Astronomers have spotted a remarkable gamma-ray burst (GRB) that's a real Incredible Hulk compared to other cosmic explosions of the same type.
What makes this GRB so special is the fact that the central engine powering it remained active for around 27 days, flaring in X-rays following the initial blast. That's 20 days longer than the longest previous GRB on record.
This record-breaking burst — which is designated GRB 220706A and was spotted on July 6, 2022 — is even more exciting because scientists don't know what the central engine powering it actually is.
"In a nutshell, GRB 220706A is an extremely late X-ray flare. X-ray flaring is common following GRBs, but previously it has always come within the first few days after we detect the burst," study team member Benjamin Gompertz, of the University of Birmingham in England, told Space.com.
"The fact that we see flaring at around 27 days in the GRB’s rest frame tells us that there’s an ongoing active process, and that’s unprecedented in the known population," Gompertz added. "The previous record for delayed flaring was about six days after the GRB."
What created the Incredible Hulk of gamma-ray bursts?
GRBs are some of the most powerful explosions in the cosmos. They have been linked to the collapse of a massive star, which also triggers a supernova and creates a black hole. This results in a long-duration GRB, while short-duration GRBs are thought to be triggered when two stellar remnants called neutron stars collide.
GRB 220706A is an example of an ultra-long GRB, a rare cosmic blast of gamma rays in which the engine powering the GRB stays active for longer. They are incredibly rare, with their cause still unknown.
"We don’t know what the central engine driving this outburst is, and that’s what makes this such a compelling event," Gompertz said. "The fact that we see a supernova tells us that a star has collapsed, which is the standard power source for GRBs, but it doesn’t immediately account for the extremely late flaring or unusually bright supernova. We can interpret that through previous events to some degree."
Gompertz added that another ultra-long GRB detected in 2011, GRB 111209A, was associated with an unusually bright supernova (SN2011k) that had clear evidence for a central engine. The team behind that research determined that GRB 111209A is powered by a magnetar, a neutron star with a particular powerful magnetic field. In this case, the magnetic field is rotating at around 1,000 times per second.
"Our supernova looks a lot like that one, so a magnetar might be present here. That ties in nicely with super-luminous supernovas, which are also thought to be magnetar-driven," Gompertz said. "Another possibility could be that the GRB jet and the supernova are colliding with shells of dense material that were ejected towards the end of the parent star’s life, as it became unstable and began ejecting its outer layers. This sort of event is also seen in the super-luminous supernova population."
The team isn't quite sure how the central engine of this ultra-long GRB differs from that of others and why that resulted in 27 days of flaring.
"One possibility is that it’s a much bigger star than we usually see producing GRBs," Gompertz said. "A bigger star would take longer to fall in on itself and so provide a longer-lived source of energy to power the jet."
GRB sources may be members of a stellar population of massive, compact stars called Wolf-Rayet stars. These are 10 times wider that the sun, with around 10 times the mass of our star. However, even the collapse of such a star can't explain GRB 220706A.
"To power GRB 220706A, we estimated we need a star more like 1,000 times the radius of the sun, which would require a population of stars known as blue supergiants," Gompertz said. "On the other hand, it may be the remnant that’s different. If this star produced a magnetar, it may be that 'normal' GRBs just collapse to black holes."
The team thinks that the big question with GRBs like this one is whether they are members of the normal GRB population or a different kind of event. Having this second event, in addition to GRB 111209A, with an unusually bright supernova potentially adds weight to the idea that these are different stars. But the problem is not yet solved.
"The incredibly late flares also strain our models for how jets are powered," Gompertz said. "It will be fascinating to see how future theoretical work can incorporate a power source from a collapsing star that can last for a month!"
There could be even longer-lasting GRBs out in the cosmos that we have thus far failed to detect. Gompertz explained why such detections are tricky.
"GRBs this long are hard to detect because most of the satellites we use to find them are in low Earth orbit, which means the target is periodically blocked every 90 minutes or so," he explained. "That's fine for your typical 10- to 100-second-long GRB but becomes a nuisance when tracking much longer events. Long-lasting GRBs also seem to be more active in the X-rays rather than the gamma rays."
The University of Birmingham researcher added that newly launched missions like the Einstein Probe, which is more sensitive in X-rays than gamma rays, may be the perfect instruments to find more ultra-long GRBs.
"With any luck, we’ll get a few more of these to chase so that we can start trying to understand the incredible power sources that drive them," Gompertz concluded.
A pre-peer-reviewed version of the team's research is featured on the repository site arXiv.
