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What could Punctum be?

Its brightness remained the same over several observations performed in 2023, meaning it is not a flare or some other kind of transitory phenomenon. Millimeter-wave radiation typically comes from cold objects such as young protoplanetary disks and interstellar molecular clouds. However, very energetic phenomena such as quasars and pulsars can also produce radio waves through synchrotron radiation, wherein charged particles moving at close to the speed of light spiral around magnetic field lines and radiate radio waves.

What we do know about Punctum is that based on how strongly polarized its millimeter light is, it must possess a highly structured magnetic field. And so, Shablovinskaia believes what we are seeing from Punctum is synchrotron radiation. Objects with strong polarization tend to be compact objects, because larger objects have messy magnetic fields that wash out any polarization.

Perhaps that synchrotron radiation is being powered by a magnetar, the team believes, which is a highly magnetic pulsar. However, while a magnetar’s ordered magnetic field fits the bill, magnetars (and regular pulsars for that matter) are much fainter at millimeter wavelengths than Punctum is.

Supernova remnants such as the Crab Nebula, which is the messy innards blasted into space of a star that exploded in 1054AD, are bright at millimeter wavelengths. The trouble is that supernova remnants are quite large — the Crab Nebula itself is about 11 light-years across — whereas Punctum is clearly a much smaller, compact object.

“At the minute, Punctum really differs– it does not fit easily into any recognized classification,” said Shablovinskaia. “And truthfully, absolutely nothing like this has actually appeared in any previous millimeter studies, mostly since, till just recently, we didn’t have anything as delicate and high-resolution as ALMA.”

There is the caveat that Punctum could just be an outlier: an extreme version of an otherwise familiar object, such as a magnetar in an unusual environment, or a supernova remnant interacting with dense material. For now, though, these are just guesses lacking supporting evidence. It is quite possible that Punctum is indeed the first of a new kind of astrophysical object that we haven’t seen before simply because only ALMA can detect them.

In the case of Punctum, it is 100 times fainter than NGC 4945’s active nucleus that is being energized by a supermassive black hole feeding on infalling matter. Punctum probably wouldn’t have been noticed at all in the ALMA data if it wasn’t for its exceptionally strong polarization.

Further observations with ALMA will certainly help shed more light on what kind of object Punctum is. The observations that discovered Punctum were actually focused on NGC 4945’s bright active core; it was just happenstance that Punctum was noticed in the field of view. Future ALMA observations targeting Punctum instead would be able to go to much lower noise levels without worrying about the galaxy’s bright core being over-exposed, and it could also be observed across different frequencies.

The greatest help could potentially come from the JWST. If it can see an infrared counterpart, then its greater resolution could help identify what Punctum is.

“JWST’s sharp resolution and broad spectral variety may assist expose whether Punctum’s emission is simply synchrotron or includes dust or emission lines,” said Shablovinskaia.

For now, it’s all ifs and buts, and all we can say for sure is that astronomers have a genuine mystery on their hands that has so far left them feeling flummoxed.

“In any case,” concluded Shablovinskaia, “Punctum is revealing us that there is still a lot to find in the millimeter sky.”

A paper explaining the discovery of Punctum has actually been accepted by the journal Astronomy & & Astrophysics, and a pre-print is readily available on astro.ph.

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