s not likely, but given that the Arecibo data was collected during a study of pulsars, this is the obvious explanation that needs to be ruled out.
• They’re dying black holes. This is really kind of an exciting possibility, because we don’t really know what the life cycle of a black hole is. Astronomers have a widely-accepted (albeit unproven) theory about how they probably form, but we don’t really know if they age and die, much less how. (And if Lee Smolin is right, every dying black hole is technically an exploding universe.)
• They’re neutron stars having loud sex, essentially: a neutron star merger so bright that we can see it 9 billion years away.
• They’re magnetars, a theoretical subcategory of neutron stars that have already been accused of emitting gamma rays. Since nobody knows what they are or how they work, magnetars can be blamed for a wide range of unexplained phenomena. (“Who farted?” “It wasn’t me—it was a magnetar!”)

I wish I could say “we’ll know more soon,” but the technology to relate these phenomena to non-radio observations doesn’t really exist yet, so we probably won’t know much more soon. This is a mystery that might stick around a while.