On the night of June 19-20, 2021, visible light from a source 10 billion light-years away from Earth was captured by three telescopes. Two of them – robotic telescopes – are operated by Czech institutions – the D50, located in Ondřejov, is managed by the Astronomical Institute of the Czech Academy of Sciences, while the other, FRAM-ORM, is located on the Spanish island of La Palma. The third telescope, Mini-MegaTORTORA, is installed in Nizhny Arkhyz, Russia. An international team has published a study of this extraordinary source in the May issue of Nature Astronomy.
The picture shows our idea of a gamma-ray burst. These are powerful bursts of gamma-ray energy that last less than a second, but can also last up to several minutes. In this short time, they release a huge amount of energy, making them the most energetic event in the universe. They are thought to be mostly associated with the explosion of stars that collapse into black holes. The explosion ejects two jets of very fast-moving material, as shown in the illustration. If the jet is heading towards Earth, we see a brief but powerful gamma-ray burst.
The gamma-ray burst (GRB) of June 19, 2021 is most likely of the type where a massive star reaches the final stage of its evolution and collapses under its own gravity. In the event of a collapse, it is thought to lead to the formation of a rapidly rotating black hole, which is then impinged upon by the material of the stellar envelope. This process causes the black hole to produce two strong, narrow jets that puncture the remaining stellar envelope. The jets last for tens of seconds, and observers on Earth have found that during this time they are the most energetic sources from space.
The light was captured only 28 seconds after the detection of the long gamma-ray burst GRB 210619B by the orbiting Swift satellite. This is one of the rare cases where visible light has been observed during and very shortly after one of these extremely bright gamma-ray bursts.
The optical light emission was so bright that it was seen with relatively small optical telescopes by research teams from two institutes of the Czech Academy of Sciences, the Institute of Physics and the Astronomical Institute, together with Italian colleagues from the Gran Sasso Science Institute and the National Institute for Astrophysics and scientists from the Special Astrophysical Observatory of the Russian Academy of Sciences.
"We have been searching intensively for these phenomena in the night sky for more than three decades, trying to understand what exactly causes these powerful gamma-ray bursts. That is, what the structure of the jet is and how its energy is converted into a gamma-ray burst. Our High Energy Astrophysics group at Ondřejov specialises in fast optical observations using robotic telescopes. The first telescope capable of responding immediately to gamma-ray burst alerts was put into operation in 1997 and since then we have been involved in several robotic telescope projects together with colleagues from the FZU," says Martin Jelínek, a researcher at the Astronomical Institute of the Czech Academy of Sciences, who has been studying these events for more than 20 years.
Capturing a gamma-ray burst that lasts only a few seconds is an extremely challenging task. Most of the findings so far are based on data ranging from hundreds of seconds to days after the explosion. What has really been lacking is the capture of multi-band emission during gamma-ray bursts, which has now been achieved thanks to the extremely fast response of small robotic telescopes. "They allowed us to observe the evolution of the GRB emission in different colours and to search for a common optical-gamma-time structure of the burst. To our surprise, while the gamma rays showed multiple bursts with second-long duration, the optical emission was smooth and simply faded with time," commented Sergey Karpov, a researcher at the Institute of Physics of the Czech Academy of Sciences, who performed a multicolour analysis for the paper in Nature Astronomy.
The measurements confirmed the correctness of the theory about one type of optical counterpart to gamma-ray bursts formulated by Peter Mészáros and Martin Rees in the late 1990s. "Our detailed modelling of this optical flash, together with X-ray and high-energy data provided by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory, allowed us to discover an extremely fast and magnetized jet expanding into a surprisingly low-density environment. These conditions are ideal for producing such a bright optical flash," explained Om Sharan Salafia, researcher at the National Institute for Astrophysics and co-author of the study.
The use of optical telescopes capable of responding as fast, or even faster, than those that helped produce the results of the study could in the future contribute to an even better understanding of the entire process of gamma-ray burst formation and its optical counterpart.
Gamma-ray bursts that last only a few seconds are routinely detected by space observatories such as the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. These brief flashes are produced by two different types of turbulent astrophysical processes: the extinction of rapidly rotating massive stars or the merger of two very compact objects known as neutron stars, which are the size of, say, a regional city but have a mass greater than our Sun.
Source: Gran Sasso Science Institute