Astronomers have succeeded in uncovering kilonova traces from the leftover glow of gamma-ray bursts that occur not too far from us. Gamma ray bursts, or SSGs, are large bursts or bursts of high-energy light that have been observed in distant galaxies. A gamma ray burst is an incredibly bright cataclysmic event in the Universe.
Long and Short
A gamma-ray burst is a transient event because the resulting explosion produces a sudden increase in brightness which then dims. SSG usually occurs for 10 milliseconds to several hours. However, the residual light from the burst can still be observed for weeks or months, although it is very faint and difficult to observe. The residual light of gamma rays can be observed at visible, X-ray, and radio wavelengths.
Based on its duration, SSG can be categorized into two types namely short duration and long duration SSG. Long duration SSGs usually last from two seconds to several minutes. Meanwhile, short duration SSGs last from a few milliseconds to two seconds!
Of the two types of gamma-ray bursts, long duration SSGs are more common and are thought to originate from the explosion of a massive star with a mass of more than 10 times the mass of the Sun. We know this huge explosion as a supernova. Not only supernovae, long duration SSGs can also be detected when hypernova explosions that are more powerful than supernovae occur. Meanwhile, short duration SSGs are thought to occur when two compact objects collide, such as in the collision of a pair of neutron stars or from a pair of neutron stars and black holes. These compact object collision events we know as kilonovas
Gamma Ray Bursts Near Earth
GRB 211211A. These gamma-ray bursts were identified on December 21, 2021 by the Burst Alert Telescope (BAT) attached to NASA's Neil Gehrels Swift Space Observatory and the Fermi Gamma Ray Telescope. The distance of these gamma-ray bursts is close. Only 1.14 billion light years. When observed, this GRB or SSG ended in 51.37 seconds. That means, GRB 211211A is a long duration gamma-ray burst that is close to Earth.
In general, the SSGs that died were either very far away or came from the young Universe. So ancient, it took the light from the gamma-ray bursts more than six billion years to reach Earth. The most distant, light bursts of gamma rays that began their journey nearly 13 billion years ago when the Universe was very young.
Kilonova's Light Trail
The close distance of GRB 211211A makes this event a target for observation to obtain more detailed information regarding the long duration SSG. Further observations were made with ground-based telescopes and space telescopes. Among them are the Gemini North Telescope in Hawaii and the Gemini South Telescope in Chile.
Observations with these two telescopes were carried out by two different teams of astronomers. The first was a team led by Jillian Rastinejad, Northwestern University, who made observations with the Gemini North telescope, the Nordic Optical Telescope (NOT), the Calar Alto Observatory, and the Karl Jansky Very Large Array (VLA). The second team was led by Eleonora Troja of the University of Rome Tor Vergata who observed the SSG remnant with the Gemini South telescope. The results of these two teams were quite surprising. The two of them discovered that the long duration of GRB 211211A did not come from a supernova explosion, but instead came from a kilonova event.
Observations by Jillian Rastinejad with the Gemini North telescope revealed the presence of infrared light at the SSG location. This result is the first strong evidence that GRB 211211A did not originate from a supernova but from a kilonova or macronova.
Kilonova's Fingerprint
Kilonova. The collision event of the pair of compact objects has its own fingerprint when observed. The fingerprint is in the difference in brightness when observed at infrared wavelengths with observations in visible light.
Observations made by Jillian Rastinejad and team showed a light source that dimmed rapidly three days after the eruption occurred. Further observations with the Gemini North telescope succeeded in detecting a source with a brightness of 22.4 magnitude in the K band, which indicates that a burst of gamma rays is brighter in the infrared than visible light.
This difference in brightness comes from the heavy elements ejected during the kilonova. The heavy elements block visible light but pass infrared light unhindered. Therefore, when Jillian Rastinejad and her team detected GRB 211211A in near-infrared light, they concluded that the SSG came from a kilonova.
Or more precisely, GRB 211211A is a gamma-ray burst from the collision of a pair of neutron stars that are currently merging!
Astronomers estimate that the combined pair of neutron stars has a mass of about 1.4 solar masses and that when the collision occurred, the two ejected about 0.02 solar masses of heavy elements.
Interestingly, this long duration SSG bears a resemblance to the mysterious long duration SSG without the presence of a previously detected supernova. Other observations with the Gemini South Telescope by Eleonora Troja and team came to the same conclusion. Two different teams made observations with two different telescopes and the results were the same. A kilonova, or collision event of two merging neutron stars, can produce long-duration gamma-ray bursts.
This evidence also gives a new paradigm to the theory that has been produced so far. Kilonova not only produces short duration SSGs, but also long duration ones.
Cool Facts
The long-duration SSG from a kilonova could be a new method for studying the formation of gold and other heavy elements in the Universe. Kilonova under extreme conditions can produce gold, platinum, and thorium. If so, astronomers could identify areas where heavy elements formed and look for kilonova trails after long duration gamma-ray bursts.
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