2023-07-19 23:44:47
Enlarge/ Most explanations for this phenomenon involve a neutron star, pictured above. These explanations are uniformly awesome. On Wednesday, researchers announced the discovery of a new astronomical mystery. The new object, GPM J1839–10, behaves like a pulsar, sending out regular bursts of radio energy. But the physics that drive pulsars means that they will stop emitting if they slow down too much, and almost all pulsars we know flicker 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 can power this. Persistent Transient GPM J1839–10 was detected while searching for transient objects in the galactic plane – something that isn’t there when you first look, but appears the next time you look. The typical explanation for a transient object is something like a supernova, where a massive event gives something an exponential increase in brightness. They are found at the high end of the radio spectrum, fast radio bursts, but they are also very short and therefore difficult to detect. In any event, GPM J1839–10 turned up in the search in a somewhat unusual way: it appeared in passing twice during the same night of observation. Instead of delivering a short burst of massive energy, like a fast radio burst, GPM J1839–10 was much lower in energy and took a 30-second blast. Tracking observations showed that the object looped regularly, with a periodicity of about 1,320 seconds (more commonly known as 22 minutes). There is a roughly 400-second window centered around this periodic period, and an explosion can appear anywhere within the window and last anywhere from 30 to 300 seconds. When active, GPM J1839–10’s intensity can vary, with many sub-bursts in the main signal. Occasionally, a window will also pass without any gusts. A search of the archival data showed that signals had been detected at the site as early as 1988. So anything that produces this signal is not truly transient, in the sense that the phenomenon that produces these bursts is not a one-time event. The list of known objects that can produce this type of behavior is short and consists of precisely zero items. Good for Nothing The most visible isotope 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 be aligned with their axis of rotation. As a result, the star’s rotation can sweep the poles through line of sight to Earth, creating the perception of a burst of radio waves when one of the magnetic poles aligns with Earth. But the pulsating flashes recur quickly, with an interval between them ranging from a minute to a few milliseconds. More importantly, physics dictates the gap to be fast. 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 darkens, which is why we don’t see any that take longer 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 a dense magnetic field that is prone to energetic explosions. But these explosions also generate more energetic photons, and the researchers examined GPM J1839-10’s site with an X-ray telescope and saw nothing. Also, magnetars are thought to be spinning faster than the 22-minute gap indicates, so they’re likely out as well. Another alternative is a white dwarf with an exceptionally strong magnetic field. They are much larger objects and therefore take much longer than the rotation of a neutron star. But we’ve seen thousands of them in the Milky Way and nothing like it. Only one has periodic emissions, and it produces much less energy than GPM J1839–10. Although we have expanded the list of possible sources to include other organisms that we do not understand, we are still falling short. The same team had identified a slow transient radio transmitter, GLEAM-X J162759.5-523504.3, a few years earlier. But it was only active for two months before disappearing, which is a far cry from the 25-year-old GPM J1839–10 eruption. And now? So, given that all possible explanations are terrible, where do we go from here? The good news is that these things will be so hard to spot that there may be a lot that we overlooked. The bad news is that they can still be hard to spot. The duration of the burst – up to 300 seconds – and the gap between bursts means that short-term views are likely to see something there the whole time or miss it entirely. We would really need the material to stare at one area of space for half an hour or more, its view split into multiple exposures, to make sure it was captured in both its in and out state. 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 of interest in other wavelengths. Since it was located on the galactic plane, that would also be difficult. Nature, 2023. DOI: 10.1038/s41586-023-06202-5 (about DOIs).
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