By combing through a collection of data, researchers may have discovered evidence that we've already observed the first "blitzar," a bizarre astronomical event caused by the sudden collapse of an overly massive neutron star. The event is driven by an earlier merger of two neutron stars; this creates an unstable intermediate neutron star, which is kept from collapsing immediately by its rapid spin. In a blitzar, the strong magnetic fields of the neutron star slow down its spin, causing it to collapse into a black hole several hours after the merger.
That collapse suddenly deletes the dynamo powering the magnetic fields, releasing their energy in the form of a fast radio burst. The researchers who performed the analysis suggest that this phenomenon could explain the non-repeating forms of these events.
Too big to live
How big can a neutron star get before it collapses into a black hole? We don't have a good answer, in part because we're not sure what happens to the bizarre forms of matter inside one of these massive objects. We don't even know if the neutrons that give the star its name survive or fall apart into their component quarks. It's one of those annoying questions where the answer includes the phrase "it depends."
The big thing it depends on is how fast the neutron star is spinning. A fast enough spin can counteract the pull of gravity on the neutron star's outer layers, keeping something that's too heavy to survive around for a bit. If the spin slows down, the whole thing will be rapidly crunched into a singularity. The simplest way to slow one of these stars down is through its magnetic field, which will interact with charged particles in the environment, creating a drag on the object's spin.
These are the conditions that take a neutron star merger and create a blitzar. If neutron stars are heavy enough, their merger will create an object that's above the mass limit that should cause it to collapse into a black hole. But the collision is also likely to set the object spinning fast enough that it cannot collapse. Their churning, superfluid interiors can also host a dynamo that supports an intense magnetic field, potentially making the object a magnetar but definitely slowing its spin. The dynamics of this balance are such that the blitzar should occur within hours of the neutron star merger.
Once the collapse happens, the dynamo that created the magnetic fields vanishes along with the rest of the neutron star. There's a lot of energy wrapped up in that field, and the loss of the neutron star releases it in a process that the new paper refers to as "shedding the magnetosphere." That burst of energy is something we can potentially detect.
Waves and bursts
Or make that "potentially have already detected." For a while now, we've been detecting bursts of energy that originate from an area with intense magnetic fields. These fast radio bursts (FRBs) sometimes repeat, and they have been associated with intensely magnetic neutron stars called magnetars. But there have also been a number of FRBs that don't seem to repeat at all, suggesting that the conditions that produced them may destroy their source. That is consistent with a blitzar.
For the new work, a team of researchers took advantage of data released by two types of observatories. One is the LIGO/VIRGO gravitational wave detection collaboration, which can identify the gravitational signals produced when massive objects, including neutron stars, merge. The second is the Canadian Hydrogen Intensity Mapping Experiment, an observatory designed for something else that turned out to be exceptionally good at locating FRBs.
The search was relatively simple: The researchers looked for events picked up by both the observatories that occurred at roughly the same time and in the same region of the sky, with the FRB coming less than a day after the gravitational waves. Out of 21 neutron star mergers detected by gravitational waves, a grand total of one was matched with an FRB, with FRB 20190425A coming about 2.5 hours after GW190425.
Until we get more detectors online, gravitational wave events can only be localized to within a broad stripe of the sky, so all we can say for certain is that the FRB occurred within an area that had a 70 percent chance of being where the neutron star merger occurred. But it was also the right distance, and we don't detect a lot of neutron star mergers. As a result, the researchers estimate the probability of this co-localization occurring by chance as being only 0.004.
Limits on the blitz
If this research is correct, the events we've been lumping together as FRBs are actually the product of two different events. The repeating events occur in the environment around a magnetar. The one-shot events are triggered by the death of a highly magnetized neutron star within a few hours of its formation.
If FRB 20190425A4/GW190425 does represent a blitzar, we already have a decent idea of what the physics of the event should look like, as astrophysics has spent a fair amount of computer time modeling them. As a result, even this single event can put some limits on the processes involved. The gravitational wave detectors indicate that the pre-merger neutron stars likely weighed 1.35 and 2.0 times the mass of the Sun, with the post-merger object being a bit over 3.2 solar masses. Based on that information, the researchers constrained the maximum neutron star mass at somewhere between 2.6 and 3.0 times the mass of the Sun. Any bigger than that and it collapses.
The more of these events we see, the better constraints we'll have on the physics of these objects. Because both of these events are highly unpredictable, we've put together a number of observatories that constantly gather data to ensure we catch one when it happens. So if blitzars are regular occurrences, it shouldn't take long for us to see more of them.
Nature Astronomy, 2023. DOI: 10.1038/s41550-023-01917-x (About DOIs).
