A team of scientists has used multiple space- and ground-based telescopes, including the NASA/ESA/CSA James Webb Space Telescope, to observe an exceptionally bright gamma-ray burst, GRB 230307A, and identify the neutron star merger that generated the explosion that created the burst. Webb also helped scientists detect the chemical element tellurium in the aftermath of the explosion.
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.
"Just over 150 years since Dmitri Mendeleev wrote down the periodic table of elements, we are now finally in a 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 United Kingdom, lead author of the study.
While neutron star mergers have long been theorised as being the ideal 'pressure cookers' to create some of the rarer elements substantially heavier than iron, astronomers have previously encountered a few obstacles to 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 1000 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 detected. It is an example of how satellites and telescopes work together to witness changes in the Universe as they unfold.
After the initial detection, an intensive series of observations from the ground and from space swung into action to pinpoint the source on the sky and track how its brightness changed. These observations in the gamma-ray, X-ray, optical, infrared, and radio showed that the optical/infrared counterpart was faint, evolved quickly, and became very red - the hallmarks of a kilonova.
"This type of explosion is very rapid, with the material in the explosion also expanding swiftly," said Om Sharan Salafia, a co-author of the study at the INAF Brera Observatory in Italy. "As the whole cloud expands, the material cools off quickly and the peak of its light becomes visible in the infrared, and becomes redder on timescales of days to weeks."
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.
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 travelled 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 thanks to the increasing number of opportunities to have space and ground-based telescopes working in complementary ways to study changes in the Universe.
"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 United Kingdom. "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."
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 region of the spectrum 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.
Though astronomers have theorised neutron star mergers to be the ideal environment to create chemical elements, including some that are essential to life, these explosive events - known as kilonovas - are rare and rapid. Webb's NIRSpec (Near-Infrared Spectrograph) acquired a spectrum of GRB 230307A's kilonova, helping scientists secure evidence of the synthesis of heavy elements from neutron star mergers.
With Webb's extraordinary ability to look further into space than ever before, astronomers expect to find even more kilonovas and acquire further evidence of heavy element creation.
The spectrum is plotted as a line graph of brightness versus wavelength of light (microns). The spectral lines range in wavelength of light along the x-axis, with the first tic labelled as '1.0' and the last tic labelled as '5.0', and in brightness, with the level of brightness becoming greater moving higher along the y-axis. The Webb spectral line is white and jagged. About a third of the way across the graph, there is a distinct peak between 2.0 and 2.5 microns.
After 2.5 microns, the spectral line slopes gradually up to the right. The model spectral line is red and smoother than the Webb data. The model's spectral line at 1.0 micron begins low (dim) and flat before peaking between 2.0 and 2.5 microns, similar to the Webb data. The area below the model spectral line is shaded red and labelled "Tellurium T E." The model spectral line then descends after 2.5 microns and follows the general trend of the Webb data.
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