GAMMA-RAY BURSTS · Probing star formation throughout the Universe with Gamma-Ray Bursts

Final Activity Report Summary - GAMMA-RAY BURSTS (Probing star formation throughout the universe with gamma-ray bursts)

Gamma-ray bursts (GRBs) are short-lived bursts of gamma-ray photons, the most energetic form of light. Lasting anywhere from a few milliseconds to several minutes, GRBs shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun, making them briefly the brightest source of cosmic gamma-ray photons in the observable Universe. GRBs are detected roughly once per day from random directions of the sky. In 1997, astronomers finally succeeded in finding the signature of a GRB at optical wavelengths. This was the smoking gun from the GRB itself and is known as the afterglow, a rapidly fading object seen at all wavelengths from radio to X-rays. Ground-based observations of this afterglow revealed its location in the distant early Universe. Today, a slew of satellite observations, follow-up ground-based observations, and theoretical work have allowed astronomers to link GRBs to very energetic supernovae (caused by the collapse of a massive star to a black hole) in distant galaxies. Due to their immense brightness, GRBs can be seen across great distances, even through the dust and debris that can exist within their host galaxies. This implies we can use GRBs to locate galaxies and stars at the very edge of the Universe. Since light can only travel at a finite speed, this also means we are exploring the infant Universe. Hence, GRBs can actually provide us with information when the first stars were born. This project has been extremely successful in systematically searching for these far away GRBs, and they use them to probe the very early Universe and the start of star formation. For instance, we have located and observed the highest redshift (measure of distance) GRB ever detected. It represents an epoch when the Universe was less than 1 billion years old. This was achieved with the aid of one of the largest telescopes in the world, the 8-meter Very Large Telescope (VLT), and the Swift satellite that is built to be fast. In fact, its name is not an acronym but a reference to the fact that it can move to GRB positions very quickly. We have also carried out the largest survey to date on GRB host galaxies using the VLT. Among the preliminary results is that hosts are mostly subluminous compared to galaxies randomly selected in the sky and contain very little dust. The majority of them are also actively forming stars. In addition, this survey provides a coherent sample for complementary Hubble Space Telescope studies and future James Webb Space Telescope observations. In a nutshell, this research has shed light on when the first stars were born, elucidated the distance distribution of GRBs, and characterised the GRB host galaxies.

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