A radio signal has been detected from near the center of the galaxy that brightens and disappears unpredictably. Astronomers at first thought it was an unusual pulsar, and still consider this the most likely explanation, but only because they have nothing else. Nevertheless, the investigators are fairly sure it's not aliens.
The universe contains many strange, unexplained objects. However, you know something is really odd when astronomers are reduced to saying it is a bit like a class of other objects they can't explain, but so unusual it may not even belong with them.
That is the situation for ASKAP J173608.2-321635, whose existence and behavior have been reported in the Astrophysical Journal. Finding a physical explanation could open new doors in physics.
The Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope is a new instrument capable of imaging large areas of the sky in considerable detail. Naturally the center of the Milky Way was among the first targets, with multiple images taken months apart. University of Sydney PhD student Ziteng Wang was one of those tasked with seeking changes and he hit the jackpot, finding a radio signal that was bright in one image, but lacked a counterpart in earlier versions.
“The strangest property of this new signal is that it is has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time,” Wang said in a statement. Non-astronomers might consider other features even stranger. “The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it,” Wang added.
ASKAP J173608.2-321635 was spotted six times over nine months, staying “on” for periods ranging from a single day to weeks. Neither optical telescopes nor the Parkes Radio Telescope could find anything at the location, but South Africa's MeerKat observatory could. Wang's supervisor, Professor Tara Muphy, told IFLScience Parkes may have looked at the wrong time,
From our viewpoint, ASKAP J173608.2-321635 lies 4 degrees from the galactic center. This means, “It is too far away to be related to supermassive black hole, but close enough to be in a high density region of stars, where many stars are dying, making it a good place to look for unusual objects,” Murphy told IFLScience. Since we do not know ASKAP J173608.2-321635's distance, its alignment with the galactic center could be a coincidence – it might be much closer or further away, but Murphy considers both unlikely.
“The only things have found like it all are also within 6 degrees of the galactic center,” Murphy said. These are the so-called Galactic Center Radio Transits (GCRTs), several of which are known to also turn on and polarized light signals erratically. However, ASKAP J173608.2-321635 is sufficiently different from the other GCRTs that it may belong in a new class, but GCRTs' cause is similarly mysterious.
Predictably, Murphy says people have asked if the team have stumbled on an alien civilization's radio beacon. However, she says the broad emission band is unlike anything humans have made, or what we would expect from aliens.
“The most likely thing is that it is some kind of obscure neutron star with properties that mean it is evading our detection much of the time,” Murphy added, but she can't explain how it does that.
Eventually Murphy hopes the enormous power of the SKA telescope will help us solve this mystery (like so many others). In the meantime, she told IFLScience there will be regular checks to see what ASKAP J173608.2-321635 is doing, hoping to view it in many wavelengths at once. Meanwhile, ASKAP will survey a much wider part of the sky in the hope of finding enough similar objects that statistical analysis becomes possible.