Milky Way from Segara Anak, Indonesia. Image: Abdul Azis via Getty Images
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Something near the center of our galaxy, the Milky Way, is spitting out bizarre radio signals that have baffled scientists, a new study reports.
The unidentified radio source blinks for weeks at a time only to suddenly turn off, a pattern that doesn’t line up any known space objects. The discovery is just the latest oddity that has been revealed by increasingly sensitive radio telescopes, which have exposed subtleties on puzzles rapid radio bursts and identified huge formations called Odd Radio Circles.
Scientists led by Ziteng Wang, a doctoral student in astronomy at the University of Sydney in Australia, first spotted the strange object in observations captured by the Australian Square Kilometer Array Pathfinder (ASKAP), a sophisticated radio network located in Western Australia.
The mysterious source, known as ASKAP J173608.2-321635, “may represent part of a new class of objects discovered through radio imaging readings,” according to a study to appear in The Journal of Astrophysics, that was posted to the arXiv preprint server.
“The strangest property of this source is that it is highly polarized,” Wang said in an email. “Our eye cannot distinguish circularly polarized light from unpolarized light, but ASKAP has the equivalent of polarized sunglasses to filter it out. This kind of source is really rare.
“To add to the mystery, the source of the radio signals turns on and off irregularly,” he continued. “The brightness of this source can change dramatically, fading in a single day, but sometimes it can last for a few weeks. “
The team saw the source ignite six times in ASKAP data from January to September 2020, with signal strength varying by a factor of 100. To better read the puzzling object, the researchers asked for observation time on a wide range of powerful telescopes currently in use. They were able to spot it last February with the MeerKAT telescope from South Africa and again in April with the Australian Telescope Compact Array.
Wang and his colleagues also looked for counterparts of radio signals in other wavelengths, such as infrared light or x-rays, using two NASA space telescopes: the Neil Gehrels Swift Observatory and the Chandra X-ray observatory. Interestingly, no sign of ASKAP J173608.2−321635 has appeared in these bands of the electromagnetic spectrum, further compounding the dilemma of its source.
“As the source was discovered using radio telescopes and was not seen at other wavelengths, further follow-up observations at other wavelengths would help us reveal the nature of the source, ”Wang said. “For example, a higher resolution observation would help us locate the source more precisely and could potentially help us find the counterpart in optical wavelengths.”
The team considered many possible origins for radio signals, but each explanation has its own drawbacks.
Some low mass stars have been observed spitting out radio polarized flares, but unlike ASKAP J173608.2−321635, they flash at other wavelengths. The source signals are also reminiscent of those emitted by pulsars or magnetars, which rapidly spin dead stars. However, these objects normally produce some sort of steady rhythm, while the new source may blink for weeks, turn off in a day, and appear absent for months.
“We thought this source could be a pulsar (a dead star) or a flaming star based on what we saw in the VAST data,” Wang said, referring to ASKAP’s VAST survey. “But other observations have shown that these two scenarios cannot explain the behavior of the source.”
Another tempting possibility is that ASKAP J173608.2-321635 is an unidentified object type called Galactic Center Radio Transient (GCRT). Scientists first spotted these short-lived radio events 20 years ago, and a handful of them have been identified near the galactic center since then.
The ASKAP J173608.2−321635 and GCRTs both emit highly polarized radio light with no x-ray counterpart, but the new source also differs from known GCRTs in the patterns and timescales of its signals. It is not clear either that the GCRT even share a common origin, which makes difficult the correspondence between ASKAP J173608.2−321635 and these little understood phenomena.
“We haven’t seen how the source comes on – if we get a chance to see this, we may be able to find out more about this source,” Wang noted.
Ultimately, no known object fully explains the strange signals. Future observations could shed light on this mysterious object at the center of the Milky Way – and possibly find more objects like this – which “may help us better understand extreme astrophysical phenomena,” according to the study. .
“If this source is an example of a previously unknown class of objects, it would be interesting to study these types of sources to better understand their origin,” Wang concluded. “Maybe we could use this type of source as a clue to research something exciting, like the expansion of the universe or the fate of the stars.”