What does the universe do when it violates its own rules? Last July, astronomers saw a gamma ray event so bizarre that they have left the astrophysics world grasping for answers. Outside our galaxy, the blast was not only incredibly intense but also continued in bursts for more than 24 hours an astonishing departure from the briefness of previously known gamma ray bursts, which last either in milliseconds or, if that’s too long, a few minutes.
Gamma-ray bursts (GRBs) are some of the most violent phenomena in the universe, releasing more energy in seconds than the Sun will emit over its entire 10-billion-year lifetime. They exist in two general categories: short bursts, which persist for less than two seconds and are often linked with the collision of neutron stars or a neutron star-black hole, and long bursts, which can persist for as long as hundreds of seconds and are linked with supernova explosions of massive stars. In both cases, the emission is typically an individual, non-repeating event. The new eruption, though, disrupted that routine, establishing repeated pulses in one day a first in the eyes of astronomers. The result would have been impossible without a harmony of observation among ground-based and space-based observatories, such as the Hubble Space Telescope.
Hubble’s ability to observe in visible, ultraviolet, and near-infrared light above the atmosphere that absorbs the majority of the infrared spectrum makes it an invaluable tool for the study of the consequences of GRBs. Through comparisons between light curves at various wavelengths, researchers are able to identify whether the source contains a kilonova, where heavy elements like platinum and gold scatters visible light but allows infrared to pass through, or a standard stellar collapse. The physics of GRBs involves the formation of highly collimated jets of matter and radiation that are launched at nearly the speed of light from the site of the cataclysm.
In instances like the GRB 221009A record-breaker, such jets also demonstrate new structures, such as a long, highly energetic center with wider, angled edges. Brendan O’Connor, an astronomer at George Washington University, described that event as “so much brighter and more energetic than any gamma-ray burst we’ve seen before, it’s not even close.” In that case, the jet’s energy varied with distance from its core a feature never before observed in a long GRB jet suggesting that the progenitor star’s size, density, or magnetic field may have shaped the outflow. For the July event, no such jet structure has been worked out yet, but the periodicity of the bursts permits the possibility of a totally different mechanism.
If the source is neither a collapsing star nor a normal kilonova, it might be an exotic compact object or a yet unseen process in high-energy astrophysics. The repeated emissions would imply a process capable of sustaining jet activity over long timescales, or perhaps a number of individual discrete explosions from the same source. Determining the origin will require more observations. The European-led team announcing the findings in the Astrophysical Journal Letters is working to nail down the location of the event and to search for any lingering afterglow across the electromagnetic spectrum.
Space-based X-ray telescopes such as NASA’s NuSTAR and ESA’s XMM-Newton could play a vital role in monitoring the manner in which the released matter interacts with the interstellar medium, producing secondary radiation that contains information on the energy and environment of the burst. For now, the phenomenon remains a “cosmic whodunit,” in one researcher’s words a reminder that even in the era of precision astrophysics, the universe still possesses the power to surprise with phenomena beyond the capabilities of our present models and requiring new physics to describe them.
