A team of scientists has used multiple space and ground-based telescopes, including NASA’s James Webb Space Telescope, NASA’s Fermi Gamma-ray Space Telescope, and NASA’s Neil Gehrels Swift Observatory, to observe an exceptionally bright gamma-ray burst, GRB 230307A, and identify the neutron star merger that generated an explosion that created the burst. Webb also helped scientists detect the chemical element tellurium in the explosion’s aftermath.
This graphic presentation compares the spectral data of GRB 230307A’s kilonova as observed by the James Webb Space Telescope and a kilonova model. Both show a distinct peak in the spectrum region associated with tellurium, with the area shaded in red. The detection of tellurium, which is rarer than platinum on Earth, marks Webb’s first direct look at an individual heavy element from a kilonova. Image credit: NASA, ESA, CSA, Joseph Olmsted (STSCI)
Other elements near tellurium on the periodic table—like iodine, which is needed for much of life on Earth—are also likely to be present among the kilonova’s ejected material. A kilonova is an explosion produced by a neutron star merging with either a black hole or with another neutron star.
“We only know of a handful of kilonovas with any certainty, and this is only the second one for which we have such detailed spectral information” said Tanmoy Laskar, assistant professor at the University of Utah, who is a co-author of the study.
“Just over 150 years since Dmitri Mendeleev wrote down the periodic table of elements, we are now finally in the position to start filling in those last blanks of understanding where everything was made, thanks to Webb,” said Andrew Levan of Radboud University in the Netherlands and the University of Warwick in the UK, lead author of the study.
While neutron star mergers have long been theorized as being the ideal “pressure cookers” to create some of the rarer elements substantially heavier than iron, astronomers have previously encountered a few obstacles in obtaining solid evidence.
Kilonovas are extremely rare, making it difficult to observe these events. Short gamma-ray bursts (GRBs), traditionally thought to be those that last less than two seconds, can be byproducts of these infrequent merger episodes. In contrast, long gamma-ray bursts may last several minutes and are usually associated with the explosive death of a massive star.
The case of GRB 230307A is particularly remarkable. First detected by NASA’s Fermi Gamma-ray Space Telescope in March, it is the second brightest GRB observed in over 50 years of observations, about 1,000 times brighter than a typical gamma-ray burst that Fermi observes. It also lasted for 200 seconds, placing it firmly in the category of long duration gamma-ray bursts, despite its different origin.
“This burst is way into the long category. It’s not near the border. But it seems to be coming from a merging neutron star,” added Eric Burns, a co-author of the paper and member of the Fermi team at Louisiana State University.
The collaboration of many telescopes on the ground and in space allowed scientists to piece together a wealth of information about this event as soon as the burst was first detected. It is an example of how satellites and telescopes work together to witness changes in the universe as they unfold.
After the first detection, an intensive series of observations from the ground and from space, including with NASA’s Neil Gehrels Swift Observatory, swung into action to pinpoint the source on the sky and to track its changing brightness at X-ray, optical, infrared, and radio wavelengths.
This radiation is usually powered by the interaction of the gamma-ray burst’s jet with the surrounding environment and is called the afterglow. Scientists found that the brightness of the afterglow was behaving in unexpected ways.
“None of our afterglow models made any sense,” said Laskar, who helped explore and rule out alternate models for the observed light. The radiation evolved quickly and became way too red, which are, instead, the hallmarks of a kilonova.
At later times it would have been impossible to study this kilonova from the ground, but these were the perfect conditions for Webb’s NIRCam (Near-Infrared Camera) and NIRSpec (Near-Infrared Spectrograph) instruments to observe this tumultuous environment. The spectrum has broad lines that show the material is ejected at high speeds, but one feature is clear: light emitted by tellurium, an element rarer than platinum on Earth.
The highly sensitive infrared capabilities of Webb helped scientists identify the home address of the two neutron stars that created the kilonova: a spiral galaxy about 120,000 light-years away from the site of the merger.
This image from Webb’s NIRCam (Near-Infrared Camera) instrument highlights GRB 230307A’s kilonova and its former home galaxy among their local environment of other galaxies and foreground stars. The neutron stars were kicked out of their home galaxy and traveled the distance of about 120,000 light-years, approximately the diameter of the Milky Way galaxy, before finally merging several hundred million years later. Image credit: NASA, ESA, CSA, STSCI, Andrew Levan (IMAPP, WARW)
Prior to their venture, they were once two normal massive stars that formed a binary system in their home spiral galaxy. Since the duo was gravitationally bound, both stars were launched together on two separate occasions: when one among the pair exploded as a supernova and became a neutron star, and when the other star followed suit.
In this case, the neutron stars remained as a binary system despite two explosive jolts and were kicked out of their home galaxy. The pair traveled approximately the equivalent of the Milky Way galaxy’s diameter before merging several hundred million years later.
Scientists expect to find even more kilonovas in the future due to the increasing opportunities to have space and ground-based telescopes work in complementary ways to study changes in the universe. For example, while Webb can peer deeper into space than ever before, the remarkable field of view of NASA’s upcoming Nancy Grace Roman Space Telescope will enable astronomers to scout where and how frequently these explosions occur.
“Webb provides a phenomenal boost and may find even heavier elements,” said Ben Gompertz, a co-author of the study at the University of Birmingham in the UK.
“As we get more frequent observations, the models will improve and the spectrum may evolve more in time. Webb has certainly opened the door to do a lot more, and its abilities will be completely transformative for our understanding of the universe.”
These findings have been published in the journal Nature.
Source: University of Utah
