On Wednesday, researchers announced the discovery of a new astronomical mystery. The new object, GPM J1839–10, behaves a bit like a pulsar, sending out regular bursts of radio energy. But the physics that drive pulsars means they will stop emitting if they slow down too much, and nearly every pulsar we know of flickers at least once a minute.
GPM J1839–10 takes 21 minutes between pulses. We have no idea what kind of physics or what kind of thing that can power that up.
GPM J1839–10 was discovered while searching the galactic plane for transient objects — something that isn’t there when you first look, but appears the next time you check. The typical explanation for a transient object is something like a supernova, where a massive event gives something a huge boost in brightness. They are found at the very end of the radio spectrum, fast radio bursts, but they are also very short, and somewhat difficult to spot.
In any case, GPM J1839–10 turned up in the search in a somewhat unusual way: it showed up as a transient element twice on the same night of observation. Instead of delivering a short burst of massive energy, like a fast radio burst, GPM J1839-10 had much lower energy and was spread out over 30 seconds.
Subsequent observations showed that the object looped regularly, with a periodic rate of about 1,320 seconds (more commonly known as 22 minutes). There is a window of about 400 seconds centered around that periodic period, and an explosion can appear anywhere within the window and will last anywhere from 30 to 300 seconds. During activity, the intensity of GPM J1839–10 can vary, with many sub-bursts present within the main signal. Occasionally a window would also pass without any bursts.
Research through archival data has shown that signals were detected at the site as far back as 1988. So, whatever results from this signal is not really transient, in the sense that the phenomenon that produces these bursts is not a one-time event – it just happened.
The list of known objects that can produce this type of behavior is short and consists of precisely zero items.
Not fit for anything
The most obvious analogue of GPM J1839-10 is a pulsar, a rapidly spinning magnetized neutron star. These objects release radio energy at their magnetic poles, which may not line up with their axis of rotation. As a result, the star’s rotation can sweep the poles through line of sight to Earth, creating the visualization of a flash of radio waves every time one of the magnetic poles aligns with Earth.
But pulsar flashes recur quickly, with a gap between them anywhere from about a minute to milliseconds. More importantly, physics dictates the gap he have to be quick. The magnetic field that powers the radio waves is generated by the star’s rotation. If it starts spinning too slowly, the magnetic field will drop to a point where it can no longer generate significant radio emissions. In other words, if it slows down, it gets dark, which is why we don’t see any of them taking more than a minute between pulses.
But this does not exclude neutron stars. Another option that includes them is a magnetar, which is a neutron star with an intense magnetic field that is prone to energetic explosions. But those explosions also generate more energetic photons, and the researchers examined GPM J1839-10’s site with an X-ray telescope and saw nothing. Additionally, magnetars are thought to be spinning faster than the 22-minute gap indicates, so they’re likely out there, too.
Another alternative is a white dwarf with an unusually strong magnetic field. These are much larger objects, and therefore take much longer than a neutron star to rotate. But we’ve observed thousands of them inside the Milky Way and we’ve never seen anything like this. Only one has periodic emissions, and it produces much less energy than GPM J1839–10.
Even if we expand the list of possible sources to include other organisms that we don’t understand, we still fall short. The same team had identified a slow transient radio transmitter, GLEAM-X J162759.5-523504.3, a few years earlier. But it remained active for about two months before fading from view — a far cry from the 25 years in which GPM J1839–10 exploded.
So, given that every possible explanation is shocking, where do we go from here? The good news is that these things will be so hard to spot that there could be a lot we’ve overlooked. The bad news is that they can still be hard to spot. The length of the splash—up to 300 seconds—and the gap between bursts means that short-tempo notes are likely to either see something there the whole time or miss it entirely.
We really need to have the devices stare at one area of space for half an hour or more, and have the stares broken up into multiple exposures, to make sure we’re capturing it both on and off. This involves a significant commitment to the hardware.
In the meantime, we can narrow down GPM J1839-10’s location to try to see if there is anything interesting out there at other wavelengths. Since this is within the galactic plane, that would also be a challenge.
Nature, 2023. DOI: 10.1038 / s41586-023-06202-5 (about DOIs).
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