Astronomers have detected a strange radio source coming from somewhere near
the center of the Milky Way galaxy. The signal repeats seemingly at random,
and can’t be neatly attributed to any known astronomical object, leading the
team to consider that it might be something brand new.
The signal was first picked up in data dating back to April 2019, collected
by the Australian Square Kilometre Array Pathfinder (ASKAP) Variables and
Slow Transients (VAST) survey. This huge telescope scans the skies for radio
sources that come and go, which may be associated with objects and phenomena
like pulsars, magnetars, supernovae, gamma ray bursts, and stellar flares.
But this new detection didn’t quite match any of those. Designated ASKAP
J173608.2-321635 and hailing from the direction of the galactic center, the
signal flickers on and off at apparently random intervals. ASKAP picked it
up 17 times in under two years, and whatever it is, it has a bizarre mix of
features.
“The strangest property of this source is that it is highly polarized,”
Ziteng Wang, lead author of the study, tells New Atlas. “Our eye cannot
distinguish between circularly polarized light and unpolarized light, but
ASKAP has the equivalent of polarized sunglasses to filter it out. These
kinds of sources are really rare, usually we only find 10 out of thousands
of sources polarized in one observation.
“Adding to the mystery, the source of the radio signals turns on and off
irregularly. The brightness of this source can change dramatically,
declining in a single day, but sometimes it can last for a few weeks.”
The team followed up the initial observations by checking for the source
using the Parkes telescope in Australia in April and July 2020, but it was
nowhere to be seen. In November the astronomers started searching for it
using MeerKAT in South Africa, making regular observations every few weeks –
and on February 7, 2021, it finally returned. It was also picked up in April
using the Australia Telescope Compact Array (ATCA).
But perhaps most intriguing was when the source wasn’t detected. The team
checked archival data of several radio surveys, including the Very Large
Array (VLA) and ATCA, among others, but nothing had ever been seen in that
spot before April 2019. Likewise, nothing showed up there in near-infrared
or X-ray data, which would be expected to accompany radio for some known
objects.
So what could this thing actually be? It has a lot of crossover with many
types of known objects, but it doesn’t fit any one profile perfectly.
Could it be flares from a star? The radio signals switching on and off are
star-like, as is the polarized signal. But, Wang tells us, this source’s
radio signal is far too bright to be stellar flares, and they would be
visible in infrared as well.
How about a pulsar? These dense objects form after a massive star collapses,
and as they rapidly spin they send jets of electromagnetic signals,
including radio, washing over Earth periodically like the beams of a
lighthouse. A pulsar could account for this signal’s intermittent nature,
polarization, and varying brightness – but pulsars tend to flash on a
predictable timeframe of seconds or milliseconds. The new source, however,
is random and can stay “on” for weeks at a time.
A magnetar, then? These are pulsars with extremely strong magnetic fields,
and their radio signals can be a bit more haphazard in their timing. They’re
also the prime suspect in another cosmic mystery, fast radio bursts (FRBs).
However, magnetars are known to give off X-ray emissions when active, which
weren’t detected here.
The closest match might be enigmatic signals currently referred to as
Galactic Center Radio Transients (GCRTs). As the name suggests, these are
short-lived flashing radio signals that originate from near the center of
the Milky Way – all hallmarks of the new detection. But again, it’s not that
straightforward.
“GCRTs are still a mystery,” Wang tells us. “They turn on and off
irregularly, they are highly polarized, and there is nothing in X-ray or
optical. As the source is close to the Galactic Center, this source could be
a new GCRT. However, the timescale of the burst from our source is not
consistent with that for GCRTs. [And] they are discovered in lower
frequencies. But we don’t even know if all GCRTs share a common origin, it
is hard to say.”
The team says that this source is the only one of its kind discovered so
far, which could lead to a brand new class of astronomical objects. Wang
speculates that it could be a pulsar with an ultra-long rotation period, but
that would make it very different from any known pulsar. The best fit seems
to be a GCRT, but that may just be because we don’t know much about them
either.
As with any good mystery, more observations are needed. The team says that
future work will include searching for similar signals away from the
Galactic Center, to see if its location is related to its nature or if
that’s just a coincidence.
The research has been accepted for publication in the Astrophysical Journal,
but has yet to go through the peer review process. The study is currently
available on the
ArXiv preprint
server.
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