At first, the dot looked like any other star. Then the astrophysicists switched to polarized light and found themselves looking at an entirely unexpected sight.
“Everything else disappeared, even the bright central black hole, and only this little dot remained,” Elena Shablovinskaia, an astrophysicist at Universidad Diego Portales in Chile and the Max Planck Institute for Radio Astronomy in Germany, told Gizmodo. Shablovinskaia’s team believes they’ve discovered an entirely new kind of space object, which they’ve dubbed “Punctum.”
Like its name, Latin for “point” or “dot,” Punctum is a compact, bright splotch of light in space—but it’s only visible at millimeter wavelengths and harbors an unusually organized magnetic field. A paper on Punctum has been accepted for publication in Astronomy & Astrophysics and is currently available on arXiv.
The team wanted to show that Punctum is simply an unusual example of a known phenomenon, such as a magnetar or a supernova remnant. NGC 4945, where Punctum lives, is a nearby galaxy known for hosting an active population of baby stars—so it made sense to assume Punctum was one of them. After all, NGC 4945 resides so close to the Milky Way that astrophysicists thought they had a solid grasp of the galaxy. We couldn’t have missed something “hiding in plain sight,” as Shablovinskaia put it…could we?
But the more they tried to force Punctum into established astrophysical wisdom, the more it seemed to defy it. For example, Punctum disappeared when the team checked the same region using X-ray or radio telescopes, appearing only in the millimeter spectrum with the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA’s ability to capture polarized light also revealed Punctum’s weird magnetic field to the scientists, adding to its mystery.
“So we compared its brightness, polarization, and spectrum to every extreme object we could think of—magnetars, pulsars, star-forming regions, and black hole jets,” Shablovinskaia recalled. “Nothing matched.”
But accepting that they’d struck something entirely new only raised more questions. What exactly is it? Where does Punctum—10,000 to 100,000 times brighter than magnetars and 10 to 100 times brighter than most supernovas—get its light? Could it be connected to a black hole or a neutron star?
For Shablovinskaia, the best shot at finding the answer might be to investigate Punctum’s strange polarization, “basically a fingerprint of the magnetic environment,” she said. After all, the disturbingly neat alignment of the light waves around Punctum was what alerted the team to the object’s weirdness.
“Usually, when light is produced in cosmic environments, it comes out mixed and disordered because the magnetic fields are tangled,” she explained. “If we can measure [Punctum’s magnetic field] at more wavelengths or watch how it changes over time, we can start to figure out what powers Punctum and whether it links to known astrophysical objects.”
“Punctum shows us the universe can still surprise us in places we thought we understood well,” Shablovinskaia said. “For me, it’s a reminder that astronomy is far from finished; we’re still just beginning to discover the full variety of cosmic objects out there.”
The new research marks another strong example of multi-messenger astronomy, in which astronomers use different kinds of signals to investigate a single source. Recently, astronomers found an ultramassive black hole using gravitational lensing. In this case, the scientists studied a specific behavior of light—polarization—alongside more conventional methods to double-check their results.
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